TWO PASS THERMAL PRINTING

Abstract: Heat accumulation in a thermal printhead is controlled by printing blocks of data along the print line in two passes when examination of the data shows the potential of excessive heating. The proportion of heat intensive or blacks parts in each block is determined by data processor 17 by examining the data for a line in memory 19. An accumulation is made assigning blocks having high heat density minus 2, blocks having intermediate heat density minus 1 and blocks having low heat density plus 1. When that figure is at minus two the next block is not printed until a second pass and the accumulation is set to zero. Where the accumulation is minus 1, print power is reduced for the next block, which is printed in the first pass. Excessive heat in the printhead results in machine damage and impaired print quality. A second pass is completely avoided where the data is such that this is unnecessary. This offers the potential of feeding ribbon only in blocks being printed, although ribbon feeding may be continued to achieve additional cooling.

Description: Technical Field This invention relates to thermal printing in a printing system in which excessive heat can accumulate in a thermal printhead. In one form of thermal printing heater elements are carried on the printhead to selectively provide the heat for thermal printing. In the form of thermal printing employed in the specific embodiment of this invention, the heat is created in a resistive transfer ribbon, with the printhead carrying electrodes which selectively provide electric current to the resistive ribbon. In systems embodying either of such forms, intensive printing can result in the printhead retaining so much heat as to interfere with printing because the print elements themselves or areas near them are so hot as to cause printing effects whether the print element is selected or not. Also, excessive heat damages the printhead and the ribbon. Such effects from the accumulation of heat must be avoided to protect the printer, as well as the quality of the printing. Background Art This invention involves the printing of images by more than one complete pass across a line of printing when this is necessary to counter the effects of residual heat in the printhead. Such multiple pass printing to avoid overheating the printhead is known prior to this invention in which a column of print elements is driven in either two or three passes as described in IBM Technical Disclosure Bulletin article entitled "All-Points-Addressable Printing With A Resistive Ribbon," Vol. 29, No. 2, July 1986 at pp. 609-610. The article discloses resistive ribbon printing of the kind employed in the preferred embodiment of this invention with a vertical column of 40 electrodes used for printing each line. To print all-points-addressable graphics in that prior system the 40 electrodes are not driven simultaneously regardless of the content of the data to be printed. Instead, at least two passes of the electrodes are made in the printing of each line, with provision for three passes when the data is particularly heavy in current-intensive, black elements, typically dark or black areas. In the two pass operation, alternating groups of two adjacent electrodes are driven to the extent that the content of the data calls for the driving of those electrodes. The printhead is returned to the start of the line and the line is traversed on a second pass with the remaining groups of two adjacent electrodes driven to the extent that the content of the data calls for the driving of those electrodes. Accordingly, at the end of two passes, each electrodes has been driven with the data and at the position on the paper being printed as would have been the case had all 40 electrodes been operated on in a single pass in accordance with the original graphic data to be printed. The data of the line is examined in continuous segments or blocks. Where such an interval of data is above a given percentage of black content, a three pass mode is entered. In the three pass mode two adjacent electrodes are driven in the first pass to the extent that the content of the data calls for the driving of those electrodes, while the next four electrodes are not driven. The remaining electrodes in the column are driven according to the same pattern of two adjacent electrodes driven and the next four not driven. In the second pass, two adjacent electrodes in each group of the four previously not driven are driven to the extent that the content of the data calls for the driving of those electrodes. In the third pass, the remaining two electrodes in each of the groups of six electrodes are driven to the extent that the content of the data calls for the driving of those electrodes. Accordingly, at the end of the three passes, each electrode has been driven with the data and at the position of paper being printed on as would have been the case had all 40 electrodes been operated in one pass in accordance with the original graphic data. US-A-4,454,516 discloses a thermal printing system in which thermal printing in one line is carried out in one or more operations dependent upon the amount of printing called for by the data. US-A-4,567,488 is a prior teaching of general interest in that it is illustrative of a thermal printing system in which the content of data defining printing near an element to be driven, including both before and after an element to be driven, is used