Patent Application
20060290769
1. Field of the Invention
The present invention relates generally to a digital printing system and, more generally, to techniques for pulsing energy to print heads in a printer.
2. Related Art
Referring to
A thermal print head element is activated by providing it with energy. Providing energy to the print head element increases the temperature of the print head element, causing either the transfer of pigment to the output medium or the formation of color in the output medium. The density of the output produced by the print head element in this manner is a function of the amount of energy provided to the print head element. The amount of energy provided to the print head element may be varied by, for example, varying the amount of power provided to the print head element within a particular time interval or by providing power to the print head element for a longer or shorter time interval.
Some conventional methods for color thermal imaging, such as thermal wax transfer printing and dye-diffusion thermal transfer, involve the use of separate donor and receiver materials. The donor material typically has a colored image-forming material, or a color-forming imaging material, coated on a surface of a substrate and the image-forming material or the color-forming imaging material is transferred thermally to the receiver material (i.e., the output medium 1608). In order to make multicolor images, a donor material with successive patches of differently-colored, or different color-forming, material may be used. In the case of printers having either interchangeable cassettes or more than one thermal head, different monochrome donor ribbons are utilized and the multiple color planes of the image are printed successively above one another. The use of donor members with multiple different color patches or the use of multiple donor members increases the complexity and the cost, and decreases the convenience, of such printing systems. It would be simpler to have a single-sheet imaging member that has the entire multicolor imaging system embodied therein.
In International Application No. PCT/US02/15868 (which corresponds to U.S. patent application Ser. No. 10/151,432, cross-referenced above), entitled “Thermal Imaging System,” there is described a direct thermal imaging system in which one or more of the thermal print heads 1604a-b can write two colors in a single pass on the single print medium 1608. The printer 1602 can write these multiple colors by addressing two or more image-forming layers of the output medium 1608 at least partially independently from the same surface so that each color can be printed alone or in selectable proportion with the other color(s).
The above-referenced patent application discloses an electronic pulsing technique that makes this result possible without modulating the heating element power supply voltage. Generally, each line printing time is divided into many subintervals. For example, referring to
Furthermore, the line printing time 104 can be divided into two segments, each containing a portion of the subintervals, as shown by the graph 200 in
In some instances this method for controlling the print head may not be completely satisfactory. For example, in wide format thermal printers in which multiple print heads are used in tandem to provide a wider format print it has been found to be advantageous to employ “screening” techniques when stitching together the image segments from each print head to form the final wider print. Examples of techniques for performing such stitching are disclosed in the above-referenced patent application entitled “Image Stitching for a Multi-Head Printer.” It is not, however, possible to accomplish effective screening using the pulse patterns just described with conventional thermal print heads.
The reason for this difficulty is that a conventional thermal print head typically has one or a small number of “strobe” signal(s) that service(s) all print head elements in the print head. The strobe signal determines the pulse duty cycle, and as a consequence all or a significant fraction of the print head elements 1606a-d in print head 1604a have the same pulse duty cycle in each subinterval; similarly, all or a significant fraction of the print head elements 1606e-h in print head 1604b have the same pulse duty cycle in each subinterval. The pulse duty cycle, in turn, determines the image-forming layer being printed, as described in the above-referenced patent application entitled “Thermal Imaging System,” and therefore it follows that during each subinterval all or a significant fraction of heating elements 1606a-d are printing on the same image-forming layer of the output medium 1608. Therefore, at any moment in time all or a significant fraction of the heating elements 1606a-d are printing the same color. This condition precludes the use of screening patterns that call for some of the heating elements 1606a-d to be printing on one image-forming layer (and therefore printing one color) while other ones of the heating elements 1606a-d are printing on another image-forming layer (and therefore printing another color).
