Vacuum workbed

Information

  • Patent Grant
  • 6322265
  • Patent Number
    6,322,265
  • Date Filed
    Thursday, April 8, 1999
    25 years ago
  • Date Issued
    Tuesday, November 27, 2001
    22 years ago
Abstract
A wide format thermal printer for printing a multicolor graphic product on a printing sheet; a vacuum workbed for supporting a sheet material for performing work operations, such as cutting, printing or plotting, thereon; a replaceable donor sheet assembly, which includes a memory, for use with a thermal printer; methods and apparatus for improved thermal printing, including methods and apparatus for conserving donor sheet and reducing the amount of time required to print a multicolor graphic product; a thermal printhead including a memory; methods and apparatus for the alignment of a sheet material for printing or performing other work operations on the sheet material; and methods and apparatus for controlling the tension of the donor sheet during printing with a wide format thermal printer. The wide format thermal printer can include provision for the automatic loading of cassettes of donor sheet from a cassette storage rack. The vacuum workbed can include provision for determining the size of the sheet material supported by the workbed, and for controlling the suction applied to the apertures in a worksurface of the workbed.
Description




BACKGROUND OF THE INVENTION




The present invention relates to methods and apparatus for printing a graphic product on sheet material in accordance with a printing program and stored data representative of the graphic product, and more particularly to methods and apparatus for printing a wide format multicolor graphic product on a printing sheet, such as a vinyl sheet for use as signage.




Known in the art are thermal printing apparatus for generating signs, designs, characters and other graphic products on a printing sheet in accordance with a printing program and data representative of the graphic product. Typically, a thermal printer interposes a donor sheet that includes donor material and a backing between a thermal printhead and the printing sheet. The thermal printhead includes an array of thermal printing elements. The thermal printhead prints by pressing the donor sheet against the printing sheet and selectively energizing the thermal printing elements of the array, thereby selectively transferring pixels of donor medium from the donor sheet to the printing sheet. Movement of the printing sheet relative to the thermal printhead (or vice versa) while pressing the donor sheet against the printing sheet with the thermal printhead draws fresh donor sheet past the thermal printhead. The printing sheet typically includes a vinyl layer secured to a backing layer by a pressure sensitive adhesive so that after printing the vinyl bearing the graphic product can be cut and stripped from the backing material and affixed to an appropriate sign board or other material for display.




The proper printing of many graphic products, such as commercial artwork or signage, can require high quality print work. Often, it is desired that the final multicolor graphic product be physically large, such as several feet wide by tens of feet long. Typically, existing thermal printers are limited in the width of printing sheet that they can print upon. For example, one popular thermal printer prints on sheets that are one foot wide. Accordingly, the final graphic product is often assembled from separately printed strips of printing sheet that must be secured to the signboard in proper registration with one another. Often, the registration is less than perfect and the quality of the final graphic product suffers, especially when backlit.




Wide format thermal printers are known in the art. For example, one wide format thermal printer currently available can accommodate a printing sheet up to three feet wide and uses four full width (i.e., three feet wide) printheads, each interposing a different color donor sheet between the printhead and the printing sheet. Accordingly, far fewer seams, if any at all, require alignment when creating the sign or other product. Also, the use of four printheads allows faster printing of the multicolor graphic product.




Unfortunately, this type of machine can be expensive to manufacture and to operate. For example, each printhead, at a typical resolution of 300 dpi, includes literally thousands of thermal printing elements, all of which are typically required to have resistances that are within a narrow tolerance range. Such a thermal printhead is difficult and expensive to manufacture, and moreover, burnout of simply a few thermal printing elements can require replacement of the entire printhead. Furthermore, donor sheet is also expensive, and the full-width printing heads can be wasteful of donor sheet when printing certain types of, or certain sections of, graphic products. For example, consider that a single color stripe one inch wide and perhaps a foot long is to be printed in center of the printing sheet. Though the printed object occupies {fraction (1/12)} of a square foot, an area of donor sheet that is three feet wide by one foot long, or three square feet, is transferred past the print head when printing the above object, and hence consumed. The printing of a wide format graphic product that includes a narrow border about the periphery of the printing sheet is another example that typically can be wasteful of donor sheet when printing with the above wide format thermal printer.




Other wide format printers are known in the art, such as wide format ink-jet printers, which can also print in a single pass. However, inkjet printed multicolor graphic products are typically not stable when exposed to the elements (e.g., wind, sun, rain) or require special post-printing treatment to enhance their stability, adding to the cost and complexity of printing with such apparatus.




Accordingly, it is an object of the present invention to address one or more of the foregoing and other deficiencies and disadvantages of the prior art.




Other objects will in part appear hereinafter and in part be apparent to one of ordinary skill in light of the following disclosure, including the claims.




SUMMARY OF THE INVENTION




In one aspect, the invention provides a vacuum workbed for supporting a sheet material to be worked upon. The vacuum workbed includes the following: a workbed having a worksurface for supporting the sheet material, the worksurface including a plurality of apertures for applying suction to the sheet material, the apertures separated into first and second zones for accommodating sheet material of different sizes and orientations; a suction source for applying suction to the apertures; a manifold for providing fluid communication between the suction source and the apertures for applying the suction thereto; and a sensor in fluid communication with the suction source for providing a signal responsive to the degree of vacuum drawn by the suction source on the apertures. The flow rate through one of the zones of apertures is restricted for producing a greater than nominal degree of vacuum when the one zone includes unblocked apertures.




In another aspect, the invention provides a vacuum workbed for supporting a sheet material to be worked upon. The vacuum workbed includes the following: a workbed having a worksurface for supporting the sheet material, where the worksurface includes a plurality of apertures separated into a plurality of zones; a suction source for applying suction to the apertures; a first manifold for providing fluid communication between the suction source and a first group of zones; and a second manifold for providing fluid communication between the suction source and a second group of zones. The first and second groups include at least one zone each. The vacuum workbed of the invention also includes a sensor in fluid communication with the suction source for providing a signal responsive to the degree of vacuum drawn by the suction source on the apertures, a first flow control valve fluidly interposed between the first group and the suction source, and a second flow control valve fluidly interposed between the second group and the suction source. The first flow control valve is fluidly interposed between the second flow control valve and the suction source.




The first group can include first and second zones and the second group can include third, fourth and fifth zones. The workbed can include first and second flow restriction elements interposed between the first and second zones, respectively, and the suction source, and third, fourth and fifth flow restriction elements, interposed, respectively, between the third, fourth and fifth zones and the suction source. The flow restriction elements are for providing a selected flow rate through the zones of apertures when unblocked.




In yet a further aspect, the invention provides a method of automatically determining the size or orientation of a sheet material supported by a workbed having suction apertures therein. The method includes the following steps: a) grouping the apertures into N groups of apertures; b) applying suction to one of the groups of apertures; c) incrementing the number of groups to which suction is applied by applying suction to an additional group and sensing the difference in the degree of vacuum attained between the application of suction prior to and subsequent to incrementing the number of groups; d) determining from the difference whether the additional group includes unblocked apertures; and when determining in the prior step that the additional group does not include unblocked apertures, repeating steps c) and d) until one of: a determination is made in step d) that the additional group does include unblocked apertures; and no groups remain.




In an additional aspect, the invention provides a method of supporting sheet materials of varying sizes for performing work operations thereon. The method includes the following steps: providing a workbed having a worksurface for supporting the sheet material, the worksurface including first and second groups of apertures; applying suction to the apertures; sensing a selected number of times the degree(s) of vacuum attained during the step of applying suction and providing a selected number of signals responsive to the degree(s) of vacuum; and determining from the selected number of signals one of the following: that all apertures are blocked; that a first group of apertures is blocked and a second group of apertures includes unblocked apertures; and that both first and second groups of apertures include unblocked apertures.











BRIEF DESCRIPTION OF THE DRAWINGS





FIG. 1

illustrates one embodiment of a wide format thermal printer according to the invention.





FIG. 2

illustrates one embodiment of the printhead carriage of the wide format thermal printer of FIG.


1


.





FIG. 3

is a perspective view of the cassette storage rack of the wide format thermal printer of FIG.


1


and of a donor sheet cassette mounted on the rack.





FIG. 4A

is a cutaway view of the upper portion of the wide format thermal printer of

FIG. 1

, including a front elevational view of the printhead carriage of FIG.


2


.





FIG. 4B

is side elevational view of the donor sheet handling apparatus, including a cassette receiving station, for slidably mounting to the base structure of the printhead carriage of FIG.


2


.





FIG. 5

is a top view of the wide format thermal printer of

FIG. 1

showing the work surface, the printhead carriage of

FIG. 2

, one of the magnetic clamps and the cassette storage rack including four (4) cassette storage trays.





FIGS. 6A and 6B

, illustrate cross-sectional and end views, respectively, of one of the magnetic clamps, including the keeper, of the wide format thermal printer of FIG.


1


.





FIG. 7

illustrates a top view of the work surface of the workbed of the wide format thermal printer of

FIG. 1

showing suction apertures in the worksurface for selectively securing the printing sheet to the worksurface.

FIG. 7

is drawn as if the workbed is transparent such that the apparatus below the workbed is readily visible.





FIG. 8

illustrates suction apparatus for selectively applying suction to the suction apertures in the worksurface illustrated in FIG.


7


.





FIGS. 9A and 9B

schematically illustrate alternative embodiments of he apparatus illustrated in

FIGS. 7 and 8

.





FIG. 10A

illustrates a donor sheet assembly for loading into the donor sheet cassette shown in FIG.


3


.





FIG. 10B

illustrates a front view of the donor sheet assembly of FIG.


10


A.





FIG. 11A

illustrates the supply core tubular body of the donor sheet assembly of

FIGS. 10A and 10B

.





FIG. 11B

is an enlarged view of the drive end of the supply core tubular body shown in FIG.


11


A.





FIG. 11C

is an end view of the supply core tubular body of

FIG. 11A

, taken along line C—C in FIG.


11


A.





FIG. 11D

is an end view of the supply core tubular body of

FIG. 11A

, taken along the line D—D in FIG.


11


A.





FIG. 12

is a front view of the donor sheet cassette of

FIG. 3

with the cover removed.





FIGS. 13A and 13B

show front and side views, respectively, of the donor sheet cassette cover of the donor sheet cassette of FIG.


12


.





FIG. 14

illustrates the donor sheet cassette cover of

FIGS. 13A and 13B

mounted to the donor sheet cassette of FIG.


12


.





FIG. 15A

illustrates method and apparatus for more economically providing donor sheet to the wide format thermal printer of FIG.


1


and for reducing the cost of printing a given multicolor graphic product.





FIG. 15B

is a flow chart illustrating one sequence for reading data from and writing data to the memory element mounted with core tubular body of FIGS.


11


.





FIG. 16A

illustrates the edge of the printing sheet when the printing sheet is skewed relative to the printing sheet translation (X) axis of the wide format thermal printer of FIG.


1


.





FIG. 16B

illustrates the effect of translating the skewed printing sheet of

FIG. 16A

in one direction along the printing sheet translation (X) axis.





FIG. 16C

illustrates the effect of translating the skewed printing sheet of

FIG. 16A

in the opposite direction along the printing sheet translation (X) axis.





FIGS. 17A and 17B

show top and elevational views, respectively, of selected components of the wide format thermal printer of

FIG. 1

, and illustrate an edge sensor and a reflective strip for detecting the location of the edge of the printing sheet shown in

FIGS. 16A-16C

.





FIG. 17C

illustrates one technique for determining the skew of the printing sheet from measurements made with the edge sensor of

FIGS. 17A and 17B

.





FIG. 18

illustrates selective actuation of the translatable clamps of the translatable clamp pair of the wide format printer for aligning the printing sheet.





FIG. 19A

illustrates a side elevational view of a printhead assembly of the present invention.





FIG. 19B

illustrates of view of the printhead assembly of

FIG. 19A

taken along line


19


B—


19


B of FIG.


19


A.





FIG. 20

illustrates the technique of Y axis conservation for reducing the amount of donor sheet consumed by the wide format thermal printer of the present invention.





FIGS. 21A and 21B

illustrate alternative techniques for printing with the wide format printer of the present invention, where

FIG. 21B

illustrates the technique of X axis conservation for consuming less donor sheet than the technique of FIG.


21


A.





FIG. 22A

illustrates two banners to be included in the multicolor graphic product printed by the wide format thermal printer of the present invention.





FIG. 22B

illustrates textual objects to be included with the banners of

FIG. 22A

in the multicolor graphic product to be printed by the wide format printer of the present invention.





FIG. 22C

illustrates the placement of textual objects of

FIG. 22B

over the banners of

FIG. 22A

in the multicolor graphic product such that portions of the banners are “knocked out.”





FIG. 22D

illustrates one of the banners of

FIG. 22C

including those “knocked out” portions that are not printed when printing the banner.





FIG. 23

illustrates a technique for printing with the wide format thermal printer for reducing the time it takes to print a multicolor graphic product on the printing sheet.





FIG. 24A

is a flow chart illustrating one data processing technique for determining those objects of the multicolor graphic product that are part of a selected color plane and for generating print slices corresponding to the selected objects.





FIG. 24B

is a flow chart illustrating one data processing technique for combining the print slices in accordance with the flow chart of FIG.


24


A.





FIG. 25A

is a flow chart illustrating additional steps, including selecting the direction of translation of the printing sheet for reducing the time for printing the multicolor graphic product in accordance with FIG.


23


and for dividing the print swipes into print swaths.





FIG. 25B

is a flow chart illustrating additional steps including a technique for processing data so as to refrain from printing the knocked-out areas of

FIGS. 22A-22D

.





FIG. 25C

is a flow chart indicating the printing of the selected color plane on the printing sheet in print swaths, including performing the Y axis, conservation shown in

FIG. 20

for each print swath.





FIG. 26

is a flow chart illustrating one procedure for processing data in accordance with the flow chart of

FIG. 25C

to create subswaths for performing the Y axis donor sheet conservation illustrated in FIG.


20


.





FIG. 27A

illustrates an example of a multicolor graphic product to be printed by the wide format thermal printer of the present invention.





FIG. 27B

illustrates the creation of bounding rectangles around those objects of the multicolor graphic product of

FIG. 27A

which are to be printed in the selected color plane.





FIG. 27C

illustrates combining two slices, which correspond to the bounding rectangles of

FIG. 27B

, to form a combined slice.





FIG. 27D

illustrates combining the combined slice of

FIG. 27C

with another slice of

FIG. 27C

to form a combined slice.





FIG. 27E

illustrates combining the combined slice of

FIG. 27D

with another slice of

FIG. 27D

to form a combined slice.





FIG. 27F

illustrates increasing the width of the combined slice of

FIG. 27E

to be an integral number of printing widths of the thermal printhead of the wide format thermal printer of the present invention.





FIG. 27G

illustrates combining the slice of

FIG. 27F

having the increased width with another slice of

FIG. 27F

to form a combined slice.





FIG. 27H

illustrates dividing the slices of

FIG. 27G

into print swaths.





FIG. 27I

illustrates counting consecutive blank rows in one of the print swaths of

FIG. 27I

in accordance with the flow chart of FIG.


26


.





FIG. 27J

illustrates the formation of sub swaths as result of the counting of the consecutive blank rows in FIG.


27


I and in accordance with flow chart of FIG.


26


.





FIG. 28

is a flowchart illustrating the steps followed to energize the take-up motor and the brake to provide a selected tension on the donor sheet.





FIG. 29

schematically illustrates in

FIGS. 29A and 29B

one example of the on board controller


22


A and the interfacing of the on board controller


22


A with other components of the wide format printer


10


.











DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS





FIG. 1

illustrates one embodiment of a wide format thermal printer


10


according to the invention. The wide format thermal printer


10


includes a base structure


12


that supports a workbed having a work surface


14


for supporting a printing sheet


16


onto which a multicolor graphic product is to be printed. A guide surface


20


can be provided for guiding the printing sheet


16


as it travels from the printing sheet supply roll


17


to the work surface


14


. A printing sheet drive motor, indicated generally by reference numeral


18


, can be provided at the other end of the printing sheet supply roll


17


for rotating the printing sheet supply roll


17


. The wide format thermal printer


10


prints the multicolor graphic product onto the printing sheet


16


in separate color planes and responsive to a controller(s), such as the non-board controller


22


A, and responsive to machine readable data representative of the graphic product. The machine readable data can be stored either on the on-board controller


22


A or on additional controllers (not shown in

FIG. 1

) located remote to the wide format thermal printer


10


and in communication with the on-board controller


22


A. Reference numeral


22


is used herein to generally refer to the controller(s), whether on-board or otherwise, associated with the wide format thermal printer


10


. The printing sheet


16


exits the printer


10


at the other end of the work surface


14


.




The wide format thermal printer


10


prints each color plane by interposing a section of a donor sheet (not shown in

FIG. 1

) corresponding to the color of the section of donor sheet interposed between the thermal printhead


24


and the printing sheet


16


. The multicolored graphic product is printed on the printing sheet


16


in individual print swaths, as indicated by reference numeral


28


, that extend along a print axis, also referred to herein as the “Y-axis”, and have a selected printing width, or swath width, along a printing sheet translation axis, also referred herein as the “X-axis”. The print (Y) axis and the printing sheet translation (X) axis define a plane substantially parallel to the plane of the work surface


14


of the workbed. The thermal printhead


24


presses the striction of donor sheet against the printing sheet


16


and selectively energizes an array of thermal printing elements


26


, which extends along a printing sheet translation (X) axis, as the thermal printhead


24


is translated in the direction of the print (Y) axis. The array of thermal printing elements is energized responsive to the machine readable data and the controller(s)


22


.