to define the extent to which the element is driven. Disclosure of the Invention The present invention relates to a thermal printing system comprising : a printhead with print elements for thermal printing in a column mounted perpendicular to a line to be printed by said elements by movement of said print head along said line; determination means to determine from data to be printed the existence of a predetermined high proportion of data for which said elements are activated; and means responsive to said determination means to cause printing by said print elements of alternating segments of data along a line passed across by said printhead, the segments not printed in a first pass of said line being printed in a second pass, characterized in that it comprises : examining means to examine data to be printed in said lines in adjoining segments, each segment comprising substantially all data of each column to be printed along the length of said line; accumulating means to accumulate the results provided for by said examining means and to determine at least a low, an intermediate, and a high proportion of data for which said elements are activated; means to activate said print elements either at an initial power or at at least one power level lower than said initial power; and means responsive to said accumulating means to cause printing of alternating ones of said examined segments when said accumulating means have determined a high proportion of data, to cause printing of segments not printed after said alternating is conducted, by passing said print elements over said segments not printed a second time, and to cause printing at said lower power level when said accumulating means have determined an intermediate proportion of data. More particularly, this invention employs the examination of continuous intervals or segments of data in a line with the printing of all data in the interval except where the examination indicates a predetermined concentration of heat-intensive, high density (typically black) elements. At such a concentration, the next interval is not printed until a second pass. The following interval is normally then printed in full. Thus, each interval is printed using all the electrodes simultaneously to the extent the content of the data calls for driving all of those electrodes; although where the data is sufficiently heat-intensive, some intervals are omitted in the first pass. To control the heat accumulation, all-point-addressable graphic images are stored in bit-for-bit maps and the bit density is obtained by counting the black or heat-intensive bits. In the preferred embodiment, the bits in segments or blocks of six-tenth of an inch (0.6 inch, approximately 1.52 cm) in length along the printing line are summed. Where the images are in the form of characters, the segments are six characters. Preferably, the character information itself includes the bit density information. The first block of data is printed on the first pass at a predetermined initial print power from the printhead. A power reference is accumulated by incrementing down by 1 the reference for any block having between, for example, 45 percent and less than, for example, 67 percent heat-intensive bits. The power reference is decremented 2 for any block have having a higher proportion of heat-intensive bits. For a block less than, for example, 45 percent heat-intensive bits, the power reference is incremented by 1, but not higher than the initial power reference level. Power for each block after the first or left block in each line is set by the power reference status resulting from the previous blocks. When the power reference is down 2 or more, a second printing pass is specified and the next block is not printed in the first pass. When the power reference is down 1, the next block is printed at a predetermined print power less than the initial print power. When a block is skipped because the two pass mode has been entered, the power reference is reset to the initial value and the next block is printed at initial print power. In the second pass all blocks skipped in the first pass are printed at initial print power as the printer traverses across the same print line a second time (which could be in the reverse direction from the first pass if the printer operates bidirectionally). A major advantage of the present invention is that a second pass is completely avoided where the data is such that this is unnecessary. Also, the skipped blocks may be large enough to permit withdrawal of the printhead and disablement of ribbon feed in certain systems. Accord ingly, although two passes are made, ribbon usage may be nearly the same as for one pass and the skipped blocks may be traversed at the non-printing speed of the system, which normally is higher than the printing speed. Additionally, the full width of a line need not be traversed on the second pass. Changing the print power level as a function of print density minimizes the number of blocks which must be skipped. In a specific system it may be desirable to feed ribbon during skipped blocks to facilitate cooling by moving cool ribbon under the printhead. Also, when the printhead is left on the platen, the platen serves as a heat sink or dissipater. Similarly, in a specific system disabling ribbon feed may be undesirable because initiating ribbon feed at the start of each block printed may cause uneven printing. Brief Description of Drawings The details of