It has been found, however, that some useful screening patterns require the print heads 1604a-b to print in just this way. For example, in the above-referenced patent application entitled “Image Stitching for a Multi-Head Printer,” there is described a screening technique for use with a method for stitching image segments to make the stitching method more insensitive to any misregistration of the dots. In general, the technique disclosed therein introduces a pattern of time delays into the rows of the image so that the pixels do not lie on a rectangular grid. Instead, the pixels in a row have a repeated pattern of displacements from the nominal (default) position of the row in the transport direction (“down-web”). In one embodiment, for example, the first pixel in the row is undisplaced, the second pixel is displaced down-web by ⅓ of a row spacing, the third is displaced by ⅔ of a row spacing, the fourth is undisplaced, and the pattern repeats. There are, then, three types of pixels in the row. The first, fourth, seventh, etc., are undisplaced pixels, the second, fifth, eighth, etc., are displaced down-web by ⅓ of a row and the third, sixth, ninth, etc., are displaced down-web by ⅔ of a row.
The use of such patterns may reduce the dependence of printing density in the stitch on the registration of the pixels. Furthermore, such patterns can be used to improve the tolerance to misregistration of colored dots formed on an imaging medium that has multiple superimposed color-forming layers in different planes, such as where one or more color-forming layers are arranged on a first side of a transparent substrate and at least one color-forming layer is arranged on a second side of the substrate. However, the down-web displacement of the pixels may cause the first time segment of some pixels to overlap the second time segment of others, requiring that some pixels be supplied with a low duty-cycle strobe pulse at the same time that others are being supplied with a high duty-cycle strobe pulse. As described above, the use of a single or a small number of strobe signal(s) for all print head elements in a print head may make it impossible to provide such varying pulse duty cycles across print head elements in the same subinterval. What is needed, therefore, are improved techniques for performing screening in a printer that can write two colors in a single pass on a single print medium.
Note further that power is typically provided simultaneously to multiple print head elements in a print head. Ordinarily, the printer power supply is chosen to satisfy the “worst case” demand represented by the supply of power to all of the print head elements simultaneously. This typically results in the choice of a larger and more expensive power supply than would be required to fulfill the “average” power demand. Power supplies may be chosen to satisfy this peak power requirement even when the average power provided to the print head elements is low, as is the case, for example, when there are repeated segments with low duty-cycle printing. What is further needed, therefore, are improved techniques for performing screening in a printer to reduce the peak power requirements.
In one aspect of the invention there is disclosed a multicolor thermal imaging system wherein different heating elements on a thermal print head can print on different color-forming layers of a multicolor thermal imaging member in a single pass. The line-printing time is divided into portions, each of which is divided into a plurality of subintervals. All of the pulses within the portions have the same energy. In one embodiment, every pulse has the same amplitude and duration. Different colors are selected for printing during the different portions by varying the fraction of subintervals that contain pulses. This technique allows multiple colors to be printed using a thermal print head with a single strobe signal line. Pulsing patterns may be chosen to reduce the coincidence of pulses provided to multiple print head elements, thereby reducing the peak power requirements of the print head.
Other features and advantages of various aspects and embodiments of the present invention will become apparent from the following description and from the claims.
In one aspect of the invention there is disclosed a multicolor thermal imaging system wherein different heating elements on a thermal print head can print on different color-forming layers of a multicolor thermal imaging member in a single pass. The line-printing time is divided into portions, each of which is divided into a plurality of subintervals. All of the pulses within the portions have the same energy. In one embodiment, every pulse has the same amplitude and duration. Different colors are selected for printing during the different portions by varying the fraction of subintervals that contain pulses. This technique allows multiple colors to be printed using the same strobe pulses. Pulsing patterns may be chosen to reduce the coincidence of pulses provided to multiple print head elements, thereby reducing the peak power requirements of the print head.
For example, referring to
Each of the on-times 312a-b is an example of a “portion” of the line interval 304a, as that term is used herein. Note that a segment need not include an off-time. In other words, the on-time of a segment may be the entire segment, in which case the term “portion” also refers to the entire segment. Likewise, a given segment need not include an on-time. A segment may include multiple portions, alternating between on-time and off-time portions.