A printhead carriage


30


mounts the thermal printhead


24


and includes a cassette receiving station for receiving a cassette


32


of the donor sheet. The cassette


32


includes a supply roll of donor sheet, typically including a supply length of donor sheet wound on a supply core tubular body, and a take-up roll for receiving the donor sheet after it has been interposed between the thermal printhead


24


and the printing sheet


16


. The take-up roll includes the consumed length of donor sheet wound on a take-up core tubular body.




The printing drive motor


36


translates the printhead carriage


30


, and hence the thermal printhead


24


, in the direction of the print (Y) axis by rotating the printhead ball screw


38


. The printhead guide rails


40


guide the thermal printhead


24


as it travels in the direction of the print (Y) axis. A pair of translatable clamps, indicated generally by reference numeral


42


, translate the printing sheet


16


in the direction of the printing sheet translation (X) axis between the printing of print swaths such that adjacent print swaths align to print a color plane of the multicolor graphic product. The first and second clamps,


44


and


46


respectively, are each movable between clamped and unclamped conditions relative to the printing sheet


16


supported on the work surface


14


and each extend from a first end


50


to a second end


52


across the work surface


14


and parallel to the print (Y) axis. The print swath


28


shown as being printed in

FIG. 1

extends parallel to the print (Y) axis in an area between the clamps


44


and


46


.




The clamp pair fixture


54


A mechanically couples the first ends


50


of the clamps


44


and


46


to one another such that the clamps


44


and


46


are substantially fixedly spaced from one another in the direction of the printing sheet translation (X) axis. A guide rod


56


supports and guides the clamp pair fixture for translation in the direction of the printing sheet translation (X) axis. The clamp actuator


58


is coupled to the clamp pair fixture


54


A via the ball screw


60


for rotating the ball screw and translating the clamp pair


42


parallel to the printing sheet translation (X) axis. The second ends of the clamps


52


are also mechanically coupled by a clamp pair fixture supported by a guide rod (both not shown in FIG.


1


). An additional actuator may be provided for translating the second ends


52


of the clamps


44


and


46


independently of the first ends


50


of the clamps


44


and


46


. Independent translation of the first and second ends of the clamps can be particularly advantageous when aligning the printing sheet


16


to the work surface


14


, as discussed in more detail below.




In the process of printing a particular color plane on the printing sheet


16


, the clamp pair


42


reciprocates back and forth in the direction of the printing sheet translation (X) axis between first and second positions. For example, after the thermal printhead


24


prints a print swath, the clamp pair


42


clamps the printing sheet


16


and moves to a second position to translate the sheet a distance typically equal to the width of one print swath


28


. The clamp pair


42


then returns to its original position so as to be ready to translate the printing sheet


16


again after the next swath is printed. The thermal printhead is then translated in the direction of the print (Y) axis and prints the next swath. The above cycle repeats until a complete color plane is printed on the printing sheet. Preferably, only one clamp of the clamp pair


42


clamps the printing sheet at time, and the printing sheet


16


is pulled by the clamp pair


42


rather than pushed. For example, when translating the printing sheet away from the supply roll


17


, the clamp


44


is in the clamped condition for clamping the printing sheet


16


and the clamp


46


is in the unclamped condition. If translating the printing sheet


16


in the opposite direction from that described above, the clamp


46


clamps the printing sheet and the clamp


44


is in the uncamped condition.




According to the invention, the wide format printer


10


can print the multicolor graphic product on the printing sheet


16


by translating the printing sheet in both directions in the direction of the printing sheet translation (X) axis. For example, when printing one color plane, the translatable clamp pair


42


translates the printing sheet in one direction along the printing sheet translation (X) axis between successive print swaths, and when printing a different color plane, the translatable clamp pair can translate the printing sheet


16


in the opposite direction between successive print swaths. Additionally, it can be advantageous to translate the printing sheet in both directions along the printing sheet translation axis when printing a single color plane. For example, one portion of the color plane can be printed by translating the printing sheet in one direction along the printing sheet translation (X) axis between successive print swaths and another portion printed by translating the printing sheet in the opposite direction between successive print swaths.




Prior art printers that print in separate color planes often avoid printing in both directions due to the difficulty of providing proper registration between the color planes. One technique known in the art is to print a registration mark at one end (along the printing sheet translation (X) axis) of the printing sheet, and print each color plane starting at that registration mark and proceeding towards the opposite end of the printing sheet. Thus the printing sheet must be “rewound” between successive color planes so that the printing of the next plane can also start at the registration mark. The present invention advantageously allows printing in both directions, avoiding the need to “rewind” the printing sheet.




The wide format thermal printer


10


also includes apparatus (not shown) for securing the printing sheet


16


to the work surface


14


of the workbed when printing on the printing sheet


16


and releasing the printing sheet


16


from the work surface


14


when translating the printing sheet


16


in the printing sheet translation (X) axis. Such apparatus for securing the printing sheet can include suction apertures formed in the work surface


14


of the workbed and a suction source coupled to the suction apertures for applying suction to the printing sheet


16


, and/or, as understood by one of ordinary skill in the art, electrostatic apparatus or mechanical clamps for clamping the printing sheet


16


to the work surface


14


. The preferred apparatus for securing the printing sheet is described in more detail below.




The wide format printer can include a cassette storage rack


55


for storing cassettes


32


that are not in use. The cassette storage rack


55


extends generally parallel to the print (Y) axis and can mount a plurality of donor sheet cassettes


32


in a row. As discussed in more detail below, the cassette receiving station of the printhead carriage


30


can include a translatable engaging element for engaging a donor sheet cassette


32


stored on the cassette storage rack


55


and transporting the cassette


32


between the cassette receiving station and the cassette storage rack


55


. The printhead carriage


30


includes donor sheet handling apparatus for, in conjunction with the cassette


32


, interposing a section of the donor sheet between the thermal printhead


24


and the printing sheet


16


supported by the work surface


14


. The cassette storage rack


55


can include donor sheet cassettes


32


that include spot color donor sheet, such that the wide format printer of the present invention can advantageously print an enhanced multicolor graphic product by easily incorporating both spot and process colors into the final printed multicolor graphic product.




The wide format thermal printer


10


can also include a user interface


61


for controlling the basic operating functions of the printer


10


. Typically, however, the printer


10


is controlled from a remote controller


22


, e.g., a workstation, that communicates with the on-board controller


22


A. Preferably the wide format thermal printer also includes squeegee bars


62


(only one of which can be shown in

FIG. 1

) for pressing against the printing sheet


16


for cleaning the printing sheet


16


and for providing a selected drag on the printing sheet


16


when the sheet


16


is translated in the direction of the printing sheet translation (X) axis. The squeegee bars can include brushes


63


that can be electrically grounded for dissipating static charge. Typically, the squeegee bars are operated by actuators (not shown), such as solenoids, that are controlled by the controller(s)


22


for selectively lifting the squeegee bars


62


away from the printing sheet material. The other squeegee bar is typically located at the opposite end (in the direction of the printing sheet translation (X) axis) of the work surface


14


, and each includes an independently controllable actuator.




Preferably, the printing sheet


16


forms a hanging loop


64


between the printing sheet and the guide surface


20


. The hanging loop


64


helps maintain proper tension on the printing sheet


16


, such that it is properly translated by the translatable clamp pair


42


. The hanging loop optical sensor


66


sensing the presence of a proper hanging loop


64


and a printing sheet supply roll motor


16


(not shown) responsive to the hanging loop optical sensor


66


, rotates the printing sheet supply roll


17


accordingly to maintain the proper hanging loop




For simplicity, the wide format printer


10


and its various components, such as the printhead carriage


30


, the donor sheet cassette


32


, and the cassette storage rack


55


, are indicated very generally and schematically in FIG.


1


. The ensuing description and FIGURES provide additional detail and description of the wide format printer


10


, and in particular of the printhead carriage


30


and the donor sheet cassette


32


.





FIG. 2

illustrates a preferred embodiment of the printhead carriage


30


. The printhead carriage


30


includes a base structure


68


that receives the printhead guide rails


40


and the printhead ball screw


38


for translation of the base structure


68


parallel to the print (Y) axis. The base structure


68


pivotably mounts a cantilever arm


72


for pivoting about a pivot pin


70


that extends along a pivot axis that is generally parallel to the printing sheet translation (X) axis and perpendicular to the print (Y) axis. A second pivot pin


76


couples the pivot actuator


74


to the base


68


and to the other end


78


of the cantilever arm


72


. The pivot actuator


74


is typically a stepper motor that rotates a lead screw


80


that is received by the threaded nut


82


. The threaded nut


82


attaches to a support


99


that defines a slot


88


for engaging a pin


90


coupled to the end


78


of the cantilever arm


72


. A bias spring


92


is inserted between the end


78


of the cantilever arm


72


and an upper surface of the support


86


. The cantilever arm


72


mounts the thermal printhead


24


. The pivot actuator


74


raises and lowers the printhead by pivoting the cantilever arm


72


. The bias spring


92


allows the pivot actuator


74


selectively advance the lead screw


80


, after the printhead


24


has contacted the printing sheet


16


, for pressing the donor sheet between the thermal printhead


24


and the printing sheet


16


with a selected pressure.




The base structure


68


mounts a donor sheet handling apparatus


94


that includes a cassette receiving station


96


. The cassette receiving station


96


includes a take-up shaft


100


and take-up shaft drive elements


102


rotationally coupled to a take-up drive motor


104


. The supply shaft


106


includes supply shaft drive elements


108


that are rotationally coupled to a magnetic brake (not shown) mounted behind the cassette receiving station


96


.




The cassette receiving station


96


is adapted for receiving a donor sheet cassette


32


, such that a section of the donor sheet threaded between supply and take-up rolls of the cassette is positioned under the thermal printhead


24


for being interposed between the printhead


24


and the printing sheet


16


. The supply shaft and take-up shaft drive elements


108


and


102


engage drive elements mounted with the donor sheet cassette


32


and are rotationally coupled to the supply and take-up rolls of the donor sheet cassette


32


. One of ordinary skill in the art, apprised of the disclosure presented herein, understands that the present invention can be practiced by manually loading a donor sheet cassette


32


onto the cassette receiving station


96


. That is, a donor sheet cassette


32


would be selected from the cassette storage rack


55


, which need not be mounted on the wide format thermal printer


10


, and the cassette placed onto the receiving station


96


for printing the color plane of the multicolor graphic product corresponding to the color of the donor sheet mounted within the cassette


32


. Furthermore, one of ordinary skill in the art also understands that the supply and take-up rolls of donor sheet can be mounted directly on the take-up and supply shafts,


100


and


106


, respectively, and appropriate guide apparatus, such as pins, arranged with the cassette receiving station


96


, for aiding in interposing the donor sheet between the thermal printhead


24


and the printing sheet


16


.




However, one of the advantages of the present invention is that it can provide for relatively unattended printing of several or all of color planes of the multicolor graphic product. Accordingly, provision is made for the automatic loading and unloading of donor sheet cassettes


32


to and from the cassette storage rack


55


. The cassette receiving station


96


mounts a cassette transport apparatus


112


that extends from the receiving station


96


toward the cassette storage rack


55


. The cassette transport apparatus


112


includes a translatable engaging element


114


that can be translated to the far end of the cassette transport apparatus


112


for engaging a donor sheet cassette


32


stored on the cassette storage rack


55


. The engaging apparatus


114


is carried by a toothed drive belt


116


that is mounted by a belt support bed


118


. The belt drive motor


120


is coupled to the toothed drive belt


116


for moving the toothed drive belt


116


about the belt support bed for translating the engaging tab


114


away and toward the cassette receiving station


96


.




The base structure


68


slidably mounts the cassette receiving station


96


via a pair of slides, one of which is visible in FIG.


2


and indicated by reference numeral


122


. The cassette receiving station


96


can thus slide up and down in the direction of the Z axis, as indicated by the arrows


124


. To move the cassette receiving station


96


upward, the pivot actuator


74


pivots the cantilever arm


72


upward such that the cantilever arm


72


contacts the cassette receiving station


96


. Further movement of the cantilever arm


72


upward by the pivot actuator


74


then moves the cassette receiving station


96


upward along the slides, such as slide mount


122


, moving the belt support bed


118


upward. As a result of this upward movement, when the cassette engaging element


114


is at the end of the belt support bed


118


and is correctly positioned, along the print (Y) axis, under a donor sheet cassette


32


on the cassette storage rack


55


, the cassette engaging element


114


engages that donor sheet cassette


32


.




To retrieve a donor sheet cassette


32


and mount the cassette onto the cassette receiving station


96


, the printing drive motor


36


is instructed to drive the printhead carriage


30


such that it is opposite a selected donor sheet cassette


32


stored on the cassette storage rack


55


. The belt drive motor


120


then drives the toothed drive belt


116


to translate the translatable engaging element


114


to the end of the belt support bed


118


, such that the translatable engaging element


114


is positioned under a donor sheet cassette


52


. Next, the pivot actuator


74


pivots the cantilever arm


72


upward such that the cantilever arm


72


contacts and drives the cassette receiving station


96


upward so that the translatable engaging element


114


engages a notch in the donor sheet cassette


32


. The belt drive motor


120


then drives the toothed drive belt


116


in the opposite direction, such that the donor sheet cassette


32


is drawn towards the cassette receiving station


96


. As the donor sheet cassette


32


is drawn towards the cassette receiving station


96


, the shaft drive elements


102


and


108


are slightly rotated so that they properly engage drive elements mounted with the donor sheet cassette


32


. The belt drive motor


120


thus pulls the donor sheet cassette towards the cassette receiving station


96


until it is properly mounted with the station and engages the shaft drive elements


102


and


108


. The procedure is reversed for returning a donor sheet cassette


32


to the cassette storage rack


55


.




After retrieving a selected donor sheet cassette


32


, the pivot actuator


74


lowers the cantilever arm


72


such that the printhead


24


presses a section of the donor sheet against the printing sheet


16


supported by the work surface


14


. Stops are included for limiting the downward travel of the cassette receiving station


96


.




Note that the cantilever arm


72


can include provision for cooling the thermal printhead


24


. The cantilever arm


72


can mount a blower


126


that draws air into the cantilever arm


72


, as indicated by reference numeral


128


. Internal cavities in the arm channel the air towards the printhead


24


, as indicated by reference numeral


130


. The air then exits the cantilever arm


72


, as indicated by reference numerals


132


, after being blown over cooling fins


133


, which are in thermal communication with the thermal printhead


24


. Additional detail on thermal printhead


24


and the thermal management thereof is given below.





FIG. 3

is a perspective view of the cassette storage rack


55


and donor sheet cassettes


32


. The cassette storage rack


55


includes individual cassette storage trays, such as tray


134


, each for storing a donor sheet cassette


32


. Cassette storage trays


134


can pivot backwardly for accessing a donor sheet cassette


32


, such as donor sheet cassette


32


B, for removing the donor sheet therefrom or for adding the donor sheet thereto. As described in more detail below, the donor sheet cassettes


32


are refillable precision donor sheet cassettes that accept replaceable donor sheet assemblies that include supply and take-up rolls. Each of the cassette storage trays


134


include a back portion


136


and a seat portion formed by legs


138


for supporting a donor sheet cassette


32


.




The donor sheet cassette


32


A is now described in additional detail to further illustrate the invention. The donor sheet cassette


32


A includes an upper portion


140


and a lower portion, indicated generally by reference numeral


142


. The upper portion


140


houses a take-up roll


150


of spent donor sheet that is wound about a take-up core tubular body and houses a supply roll


152


of a supply length of donor sheet wound about a supply core tubular body. The lower portion


142


includes four (4) legs


144


that extend downwardly from the upper portion


140


. Tho lower portion


142


serves to position the donor sheet


153


such that it is interposed between the thermal printhead


24


and the printing sheet


16


. The legs


144


form a rectangular “box” of the donor sheet


153


, and the thermal printhead


24


fits into the “box”, as indicated by reference numeral


158


, as the donor sheet cassette


32


is loaded onto the cassette receiving station


96


. Thus the donor sheet cassette


32


of the present invention includes structure for precisely guiding the donor sheet


153


, as in contrast to much of the prior art, wherein the cassettes are non-precision structures, typically made of plastic, that simply roughly position the donor sheet for positioning by precision guiding apparatus fixedly mounted with the printer.




The upper portion


140


includes a handle


146


and a cover


148


. The donor sheet supply roll


152


includes a supply length of the donor sheet


153


that is wound about a core tube (not shown). The cover


148


rotationally mounts torque transmission elements


154


A and


154


B, for transmitting torque from the take-up and supply shafts,


100


and


106


, respectively, of the cassette receiving station


96


to the take-up and supply rolls,


150


and


152


. The donor sheet casssette


32


A includes a transfer apparatus for transferring the donor sheet


153


from the supply roll


152


to the take-up roll


150


, such that it can be interposed between the thermal printhead


24


and the printing sheet


16


. The donor sheet transfer apparatus includes a donor sheet take-up roll mounting shaft and a donor sheet supply roll mounting shaft, which mount the take up and supply rolls


150


and


152


, respectively, and which are not visible in FIG.


3


. The donor sheet transfer apparatus also includes guide rollers


156


, including those supported by the legs


144


, for guiding the donor sheet


153


from the supply roll


152


, to the take-up roll


150


, such that the lower section


153


A of the donor sheet


153


is interposed between the thermal printhead


24


and the printing sheet


16


. When printing, and as the pivot actuator


74


presses the thermal printhead


24


against the printing sheet


16


, as the printing drive motor


36


translates the thermal printhead


24


along the print (Y) axis, fresh sections


153


of the donor sheet


153


are drawn past the thermal printhead


24


from the supply roll


152


, and the consumed donor sheet is wound on the take-up roll


150


.




As described briefly above, the legs


144


of the lower section


142


of the donor sheet cassette


32


A are spaced such that the thermal printhead


24


can fit therebetween for pressing the lower section


153


A of the donor sheet


153


against the printing sheet


16


, Reference numeral


158


indicates how the thermal printhead


26


extends between the legs


144


when the donor sheet cassette


32


A is received by the donor sheet cassette receiving station


94


, shown in FIG.


