Patent Application
20110074904
The present invention relates to measurement methods and apparatuses, and more particularly to a method and apparatus to maintain a desired peel location of a first web from a second web located in a printer.
A typical multi-color dye donor web that is used in a dye transfer or thermal printer has a repeating series of three different rectangular-shaped color sections or patches such as a yellow color section, a magenta color section and a cyan color section. In addition, there may be a transparent colorless laminating section immediately after the color sections.
Each color section of the dye donor web consists of a dye transfer area which is used for dye transfer printing and a pair of opposite longitudinal edge areas alongside the dye transfer area which are often not used for printing. The dye transfer area may be about 152 mm wide and the two longitudinal edge areas may each be about 5.5 mm wide, so that the total web width is approximately 163 mm.
To make a multi-color image print using a thermal printer, a motorized donor web take-up spool draws a longitudinal portion of the dye donor web off a donor web supply spool in order to successively move an unused single series of yellow, magenta and cyan color sections over a stationary liner array (bead) of selectively heated resistive elements on a thermal print head between the supply and take-up spools. Respective color dyes within the yellow, magenta and cyan color sections are successively heat-transferred line-by-line, via the selectively heated resistive elements, onto a dye receiver medium such as a paper or transparency sheet or roll, to form the color image print. The selectively heated resistive elements often extend across the entire width of a color section, i.e. across the dye transfer area and the two longitudinal edge areas comprising that color section.
As each color section is drawn over the selectively heated resistive elements, it is subjected to a longitudinal tension particularly by the forward pulling force of the motorized donor web take-up spool. Since, the dye transfer area in the color section is heated by the resistive elements the web is weakened, making the web vulnerable to being longitudinally stretched if too much tension is applied. Consequently, too much longitudinal tension will stretch the donor web in the dye transfer area which in turn causes some creases or wrinkles to develop in the dye transfer area. As the dye donor web is pulled by the motorized donor web take-up spool over the selectively heated resistive elements, the creases or wrinkles tend to spread from a trailing (rear) end portion of a used dye transfer area at least to a leading (front) end portion of the next dye transfer area to be used. The line artifacts printed on the dye receiver medium, although they may be relatively short, are quite visible. This indicates that too much tension on the dye donor web will result in creases or wrinkles being created in an unused dye transfer area and line artifacts being printed on the dye receiver medium during the dye transfer process.
More significantly, as each color section is drawn over the selectively heated resistive elements too little tension will cause the web to be slack. Decreasing the tension further will cause more slackness in the web. This will result in improper peeling or delamination of the dye donor web from the receiver web. Improper peeling ranges from the peel position of the web shifting from the desired location at the peel bar to the extreme of not delaminating at all and causing a printer jam. When the peel location of the dye donor web is not at its desired location there is a high probability for defects to occur in the printed image. These defects include spot defects, creases, sticking defects and streaks. Many of these defects are due to the fact that the donor web will selectively stick to the receiver web at specific locations if there is not enough web tension to maintain the peel location. Spot defects are regions of low and high print density caused by micro folds in the donor sheet due to too little tension in the donor web. Sticking defects are due to the detachment of the dye layer in a thermal donor from the PET (polyethylene terephthalate) support and transfer of the dye layer to the receiver during the peel process following printing. This is a serious and unacceptable problem for the customer because it results in high density dye specs being scattered across the face of the receiver.
Thus, there is a need to maintain a desired peel location of a first web from a second web in thermal printers. The first web is usually a donor media (“dye donor web”) containing the colorants that are thermally transferred to the receiver media (second web). The receiver media is usually the final hardcopy print. The transfer process needs to be carefully controlled so that the correct amount of colorant(s) is transferred to produce a high quality image on the receiver material. During the printing process the two webs are brought in contact at the print head where thermal lamination occurs during the dye transfer process. After lamination, the two webs must be separated from each other in a controlled fashion. This separation is achieved by applying a known force to the laminated layers at a fixed location known as the peel location. This force is usually applied by tensioning the webs and forcing the webs to travel in different directions as they pass the desired peel location. The desired peel location is at a peel bar. If the force applied to the webs is insufficient then the webs will not separate at the desired peel location. When this occurs the quality of the print can be adversely affected and in severe cases the webs stay laminated together and cause the printer to jam. If the force applied to the web is too large then the webs may deform and introduce printing artifacts.