Line interval 304a includes pulses 310a-h, all of which have the same energy. In the particular example illustrated in
Segment 308a is divided into subintervals 306a-g. Portion 312a contains subintervals 306a-d and portion 314a contains subintervals 306e-g. Pulses 310a-d having the same energy are provided in portion 312a of the first segment 308a. Although in the particular example illustrated in
Segment 308b is divided into subintervals 306h-z. In the second segment 308b, on-time portion 312b contains subintervals 306h-w and off-time portion 314b contains subintervals 306x-z. In the particular example illustrated in
Although in the example illustrated in
Since the thermal time constant of the print head is typically much longer than the length of one of the subintervals 306a-z, the average power in portion 312b of the second segment 308b is approximately ¼ of the average power in portion 312a of the first segment 308a. In other words, the average power in the portion 312b is reduced not by varying the duration of individual pulses but by selecting the fraction of subintervals in the portion 312b in which the print head element is pulsed. The average power provided in the first on-time portion 312a thereby selects a first one of the color-forming layers in the output medium 1608 for printing, while the average power provided in the second on-time portion 312b thereby selects a second one of the color-forming layers in the output medium 1608 for printing.
Note that the scheme described above with respect to
This, in turn, enables arbitrary time delays to be applied to pixels printed during the line times 304a-b, allowing screening to be applied to the image to improve the joining of image segments, to reduce the effect of misregistration of images printed on the front and back sides of a transparent substrate, or to reduce the peak power demand of the printer. To understand how the modulation of average power using selective pulsing enables screening to be performed, recall that in the above-referenced patent application entitled “Image Stitching for a Multi-Head Printer,” screening techniques are disclosed in which print head elements printing different colors may be active at the same time. In systems in which multiple colors are printed by varying the average power provided to print head elements, printing multiple colors at the same time requires the ability to provide different average power levels to different print head elements at the same time. It is not possible to achieve this result by varying the pulse duty cycle of individual pulses in systems that use a single pulse strobe signal. The techniques disclosed above, however, enable the average power provided to a print head element to be varied by varying the fraction of pulses provided to the print head element in a given time period, even when all pulses share the same pulse duty cycle as dictated by the use of a single strobe signal. The techniques disclosed herein therefore enable screening techniques, such as those disclosed in the above-referenced patent application entitled “Image Stitching for a Multi-Head Printer,” to be used even in multicolor printers that use a single pulse strobe signal for each print head.
Referring to
The method 400 identifies a common energy for all pulses (step 402). Recall, for example, that the pulses 310a-h in
The method 400 enters a loop over each segment S in a line interval (step 404). For example, referring again to
The method 400 identifies an average power PAVG to be provided to a corresponding print head element during segment S to select the color-forming layer identified in step 406 (step 408). Techniques for performing step 408 are disclosed, for example, in the above-referenced patent application entitled “Thermal Imaging System.”
The method 400 identifies a pattern of pulses that produces (approximately) the average power PAVG, subject to the constraint that each of the pulses has the common energy identified in step 402 (step 410). Note that any pattern satisfying the specified constraints may be selected in step 410. The pulse pattern may be a pattern that only occupies subintervals in a designated “on-time” portion of a segment, such as on-time portion 312a or 312b in
Since the average power PAVG varies from color-forming layer to color-forming layer, the pulse pattern selected in step 410 for a first color-forming layer will differ from the pulse pattern selected in step 410 for a second color-forming layer, as a result of the constraint that pulses in the patterns have the same energy. In particular, such pulse patterns will differ in the fraction of subintervals that contain pulses, as illustrated by the example in
The method 400 provides the identified pulse pattern to the corresponding print head element to select the color-forming layer identified in step 406 and therefore to print the appropriate color (step 412). The method 400 repeats steps 406-412 for the remaining segment(s) in the line interval (step 414).
Note that although in the example illustrated in
It should be appreciated, in accordance with the teachings of the above-referenced patent applications, that each of the segments 308a-b may correspond to a different color to be printed. For example, the pulses 310a-d provided in the first segment 308a may be used to print on a yellow image-forming layer of the print medium 1608, while the pulses 310e-h provided in the second segment 308b may be used to print on a cyan image-forming layer of the same print medium 1608.
In the example illustrated in
It may appear to be a limitation of the techniques disclosed above that 1-out-of-N pulsing does not allow the selection of an arbitrary value for the average power. That is to say, 1-out-of-2 pulsing reduces the average power by 2 (i.e., to PMAX/2), 1-out-of-3 pulsing reduces the average power by 3 (i.e., PMAX/3), and in general 1-out-of-N pulsing reduces power by N (i.e., to PMAX/N) . Solely using 1-out-of-N pulsing, therefore, does not allow for reduction of average power to values other than PMAX/N for single integral values of N. If finer adjustment is desired, it may be obtained using any of a variety of techniques involving the issuance of more irregular pulse streams.