2


. Reference numeral


160


indicates how the spacing of the legs


144


also allows the cassette transport apparatus


112


to fit between the legs such that the translatable engaging element


114


may engage a slot formed in a lower wall of the upper portion


140


of the donor sheet cassette


32


A. The location of the slot is indicated generally by the reference numeral


162


in FIG.


3


.




Partially shown in

FIG. 3

are the following: the base structure


68


of the printhead carriage


30


; the take-up drive motor


104


; the magnetic brake


110


that is rotatably coupled to the supply shaft


106


; the pivot actuator


74


; the pivot actuator housing


84


; the pivot actuator threaded nut


82


; and the bias spring


92


.





FIGS. 1-3

are discussed above to generally and schematically illustrate many of the salient features of the wide format printer of the present invention. Additional detail is provided in the FIGURES and discussion presented below.





FIGS. 4-5

illustrate additional views of the apparatus shown in

FIGS. 1-3

.

FIG. 4A

is a cutaway view of the upper portion of the wide format thermal printer


10


, including a front elevational view of the printhead carriage


30


.




With reference to

FIG. 4A

, note that separate drive actuators


58


A and


58


B, respectively, independently drive the first and second ends of the translatable clamp pair


42


. Only the clamp


44


of the translatable clamp pair


42


is shown in

FIG. 4A

, and the clamp


44


is cutaway to illustrate full detail of the printhead carriage


30


The work surface


14


is defined by a workbed


13


, shown in cross-section in FIG.


4


A. The reference character “A” indicates a space between the cantilever arm


72


and the cassette receiving station


96


. The pivot actuator


74


has pivoted the cantilever arm


72


downward such that it does not contact the cassette receiving station


96


, and mechanical stops have limited the downward travel of the cassette receiving station. Also indicated in

FIG. 4A

, by reference numeral


408


, is the mounting axis, along which a trunnion pin is preferably disposed for coupling the thermal printhead


24


to the cantilever arm


72


. The thermal printhead


24


is described in more detail below.





FIG. 4B

illustrates a side elevational view of the donor sheet handling apparatus


94


including the cassette receiving station


96


that is slidably mounted to the base structure


68


of the printhead carriage


30


. Shown are the take-up drive motor


104


, the magnetic brake


110


, as well as the translatable cassette engaging element


114


. A boss


168


is formed at the base of the supply shaft


106


.





FIG. 5

is a top view of the wide format thermal printer


10


showing the work surface


14


, the printhead carriage


30


, the clamp


46


, and the cassette storage rack


55


, including four (4) cassette storage trays


134


. Note that the work surface


14


can include suction apertures


176


. Suction is selectively applied to the suction apertures


176


for securing the printing sheet


16


to the work surface


14


when printing on the printing sheet


16


and releasing the printing sheet


16


from the work surface


14


when translating the printing sheet


16


with the translatable clamp pair


42


. The workbed


13


typically includes a platen


275


, against which the thermal printhoad


24


presses the donor sheet and printing sheet


16


.





FIGS. 6A and 6B

illustrate cross-sectional and end views, respectively, of the magnetic clamp


44


, including the keeper


45


. Screws


164


attach the ears


173


of the magnetic clamp


44


to the clamp pair fixtures


54


A and


54


B. The pins


166


guide the keeper


45


and pass through apertures


49


in the keeper


45


. The clamp


44


is placed in the clamped condition by energizing the magnetic coils


172


disposed within the clamp


44


via the connector


174


to attract the keeper


45


so as to clamp the printing sheet


16


between the keeper


45


and a clamping surface of the clamp


44


.




The present invention is deemed to include many additional features and aspects. These features and aspects are now described in turn. The order of discussion is not intended to bear any relation to any relative significance to be ascribed to the features or aspects of the invention.




Vacuum Workbed




The wide format thermal printer


10


of the present invention is intended to be used with a variety of widths of printing sheets


16


. “Width”, in this context, refers to the dimension of the printing sheet along the print (Y) axis. Narrow printing sheets may not cover all of the suction apertures


176


in the worksurface


14


of the workbed


13


, which are provided for securing the printing sheet


16


to the worksurface


14


. To ensure that sufficient suction is applied to apertures blocked by the printing sheet


16


to secure the printing sheet


16


to the worksurface, it is often necessary to isolate many if not all of the unblocked apertures from the suction source


210


. It is known in the art to arrange the apertures


176


in independent zones and for an operator to manually isolate, such as by turning valves or causing operation of solenoids, selected zones so as to not apply suction to those apertures not blocked by the printing sheet


16


.




Furthermore, it is known for the operator, based upon observation of the width of the printing sheet


16


, to manually inform the controller


22


B of the width of the printing sheet


16


, such as by data entry to the controller using a keypad. Knowledge of the width of the printing sheet


16


can be advantageous for a number of reasons. First, the array of thermal printing elements


26


is not to be energized when dry. That is, the array of thermal printing elements


26


of the thermal printhoad


24


should not be energized when the thermal printhead


24


is not pressing donor sheet


153


against the printing sheet


16


. Running the thermal printhead


24


“dry” risks ruining the typically expensive thermal printhead


24


, as the thermal printing elements of the array


26


can overheat and change their printing characteristics. Accordingly, it is useful to know the width of the printing sheet


16


for imposing a limit on the travel of the thermal printhead


24


along the print (Y) axis.




According to the invention, there is provided a simple system for accommodating various widths of printing sheets


16


without the need for an operator of the wide format thermal printer


10


to observe which zones of apertures


176


are not blocked by the printing sheet


16


and to then manually operate valves so as to isolate those apertures from a suction source. The system of the invention can also automatically determine the width of the printing sheet


16


.





FIG. 7

illustrates a top view of the work surface


14


of the workbed


13


.

FIG. 7

is drawn as if the workbed


13


is transparent such that the apparatus below the workbed


13


is readily visible. The clamps


44


and


46


are shown as cutaway and the thermal printhead


24


is illustrated on the right-hand side of

FIG. 7

so as to indicate the location of the print swath


28


relative to the apertures


176


. The worksurface


14


of the workbed


13


can be curved. For example, the worksurface


14


of the vacuum workbed


13


can be the cylindrical worksurface of a drum platen.




The dotted lines indicate plenums formed in the workbed


13


below the worksurface


14


and in fluid communication with those apertures


176


surrounded by a particular dotted line. Reference numerals


186


and


188


indicate manifolds for applying suction to the apertures, and the circles within the dotted lines indicate fluid communication between a manifold and the plenum indicated by the dotted line.




For example, the manifold


186


fluidly communicates with plenum indicated by the reference numeral


180


, as indicated by the circle


184


, and hence, taking note of the additional circles shown in

FIG. 7

, fluidly communicates with the apertures indicated by the reference letters A and B. The manifolds


186


and


188


can be fabricated from suitable lengths and couplings of plastic pipe or tubing.




According to the invention, the apertures


176


are organized into zones, which can correspond to different widths of the printing sheet


16


disposed upon the worksurface


14


of the workbed


13


. Reference numeral


194


indicates a dividing line between zone I and zone II; reference numeral


196


indicates a dividing line between zone II and zone III; reference number


198


indicates a dividing line between zone III and zone IV; and reference number


200


indicates a dividing line between zone IV and V. The apertures


176


included in each zone are further delineated by reference letters A-E. Zone I includes the plenums, and suction apertures in fluid communication therewith, indicated by reference letters A; Zone II is similarly indicated by reference letters B, and zones III, IV and V are indicated by reference letters C, D and E, respectively,

FIG. 7

is to be viewed in conjunction with

FIG. 8

, and the circles


204


and


206


indicate fluid communication with the apparatus shown in

FIG. 8

for applying suction to the manifolds


186


and


188


.




Shown in

FIG. 8

are the following: a suction source


210


, which can be a mechanical evacuation pump, manifold


212


that includes elbows, such as elbow


214


, and tubing sections, such as tubing section


216


; a vacuum sensor


220


for providing an electrical signal responsive to the degree of vacuum drawn by the suction source on the apertures; the muffler


222


that provides an orifice for providing for a selected fluid leakage from the atmosphere to the suction source


210


; and first and second flow control valves


224


and


226


, respectively. Reference numerals


204


and


206


indicate where the apparatus, shown in

FIG. 8

, interconnects with the first and second manifolds


186


and


188


, shown in FIG.


7


. The controller


22


B in

FIG. 8

receives signals produced by the vacuum sensor


220


and is in electrical communication with the flow control valves


224


and


226


for controlling thereof. The controller


22


B, shown in

FIG. 8

, can be the on-board controller


22


A or an off-board controller.




With reference to

FIG. 7

, the zones can be further organized into groups. In the embodiment shown in

FIGS. 7 and 8

, the first group includes zones I and II and includes the apertures


176


in fluid communication with the manifold


186


. The second group includes zones III, IV and V, and the apertures in fluid communication with the manifold


188


. The first vacuum manifold


186


provides fluid communication between the suction source


210


and the first group of apertures (zones I and II), and the second manifold


188


provides fluid communication between the suction source


210


and the second group of apertures (zones III, IV and V).




The first vacuum manifold


186


includes a first flow restriction element


190


A interposed between the suction source


210


and the apertures


176


of zone I, and a second fluid flow restriction element


190


B interposed between the suction source and the apertures


176


of zone II. Similarly, the second vacuum manifold


188


can include fluid flow restriction elements


190


C,


190


D and


190


E. The flow restriction element


190


C is interposed between the suction source


210


and zone III, fluid flow restriction element


190


D is interposed between the suction source and the apertures


176


of Zone IV, and fluid flow restriction element


190


E is interposed between the fluid restriction element


190


D and the apertures


176


of Zone V. The flow restriction elements


190


restrict the flow rates through the zones of apertures for providing selected differences in the degree of vacuum attained, and hence In the signals provided to the controller


22


B by the vacuum sensor


220


, when the apertures


176


of the different zones are unblocked.




In a preferred embodiment, the apparatus of

FIGS. 7 and 8

operates as follows: the controller


22


B energizes the suction source


210


. Initially, the flow control valve


224


and the flow control valve


226


are “closed” and the vacuum sensor


220


provides a signal indicative of a high degree of vacuum. Next, the controller


22


B opens the flow control valve


224


to apply suction to the first group of apertures, that is the apertures


176


of zones I and II. If the printing sheet


16


is only wide enough to cover zone


1


, leaving the apertures of zone II unblocked, the vacuum sensor


220


senses a difference in vacuum from that sensed when the switches were closed, the magnitude of the difference being responsive to the flow restriction element


190


B. The difference in signal level indicates to the controller


22


B that the apertures of one of the zones, typically zone II, are unblocked. If a difference in vacuum is sensed after the flow control valve


224


is opened, the controller typically does not proceed to open flow control valve


226


, as the printing sheet extends from left to right in FIG.


7


and the apertures in zones III, IV and V are unblocked. Note that the flow restriction element


190


A can be included in the manifold


186


for limiting the flow when the apertures of both zones I and II are unblocked, or for facilitating detection of which of the zones is unblocked, creating a first level, or degree, of vacuum when zone I is unblocked and zone II is blocked and different degree of vacuum for indicating that zone I is blocked and zone II is unblocked.




Alternatively, if the printing sheet


16


placed upon the work surface


14


blocks the apertures of both zones I and II, there is little or no change in the level of vacuum attained by the suction source


210


and hence sensed by the vacuum sensor


220


, except perhaps for a transient response as the manifold


186


is initially evacuated, Thus no change in the signal produced by the vacuum sensor


220


indicates to the controller


22


B that all of the apertures


176


of zones I and II are blocked, and that the printing sheet


16


is at least wide enough to cover zones I and II.




The controller


22


B next opens the flow control valve


226


to apply suction to the second group of apertures, that is the apertures


176


of zones III, IV and V. Should the level of vacuum also change very little compared to that attained when both flow control valves


224


and


226


were closed, the printing sheet


16


is determined to extend past all of the zones. If the printing sheet is wide enough to cover zones I and II, but not all of zones III, IV and V, for example, if it is wide enough to only cover zones III and IV, upon opening flow control valve


226


, the level of vacuum attained by the evacuation source and, hence, the signal responsive to that level of vacuum provided by the sensor


220


to the controller


22


B, will be different than those levels and signals previously obtained. How different depends on how many of zones III, IV and V are unblocked. The flow restriction elements


190


C and


190


D and


190


E are interposed in the manifold


188


such that different vacuum levels will be attained by the evacuation source responsive to the number of zones containing unblocked apertures. For example, if the flow restriction elements were not included, uncovering any one of the zones may be sufficient to significantly reduce the vacuum attained by the evacuation source


210


to the same nominal level. Restricting the flow through the zones of apertures ensures that the vacuum decreases as zones are unblocked in discrete steps and signals can be provided, by the vacuum sensor


220


to the controller


22


B, that are responsive to the number of zones are unblocked.




The number of zones and groups described above are merely exemplary and the invention can be practiced with other numbers of zones and groups, as is understood by one of ordinary skill in the art, in the light of the disclosure herein. Typically, suction is successively applied to the groups of apertures until it is determined that one of the groups includes unblocked apertures or until all of the groups have had suction applied thereto, that is, until no groups remain. The five (5) zones shown in

FIG. 7

correspond to the five (5) widths of printing sheets


16


that are commonly expected to be used with the wide format printer


10


of the invention. Grouping of the zones into first and second groups reduces the number of separate signal levels that are to be sorted by the controller


22


B for a given total number of zones. In practice, the flow restriction elements


190


can be realized by judicious choice of the hardware used to construct the manifolds


186


and


188


. For example, it has been found that elbows typically used for interconnecting sections of tubing can be selected to function as the flow restriction elements


190


. According to the invention, the flow restriction elements can be selected for both ensuring separate signal levels for identifying the zones having unblocked apertures, and also for ensuring that those apertures within a group and which are blocked provide adequate suction for securing the printing sheet to the workbed even when the other apertures of the group are unblocked.




However, as understood by one of ordinary skill in the art, apprised of the disclosure herein, the vacuum apparatus and method described above is not limited to use with printers, but can be of advantage in many other instances as well. For example, in the garment industry, sheet materials, such as layups of cloth, are often cut into selected shapes on a table that mounts a numerically controlled cutting implement. The sheet material is often secured to the table via the application of suction to apertures in the surface of the table, and knowledge of the width of the sheet material and constraining the travel of the cutter is also of importance, for reasons similar to those discussed above. This is but one example of an additional environment where the present invention can be useful. In general, the invention is deemed useful in many environments where a workbed includes a worksurface for supporting a sheet material on which work operations are to be performed, such as by translatable workhead mounting a pen, cutter or printhead or other work implement.





FIGS. 9A and 9B

illustrate two embodiments of the invention.

FIG. 9A

corresponds to the arrangement of hardware shown in

FIGS. 7 and 8

, whereas

FIG. 9B

illustrates an alternative embodiment. Note that in

FIG. 9B

the zones and groups are arranged more in “parallel” with respect to the suction source


210


than the arrangement depicted in FIG.


9


A.




Briefly returning to

FIG. 7

, as is known in the art of thermal printing, the workbed


13


typically includes a platen for supporting the printing sheet material


16


as it is printed upon by the thermal printhead


24


. For example, reference numeral


275


in

FIG. 7

indicates the area of the workbed


13


typically occupied by the platen, which can be a rectangular, hard, antistatic rubber material that is fitted to the workbed


13


so as to extend along the print (Y) axis. The upper surface


276


of the platen is typically substantially flush with the rest of the worksurface


14


, and includes those vacuum apertures shown as within the area


275


of FIG.


7


.




Donor Sheet Assembly





FIG. 10A

illustrates a donor sheet assembly


228


for loading into the donor sheet cassette


32


. The donor sheet assembly


228


includes a length of donor sheet


229


wound about a supply core having a tubular body


230


. The supply core


230


extends along a longitudinal axis


231


from a base end


233


to a drive end


234


and has a central opening


232


therethrough. Reference numeral


236


generally indicates drive elements and a memory element located substantially at the drive end of the supply core body


230


. The drive elements and memory element are both described in more detail below.




The donor sheet assembly


228


can also include a take-up core having a tubular body


235


having a central opening


232


therethrough. As shown in

FIG. 10A

, the take-up core body


235


can be packaged with the length of donor sheet


229


wound about the supply core body


230


.

FIG. 10B

illustrates a front view of the donor sheet assembly


228


of FIG.


10


A. Reference numeral


240


indicates that a free-end of the length of donor sheet


229


can be attached to the take-up core tubular body


235


for facilitating insertion of the assembly


228


into, and use of the assembly


228


with, the donor sheet cassette


32


. The donor sheet assembly


228


can be wrapped in cellophane or some other appropriate packaging material to protect the length of donor sheet


229


and to hold the assembly


22


B together. The take-up core body


235


also includes drive elements disposed at one end thereof, as indicated generally by the dotted lines


236


A. Typically, the take-up core body


235


does not include a memory element disposed therewith.





FIGS. 11A through 11D

illustrate additional details of the supply core body


230


. As shown in

FIG. 11A

, supply core tubular body includes drive elements


242


located within the central opening


232


and substantially at the drive end


234


of the supply core body


230


, and that generally extend along and radially of the longitudinal axis


231


. As shown in additional detail in

FIG. 11B

, which is an enlarged view of the drive end


234


of the supply core body


230


shown in

FIG. 11A

, the drive elements can include drive teeth


242


that extend from a base end


244


to a front end


245


. The base end


244


is adjacent an annular support


246


. Retaining elements


247


, which can be spring fingers integral with the supply core body


230


, hold the memory element


300


in place against the annular support


246


, inboard of the drive elements


242


. The memory element


300


includes a data transfer face


302


facing the base end


233


of the supply core body


230


and a back face


303


facing the drive end


234


of the supply core body


230


. The data transfer face


302


is substantially perpendicular to the longitudinal axis


231


.