Different donor and receiver materials will have different binding forces when thermally laminated together and will therefore require different levels of separation force in order to ensure separation at the desired peel location in a thermal printer. Furthermore changing environmental conditions such as ambient temperature and humidity can also cause the binding forces to change for a given set of donor and receiver webs. Product variability resulting from material variations can also affect the binding forces. All sources of variation in the binding forces between a pair of donor and receiver webs will require different levels of separation force in order to ensure separation at the desired peel location in a thermal printer.
U.S. Pat. No. 6,315,471 by C. Hsieh and C. Chung entitled “Apparatus for Controlling Ribbon Tension in a Thermal Printer” describes an apparatus and method for controlling the tension on the web by pulse width modulation (duty cycle control) by monitoring the input and output diameters of the web on the supply and take-up reels and setting up a transforming table for varying the pulse width modulation as a function of web diameters to keep uniform tension on the ribbons (web).
U.S. Pat. No. 6,082,914 by G. Barrus and K. Moore entitled “Thermal Printer and Drive System for Controlling Print Ribbon Velocity and Tension” describes a thermal printer having a supply of media with a rotatable platen on which the media is moved for printing by a thermal printing head. A supply spindle supplies print ribbon from a supply spool mounted thereon, and a take-up spindle takes up the used print ribbon on a take-up spool. The spindles are each driven by a motor and controlled by a controller which detects the Back EMF (BEMF) of the motors, and calculates the velocity of the spindles, spool, and print ribbon to control each motor based on the BEMF.
Commonly assigned U.S. Pat. No. 6,859,221 by Z. J. Gao, R. F. Mindler and J. F. Corman entitled “Preventing Crease Formation In Donor Web In Dye Transfer Printer That Can Cause Line Artifact On Print” describes a method of preventing crease formation in a dye transfer area of a dye donor web that can cause a line artifact to be printed on a dye receiver during a dye transfer from the dye transfer area to the dye receiver in a dye transfer printer by controlling the heat distribution over the dye transfer area.
Commonly assigned U.S. Pat. No. 6,977,669 by Po-Jen Shih et al. entitled “Preventing Crease Formation in Donor Web in Dye Transfer Printer That Can Cause Line Artifact On Print” describes a thermal printer which employs the method described in commonly assigned U.S. Pat. No. 6,859,221.
U.S. Pat. No. 6,922,205 entitled “Color Thermal Printer And Color Thermal Printing Method” by M. Shusuke describes a thermal printer which conveys a recording sheet at a certain speed by keeping tension applied to a conveyor roller pair within a range designed not to influence the conveyance speed.
None of the prior art can ensure that the location of the peel location is correct and that the thermal printer is working at its designed print resolution. Thus there is a need for a thermal printer that includes a sensor system which can determine the actual peel location of the first web and the second web and to adjust the web tensions so that the peel location will be maintained at the desired peel location within desired tolerance limits for various combinations of thermal web media. Such a thermal printer will tolerate broader variations in manufacturing of the media which affect the tension requirements. Thus, the media may not need to have as tight manufacturing tolerances which would lead to less waste in media manufacturing. This thermal printer will also be able to accommodate for changing environmental conditions which change the peel force and will also result in decreased incidence of machine jams. With such a sensor system installed in a thermal printer, the web tension for new web materials can be automatically adjusted to enable for use in an existing printer.
The need is met according to the present invention by providing a method for maintaining a peel location for peeling a first web from a second web in a thermal printer. A preferred embodiment of the present invention comprises a method for maintaining a peel location of a first web from a second web in a thermal printer. An optical probe, that includes a light source and a photodetector, transmits light from the optical probe toward the first web. The web reflects a portion of the transmitted light onto the photodetector, which then outputs an electrical signal which is compared with a preselected signal level and the difference between them provides an indication as to how much adjustment the peel location requires. Adjusting the peel location may comprise changing environmental characteristics of the first web or the second web or adjusting a tension of the first or second web. The difference between the measured electrical signal levels is related to a physical distance of the first web from the desired peel location. The optical probe is pointed at one of the webs in a region after the desired peel location.