For example, in one embodiment of the present invention, 1-out-of-N pulsing is used, but the value of N may vary within a line interval. Referring to
Techniques may be applied to obtain other desired average power levels. Let PAVG be the desired average power level. For example, consider a case in which it is desired to obtain an average of 38%, i.e., in which PAVG=0.38Pmax. Since 38% is intermediate between 1-out-of-2 (50%) and 1-out-of-3 (33%), the pulse rate may be restricted to a choice between 1-out-of-3 pulses and 1-out-of-2 pulses (i.e., in which N is restricted to be equal to either 2 or 3). This can be accomplished by keeping track of the average power so far, and applying the following rule: if the average power so far is above the target power of 0.38Pmax, then the next pulse sequence should be 1-out-of-3, so as to lower the average; if the average power so far is below the target power, then the next sequence should be 1-out-of-2, so as to raise the average.
Assume, for example, that the first pulse sequence uses 1-out-of-2 pulsing. The result of applying the above-described rule in this case is illustrated by the graph 600 in
Note that the set of pulse sequences shown in Table 1 is not necessarily perfectly repetitious. After the sequence of twenty-one subintervals shown in Table 1, eight pulses have been issued with a net fraction of 8/21, or 0.381Pmax, which is very close to the desired target of 0.38Pmax. Note also that the benefits of such averaging may only be obtained if averaging is performed over a time interval shorter than the thermal relaxation time of the print head.
Referring to
(1/NH)*PMAX<PAVG<(1/NL)*PMAX.
The method initializes a “pattern list” to an empty list (step 436). A pattern list is a representation of a sequence of values of N that are used in a pulse pattern. For example, the pattern list (2,3) indicates a pattern in which a 1-out-of-2 (N=2) pulse sequence is followed by a 1-out-of-3 (N=3) pulse sequence. The method initializes a count S of the cumulative subintervals traversed so far to zero (step 438). Similarly, the method initializes a count T of cumulative pulses included so far to zero (step 440). The method initializes the value of N to NL (step 442). This choice is arbitrary; N may instead be initialized to the value of NH. It may be advantageous, however, to select NL as the initial value of N when beginning with a print head at room temperature.
The method adds the current value of N to the pattern list (step 444). Assuming, as in the case of
Otherwise, the method increases the value of S by the current value of N (step 448). In the present example, S=2 after performance of step 448. The method increments the value of T by 1, since one pulse has been added to the current pulse pattern in step 444 (step 450).
The method identifies the average power P in the current segment as (T/S)*PMAX (step 452). In the present example, T=1 and S=2, so the average power is P=(½)*PMAX, as indicated in the “Net Percent of Pmax” column of the first row of Table 1.
The method determines whether the value of P corresponds to an average power that is less than the value of PAVG identified in step 408 of
Since the pattern is not complete (step 446), the method assigns the value of 5 to S (step 448), and assigns the value of 2 to T (step 450). The average power at this point is therefore ⅖ of PMAX or 0.40*PMAX, as indicated in the “Net Percent of PMAX” column of the second row of Table 1 (step 452). Since this value is still greater than PAVG (0.38), the method assigns the value of NH (i.e., 3) to N (step 458). The method adds the value of N to the pattern list, at which point the pattern list is (2,3,3), as indicated by portions 602a-c in
If the pattern is not complete (step 446), the method assigns the value of 8 to S (step 448), and assigns the value of 3 to T (step 450). The average power at this point is therefore ⅜ of PMAX or 0.375* PMAX, as indicated in the “Net Percent of PMAX” column of the third row of Table 1 (step 452). Since this value is less than PAVG (0.38), the method assigns the value of NL (i.e., 2) to N (step 456). The method adds the value of N to the pattern list, at which point the pattern list is (2,3,3,2), as indicated by portions 602a-d in
It should be appreciated that subsequent iterations of the loop in steps 444-458 produce pulses corresponding to the remaining portions 602e-i shown in
In the examples described above, the average power provided to a print head element is varied by varying the pattern of fixed-duration pulses provided to the print head element. As will now be described in more detail, in one embodiment of the present invention pulse patterns are provided to a plurality of print head elements in a manner which reduces the peak power requirements of the print head. Such power requirement reduction may be obtained while obtaining some or all of the benefits provided by the screening techniques disclosed above, such as the ability to obtain relative insensitivity to misregistration among the outputs produced by multiple print heads.