FIGS. 11C and 11D

show end views of the supply core body


220


taken along section lines C—C and D—D, respectively of FIG.


11


A. Note that the drive elements


242


are recessed from the drive end


234


of the supply core body


230


, as indicated by reference numeral


250


in FIG.


11


B. The take-up core body


235


also includes drive elements substantially similar to those shown with the supply core body


230


.





FIGS. 12

,


13


A,


13


B and


14


show additional details of the donor sheet cassette


32


.

FIG. 12

is a front view of a donor sheet cassette


32


with the cover


148


removed. Shown are the upper portion


140


of the donor sheet cassette


32


and the lower portion


142


. The take-up inner shaft


256


rotationally mounts a take-up shaft


255


for mounting the take-up core body


235


for having spent donor sheet wound thereon, as indicated by reference numeral


150


shown in FIG.


3


. The take-up shaft


255


fits through the central opening


232


of the take-up core


235


. An inner supply shaft


257


rotationally mounts a supply shaft


258


for receiving the supply core body


230


.

FIG. 3

as discussed above, illustrates how the donor sheet is threaded between the supply core body


230


and the take-up core body


235


. The inner supply shaft


257


also mounts at the front thereof a data transfer element


304


, described in more detail in

FIG. 14

, for transferring data between the controller(s)


22


and the memory element


300


associated with the donor sheet. Note the slot


162


A for receiving the translatable engaging element


114


that is mounted by the toothed drive belt


116


of the cassette transport apparatus


112


. (See FIG.


2


). The donor sheet cassette


32


includes threaded holes


262


for receiving screws for holding the cover


148


to the donor sheet cassette


32


, and a guide holes for recieving the pins


268


, shown in

FIGS. 13A and 13B

, of the cover


148


.





FIGS. 13A and 13B

show front and side views of the donor sheet cassette cover


148


. The cover


148


includes bearings


274


that mount a take-up torque transmission element


154


A and a supply torque transmission element


154


B, each having male and female ends,


276


and


278


, respctively. The supply torque transmission element


154


B, which is substantially identical to the take-up roll torque transmission element


154


A, is shown in cross-section. The male ends


276


includes an external drive element(s)


280


and the female ends


278


include internal drive elements


282


. The torque transmission elements


154


couple the drive elements of core bodies


230


and


235


to the shaft drive elements


102


and


108


of the cassette receiving station


96


. The cover also includes through holes


266


through which the mounting screws past for securing the cover


148


to the donor sheet cassette


32


. Also included are the guide pins


268


which are received by the apertures


262


A, shown in FIG.


12


.





FIG. 14

illustrates the donor sheet cassette cover


148


mounted to the donor sheet cassette


32


. The supply shaft


258


is shown cut-away. The rear shaft bearings


290


A and front shaft bearings


290


B rotationally mount the supply shaft


258


to the inner supply shaft


257


, and the take-up shaft


255


is similarly mounted to the take-up inner shaft


256


. The core tubular bodies


230


and


235


and length of donor sheet wound thereon and therebetween are omitted from

FIG. 14

for simplicity; however, the memory element


300


is included and is shown mating with the data transfer element


304


of the supply shaft


258


. Communication elements(not shown) at the back of the donor sheet cassette


32


communicate data to and from the memory element


300


via the data transfer element


304


. The communication elements communicate with the storage trays


134


via conducting tabs located on the donor sheet cassette body for transferring data to and from the memory elements


300


and the controller(s)


22


.




The methods and apparatus of the present invention are intended to increase the economy and efficiency of existing thermal printers, in part by reducing the amount of donor sheet required to print a given multicolor graphic product on the printing sheet


16


. The refillable donor sheet cassette


32


receives the donor sheet assembly


228


that can include relatively long lengths of donor sheet wound about the supply core body


230


. This helps to realize the economic benefit of obtaining the donor sheet in bulk, and for allowing for the completion of more print jobs between reloading the donor sheet cassette. Typically, the donor sheet assembly


228


will include a length of donor sheet


229


that can be up to or greater than 500 meters. Use of a refillable donor sheet cassette


32


also avoids the cost or waste and recycling problems associated with the use of plastic disposable cassettes. When refilling the donor sheet cassette


32


, the cover


148


is removed and the used supply and take-up core bodies removed, and a new donor sheet assembly


228


inserted into the cassette. Preferably, the spent donor sheet, now wound about the take-up core body


235


, and the used supply core body


230


are recycled, and in particular, the used supply core body


230


can be returned for reading of data written on the memory element


300


by the wide format thermal printer


10


. The used supply core body can have a fresh length of donor sheet


229


wound thereon and the new data written to the memory element


300


. The reading and writing of data to and from the memory element


300


is now described in more detail.




Typically, the wide format printer


10


prints a color plane of the multicolor graphic product responsive to the data read from the memory element


300


mounted with the donor sheet assembly


228


to be used in printing that color plane. Many types of information can be stored on the memory element


300


. Typically included is data characteristic of the donor sheet. For example, as there are a variety of colors of donor sheet, including spot and process colors, and as there are known to be at least sixty (60) different types of donor sheets, it is typically important that the wide format thermal printer


10


be aware of the color and type of donor sheet being used such that printing parameters such as the energization of the thermal printing elements


26


or the pressure with which the thermal printhead


24


presses the donor sheet against the printing sheet


16


, can be adjusted accordingly. The stored information, therefore, can include data representative of at least the color and type of the donor sheet, including, for example, information relating to the type of finish on the donor sheet, whether the donor sheet is resin based or wax based, and the class of the ink donor material on the donor sheet.




Other data characteristic of the donor sheet stored on the memory element


300


can include the average color spectra reading, such as the LAB value, for the length of donor sheet


229


. Typically, a particular manufactured lot of donor sheet is tested to determine this color spectra value, and all memory elements


300


included in donor sheet assemblies


228


that include a length


229


from that lot store substantially identical color spectra information. The color spectra reading is used in the printing process, either by the wide format thermal printer


10


or in preprocessing of data representative of the multicolor graphic image, to account appropriately for variations in the manufacturing processes that result in different color spectra values. For example, the RIP (raster image processing) computations can be varied in accordance with different color spectra data. Furthermore, the wide formal thermal printer


10


can vary the voltage applied for energizing the array of thermal printing elements


26


responsive to variations in the value of the color spectra value read from the memory element


300


.




The memory element


300


can also include data representative of information pertaining to the specific opacity/transparency value for the length of donor sheet


229


included in the donor sheet assembly


228


. The wide format thermal printer


10


can use this information to adjust how the donor sheet is printed to maximize performance and color.




Data representative of the “firing deltas” to be used in energizing the array of thermal printing elements


26


to optimally print with a particular length of donor sheet


229


can also be stored on the memory element


300


. The term “firing deltas” refers to variations in printing parameters for improving printing with a particular donor sheet. For example, the firing deltas can include data for varying the voltage and/or power applied to thermal printing elements, the time that the thermal printing elements are energized, and the pressure with which thermal printhead presses the donor sheet against the printing shoot.




Data representative of the length of the length of donor sheet


229


originally wound during the donor sheet assembly


228


can also be stored in the memory element


300


. Typically, the length is stored in centimeters. This length is used to track the remaining length of unused donor sheet wound on the core tube


230


. As the wide format thermal printer


10


prints a color plane, the donor sheet is interposed between the printhead and the printing sheet


16


and the thermal printhead


24


is translated along the print axis, drawing the donor sheet past the printhead


24


. From this process, the wide format printer can track the length of donor sheet drawn past the thermal printhead


24


, and hence can determine the length remaining on the supply core body


230


.




The memory element


300


can also include data representative of the supply side roll diameter, that is, the diameter of the length of donor sheet


229


originally wound on the supply core body


230


. This diameter is not uniquely determined by the length of donor sheet


229


. The diameter can vary significantly with the color of the donor sheet and other characteristics of the donor sheet. The diameter should be accurately tracked and recorded when the length of donor sheet is wound on the core


230


and this information is used by the wide format thermal printer


10


to accurately estimate and control the tension applied to the donor sheet while printing, as described below.




The memory element


300


can include a “read only” portion for storing data representative of the manufacturer of the donor assembly


228


of the donor sheet. Such data can be stored on the memory element by the manufacturer of the memory element


300


, and can be read by the wide formal thermal printer


10


upon loading of the donor sheet assembly


228


into a donor sheet cassette


32


that is mounted on the cassette storage rack


55


. An operator of the wide format thermal printer


10


can be informed when a donor sheet assembly


228


that is not warranted or whose quality cannot be guaranteed is to be used on the wide format thermal printer


10


.




The memory element


300


can also store data representative of a lot code assigned to each manufacturing run of donor sheet produced by the manufacturer. This lot code will allow any performance problems reported by customers to be tracked back to an original lot. If problems are being reported with the donor sheet of a particular lot, the remaining unused donor sheet of that lot may be removed from service to avoid future problems.




The memory element


300


can also include information representative of a “born-on date” of the length of donor sheet


229


. This information is the actual date of the manufacture of the donor sheet assembly


228


, that is, the date that the length of donor sheet


229


was wound onto the supply core body


230


. This “born-on date” can be significantly different than other dates of importance, such as, a “lot code” date typically included with the lot code information described above. For example, it can be beneficial to energize the thermal printing elements differently when printing with older donor sheet lengths


229


, and whether the donor sheet has aged before or after being wound on the supply core body


230


can be of importance. The “born on” date can be checked to see if a selected shelf life of the donor foil assembly


228


has been exceeded.





FIG. 15A

illustrates one method for more economically providing donor sheet to the wide format thermal printer


10


and for reducing the cost of printing a given multicolor graphic product on the printing sheet


16


. A donor sheet assembly


228


can be prepared from a master roll


344


that is sliced by cutters


348


into number of “slices” A, B, C, D, and E that are then wound onto the five individual core bodies


230


A through


230


E. The master roll


334


includes a length of donor sheet having a width (W), as indicated by reference numeral


346


. The individual slices of donor sheet have a width


350


that is smaller than the width


346


of the master roll


344


. In the example shown in

FIG. 15A

, the width


350


is approximately one-fifth (⅕) of the width of the donor sheet


346


on the master roll


344


. Although four (4) cutters


348


are shown in

FIG. 15A

, typically two (2) additional cutters are positioned at the edges of the donor sheet and trim off a scrap width of the donor sheet material. The core bodies


230


A-E are then incorporated into donor sheet assemblies


228


. According to the invention, data representative of the “slice position” is stored on the memory element


300


to account for variations of properties across the width


346


of the donor sheet. For example, the stored information can indicate whether the length of donor sheet


229


is from slice position “A”, “B”, “C”, “D” or “E”. This information can also allow any problems reported with donor sheet assemblies


228


to be tracked to the manufacturing process and can allow better monitoring of that process for improvement thereof.




The above are examples of data characteristic of the donor sheet. One of ordinary skill in the art, in light of the disclosure herein, can envision other data characteristic of the donor sheet and that can be advantageously stored on the memory element


300


. Additional examples are given below.




Other information that can be stored on the memory element


300


can include a revision code. The revision code will inform software running on the controller(s)


22


how many data fields are present in the memory element


300


and the format of the data fields. This revision code is updated each time a change is made to the amount or type of data that is being stored on memory elements


300


provided with donor sheet assemblies


228


. Many revisions are likely be made over time and it is appropriate that the controller(s)


22


understands what data is actually on a particular memory element


300


.




Data can be stored on the memory element


300


before or after mounting the memory element with the supply core body


230


. When recycling previously used supply core tubular bodies, the memory elements


300


are likely not removed from the core bodies, and new data can be written to the memory element


300


by inserting a probe having a data transfer element into the central opening of the supply core body


230


at the base end


233


thereof such that the probe data transfer element contacts the data transfer face


302


of the memory element


300


.




Typically, the data described above is stored on the memory element


300


between the time of manufacture of the donor sheet assembly


228


and the first use of the donor sheet assembly


228


with a wide format thermal printer


10


. However, the invention also provides for the wide format thermal printer


10


to write to the memory element


300


before, during or after printing a multicolor graphic product.




As described above, the amount of donor sheet used when printing can be tracked by the wide format thermal printer


10


(i.e., by the controller(s)


22


). Accordingly, after a particular color plane has been printed, or after it is determined that the wide format thermal printer is through printing with that particular donor sheet cassette


32


, the wide formal thermal printer


10


can write data representative of the amount of donor sheet remaining on the supply core body


230


to the memory element


300


. The remaining length of information can be important for planning jobs so that the wide format thermal printer


10


, before loading a particular donor sheet cassette to the cassette receiving station


96


, can ensure that it will not run out of donor sheet while printing a print swath. Running out of donor sheet during printing a print swath usually destroys the multicolor graphic product. Furthermore, the color fidelity of the donor sheet can vary from lot to lot, and it is a good idea for the wide format printer


10


to be able to predict when there is not enough donor sheet in the donor sheet cassette


32


to complete a particular print job. A warning can be provided to an operator of the wide format thermal printer


10


, such as via a display associated with the controller


22


. The remaining length information is also typically stored in centimeters. It is initially set by the manufacturer of the donor sheet assembly


228


to match the manufactured length information, and decremented by the wide format thermal printer


10


as donor sheet is consumed.




The wide format thermal printer


10


can also write other information to the memory element


300


. This information can include, for example, the following: (1) the number of donor sheet-out/snaps. (This information is used to track the number of times that use of a particular donor sheet assembly results in an unexpected out-of-donor-sheet condition); (2) the number of times the donor sheet assembly


228


is used for printing. (Preferably, this information reflects the number of times donor sheet cassette


32


including the donor sheet assembly


228


is picked-up and used actively for printing during a job. If a donor sheet is not used, but is mounted in one of the several donor sheet cassette storage locations on the cassette storage rack


55


, the information is not changed. Furthermore, the length used to-date, that is, the original length of donor sheet minus the length remaining, divided by the number of times used, yields information representative of the average size of the print jobs being printed by the wide format thermal printer


10


); (3) the date of the first use of the donor sheet assembly


228


for printing; and (4) the date of last use. This latter date is updated each time the donor sheet assembly


228


is used for printing.




Data representative of information related to the usage of the wide format thermal printer


10


on which the donor sheet assembly


228


is mounted and of the usage of the donor sheet assembly


228


can also be written on the memory element


300


. This information can include: (1) the number of different wide format thermal printers


10


on which the donor sheet assembly has been used; (2) the serial number of the wide format thermal printers


10


with which the donor sheet assembly


228


has been used; (3) the total number of hours on the printhead


24


that was last used to print with the donor sheet assembly


228


; (4) the total travel distance accumulated along the printing sheet translation (X) axis of the wide format thermal printer


10


used to print with the donor sheet assembly


228


; (5) the total distance that a wide format thermal printer


10


has translated all printheads


24


installed in the wide format printer


10


, as well as the total distance that the particular thermal printhead


24


now installed has been translated; (6) the average steering correction used by the wide format thermal printer when translating the printing sheet


16


in one direction along the printing sheet translation axis; and (7) the average steering correction used when translating the printing sheet


16


in the opposite direction along the printing sheet translation (X) axis. Steering correction refers to maintaining alignment of the printing sheet


16


relative to the worksurface


14


during printing of the multicolor graphic product, and is elaborated upon below.




Much of the data described above can be very useful in tracking the performance of the wide format thermal printers and donor sheet assemblies for diagnosis of problems, for improving the printers and the donor sheet assemblies, for determining when warranty claims are valid, and for limiting the extent of any problems that should occur.





FIG. 15B

is a flow chart illustrating one sequence that can be followed in reading of data from, and writing of data to, the memory element


300


. In Block


351


, data is read from the memory element


300


mounted with a supply core body


230


that is mounted within a donor sheet cassette


32


on the cassette storage rack


55


. In block


352


, selected printing parameters, such as the desired tension to be applied to the donor sheet, or the proper energization of the array of thermal printing elements


26


, are determined as a function of the data read from the memory element


300


. Next, as indicated by block


353


, the donor sheet cassette


52


is removed from the cassette storage rack


55


and mounted on the cassette receiving station


96


, and as indicated by block


354


, the color plane corresponding to the donor sheet in the donor sheet cassette is printed on the printing sheet


16


. During printing, selected printing parameters, such as the distance traveled along the print (Y) axis by the thermal printhead


24


while pressing donor sheet against the printing sheet material


16


, are monitored. Proceeding to block


355


, the donor sheet cassette


32


is returned to the cassette storage rack


55


. As indicated by block


356


, the selected data on the memory element


300


is updated responsive to the monitored printing parameters. For example, the data field corresponding to the length of donor sheet remaining on the supply core body


230


can updated (e.g., decremented) to account for the length of donor sheet consumed in block


354


. The length of donor sheet consumed can be determined from the printing parameter monitored above, that is, from the distance traveled by the thermal printhead


24


while pressing the donor sheet against the printing sheet material. The steps shown in

FIG. 15B

are typically all accomplished via the controller(s)


22


, and are repeated for each of the color planes of the multicolor graphic product printed on the printing sheet


16


by the wide format thermal printer


10


.




Printing Sheet Alignment And Tracking




With brief reference to

FIG. 1

, note that the edge


19


of the printing sheet


16


is illustrated as substantially parallel to the printing sheet translation (x) axis. As understood by those of ordinary skill, such substantial parallelism is desirable so as to avoid “skew” errors in the multicolor graphic product, such as adjacent print swaths not aligning properly.