Another preferred embodiment includes a method for peeling a layer of media from a surface. The media typically is provided on a supply roll and is collected on a take-up roll by rotating the take-up roll via a drive motor. Between traveling from the supply roll and being taken up by the take-up roll the media can be adhered to a surface using a heat source. The media is then peeled from the surface also by using the take-up roll drive motor which rotates the take-up roll for drawing the media away from the surface. An optical probe proximate the peeled media includes a light source and at least one photodetector. By transmitting light from the light source toward the media the photodetector detects a portion of the transmitted light that is reflected by the media. In response, the photodetector outputs an electrical signal corresponding to an amount of reflected light detected. By monitoring an output level of the electrical signal the media can be peeled from the surface at a preferred peel location by adjusting a speed of the take-up roll drive motor in response to the output level of the electrical signal. The monitoring includes determining a difference between the output level of the electrical signal and a preselected electrical reference signal. This difference corresponds to a distance between the media and the optical probe. Therefore, the adjusting step includes increasing the distance between the media and the optical probe if so indicated by the difference in signal levels. The distance between the media and the optical probe indicates a peel location of the media being peeled from the surface. The preselected electrical reference signal represents about an ideal distance between the media and the optical probe. Adhering the media to the surface includes heating selected areas of the media against the surface. Different ones of the rollers can be rotated for moving the media.
Another preferred embodiment of the present invention includes a method to maintain a preselected peel location around a peel bar of a first web from a second web in a thermal printer. An optical probe is provided which includes a light source and at least one photodetector. The probe is faced toward the first web proximate the peel location. A load cell on the peel bar measures a web tension on the first web. Light is transmitted from the optical probe to the first web. A portion of the transmitted light is reflected off one or more surfaces of the first web onto the photodetector. Electrical output signal levels of the photodetector are measured when the first web is positioned at the preselected peel location to form a reference signal. Repetitively measuring the electrical output signal levels of the photodetector as a function of time and computing the difference between the measured electrical output signal levels and the reference signal while repetitively measuring a tension on the peel bar as a function of time provides information above which signal levels correspond to an ideal peel location. Having gathered this information, it is possible to adjust the tension of the first web in a manner corresponding to the difference between the measured electrical output signal levels of the photodetector and the reference signal while maintaining the tension on the peel bar in a safe range. Adjusting a speed of a roller, via controlling a drive motor speed, that takes up the first web is one way to accomplish this adjustment.
The present invention enables the design of improved thermal printers which provide improved performance in providing less printer defects and decreased incidence of machine jams. Furthermore the printer better accommodates for changing environmental conditions which change the peel force and better tolerates variations in manufacturing of the media which affect the tension requirements.
These, and other, aspects and objects of the present invention will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following description, while indicating preferred embodiments of the present invention and numerous specific details thereof, is given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications. The figures below are not intended to be drawn to any precise scale with respect to size, angular relationship, or relative position.
In the detailed description of the preferred embodiments of the invention presented below, reference is made to the accompanying drawings, in which:
The present description will be directed in particular to methods and/or elements forming part of, or cooperating more directly with, apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.
The peel bar 40 may also include an optional load cell 45 which is used to measure the tension of the donor web 50 as it passes over the contact region with the peel bar 40. The purpose of peel bar web tension measuring load cell 45 is to keep the tension of the donor web within the safe level tension so that defects due to too much tension on the donor web 50 do not occur. This safe level tension is determined during initial printer set up. Too much tension on the donor web 50 causes an upward pressure on the peel bar 40 which is detected by the peel bar web tension measuring load cell 45. Adjustment is then made to the speed of donor web take-up spool 60.