As background, consider, for example, the case in which the pulsing techniques described above are performed without also performing screening. Assume for purposes of example that the line-printing interval is divided into two segments. The first (high-power) segment has 38 subintervals and the second (low-power) segment has 629 subintervals (the last 370 of which are part of the off-time portion of the second segment). During the low-power segment of the line interval, 1-out-of-8 pulsing (N=8) is applied.
Referring to
In
To find the total power in each subinterval, the power applied to all the heaters may be summed by summing the plots for all of the pixels in the thermal print head. To the extent that the plots 702a-o are representative of a repeating pattern in the thermal print head, the average power may be identified by averaging the plots 702a-o. The result, shown in graph 800 in
It is evident from
In one embodiment of the present invention, the required size of the power supply is reduced by distributing power more evenly over the line-printing interval to decrease peak power consumption. For example, the power may be distributed more evenly over the line-printing interval by varying the pulse sequences that are applied to the print head elements so as to reduce the sum of the pulse signals applied to the print head elements at any point in time.
In one embodiment of the present invention, the pulse sequences are varied using time shifts, but without otherwise varying the pulse patterns. Consider, for example, a three phase screening, in which the pulse patterns 902a-o applied to the first 15 pixels are as shown in
As may be seen by comparing
The example illustrated in
Referring to
The remaining peaks 1208a-c are largely a result of the coincidence of high-power intervals in regions 1104a-c (
In the present example, peak power may be further reduced, for example, by using a screening with different delays for each of the 15 heater pulse patterns. In one particular example illustrated in
To those skilled in the art, it will be apparent that the introduction of time delays into the pulse streams applied to each heater will result in slight shifts of the locations at which the corresponding pixels are printed. These shifts are less than the pixel spacing, and in general are difficult to see. However, the repeating pattern of the shifts is sometimes detectable. For example straight horizontal lines in the image take on a slight serrated pattern that may be visible in some contexts. To counteract such patterns, the image may be resampled to find interpolated image values corresponding to the points at which pixels will actually be printed. For example, if it is known that a pixel will be subjected to a time delay of one-half of a line time, then this pixel may be replaced with an interpolated value corresponding to the position halfway between the original pixel position and the next down-web pixel position. When the image data are resampled in this way, the printed image will be largely free of visible serration artifacts from the time delays.
Referring to
In general, the steps that may be taken in accordance with embodiments of the present invention to reduce power demands are not inconsistent with the types of screening patterns that result in tolerance for misregistration. For example, those having ordinary skill in the art will appreciate how to apply the power reduction techniques just described to the screening techniques disclosed in the above-referenced patent application entitled “Image Stitching for a Multi-Head Printer.”
Various examples of techniques have been described for reducing the peak power requirement on the print heads 1604a-b. More generally, the peak power requirement may be reduced in accordance with various aspects of the invention by any of the following techniques, either singly or in any combination: (1) choosing the number of time delays to be near to, but less than, the ratio of the line-printing time to the high-power segment length, but with enough “slack” to allow the time delays to be additionally advanced or delayed by one or more subintervals; (2) choosing the time delays to divide the line-printing interval nearly equally, so that the high-power segments do not overlap between any two time-delayed pulse patterns; and (3) considering any remaining power peaks that result from coincidences between the low-power segment pulses for different phases and adjustment, if necessary, of the time delays to reduce or eliminate those coincidences as much as possible. It should be noted that if there are 1-out-of-N pulses activated in the low-power segments, there is only a range of N subintervals for adjustment, and if the number of time delays exceeds N, then some overlap of low-power segment pulses is unavoidable.