FIGS. 16A-16C

illustrate the edge


19


of the printing sheet


16


when skewed relative to the printing sheet translation (X) axis. The skewing is exaggerated for purposes of illustration. In

FIG. 16A

, the edge


19


of the printing sheet


16


disposed at an angle to the edge


15


of the work surface


14


such that along the dotted line


29


B, representing the lower edge of a print swath


28


, the edges


15


and


19


are separated by a distance d


1


. (For purposes of illustration the edge


15


is taken as parallel to the printing sheet translation (X) axis.) As shown in

FIG. 16B

, as the printing sheet


16


is translated along the printing sheet translation axis (X) towards the top of the page on which

FIG. 16A

is illustrated, the distance between the edge


19


of the printing sheet


16


and the edge


15


of the working surface


14


along the dotted line


29


B has decreased to d


2


, whereas, along the dotted line


29


A, indicating the other boundary of the printing swath


28


, the distance between the edge


19


and the edge


15


is now d


1


.




Alternatively,

FIG. 16C

illustrates the change in the distances between the edges


19


and


15


as the printing sheet


16


is translated starting from the position shown in

FIG. 16A

in the opposite direction along the printing sheet translation axis (X), or towards the bottom of the page on which

FIG. 16A

is shown. Along the dotted line


29


B, the distance between the edges has now increased to d


3


and along the dotted line


29


A, indicating the upper edge of the print swath


28


, the distance between the edges


15


and


19


has increased to d


4


.




As illustrated by

FIGS. 16A-C

, when the printing sheet is skewed, the position of the edge


19


as measured along the print (Y), varies as the printing sheet is translated along the printing sheet translation (X) axis. One of ordinary skill is well aware of the problems such skew can cause with the printing of multicolor graphic product on the printing sheet


16


. As the printing sheet


16


is driven along the printing sheet translation (X) axis, the error becomes cumulative in the print (Y) axis and produces an increasing lateral position error as the printing sheet


16


moves along the printing translation (X) direction. The error can quickly become large enough to cause printing off of the edge of the printing sheet


16


. Accordingly, skew error is highly undesirable and can result in the multicolor graphic image being destroyed or in damage to the thermal printhead


24


. In a wide-format thermal printer


10


, which is intended to print large printing sheets, for example, 36″ wide along the (Y) axis by 40′ long in the (X) axis, skew error can be a problem of great concern.




According to the invention, the change in the print (Y) axis position of the edge of the printing sheet


16


as the printing sheet is translated back-and-forth along the printing sheet translation (X) axis can be used advantageously to correct the skew of the printing sheet


16


.





FIGS. 17A and 17B

show top and elevational views, respectively, of selected components of the wide format thermal printer


10


.

FIG. 17A

is a top view along the (Z) axis schematically illustrating the printhead carriage


30


, the guiderails


40


, the printing sheet


16


and the work surface


14


;

FIG. 17B

is an elevational view along the printing sheet translation (X) axis, and schematically illustrating the printhead carriage


30


, the thermal printhead


24


, the workbed


13


, the work surface


14


and the printing sheet


16


. With reference to

FIGS. 17A and 17B

, the printhead carriage


30


mounts an edge sensor


360


for detecting the location of the edge


19


of the printing sheet


16


. As shown in

FIG. 17B

, the edge sensor


360


transmits and receives a light beam


364


for detecting the edge


19


of the printing sheet


16


. The edge sensor


360


includes a transmitting portion for generating light and a receiving portion for receiving reflected light. The change. in the intensity of the reflected light received as the edge sensor passes over the edge


19


is used to determine the location of the edge


19


. A reflective strip


362


is provided for enhancing the change in the intensity of the reflected light received by the edge sensor


360


as it passes over the edge


19


of the printing sheet The edge sensor


360


is shown as located along the lower edge of a print swath


29


B. Again, this selection of location is exemplary. Note that rather than a reflection sensor, a linear array of receiving sensors, or pixels, can be located with the worksurface


14


. The array would extend along the print (Y) axis, and the number of pixels illuminated indicate the position of the edge


19


of the printing sheet


16


.




The skew of the printing sheet


16


can be determined as follows. The printhead carriage


30


is moved back and forth along the print axis so as to detect the edge


19


of the printing sheet


16


. Assume that the edge


19


is located as indicated by the distance d


1


in FIG.


16


A. The printing sheet


16


is next translated along the printing sheet translation axis by the pair of translatable clamps


42


so as to, for example, move the printing sheet


16


to the position shown in FIG.


16


B. The printhead carriage


30


is again moved back and forth along the print axis to detect the edge


19


of the printing sheet


16


, wherein the edge is located as indicated by the distance d


2


. Based on the difference in relative positions of the printhead carriage


30


corresponding to the two detections of the edge


19


, the relative change in distance, d


1


-d


2


, can be determined, and from the knowledge of the distance the printing sheet


16


was translated along the printing sheet translation axis, the slope of the edge


19


can be determined, as shown in FIG.


17


C.




The skew can be varied (e.g., reduced) by independently actuating the clamp actuators


58


A and


58


B while placing at least one of the clamps of the clamp pair


42


in the clamped condition and refraining from applying suction to the suction apertures


176


. For example, with reference to

FIG. 18

showing a top view of tho printing sheet


16


and the translatable clamp pair


42


, placing the clamp


44


in the clamped condition and actuating the right clamp actuator


58


B (not shown) more that the left clamp actuator


58


A (not shown) translates the right clamp pair fixture


54


B more than the left clamp pair fixture


54


A and moves the edge


19


of the printing sheet


16


to the position indicated by reference numeral


19


′, skewing the printing sheet as shown. Basically, the clamp


44


differentially drives spaced portions of the printing sheet, such as portions indicated by reference numerals


365


and


367


, for producing a torque on the printing sheet


16


. Of course, as the clamp


44


clamps the printing sheet


16


along a substantial length, and the particular selection of the spaced portions shown in

FIG. 17

is exemplary. As used herein, differentially driving spaced portions includes driving spaced portions on the sheet material in different directions, driving the spaced portions different distances in the same direction, and fixing one portion and driving the other portion.




Typically, an iterative procedure is followed for varying the skew of the printing sheet


16


. For example, the skew is determined as noted above, the clamp actuators independently actuated to vary the skew, the skew again measured, again varied, and so on, until the skew o the printing sheet


16


is within selected limits.




In general, independent actuation of the actuators


58


A and


58


B is used, not only to correct skew, but to “walk” the printing sheet


16


along the surface


14


of the workbed


13


so as to obtain a selected distance between the edge


19


of the printing sheet and the edge


15


of the work surface


14


or some other reference location along the print (Y) axis. Once this distance is within a predetermined range, the skew is varied as indicated above. Typically, if the edge


19


of the printing sheet


16


is within a tenth (10th) of an inch of the edge of the work surface


14


, it is not necessary to walk the printing sheet


16


. “Walking” as used herein, refers to selectively activating the actuators


58


A and


58


B to first skew the printing sheet in one direction, and then to skew the printing sheet in the other direction, thereby “walking” the printing sheet


16


. The term “aligning,” as used herein, refers to moving the printing sheet to obtain a selected skew (including no skew) and to obtain a selected distance between the edge


19


of the printing sheet and a reference location.




The location of the edge


19


relative to a reference position along the print (Y) axis can be determined with the aid of the home position sensor


360


. The home position sensor indicates when the printhead carriage


30


is at known position along the print (Y) axis, such as when the left edge of the printhead carriage


30


is aligned with the edge


15


of the work surface


14


. As understood by one of ordinary skill, another home position could be suitably selected. Use of the home position sensor


360


allows more accurate determination of the location of the edge


19


relative to the edge


15


of the edge of the worksurface


14


.




Note that the skew need not be totally eliminated, that is, it is acceptable to proceed with a selected residual skew during the printing of each color plane. However, the skew should not vary during printing. Preferably, the skew is periodically checked during the printing of each color plane of the multicolor graphic product on the printing sheet


16


and adjusted as necessary. For example, as the printhead carriage


30


translates back-and-forth along the print axis to print the print swaths, and the printing sheet is translated along the printing sheet translation axis between successive swaths, the edge sensor


360


can be used to continually monitor the skew and position of the edge


19


. If it is determined that the skew is varying during actuation of the clamp pair to translate the printing sheet, the steering is corrected, that is the actuation of the actuators


58


A and


58


B is selectively adjusted so as to maintain the predetermined skew. The actuators


58


A and


58


B are preferably stepper motors, and the controller(s)


22


can independently vary the number of steps each is instructed to turn. However, other types of actuators are also suitable, such as servomotors that include position encoders.




Note that the controller


22


can control the edge detection sensor


360


so as to detect both edges of the printing sheet


16


for determining the width of the printing sheet


16


. The controller


22


can determine the distance between the detected edges of the printing sheet


16


from the knowledge of the distance printing carriage


30


is translated.




The translatable clamp pair


42


is but one example of a drive apparatus for moving a strip or web of sheet material, i.e., the printing sheet


16


, longitudinally back-and-forth along a feed path, in this instance, the printing sheet translation (X) axis of the wide format thermal printer


10


.




Other known drive apparatus include friction, grit or grid drive systems. Drive systems find use not only in printers, but in plotting and in cutting devices. For example, in friction-drive systems, the friction (or grit) wheels are placed on one side (i.e., above) of the strip of sheet material and pinch-rollers (made of rubber or other flexible material) which are placed on the other side (i.e., below) of the strip of sheet material with spring pressure urging the pinch rollers and material toward the friction-wheels. During work operations, such as plotting, printing or cutting, the strip material is driven back-and-forth in the longitudinal or (X) direction by the friction-wheels while, at the same time a workhead including a pen, printing head or cutting blade is driven over the strip material in the lateral, or Y, direction. Friction-drive systems, in particular, have gained substantial favor with many types of printers due to their ability to accept plain (unperforated) strips of material of differing widths. Tractor-drive systems for use with perforated strips of material are known in the art, but require correct spacing of the track-drive wheels to match the spacing of the perforated strips.




One example of a friction drive system is disclosed in patent application Ser. No. 09/217,667, entitled “METHODS FOR CALIBRATION AND AUTOMATIC ALIGNMENT AND FRICTION DRIVE APPARATUS”, filed on Dec. 21, 1998, and owned-in-common with the present application, and herein incorporated by reference. Disclosed in the above-referenced application are friction drive wheels spaced in a direction parallel to the print (y) axis from each other, and which can be differentially actuated for differently driving spaced portions of the printing sheet for aligning the printing sheet


16


. The use of friction, grit or grid drive apparatus for translating the printing sheet


16


along the printing sheet translation axis, and in particular of the apparatus and methods disclosed in the above reference application, are considered within the scope of the present invention.




Described above is a technique wherein the printhead carriage


30


mounts the edge sensor


360


which, in cooperation with the reflective strip


362


, determines the skew of the printing sheet


16


. However, also disclosed in the above-referenced application are methods and apparatus wherein a light source is disposed above a sensor that includes an array of pixels extending in the direction of the print (Y) axis. The sensor is disposed with the worksurface


14


for sensing the edge


19


of the printing sheet


16


, and is spaced in the direction of the printing sheet translation (X) axis from the apparatus for driving the printing sheet (i.e., one of the translatable clamps or the friction drive wheels. Preferably, two sensors are used, one ahead and one behind the drive mechanism. The use of such sensors, as well as of other techniques and apparatus disclosed in the above reference application, are deemed within the scope of the present invention.




According to invention, reference indicia for providing a “ruler” can be provided on the printing sheet


16


and a sensor disposed for reading these indicia such that the controller(s)


22


, responsive to sensor, can track the distance the printing sheet


16


is translated along the printing sheet translation (X) axis by the clamp pair


42


or the friction wheels. For example, the “ruler” can be printed on the back side of the printing sheet


16


, that is the side facing the worksurface


14


, and read by a sensor disposed with the worksurface


14


, such the pixel array sensor discussed above.




Field Replaceable Thermal Printhead Assembly




According to the invention, the thermal printhead


24


can be mounted to the cantilever arm


72


of the thermal printhead carriage


30


(See FIGS.


2


,


4


or


5


) via the thermal printhead assembly


400


illustrated in FIG.


19


A. With reference to

FIG. 19A

, the thermal printhead


24


can include a mounting block


402


for mounting the thermal printhead circuit board


403


to the printhead assembly base


404


. A single coupling joint mounts the printhead assembly


400


, and hence the thermal printhead


24


, along the mounting axis


408


, shown in

FIG. 4A

, to the cantilever arm


72


. Preferably, the coupling joint is a trunnion joint and the base


404


defines an aperture


410


for accommodating a trunnion pin (not shown) that extends along the mounting axis


408


(in the preferred embodiment the trunnion joint axis) that is received by the cantilever arm


72


. Note that the mounting axis


408


is generally perpendicular to the direction along which the array of thermal printing elements


26


extends, and hence is generally perpendicular to the printing sheet translation (X) axis. The single coupling joint


406


advantageously provides for simple and easy removal and replacement of the thermal printhead


24


in the field, and can allow the printhead


24


to swivel for producing a more even pressure distribution on the thermal printing elements


26


.




The thermal printhead assembly


400


can also include a heating element


412


and a cooling element


414


for transferring heat with the thermal printhead


24


. The cooling element


414


can include cooling fins


133


that are mounted with the printhead assembly bane


404


. The cooling fins


133


are also shown in

FIGS. 2 and 4A

, and when the thermal printhead assembly


400


is mounted to the cantilever arm


72


, the cooling fins


133


receive air directed to them by the blower


126


mounted with the cantilever arm


72


. Preferably, the base


404


is thermally conductive for providing thermal communication between heating and cooling elements and the array of thermal printing elements


26


.




The heating element


412


and the cooling element


414


are provided for enhanced thermal management of the thermal printhead


24


and, in particular, the array of thermal printing elements


26


. Upon initial startup of the wide format thermal printer


10


, the array of thermal printing elements can advantageously be warmed by the transfer of heat from the heating element


412


such that multicolor graphic image is printed properly on the printing sheet


16


. However, during extended printing, it can be advantageous to remove heat from the array of thermal printing elements


26


and, accordingly, removal of such heat is enhanced by the cooling element


414


. The heating element


412


is typically an electrical power resistor mounted for thermal communication with the printhead assembly base


404


and, hence, with the thermal printhead


24


and array of thermal printing elements


26


.




The thermal printhead


24


receives signals via the thermal printhead connector


416


which include data representative of the multicolor graphic product to be printed on the printing sheet


16


. As is known in the art, thermal printhead


24


typically includes drive electronics for conditioning those signals


35


prior to energizing the array of thermal printing elements


26


responsive to the signals. For example, the drive electronics can convert the signals received by the connector


416


from differential type signals to single-ended signals. The thermal printhead


24


also receives power from a power supply


828


, as is known in the an, for energizing the array of thermal printing elements


26


.




According to the invention, a semiconductor element


420


is included with the thermal printhead


24


for storing data characteristic of the thermal printhead


24


. The printhead assembly base


404


mounts a semiconductor element mounting board


422


that, in-turn, mounts the semiconductor element


420


. The connector


424


provides communication between the semiconductor element


420


and the controller(s)


22


associated with the wide format thermal printer


10


. The arrangement shown in

FIG. 19A

is exemplary, and as understood by one of ordinary skill, in light of the disclosure herein, the semiconductor element


420


can be mounted adjacent the array of thermal painting elements


26


, such as on the thermal printhead circuit board


403


add/or be incorporated with the drive electronics. The term “printhead assembly,” is employed herein to aid in the above discussion; however, as understood by one of ordinary skill in the art, the printhead assembly


400


need not include all of the elements described above.




The data characteristic of the printhead stored by the semiconductor element


420


can include data representative of the resistances of the thermal printing elements


26


, such as an average resistance of the printhead elements. This resistance data can be useful in a variety of ways. For example, for proper printing of the multicolor graphic product on the printing sheet


16


, the array of thermal printhead elements


26


is selectively energized. Typically, the thermal printhead elements are energized such that a selected amount of heat is generated in each element for transferring a pixel of color from the donor sheet to the printing sheet


16


. Of course, the amount of heat generated depends, in-turn, on the current (or voltage) applied to the thermal printing element and the resistance of that element. Typically, it is more important that the manufacturer of the thermal printhead keep the individual resistances of the thermal printing elements that makeup the array of thermal printing elements


26


within a rather narrow range of tolerances than the manufacturer provide a particular resistance. Thus the average value of the resistances of the thermal printing elements can vary, and the data stored in the semiconductor element


420


allows the wide format thermal printer


10


to automatically compensate for a thermal printhead


24


that has a higher or lower average resistance than another printhead


24


. Accordingly, when the thermal printhead


24


is replaced in the field, a calibration procedure is not necessary or, if necessary, can be less difficult or time consuming and the wide format thermal printer


10


can more readily be returned to service.




Keeping the resistances of the individual thermal printing elements within narrow tolerances, for example, within one (1%) percent, typically adds to the cost and difficulty of manufacturing the thermal printhead


24


, and can alto lead to a thermal printhead


24


that is less robust than one manufactured with a wider range of tolerances. However, according to the invention, the data characteristic of the printhead can include the individual resistances of a selected plurality of the thermal printing elements. The selected plurality of the thermal printhead elements can included the individual resistances of each of the thermal printhead elements that is normally used in printing. The data representative of the resistances of the individual elements are stored in the semiconductor element


420


and each individual resistance is accounted for when energizing that element during printing. Accordingly, the manufacturer of the thermal printhead


24


need not take such extreme measures for producing a narrow range of tolerances, leading to a less-expensive thermal printhead and one that can be more robust in use.




According to the invention, the data stored on the semiconductor element


420


can include data representative of the history of use of the thermal printhead


24


, or of the printer, and is typically acquired by monitoring selected printing parameters. For example, history data can include data representative of the following: the total time of use of the wide format thermal printer


10


with the thermal printhead


24


installed thereon; the total amount of time the thermal printhead has spent pressing donor sheet against printing sheet


16


and printing; the total distance translated along the print (Y) axis by the thermal printhead


24


while pressing the donor sheet against printing sheet


16


and printing; the voltages that have applied to the thermal printing elements when energizing the thermal printing elements; and information related to the number of printing pulses (e.g. voltage pulses) that have been communicated to the thermal printing elements.