During the printing process the receiver sheet 110 is driven forward by motorized capstan roller 80 and pinch roller 90 to the beginning of the first section 1 or patch of donor web 50. The print head 30 is then lowered to make good thermal contact with donor web 50 and receiver sheet 110 over platen roller 100. The donor web 50 and the receiver sheet 110 are then both transported at the same velocity while heat is applied to the webs by the print head 30. When the printing of the first section 1 of donor web 50 is completed, by heat induced transfer of the donor web onto the receiver sheet 110 (the print), the webs are stopped and the print head 30 is raised. The receiver sheet 110 is then driven backward by motorized capstan roller 80 and pinch roller 90 to align the start of the printed region on the receiver sheet 110 with the beginning of the second section 2 of donor web 50. The print head 30 is then lowered to make good thermal contact with donor web 50 and receiver sheet 110 over platen roller 100. The donor web 50 and the receiver sheet 110 are then both transported at the same velocity while heat is applied to the webs by the print head 30. When the printing of the second section 2 of donor web 50 is completed the webs are stopped and the print head 30 is raised. This process is repeated to transfer donor sections 3 and 4 to receiver sheet 110. After printing section 4 of the donor web the print head 30 is raised and the print exits the printer. Not shown in
An optical probe 70 which measures the distance of a web from the probe tip is installed in the printer with probe tip 270 facing the web. The optical probe 70 comprises a light source which transmits light to the web. Light is reflected from the web and the reflected light is incident on at least a pair of optical sensors which have different signal profiles as a function of the distance between the web and the sensors. The ratio of the two optical sensor signals is obtained, as explained below, and the ratio is dependent upon the distance from the web to the sensors. With an accurate measurement of the distance, i.e. at one of the peel locations 130, the tension in the web can be adjusted so that the web can be brought back to the appropriate distance for the product. For example, a distance detected by the sensors may indicate that the donor web 50 is at position c or d (of
Details of an optical probe 70 embodiment are illustrated in
As shown in
In cases where the reflectivity of the web 50 is relatively constant as measured from patch to patch, it is not necessary to use a plurality of photodetectors in the optical probe 70. In this case a single photodetector can be used and its electrical output signal, such as that for photodetector D1 in
A flow chart of the page printing process 400, as described above, is briefly illustrated in
The repetitively measured ratio of the electrical output signal levels as a function of time during printing of four patches using normal print conditions is shown graphically in the plot in
In a preferred embodiment of the present invention, the monitored detector position signal can be used as the basis of a negative feedback control loop to maintain the desired peel location during printing. A flowchart of the operation of a control loop for maintaining the peel position at the desired peel location during the printing process is shown in
The thermal printer includes a controller (not shown) which is used to control web tension by regulating roller motor velocities, collecting sensor data from printer functions including photodetectors D1 and D2. A comparator is used to determine the difference between the measured electrical output signal levels of the photodetector(s) and the preferred position reference signal, which can be stored in a controller memory. The comparator could be electronic or implemented as a software program in the controller. The tension on the donor web is then adjusted by the controller regulating roller motor velocities via a feedback loop based on the magnitude of the difference measured by the comparator. Motor speed control negative feedback loops are well known and are not described further. When the photodetectors sense that the donor web position 130 is closer to the photodetectors than a preferred position, a voltage or pulse width modulated duty cycle output to roller drive motors increases in response to the photodetectors, which increases the power to the roller motor controlling spool 60, for example, thereby tightening the donor web and bringing its peel position closer to point a of the peel position as described above. Conversely, when the photodetectors sense that the donor web position 130 is further from the photodetectors and closer to the ideal position a described above, then a voltage or pulse width modulated duty cycle output to roller motors decreases, which decreases the power to the roller motor controlling spool 60. A preferred embodiment of the present invention includes duty cycle control as described in U.S. Pat. No. 6,315,471, described above.
Although the discussion of the optical probe 70 up to now has described an optical fiber probe, it is understood that the optical probe 70 may also comprise a pair of LED/photodetector pair sensors such as the Honeywell HO1160 series or HOA1397 reflective pair, Optek OPB700 series or Fairchild QRB1133 optical sensors. Reflectivity compensated optical fiber probes are commercially available from Philtec as part of their RC 100 fiber optic sensor or from MTI as part of their 2100 photonic sensor series.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. In particular, although the above discussions relate to maintaining the peel location in a printer, it is understood that the method and apparatus for maintaining a peel location applies to any peeling or separation process or device as applied between two or more substrates, sheets, or other media. The substrates may be bound together by adhesives, thermal processes or by any other method or technology. A substrate may exist naturally or by manufacture as an integrally formed single substrate that can be separated by peeling. Examples of such alternative peeling processes include solvent casting, compression rolling, thermal transfer and dry film photolithography and photoresist.
Reference is made to commonly assigned, co-pending U.S. patent application Ser. No. ______ by Marcus et al. (Docket 95883) filed of even date herewith entitled “Apparatus For Controlling Peel Position In A Printer”, the disclosure of which is incorporated herein by reference in their entireties.