For example, referring to
The method 1500 selects a first set of time shifts to apply to the default pulse patterns to reduce the coincidence of high-power segment pulses with each other (step 1504). The shifted pulse patterns 902a-o shown in
The method 1500 selects a second set of time shifts to apply to the first shifted pulse patterns to reduce coincidence of low-power segment pulses (step 1506). The pulse patterns 1102a-o shown in
The method applies the first and second time shifts to the default pulse patterns to produce a set of shifted pulse patterns (step 1508). The method provides the shifted pulse patterns to one or more print heads to produce the desired output (step 1506).
Returning to
It is to be understood that although the invention has been described above in terms of particular embodiments, the foregoing embodiments are provided as illustrative only, and do not limit or define the scope of the invention. Various other embodiments, including but not limited to the following, are also within the scope of the claims. For example, elements and components described herein may be further divided into additional components or joined together to form fewer components for performing the same functions.
Note that although in the examples described above, all of the individual pulse duty cycles are set to a single value which may be close to 100%, the common duty cycle may be lower if required by the print head specification, or if desired for some other reason.
Note that although a particular printer 1602 having a particular number of print heads 1604a-b and a particular number of print head elements 1606a-h is shown in
U.S. Pat. No. 6,661,443 to Bybell and Thornton describes a method for providing the same amount of energy to each active element in a thermal print head during each subinterval used to print an image irrespective of the number of print head elements that are active during each subinterval. The desired amount of energy may be provided to a plurality of print head elements that are active during a print head cycle by delivering power to the plurality of print head elements for a period of time whose duration is based in part on the number of active print head elements. The period of time may be a portion of the print head cycle. According to one embodiment of the present invention, the pulse duty cycle is changed from subinterval to subinterval, implementing a so-called “common mode voltage correction” by varying the pulse duration in response to the change in voltage caused by the change in the number of active print head elements, thereby maintaining a constant energy for all pulses.
The techniques described above may be implemented, for example, in hardware, software, firmware, or any combination thereof. The techniques described above may be implemented in one or more computer programs executing on a programmable computer including a processor, a storage medium readable by the processor (including, for example, volatile and non-volatile memory and/or storage elements), at least one input device, and at least one output device. Program code may be applied to input entered using the input device to perform the functions described and to generate output. The output may be provided to one or more output devices.
For example, the techniques disclosed herein may be implemented in a printer or other device having components for performing the functions illustrated by the system 1700 in
Each computer program within the scope of the claims below may be implemented in any programming language, such as assembly language, machine language, a high-level procedural programming language, or an object-oriented programming language. The programming language may, for example, be a compiled or interpreted programming language.
Each such computer program may be implemented in a computer program product tangibly embodied in a machine-readable storage device for execution by a computer processor. Method steps of the invention may be performed by a computer processor executing a program tangibly embodied on a computer-readable medium to perform functions of the invention by operating on input and generating output. Suitable processors include, by way of example, both general and special purpose microprocessors. Generally, the processor receives instructions and data from a read-only memory and/or a random access memory. Storage devices suitable for tangibly embodying computer program instructions include, for example, all forms of non- volatile memory, such as semiconductor memory devices, including EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and CD-ROMs. Any of the foregoing may be supplemented by, or incorporated in, specially-designed ASICs (application-specific integrated circuits) or FPGAs (Field-Programmable Gate Arrays). A computer can generally also receive programs and data from a storage medium such as an internal disk (not shown) or a removable disk. These elements will also be found in a conventional desktop or workstation computer as well as other computers suitable for executing computer programs implementing the methods described herein.
The present application is related to the following commonly assigned applications and patents, which are incorporated by reference herein in their entirety: U.S. patent application Ser. No. 10/374,847, filed on Feb. 25, 2003, Attorney Docket No. C-8566, entitled “Image Stitching for a Multi-Head Printer”; U.S. patent application Ser. No. 10/151,432, filed on May 20, 2002, entitled “Thermal Imaging System”, now U.S. Pat. No. 6,801,233; U.S. patent application Ser. No. 10/990,672, entitled “Method and Apparatus for Controlling the Uniformity of Print Density of a Thermal Print Head Array”; and U.S. Pat. No. 6,661,443 to Bybell and Thornton, issued on Dec. 9, 2003, entitled “Method and Apparatus for Voltage Correction.”