The semiconductor element


420


can include a processor programmed for tracking the number of printing pulses communicated to the thermal printing elements and for storing that number in the memory of the semiconductor element


420


. As is known in the art, very often more than one pulse is sent to a thermal printing element to print a pixel with that element. Accordingly, the program can include tracking the total number of printing pulses communicated to all of the thermal printing elements or can track a number related to the total number to account for multi-pulse printing of each pixel. The total printing time accumulated on the printhead assembly


400


is related to the number of printing pulses transmitted to the thermal printing elements


26


. From a knowledge of the number of printing pulses provided to the array of thermal printing elements


26


and the resolution of the multi-color graphic product, that is, the dots per inch, an approximate total time of use of the thermal printhead


24


can be determined, such as by the tracking program or by the controller(s) associated with the wide formal thermal printer


10


, and stored on the semiconductor element.




There are many different types of donor sheets and printing sheets


16


used in the graphic arts. These types of donor sheets and printing sheets


16


can produce varying amounts of wear on the thermal printhead


24


. Accordingly, the types of printing sheets and donor sheets used with the thermal printhead


24


can be tracked and the history of use data described above can include data representative of the amount of time spent printing selected donor sheets and printing sheets. Typically, the controller(s)


22


read data characteristic of the donor sheet from the memory element


300


mounted with the supply roll of the donor sheet.




The data described above can be useful in a number of ways, such as diagnosing problems with the quality of the multicolor graphic product, determining if customer claims are within a warranty, tracking use for timely performing service and maintenance. For example, data can be read from the semiconductor element


420


when testing a particular thermal printhead


24


in the field. The thermal printhead assembly


400


can be removed from the printer and the resistance profile, that is the average resistance or the resistance of individual thermal printing elements of the thermal printhead


24


, read from the semiconductor element


420


. The stored profile will typically correspond to the resistances of the thermal printing elements


26


at the time of manufacture of the thermal printhead


24


, and can be compared to actual empirical tests performed on the thermal printhead


24


when removed from the wide format thermal printer


10


. A determination that some or all of the thermal printing elements have changed their resistance can be an indication of over-stressing, that is, over-heating, of the thermal printhead. The thermal printhead can be replaced, or the controller(s)


22


associated with the wide format thermal printer


10


instructed to print the color plane of the multicolor graphic product so as to compensate for changed thermal printing elements.




The thermal printing elements


26


of the thermal printhead


24


selectively heat the donor sheet to transfer pixels of donor material, such as an ink, from the donor sheet to the printing sheet


16


. Typically, each thermal printing element corresponds to a single pixel. Depending on the nature of the multicolor graphic product to be printed, a particular thermal printing element can be energized repeatedly within a relatively short period of time, or can be energized infrequently. Furthermore, a particular thermal printing element can be surrounded by neighboring thermal elements that are relatively hot or cold, depending on the recent usage of those elements. As is known in the art, the amount of heat transferred to the donor sheet by a particular thermal printing element thus can vary as a function of the past energization of that thermal printing element and its neighbors. Print quality can be affected if the amount of energy transferred when printing similar pixels is allowed to excessively vary from pixel to pixel. Accordingly there are known in the various “hysteresis control” techniques for accounting for the past energization of a thermal printing element and its neighbors when energizing that element for printing.

FIG. 19B

is a view of the thermal printhead assembly


400


taken along the line


19


B—


19


B of FIG.


19


A. Note that the outer thermal printing elements


430


, which are located near the ends of the array of thermal printing elements


26


, have fewer neighbors than those elements


432


nearer the middle of the array of thermal printing elements


26


. According to the invention, the array of thermal printing elements


26


can include thermal elements


26


A and


26


B that are not normally used in printing. That is, print swaths, such as print swath


28


, are printed by the thermal printing elements normally used in printing, which are those elements of the array between the dotted lines defining the print swath


28


. According to the invention, selected thermal printing elements not normally used in printing are energized so as to provided additional heated neighbors for the outer thermal elements


430


to reduce any printing discrepancies between the outer thermal printing elements


430


and those thermal printing elements


432


nearer the middle of the array of thermal printing elements


26


. The thermal printing elements


26


that are heated can be energized prior to and/or during the energization of the outer thermal printing elements


430


.




In addition, it is also understood by those of ordinary skill, in light of the disclosure herein, that proper alignment of consecutive print swaths can be important to avoid or limit the visibility of “seams” running along the print (Y) axis and indicating where individual print swaths meet. Such seams can be more or less visible depending on the nature of the multicolor graphic product being printed. The translatable clamp pair


42


of the present invention can provide accurate and repeatable translation of the printing sheet


16


for limiting misalignment of the print swaths. The disclosed apparatus and methods for alignment of the printing sheet


16


along the printing sheet translation (X) axis also can contribute to reducing any misalignment of the printing swaths. For example, one technique for reducing the visibility of seams can include printing the multicolor graphic product such that print swaths used in printing one color plane are not in registration with those of another color plane. Thus any seams in the first color plane do not have the same position along the printing sheet translation (X) axis as seams in the other color plane. Another technique that may be of use is to print swaths with other than “straight” bounding edges. For example, the print swath


28


shown in

FIG. 1

is bounded by the straight edges


29


A and


29


B. The array of thermal printing elements


26


can be energized such that bounding edges of the print swath assume a meandering shape, such as a sawtooth or sinusoid. Successive print swaths thus have edges that meet in the manner of the pieces of a jigsaw puzzle.




According to another technique practiced in accordance with the invention, the distribution of pressure along the array of thermal printing elements is modified. For example, with reference to

FIG. 19B

, consider that thermal printhead


24


is about to print the print swath


28


, having just printed print swath


28


′ and deposited a slightly raised area of ink


435


on the printing sheet material


16


. The thermal printing elements


26


A, though not normally used for printing, contact the raised are of ink


435


, and the contact and/or pressure between the array of thermal printing elements


26


and the printing sheet material


16


is not uniform along the length of the array of thermal printing elements


26


. Accordingly, shims


437


can be placed between the mounting block


402


of the thermal printhead


24


as shown in

FIGS. 19A and 19B

. Typically, these shims are approximately 1 thousandths of an inch thick. The use of such shims has been found to improve the quality of the printed multicolor graphic product.




Donor Sheet Conservation




The present invention includes many features intended to provide for economical and efficient printing of the multicolor graphic product on the printing sheet


16


. It is known in the art that the donor sheet is typically expensive. Accordingly, the donor sheet assembly


228


includes a length of donor sheet


229


that can be, for example, 500 meters long, such that an operator of the wide format thermal printer can realize the economic benefits of buying in bulk.




Furthermore, the memory element


300


includes data representative of the length of unused donor sheet remaining on the supply core body


230


. Accordingly, before a particular job is started, the controller(s)


22


associated with the wide format thermal printer


10


can determine whether enough donor sheet remains on the supply core body


230


to completely print a particular color plane. Unexpectedly running out of the donor sheet during printing is a problem not unknown with prior art printers and typically destroys the multicolor graphic product, wasting the donor sheet that had been already used in printing the color planes of the multicolor graphic product. This problem can be avoided with techniques and apparatus of the present invention.




According to the invention additional methods and apparatus are provided for conserving donor sheet while printing and for reducing the amount time required to print a particular multicolor graphic product on the printing sheet


16


. The apparatus and method involve programming running on the controller(s)


22


associated with the wide format thermal printer


10


. Techniques referred to herein as X axis conservation, Y axis conservation, knockout conservation, and time conservation, are now described.





FIG. 20

illustrates the technique of Y axis conservation. Consider printing the text “MAXX”, as indicated by reference numeral


450


. The individual letters are indicated by reference numerals


452


A through


452


E. Assume for simplicity that the height of the text “MAXX” is such that it may be printed in one print swath


28


. The thermal printhead


24


prints the text


450


by pressing the donor sheet


153


against the printing sheet


16


and selectively energizing the array of thermal printing elements


26


while translating the thermal printhead


24


along the print (Y) axis. Translation of the thermal printhead


24


while pressing the donor sheet


153


against the printing sheet, causes the donor sheet to be drawn past the thermal printhead


24


. Reference numerals


454


indicate translation along the print (Y) axis with the thermal printhead down for printing the individual letters


452


A through


452


E of the text


450


. According to the invention, the thermal printhead


24


is lifted in between printing objects, such as the individual letters


452


A through


452


E, when the objects are separated by at least a selected distance in the direction of the print (Y) axis, so as to not draw the donor sheet


153


past the thermal printhead


24


when there are not any pixels to be printed. Reference numerals


456


indicate translation along the (Y) axis while the thermal printhead is lifted away from the printing sheet


16


. The pivot actuator


74


lifts the thermal printhead


24


by moving the cantilever arm


72


upward, upon instruction from the controller(s)


22


associated with the wide format thermal printer


10


.





FIGS. 21A and 21B

illustrate the use of the technique referred to as (X) axis conservation. With reference to

FIG. 21A

, consider the printing of the exclamation mark


474


having a top portion


474


A and a lower portion


474


B. The printing sheet


16


is translated in the direction indicated by reference numeral


470


. According to one technique for printing the multicolor graphic image, each of the color planes is divided into a number of print swaths, each having a swath width substantially equal to the printing width of the array of thermal printing elements


26


along the printing sheet translation (X) axis, and the printing sheet


16


is translated a distance equal to the swath width after printing each of the print swaths. Such a technique can result in the exclamation mark


474


being printed as illustrated in

FIG. 21A

, that is, in the three (3) print swaths


28


A,


28


B and


28


C. When printing the exclamation point


474


, the printhead is only down for a distance along the (Y) axis, indicated by the reference numeral


476


. However, note that the shaded areas, indicated by reference numerals


478


A, are portions of the donor sheet that are drawn past the thermal printhead


24


, but are not used for printing. The portions


478


A are simply wasted. Some waste, of course, is unavoidable. However, by translating the printing sheet


16


a selected distance


480


along the printing sheet translation axis, it is possible to print the exclamation mark


474


in fewer print swaths.




For example, as shown in

FIG. 21B

, the exclamation mark


474


may be printed in two (2) print swaths


28


C and


28


D, such that the wasted portions of the donor sheet, indicated by reference numerals


478


B, is less than the wasted portions indicated by reference numerals


478


A. Typically, (X) axis conservation involves translating the printing sheet


16


a selected amount, which can be other than an integer number of swath widths, so as to print a given portion of the color plane with a reduced number of print swaths.




The invention also includes methods and apparatus for practicing the technique referred to above as “knock-out” conservation. Consider the two (2) yellow banners, indicated by reference numeral


500


as shown in

FIG. 22A

, and also consider the text “MAXX”, indicated by reference numeral


450


and shown in

FIG. 22B. A

graphic designer may desire that the text


450


be laid-over the yellow banners


500


such that the text, if for example, printed in black, knocks out the yellow banners where the text overlays the yellow banners


500


. For example, with reference to

FIG. 22C

, the letter “A”, indicated by reference numeral


452


B, knocks out a portion of the left yellow banner


502


A, as does the letter “M”, indicated by reference numeral


452


A. These two (2) knocked out portions are shown in

FIG. 22D

, and indicated by reference numerals


506


and


508


, respectively. Because the wide format printer


10


prints in separate color planes, unless properly instructed, the printer


10


simply prints all of tho yellow banners


502


A and


5028


when printing the yellow color plane and then proceeds to print the yellow with the black text “MAXX” when printing the black color plane. However, according to the invention, the knocked out areas of the yellow banners, such as those areas indicated by reference numerals


506


and


508


in

FIG. 22D

, are determined and the printer


10


refrains from printing knocked out areas such as


508


and


506


for conserving the yellow donor sheet.




The invention also includes method and apparatus for reducing the time required to print the multicolor graphic product on the printing sheet


16


. For example, with reference to

FIG. 23

, consider that the exclamation mark


474


is the final object printed in a first color plane and that it is printed in two (2) print swaths


28


C and


28


D. Consider also that the next color plane to be printed is a green color plane that consists of the four (4) rectangular blocks


512


A through


512


D. The thermal printhead


24


finishes printing the first color plane with the printing of the print swath


28


.




The green color plane can be considered to have a near end, indicated by reference numeral


518


, and a far end, indicated by reference numeral


516


. The wide format thermal printer


10


can print the green color plane by translating the printing sheet


16


, as indicated by reference numerals S


20


and


522


such that objects nearer the far end


516


are printed first, or, alternatively, can trans late the printing sheet


16


as indicated by reference numeral


524


and


526


, such that objects nearer the near end


518


are printed first. As can be appreciated by viewing

FIG. 23

, the total distance the printing sheet


16


is translated is less when printing the color plane by printing objects nearer the near end


518


first than when printing the objects nearer the far end


516


first. Translating the printing sheet


16


a shorter distance reduces the time to print the multicolor graphic product. Because the wide format thermal printer of the present invention can print in either direction along the printing sheet translation (X) axis, one printing technique can be simply alternating printing directions as successive color planes are printed. However, as shown in

FIG. 23

, it can be more efficient to evaluate the position of the printing head when finishing a first color plane relative to the objects of the next color plane to be printed and translating the printing sheet such that the objects nearer the near end of the next color plane are print ed before the objects nearer the far end of the next color plane. This can involve printing successive color planes in the same direction. Note that printing a single color piano can involve printing while translating in both direction along the printing sheet translation (X) axis.




Before the multicolored graphic product is printed on the printing sheet


16


, machine readable data files representative of the graphic product are created. Typically, a graphic artist working at a computer workstation provides input using a keyboard and a pointing and selecting device, such as a mouse or light pen, to gene rate an image representative of the multicolor graphic product on the screen of the workstation. The workstation stores one or more data files representative of the multicolor graphic image in a memory associated with the workstation. The graphic artist incorporates bitmap images, text, and geometric shapes, as well as other objects, into the final multicolor graphic product, and can enter these objects into workstation in any order. The file created by the workstation representative of the multicolor graphic image is referred to herein as “plot file,” or alternatively at a “job file.” According to the invention the plot file is processed to separate out individual color plane data and to place the data representative of the multicolor graphic image in a form suitable for instructing the wide format thermal printer


10


to print the multicolor graphic product using the donor sheet and time conservation techniques illustrates in

FIGS. 20-23

.




Accordingly, the above techniques illustrated in

FIGS. 20-23

are implemented via appropriate software, hardware, or firmware associated with the controller(s)


22


of the present invention, and typically involve processing of the data representative of the multicolor graphic product, such as the job file. Presented below is a preferred embodiment of processing techniques, in the form of flow charts, for achieving X axis conservation, Y axis conservation, knock out conservation and printing time conservation, as illustrated in

FIGS. 20-23

above. One of ordinary skill, in light of the disclosure herein, can program the controller(s)


22


associated with wide format thermal printer


10


and/or provide the appropriate firmware or hardware so as to functionally achieve the above conservation techniques.





FIGS. 24-26

are flow charts illustrating processing data representative of the multicolor graphic product such that the wide format thermal printer


10


of the present invention prints the multicolor graphic product according to the conservation techniques illustrated in

FIGS. 20-23

.





FIGS. 27A-27I

are intended to be considered in conjunction with the discussion of

FIGS. 24-26

. Each of the

FIGS. 27A-27I

includes a coordinate axes indicating the printing sheet translation (X) and print (Y) directions. With reference to

FIG. 27A

, consider that the multicolor graphic product to be printed on the printing sheet


16


consists of the word “TEXT” printed twice. The letters represented by the reference numerals


552


A through


552


F are to be printed in one color, and that the letters “X” and “T”, represented by reference numerals


554


A and


554


B, respectively, are to be printed in a second color. Each of the letters in


552


and


554


is an object in a plot file created by the graphic artist, who may enter the objects into the plot file in any order. For simplicity, all the objects shown in

FIG. 27A

are textual characters, which are typically geometric shapes.




The data processing steps indicated in the flow charts in

FIGS. 24-26

are performed for each color plane. Typically, the order of printing color planes is predetermined by the nature of the multicolor graphic product. Typical multicolor graphic products printed by the wide format thermal printer


10


of the invention Can include process colors, such as the subtractive “CMYK” process colors and additionally, spot colors specific to a particular job and that are typically not rendered faithfully by a combination of the process colors and, hence, are printed by using a donor sheet of the desired spot color. It is known in the art that the CMYK process colors are preferably printed in a selected order. Accordingly, the multicolor graphic product can include deliberate overprints.




Reference numerals


558


A through


558


E in

FIG. 24A

indicate data processing steps wherein the job file is read to sort out those objects that are of the same color as the color plane to be printed. For each object found that is of the color plane to be printed, a bounding rectangle is created about that object, as indicated by reference numeral


558


D. For example, assume that the color plane to be printed corresponds to the color of the objects


552


in FIG.


27


A. The routine indicated by reference numeral


558


in

FIG. 24A

results in the creation of the bounding rectangles


562


A through


562


F shown in FIG.


27


B. Note that the objects


554


A and


554


B do not receive bounding rectangles because they are not of the color to be printed in this color plane. Typically objects are shapes and bitmaps. A bitmap receives its own bounding rectangle.




After the job file has been read through to sort those objects of the color of the color plane to be printed and the bounding rectangles drawn around each object, the bounding rectangles are sorted left-to-right along the printing sheet translation (X) axis, as indicated by functional block


564


. For example, each bounding rectangle


562


shown in

FIG. 27B

can be considered to have an X and Y coordinate associated therewith, such as the X and Y coordinate corresponding to the lower left-hand corner of each bounding rectangle. According to functional block


564


, the bounding rectangles are sorted such that those with the lower X coordinate are arranged in a list before those with higher X coordinates. Next, as indicated by functional block


566


, print slices are created from bounding rectangles. The term “print slice” as used herein, simply refers to a rectangular area of the color plane. Initially there is a 1 to 1 correspondence between print slice and bounding rectangles; that is, each print slice simply becomes a bounding rectangle.




Proceeding to functional block


568


, print slices that are within a selected distance of each other along the X axis are combined.

FIG. 24B

is a block diagram schematically illustrating a preferred technique for combining print slices. As indicated by functional block


570


A, a “slices changed” variable is defined and set as “TRUE.” In decision block


570


B, the slices changed variable is evaluated. If the “slices changed” is true, the “yes” branch is followed to functional block


570


C where the “slices changed” variable is set to “FALSE,” and proceeding to functional block


570


D, the current slice is selected to be the first slice from the list of slices created by functional blocks


564


and


566


. Next, decision block


570


E checks to see whether slices remain in the list to be processed, and returns to decision block


570


B if the list includes more slices to consider, as is discussed below. Proceeding to decision block


570


F, neighboring slices are compared to see if they are within a selected distance of each other along the X axis. If the slices are close, that is, they are separated by less than the selected distance, they are combined to form a new slice. For example, in

FIG. 27B

, the rectangular boxes


562


A and


562


B are now each slices. As they are very close, actually overlapping, they are combined into the new combined slice


980


in FIG.


270


.




Proceeding with functional blocks


570


H and


570


I in

FIG. 24B

, the number of slices is decremented and the “slices changed” variables is set to “TRUE.” Returning to decision block


570


E, the above procedure is repeated, and

FIG. 27D

illustrates the result of proceeding through the blocks


570


E through


570


I again. The new combined slice


580


has been compared to the next nearest slice, which is the former rectangle


562


C. Accordingly, these two are combined, as shown in

FIG. 27D

, to form the new slice


582


which will, in turn, be combined with the former rectangular box


562


D to form the combined slice


584


, shown in FIG.


27


E. Note that the combined print slice technique shown in the block diagram


570


will continue until, in going through the entire list of slices, no slices are changed. For example, whenever any slice is changed, the “slices changed” variable is set to “TRUE” and after following the “no” branch from decision block


570


E to decision block


570


B, the procedure of blocks


570


E through


570


I is again followed. This process continues until, in going through the whole list of slices, no slices are changed, at which point, the “combine slices” routine


570


is exited, as indicated by reference number


570


K.




With reference again to

FIG. 24A

, proceeding from functional block


568


to functional block


572


, the width of each slice, where “width” in this context refers to its dimension along the X axis, is “grown”, or increased, to be an integer number of printing, or swath, widths. The increase in X dimension is toward the middle of the color plane. For example, with reference to

FIG. 27F

, the right-hand boundary


585


of the slice


584


is extended to


586


such that the width of the slice


588


along the X axis corresponds to an integral number of print-head widths. The printing width is typically about 4 inches.




Returning to

FIG. 24A

, after increasing the width of each slice as necessary to be an integer number of printing width, the combine print slices procedure


570


of

FIG. 24B

is again performed, as indicated by functional block


576


. For example, the new slice


584


having the boundary indicated by reference numeral


586


in

FIG. 27F

, is now much closer to the rectangular box


562


E, now considered a slice, in FIG.


27


F. Accordingly, as shown in

FIG. 27G

, on proceeding again through the combined print slice flow chart


570


, a new slice


586


, as indicated in

FIG. 27G

, is generated. The combined print slice flow chart is followed again until reaching the “done” block


570


K.




The block diagram shown in

FIG. 24A

results in the color plane of the color to be printed being organized into a selected number of print slices where a print slice, as noted above, is a rectangular area of the color plane. With reference now to

FIGS. 25A and 25B

, reference numeral


556


refers to the generation of the print slices described above in

FIGS. 24A and 24B

.




Proceeding to functional block


594


of

FIG. 25A

the direction of motion of the printing sheet along the printing sheet translation axis during printing of the color plane is determined. This direction is determined, as indicated by FIG.


23


. That is, the left to right list created at functional block


564


is examined and compared to the known present position of the thermal printhead


24


to determine the nearer end of the color plane. The direction of translation of the printing sheet


16


is selected such that the color plane is printed from its nearer end to it farther end. Depending as on the direction selected, as indicated by reference numerals


596


to


600


, either the last print slice or the first print slice is taken as the current print slice.




Decision block


602


causes an exit to the “done” state, indicated in decision block


604


, if there remain no print slices to process in the color plane. Next, as indicated by functional block


606


, the printing sheet


16


is translated such that the thermal printhead


24


is positioned at the beginning of the current print slice location. Proceeding to functional block


608


, the print slice is subdivided into print swaths of width equal to the printing width, described above, of the thermal printhead


24


. See

FIG. 27H

, wherein the print slice


586


is now divided into print swaths


28


A,


28


B and


28


C and the rectangular box


562


F, now a print slice, is divided into a print swath


28


D. Proceeding to functional block


610


, the first print swath is set as the current print swath. As indicated by reference numeral


612


, indicating the circled “A”, the remainder of processing is described in FIG.


25


B.




With reference to

FIG. 25B

, decision block


614


checks to ensure that print swaths remain to be processed. If the answer is “NO”, reference numerals


616


referring to the circled “C” in

FIGS. 25A and 25B

, indicate proceeding back to decision block


602


of

FIG. 25A

to print other print slices. As described above, if there are no other print slices, decision block


602


leads to “done,” as indicated by block


604


, and printing of the color plane is complete.




However, as of yet, the printing of a print swath is not described. Returning to

FIG. 25B

, as indicated by block


618


, a memory region that is equal to the length and width of the print swath is set aside in a memory associated with the controllers. This is a one-to-one mapping, that is, the memory region includes one memory location for each pixel that can be printed within the print swath. Next, as indicated by functional block


620


, the print job, that is, the file created by the graphic artist, is examined again. Each object in the print job file is examined to determine if it is of the color to be printed in the color plane and whether it falls within the current print swath. Initially, as indicated by functional block


620


, the first object in the print job file becomes the current object. Decision block


622


checks to make sure there are still objects to process. Proceeding to decision block


624


, if the object is the same color as the color plane about to be printed and it falls within the current print swath, the object is “played” into the memory region, that is, binary “ONES” are inserted in the memory regions at those locations corresponding to the pixels wherein the color should be printed on the printing sheet


16


.




Assume that it is determined at decision block


624


that the current object is not of the color plane to be printed. Following the “NO” branch from decision block


624


, decision block


630


checks to see if the current object is an deliberate overprint, that is, the object is to be deliberately printed over to achieve a particular effect. If it is an overprint, as indicated by the “YES” branch of decision block


630


, decision block


628


makes the next object the current object. However, if the current object is not a deliberate overprint, then the current object is of a color that prints over the color of the color plane being printed, and a “hole” is knocked-out for the object in the memory region, that is any “ONES” in a locations corresponding to current object are changed to “ZEROS.” This corresponds to the “knock-out” conservation shown in FIG.


22


D. After all objects in the print job file are processed, the “NO” branch of decision block


622


is followed, leading to the circled “B”, as indicated by reference numeral


640


.




With reference to

FIG. 25C

, further processing is now described as indicated by decision block


642


, a check is made to determine whether the memory region created by functional block


618


is empty. If the memory region is empty, there are no objects to be printed in the current print swath. For example, all of the objects printed in the swath may have been knocked-out. If the memory region is empty, following the “YES” branch of decision block


642


leads to functional block


744


, wherein the printing sheet


16


is translated past the print swath


28


A, and as indicated by reference numeral


612


and the circled “A”, the next print swath is printed, as indicated by reference numeral


612


in FIG.


25


B.




Alternatively, if the memory region is determined in decision block


642


not to be empty, functional block


646


performs Y axis conservation for the current print swath, corresponding to lifting the printhead as illustrated in

FIG. 20. A

print swath consists of consecutive rows of pixels, where the rows extend along the printing sheet translation (X) axis, each pixel corresponding to one thermal printing element of the array of thermal printing elements


26


. Basically, each row of pixels within the print swath is examined to see if all the pixels that row are blank, and to determine when there exists consecutive blank rows. The number of consecutive blank row is counted, and, should more than a threshold number of consecutive blank rows be found, the print swath is divided into sub-swaths, where the thermal printhead


24


is lifted between subswaths. This procedure is described in detail below.





FIG. 26

is a flow chart illustrating the Y axis donor sheet conservation procedure and is considered in conjunction with FIG.


27


I. Consider print swath


28


A, shown in FIG.


27


I. Starting with functional block


647


in

FIG. 26

, the variable “looking for a blank row” is set at “TRUE.” Then, in functional block


648


, the number of blank rows are set equal to “ZERO,” Proceeding to functional block


650


, the current row is set as the first row of the swath


28


A. The first row of pixels is indicated by reference numeral


651


in

FIG. 27I

, with the individual pixels indicated by reference numerals


652


. For simplicity, the individual pixels


652


are shown as much larger than they typically are in practice. (Typically, a print swath is four (4) inches wide, and there are 1200 pixels across the width of the swath, for a resolution of 300 dpi.)




Returning again to the flow chart of

FIG. 26

, the decision block


660


checks to see whether there are more rows in the swath


28


A to process. At this point, the variable “looking for a blank row” is “TRUE,” having been set by the functional block


647


and not otherwise reset. Accordingly, proceeding along the “YES” branch to decision block


666


, each pixel of the current row is examined to determine whether the row


651


is blank. Accordingly, proceeding along the “YES” branch from decision block


666


to functional block


668


, the number of blank rows is incremented. Proceeding to decision block


670


, the number of blank rows is compared to the threshold value, and assume for the purposes of this example that this threshold value is six (6) blank rows.




The six blank rows


651


to


656


are counted by repeating the blocks


660


,


664


,


666


,


668


,


670


, and


672


. As the number of blank rows does not exceed six (6), the “NO” branch leading from decision block


670


is followed, which leads to functional block


672


, setting the next row as the current row, leading again to a decision block


660


,


664


, etc. This procedure continues through the decision and functions blocks indicated until all the six rows


651


-


656


shown in slice


28


A of

FIG. 27I

are counted. Finally, when processing the seventh (7th) row, indicated by reference numeral


674


in

FIG. 27I

, decision block


666


determines that the row is not blank, and proceeding along the “NO” branch to functional block


680


, resets the number of blank rows. The next row is made the current row according to functional block


672


and the process described above repeats.




Consider the examination of rows


680


-


688


in FIG.


27


I. In this instance, it is determined by the program represented by the flow chart of

FIG. 26

that the threshold number of blank rows is exceeded. Accordingly, when examining the row


687


in

FIG. 27I

(the seventh row), it is determined in decision block


670


that the number of blank rows is greater than the threshold value (6) and, proceeding along the “YES” branch to functional block


671


, a sub-swath is created such that after printing the “T”


552


A in swath


28


A, the thermal printhead


24


is lifted. Proceeding now to functional block


692


, the variable “looking for a blank row” is set at “FALSE,” and the next row is made the current row by functional block


672


. Basically, at this point, the counting of blank rows continues to determine when the thermal printhead


24


is to be dropped again. As the variable “looking for a blank row” is “FALSE,” when reaching decision block


664


the “NO” branch is followed, leading to decision block


694


which checks to determine whether the current row is blank. If the current row is blank, functional block


672


sets the next row as the current row. Eventually, however, after examining row


696


, the next row is found to contain pixels to be printed. The “NO” branch leading from decision block


694


is followed and, as indicated in functional block


700


, the number of blank rows is set to “ZERO.” Proceeding to functional block


702


, the variable “looking for blank rows” is set at “TRUE” and the procedure illustrated above repeats until all the rows of the swath have been examined. For the example of print swath


28


A, two (2) sub-swaths


690


and


710


are created, as shown in FIG.


27


J.




Referring back to

FIG. 25C

, after performing the print (Y) axis donor sheet conservation of functional block


646


, the first sub-swath is taken as the current swath, as indicated by functional block


712


. Proceeding to decision block


714


, a check is made to determine whether there are more sub-swaths to process. Proceeding to functional block


716


, the thermal printhead


24


is moved along the print (Y) axis to the beginning of the sub-swath position corresponding to the position indicated by reference numeral


718


in FIG.


27


J.




Proceeding to functional block


720


, the sub-swath


690


of

FIG. 27J

is now printed by translating the thermal printhead


24


along the print (Y) axis. The thermal printhead


24


is lifted at the end of the print swath indicated by reference numeral


722


. As indicated by FIG.


25


C and the loop return path


724


, the next sub-swath


710


is printed. Next the “NO” branch of decision block


714


is followed, leading to functional block


744


wherein the printing sheet


16


is moved along the printing sheet translation (X) axis past print swath


28


A to the next print swath


28


B. As indicated by reference numeral


612


, indicating the circled “A”, returning to the top of

FIG. 25B

the remaining print swaths are processed and the procedure outlined above repeats for each print swath in the color plane. The flow charts of

FIGS. 24-26

are repeated for each color plane of the multicolor graphic product, for example so as to print the objects


554


A and


554


B.

FIG. 27J

illustrates how the procedure as detailed in the above flow charts can divide the print swaths


28


B,


28


C and


28


D into individual sub-swaths


750


to


754


,


756


and


758


.




Tension Control




Proper control of the tension applied to the donor sheet section


153


A (see

FIG. 12

) during printing can help ensure that a high quality multicolor graphic product is printed on the printing sheet


16


. As understood by one of ordinary skill in the art, the tension to be applied to the donor sheet section


153


A typically varies as a function of the characteristics of the particular type of donor sheet being used to print. According to the invention, data characteristic of the donor sheet can be read from the memory element


300


mounted by the supply core body


230


prior to loading the donor sheet cassette


32


on the cassette receiving station


96


, and the desired tension determined by the controller(s)


22


as a function of the read data. Alternatively, the desired tension can be assumed to be a constant, i.e., the same for all donor sheets. This assumption is often justified.




The desired tension is applied to the donor sheet by selectively energizing the take-up motor


104


and the magnetic brake


110


. As is also known in the art, the radius of the length of donor sheet


229


wound on the supply core body


230


(i.e., the radius of the supply roll of donor sheet) and the radius of any donor sheet wound about the take-up core body


235


(i.e., the radius of the take-up roll) need to be determined and taken into account to determine the proper energization of the take-up motor


104


and the magnetic brake


110


.




It is known in the art to determine the overall radius of a known length of donor sheet wound on the supply core body


230


from a knowledge of the radius of the core body and the thickness of the donor sheet. See for example U.S. Pat. No. 5,333,960 issued Aug. 2, 1994, and herein incorporated by reference. According to the invention, however, the thickness of the donor sheet need not be known to determine the overall radius of a remaining length of donor sheet wound on a core body.




In the present invention, the controller(s)


22


can track the length of donor sheet used, i.e., the length transferred past the thermal printhead


24


, by tracking the distance translated by the thermal printhead


24


along the print (Y) axis with the thermal printhead


24


pressing the donor sheet against the printing sheet


16


. The length of donor sheet remaining on the supply roll is determined as the original length wound on the supply core body minus the length used as tracked above The length of donor sheet wound on the take-up core body is equal to the length tracked above, or the original length wound on the supply core body


230


minus the length remaining on the supply core body


230


.




According to the invention, the radius of the supply roll of the donor sheet can be determined responsive to data read from the memory element


300


. For example, the controller(s)


22


can approximate the current radius of the supply roll from data representative of the following: 1) the remaining length of the donor sheet on the supply core body; 2) a known length of donor sheet wound on the supply core body


230


; 3) the radius of the supply roll when the known length is wound on the supply core body


230


; and 4) the radius of the core tubular body. Typically, items 1)-3) are read from the memory element, and item 4) is fixed and stored by a memory associated with the controller. Item 1), the remaining length, is written to the memory element


300


when the donor sheet cassette


32


is returned to the cassette storage rack


55


after printing a color plane or a portion thereof. The known length and known radii typically are the original length of donor sheet wound on the supply core body


230


, and the radius corresponding to the original length, and these are written to the memory element


300


at the time of manufacture of the supply roll. The radius r


c


of the core supply core body


230


and the radius R of the supply roll of donor sheet are shown in FIG.


15


A.




According to the invention, the radius of the supply roll can be determined from the equations I and II below, or directly from equation III, which is obtained by combining equations I and II. The terms used in the equations are defined below.




L


f


=a known length of donor sheet wound on the core body




R


f


=the known radius of the length L


f


of donor sheet wound on the core body




r


c


=the radius of the core body




l


c


=the length of the donor sheet that when wound into a roll would have the radius r


c






L=a second known length of donor sheet wound about the core body




R=the radius of the length L of donor sheet wound on the core body, unknown and to be determined












L
f

+

l
c



l
c


=


R
f
2


r
c
2






Equation





I








L
+

l
c




L
f

+

l
c



=


R
2


R
f
2






Equation





II






R
=




r
c
2



(

1
-

L

L
f



)


+


L

L
f








R
f
2








Equation





III













Once the radius of the supply roll is determined, the brake


110


is energized by providing the energization E to the take-up motor according to Equation IV, where:




E=the energization provided to the take-up motor (or brake) to provide desired tension




E


thresh


=the threshold energization that must be provided to the take-up motor to overcome friction (or to the brake to initiate braking)




E


c


=the energization of the motor (or brake) needed to provide a known tension for a known radius (the “known” radius used is r


c


)




T


d


=desired tension to be applied to donor sheet (such as determined from data read from the memory element)




T


k


=tension applied to the donor sheet at energization E


c


and known radius r


c












E
=



(


E
c

-

E
thresh


)



R

r
c









T
d


T
k



+

E
thresh






Equation





IV













The tension T


k


which is the tension applied to the donor sheet when a known energization E


c


is applied to the brake


110


and the supply roll has the known radius r


c


, can be determined empirically, such as by using a spring gauge, taking into account the typical translation speed (e.g., 2 inches/minute) of the printhead carriage


30


when printing along the print (Y) axis. This data is typically stored in a memory associated with the controller


22


.




The above equations are also used for the energization of the take-up motor


104


. Note that the thermal printhead


24


, when pressing the donor sheet against the printing sheet


16


, largely isolates the brake


110


from the take-up motor


104


, such that the tension in the donor sheet between the thermal printhead


24


and the supply roll is affected largely by the brake rather than the take-up motor, and the tension on the donor sheet between the thermal printhead


24


and the take-up roll is affected mostly by the energization of the take-up motor


104


, rather than by the brake.




The threshold energization of the take-up motor


104


and the brake


110


can be determined as follows: After mounting a new donor sheet cassette


32


onto cassette receiving station


96


, the take-up motor


104


is be rotated in the reverse direction to create some slack in the donor sheet. Next, take-up motor is increasingly energized for forward rotation until the take-up motor just begins to rotate. The take-up motor threshold energization level corresponds to the energization at which this onset of rotation is noted.




A threshold energization for the brake can be determined in a similar manner. For example, after generating the slack in the donor sheet and determining E as noted above, the take-up motor


104


is further rotated to remove the slack previously introduced, and the energization of the take-up motor is further increased such that rotational sensor or encoder again indicates the onset of rotation of take-up roll. The brake is now increasingly energized until the rotation ceases, and this energization level corresponds to the threshold energization when using the equations above to determine the energization of the brake to provide the desired tension. Typically, the threshold energization do not vary significantly from donor sheet cassette to donor sheet cassette.





FIG. 28

is a flowchart illustrating the steps followed to energize the brake


110


(or the takeup motor


104


) to provide a selected tension on the donor sheet. As indicated by block


770


, the original length of donor sheet wound on the supply core body


230


, the original radius of the of the length of donor sheet wound on the supply core body, and the length of donor sheet remaining on the supply core body


230


are read form the memory element


300


. Proceeding to block


772


, the radius corresponding to the length of donor sheet wound on the supply core is determined as a function of the data read from the memory element and the radius of the core tube, which is typically fixed and stored in a memory associated with the controller


22


. Proceeding to block


774


, the desired tension is determined. If necessary, additional data can be read from the memory element, and, for example, look up tables consulted to determine the desired tension corresponding to the donor sheet. As indicated in block


778


, the donor sheet cassette containing the donor sheet wound on the core body is loaded onto the cassette receiving station


96


. The energization to be applied to the take-up motor and the brake are each determined in accordance with Equation IV presented above. Proceeding to block


780


, the energization is applied to the brake to provide the desired tension.




The donor sheet can spool onto the take-up core differently than the unused donor sheet spools on the supply core body


230


, due to the ink material transferred from the donor sheet to the printing sheet


16


during printing, among other factors. However, as with energizing the brake


110


, a known radius corresponding to a known length of donor sheet wound on the take-up core body suffices to determine the proper energization of the take-up motor


104


, and both are typically determined empirically. A rotation sensor, such as the encoder indicated by reference numeral


875


in

FIG. 4B

, is typically coupled to the take-up motor


104


, and is included in the present invention to determine when the donor sheet has broken. The encoder will indicate an excessive number of rotations per unit time.) According to another technique that can be practiced in accordance with the invention, the change in the radius of the takeup roll can be tracked by noting the length of donor sheet used, as described above, as well as the number rotations of the take-up roll, as determined by a rotation sensor or encoder


875


.




Preferably, the invention includes the magnetic brake


110


coupled to the supply roll for tensioning the donor sheet between the supply roll and the thermal printhead


24


. However, as is known in the art, a mechanical brake can also be used. For example, a spring-biased arm mounting a friction pad can be disposed such that the friction pad rests against the supply roll, such as against the outer layer of donor sheet wound on the supply roll.





FIG. 29

schematically illustrates one example of the on-board controller


22


A and the interfacing of the on board controller


22


A with other components of the wide format printer


10


. The on board controller


22


A can include an IBM compatible pc


800


in communication with the Digital Signal Processor (DSP)


802


, which handles much of the standard, lower level functionality of the wide format printer


10


. The IBM compatible pc can include the Pentium MMX processor


801


, and the typical other standard hardware, such as the mouse keyboard and video interfaces


804


; the printer port


806


; the hard drive


808


; the CD ROM drive


810


; the floppy disk drive


812


; and the random access memory (RAM)


814


. Also included are the following: the serial port


816


in communication with the data transfer element(s)


304


for communication with memory elements


300


mounted in donor sheet apparatus


228


received by donor sheet cassettes


32


on the cassette storage rack


55


; the second serial port in communication with the user interface


61


; and the communication interface


822


for communicating with other controller(s)


22


.




The DSP


802


communicates with the printhead power supply


828


that provides the electrical power for energizing the thermal printing elements of the thermal printhead


24


. As is known by ordinary skill in the art, considerable power can be required to properly energize the thermal printing elements, and the printhead power supply often includes a large storage capacitor(s) for enhancing power deliver to the thermal printing elements. The storage capacitor or capacitors can be located proximate to thermal printhead


24


, rather than with the printhead power supply


828


, for reducing the effects of the inductance of the power leads running from the printhead power supply


828


to the thermal printhead


24


. The DSP also communicates with the semiconductor element


420


mounted with the thermal printhead


24


, communicates print data representative of the multicolor graphic product to the thermal printhead


24


for selectively energizing the thermal printing elements, and communicate with the rotary sensor or encoder


830


coupled to the take-up shaft


100


for sensing rotation thereof.




The wide format thermal printer


10


can also include the driver board


834


and the five (5) motor drivers


840


for driving those motors or actuators of the wide format thermal printer


10


that preferably are stepper motors. For example, as indicated by

FIGS. 29A AND 29B

, the printing drive motor


36


, left and right clamp actuators


58


A and


56


B, respectively, the pivot actuator


74


, and the belt drive motor


120


are preferably stepper motors and can be driven by the driver board


834


in combination with the motor driver boards


840


.




As understood by those of ordinary skill in the art, the wide format thermal printer of the present invention can include various sensors, detectors, interlocks, etc., that are known to be useful for safe and efficient use of the wide formal thermal printer and that are often employed on printers or plotters known in the art. Sensors are often included with stepper and other motors to indicate “home” and “end” positions of the motors or the apparatus driven by the motors. The driver board


834


communicates with such sensors and interlocks. As indicated by reference numerals


845


and


847


, the driver board


834


can also communicate with the home position sensor


366


described in conjunction with aligning and tracking the printing sheet


16


, the edge sensor


360


and the hanging loop optical sensor


66


. As indicated by reference numeral


850


, the driver board


834


also drives the clamps


44


and


46


between the clamped and unclamped conditions, as well the dc motors or actuators of the wide format thermal printer


10


, such as the take-up motor


104


and the brake


110


, and the squeegee


62


actuators. The vacuum sensor


220


and flow control valves


224


and


226


can also be driven by the driver board


834


.



Claims
  • 1. Vacuum workbed for supporting a sheet material to be worked on, comprising:a workbed having a worksurface for supporting the sheet material, the worksurface including a plurality of apertures for applying suction to the sheet material, said apertures separated into first and second zones for accommodating sheet material of different sizes and orientations, with said zones not being in fluid communication with each other; a suction source for applying suction to the apertures; a manifold for providing fluid communication between said suction source and said apertures for applying suction thereto; a sensor in fluid communication with said suction source for providing a signal responsive to the degree of vacuum drawn by said suction source on the apertures; and means for restricting the flow rate through one of the zones of apertures for reducing degree of vacuum loss when said one zone includes unblocked apertures.
  • 2. The vacuum workbed of claim 1 wherein said means for resricting the flow rate includes a flow restriction element interposed between said one of the zones and said suction source.
  • 3. The vacuum workbed of claim 2 including a second flow restriction element interposed between the other of the zones and the suction source.
  • 4. The vacuum workbed of claim 1 wherein said means for restricting the flow rate restricts the flow rate through said one of the zones such that adequate suction is provided through the other of said zones when said one of said zones includes unblocked apertures.
  • 5. The vacuum workbed of claim 1 wherein said worksurface of said workbed is a flat worksurface.
  • 6. The vacuum workbed of claim 1 wherein said worksurface of said workbed is a curved worksurface.
  • 7. The vacuum workbed of claim 1 wherein said worksurface of said vacuum workbed is a cylindrical worksurface of a drum platen.
  • 8. The vacuum workbed of claim 1 wherein said suction source is a mechanical evacuation pump.
  • 9. The vacuum workbed of claim 1 including an orifice for communication with the atmosphere for providing a selectad leakage to said suction source.
  • 10. The vacuum workbed of claim 1 including a flow control valve disposed for controlling the application of suction to said zones.
  • 11. Vacuum workbed for supporting a sheet material to be worked on, comprising:a workbed having a worksurface for supporting the sheet material, the worksurface including a plurality of apertures for applying suction to the sheet material, said apertures separated into first and second zones for accommodating sheet material of different sizes and orientations; a suction source for applying suction to the apertures; a manifold for providing fluid communication between said suction source and said apertures for applying suction thereto; a sensor in fluid communication with said suction source for providing a signal responsive to the degree of vacuum drawn by said suction source on the apertures; means for restricting the flow rate through one of the zones of apertures, and wherein signals responsive to the number of said zones having unblocked apertures are produced by said sensor.
  • 12. Vacuum workbed for supporting a sheet material to be worked on, comprising:a workbed having a worksurface for supporting the sheet material, the worksurface including a plurality of apertures for applying suction to the sheet material, said apertures separated into first and second zones for accommodating sheet material of different sizes and orientations; a suction source for applying suction to the apertures; a manifold for providing fluid communication between said suction source and said apertures for applying suction thereto; a sensor in fluid communication with said suction source for providing a signal responsive to the degree of vacuum drawn by said suction source on the apertures; means for restricting the flow rate through one of the zones of apertures, and wherein signals responsive to the degree of vacuum are produced by said sensor for determining the zones that include unblocked apertures.
  • 13. Vacuum workbed for supporting a sheet material to be worked on, comprising:a workbed having a worksurface for supporting the sheet material, the worksurface including a plurality of apertures for applying suction to the sheet material, said apertures including apertures separated into first and second zones; a suction source for applying suction to the apertures; a manifold for providing fluid communication between said suction source and said apertures for applying suction thereto; a sensor in fluid communication with said suction source for providing a signal responsive to the degree of vacuum drawn by said suction source on the apertures; means for restricting the flow rate through one of the zones of apertures, and wherein said workbed includes first and second groups of apertures, a first manifold for providing fluid communication between said suction source and said first group of apertures, and wherein said second group includes said first and second zones of apertures.
  • 14. The vacuum workbed of claim 13 including first and second flow control valves fluidly interposed between said suction source and first and second groups of apertures, respectively.
  • 15. Vacuum workbed for supporting a sheet material to be worked upon, comprising:a workbed having a worksurface for supporting the sheet material, the worksurface including a plurality of apertures separated into a plurality of zones; a suction source for applying suction to the apertures; a first manifold for providing fluid communication between said suction source and a first group of zones; a second manifold for providing fluid communication between said suction source and a second group of zones, said first and second groups including at least one zone each; a sensor in fluid communication with said suction source for providing a signal responsive to the degree of vacuum drawn by said suction source on the apertures; a first flow control valve fluidly interposed between said first group and said suction source; a second flow control valve fluidly interposed between said second group and said suction source; and wherein said first flow control valve is fluidly interposed between said second flow control valve and said suction source.
  • 16. The vacuum workbed of claim 15 including means for restricting the flow rate through one of said two zones of apertures for producing a selected degree of vacuum when said one zone includes unblocked apertures.
  • 17. The vacuum workbed of claim 16 wherein said means for restricting the flow rate includes a flow restriction element interposed between said one of said two zones and said suction source.
  • 18. The vacuum workbed of claim 16 wherein said means for restricting the flow rate includes a flow restriction element interposed between said first zone of said second group and said suction source for restricting the flow rate through said first zone of said second group.
  • 19. The vacuum workbed of claim 15 wherein said first group includes first and second zones, said second group includes third, fourth and fifth zones, and wherein said workbed includes first and second flow restriction elements interposed between the first and second zones, respectively, and the suction source, and third, fourth and fifth flow restriction elements, interposed, respectively, between the third, fourth and fifth zones and said suction source, said flow restriction elements for providing a selected flow rate through said zones of apertures when unblocked.
  • 20. The vacuum workbed of claim 19 wherein the fourth flow restriction element is interposed between both said fourth and fifth zones and said suction source.
  • 21. The vacuum workbed of claim 15 wherein said zones are arranged in a linear array.
  • 22. The vacuum workbed of claim 15 wherein said worksurface of said workbed is a flat worksurface.
  • 23. The vacuum workbed of claim 15 wherein said worksurface of said workbed is a curved worksurface.
  • 24. The vacuum workbed of claim 15 wherein said worksurface of said vacuum workbed is a cylindrical worksurface of a drum platen.
  • 25. The vacuum workbed of claim 15 wherein said suction source is a mechanical evacuation pump.
  • 26. The vacuum workbed of claim 15 including an orifice in communication with the atmosphere for providing a selected fluid leakage to said suction source.
  • 27. A method of automatically determining the size or orientation of a work piece supported by a workbed having suction apertures therein, comprising the steps of(a) grouping the apertures into N groups of apertures; (b) applying suction to one of the groups of apertures; (c) incrementing the number of groups to which suction is applied by applying suction to an additional group and sensing the difference in the degree of vacuum attained between the application of suction prior to and subsequent to incrementing the number of groups; (d) determining from the difference whether the additional group includes unblocked apertures; and when determining in the prior step that the additional group does not include unblocked apertures, repeating steps (c) and (d) until one of: a determination is made in step (d) that the additional group does include unblocked apertures; and no groups remain.
  • 28. The method according to claim 27 including ceasing the application of suction to an additional group determined to include unblocked apertures.
  • 29. The method of claim 27 wherein the step of determining includes determining that substantially all the apertures of the additional group are unblocked, steps c and d being repeated until such a determination is made in step (d).
  • 30. The method of claim 27 including the step of organizing the apertures into M zones, where M is greater than N, such that at least one group includes more than one zone, and wherein the rate of flow through the one zone is restricted such that application of suction to the group having the one zone unblocked allows sufficient suction to be drawn on the blocked apertures of the workbed for securing the work piece to the workbed for the performance of the work operations thereon.
  • 31. The method of claim 27 including the step of organizing the apertures into M zones, where M is greater than N, such that at least one group includes more than one zone,restricting the rate of flow through selected zones of the at least one group such that the degree of vacuum attained when applying suction to the group is indicative of the number of zones of the one group having unblocked apertures; and determining from the degree of vacuum attained the number of zones of the one group having unblocked apertures.
  • 32. A method of supporting sheet materials of varying sizes for performing work operations thereon, comprising the steps of:providing a workbed having a worksurface for supporting the sheet material, the worksurface including first and second groups of apertures; applying suction to the apertures; sensing a selected number of times the degree(s) of vacuum attained during the step of applying suction and providing a selected number of signals responsive to the degree(s) of vacuum; and determining from the selected number of signals one of the following: that all apertures are blocked; that a first group of apertures is blocked and a second group of apertures includes unblocked apertures; and that both first and second groups of apertures include unblocked apertures.
  • 33. The method of claim 32 wherein the step of applying suction includes the step of applying suction to the first group of apertures only and the step of applying suction to both groups of apertures;wherein the step of sensing includes the steps of sensing a degree of vacuum attained in the applying of suction to the first group of apertures and sensing the degree of vacuum attained in the applying of suction to both groups of apertures and providing first and second signals responsive thereto, respectively; and wherein the step of determining includes determining the difference between the first and second signals.
  • 34. The method of claim 33 wherein the step of applying suction includes:providing a suction source; providing a first manifold for providing fluid communication between said first group of apertures and the suction source; providing a second manifold for providing fluid communication between the second group of apertures and the suction source; and providing first and second flow control valves interposed between the suction source and the first and second groups, respectively, for controlling the application of suction to the apertures.
  • 35. The method of claim 34 wherein the step of providing flow control valves includes fluidly interposing the first flow control valve between the suction source and the second flow control valve.
  • 36. The method of claim 32 wherein the step of determining includes determining a selected difference between the first and second signal for indicating that the second group of apertures is not blocked by the sheet material; and further including the step of:applying suction to first group of apertures only.
  • 37. The method of claim 32 wherein the step of providing a workbed having worksurface includes providing a workbed having a flat worksurface.
  • 38. The method of claim 32 wherein the step of providing a workbed having a worksurface includes the step of providing a workbed having a curved worksurface.
  • 39. The method of claim 32 wherein the step of providing a workbed having a worksurface includes the step of providing a workbed having a cylindrical worksurface.
  • 40. The method of claim 32 wherein the step of providing a workbed includes providing a workbed having first and second groups of apertures wherein a selected group includes a plurality of zones of apertures, and including the step of:restricting the rate of flow through at least one of the zones of apertures such that a signal is produced by the sensor, when suction is applied to the selected group, responsive to the degree of vacuum attained for distinguishing between when one of the zones of the selected group includes unblocked apertures and when both of the zones of the selected group include unblocked apertures.
  • 41. The method of claim 40 including the step of:restricting the rate of flow through selected zones of the second group of apertures such that, when the first group of apertures is blocked and suction is applied to the second group of apertures, the signal produced by the sensor is responsive to the number of the zones of the sound group including unblocked apertures.
  • 42. The method of claim 32 wherein the step of providing a workbed includes providing a workbed wherein the first group of apertures includes first and second zones and wherein the step of applying suction includes applying suction to the first zone via a first flow restriction element for reducing the flow through the first zone and applying suction to the second zone via a second flow restriction element for reducing the flow through the apertures of the second zone.
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