this invention will be described in connection with the accompanying drawing, in which : Fig. 1 is illustrative of a printing system employing this invention; Fig. 2 is an idealized logic or truth table illustrating implementation of this invention; Fig. 3 is illustrative of the printing of an all black line; and Fig. 4 is illustrative of the printing of line having less than all black. Best Mode For Carrying Out The Invention Fig.1 is illustrative of a printing system employing this invention. The preferred system is a resistive ribbon printer 1 in which a ribbon 2 has a resistive outer layer against which a printhead 3 presses during printing. Printhead 3 supports 40 electrodes 5 in a vertical column which is perpendicular to a line of printing on paper 7 held on platen 9 between printhead 3 and platen 9. The outer surface of ribbon 2 opposite printhead 3 and adjacent the paper 7, carries a thermal ink which becomes flowable in response to heat created in ribbon 2 by electrical current driven into ribbon 2 by electrodes 5 of printhead 3. Printhead 3 is mounted on carrier 11 and moved across paper 7 parallel to platen 9, as suggested by the arrows on carrier 11. Electrodes 5 are driven or not driven with respect to the position of printhead 3 on paper 7 so as to define selected graphic images or characters. This printing system as just described is known and is described in a number of patents assigned to the assignee of this invention, for example, as the printing system of US-A-4,467,363 and US-A-4,575,731. The printing system has a font memory 13, which preferably contains data as described in the foregoing patent 4,467,363, but which additionally contains for each stored character a separate, four bit code specifying the relative density of black picture elements (pels) or dots to all of the dots in the character. This density is divided linearly into sixteen categories. Thus, for example, a character specifying slightly more than one-half of its pels black would carry an 8, while one having virtually no black would carry a zero. More specifically, the code number for each character density range is essentially as follows : Pel density : 0 to 6 percent, Code 0; Pel density 6 - 12 percent, Code 1; Pel density 12 to 18 percent, Code 2; Pel density 18 to 24 percent, Code 3; Pel density 24 to 30 percent, Code 4; Pel density 30 to 36 percent, Code 5; Pel density 36 to 42 percent, Code 6; Pel density 42 to 48 percent, Code 7; Pel density 48 to 54 percent, Code 8; Pel density 54 to 60 percent, Code 9; Pel density 60 to 66, Code 10; Pel density 66 to 72 percent, Code 11; Pel density 72 to 78 percent, Code 12; Pel density 78 to 84 percent, Code 13; Pel density 84 to 90 percent, Code 14; and Pel density 90 to 100 percent, Code 15. When the pel density of a character is exactly on a boundary, such as exactly 48, it is assigned the higher code. The information given is necessarily approximate as density information is given only within the sixteen categories. Additionally, although the printing may be done in proportional spacing or reduced in pitch, this is ignored by the density information. The paper feed system 15 has the capability of feeding paper in increments of the spacing of eight electrodes, which is approximately 0.084 cm (1/30 inch) in the typical system where forty electrodes cover one line, which is approximately 0.42 cm (1/6 inch). Such paper feed may be entirely conventional, as by a direct drive from a conventional stepper motor. The system is controlled by a central data processor 17, typically a microprocessor, but which also may be special-purpose logic. Data to be printed as a line is stored in a memory 19, which typically is an integral part of data processor 17. Where that data is character data, a six-bit code defining the character is stored for each character along, with the foregoing four-bit density code. Where that data is allpoint-addressable graphics data, it is stored in memory 19 in a bit map, meaning that, for example, a binary 1 is stored in memory for each electrode to be driven in a bit position and a binary 0 is stored for each electrode not driven, with those ones and zeros in relative position and in exact number of pels in the line to be printed. Figure 2 is a table illustrating the logical or definitional constraints responded to in this embodiment. In the figures, CumRef should be understood as an abbreviation for cumulative reference, a numerical value which is initially zero and which is accumulated in the system and is responded to by the system to reduce print current to a present amount less than initial. Figure 2 is idealized in its statements of the proportion of heat intensive elements since in practice, as indicated above, the information used in the character mode is approximate. The block status information in Figure 2 is descriptive only to facilitate understanding of how the printing of the current block is influenced by the prior blocks. This implementation is, in fact, fully defined by the numerical value of the cumulative reference. As Fig. 2 illustrates, a line is printed by examining the density of heat-intensive bits in separate, adjoining segments or blocks. The initial block and all blocks immediately after a block not printed are printed at a preset, initial power. Blocks after printed blocks are not printed when the cumulative reference for the previous blocks is minus 2 and are printed at reduced power when the cumulative reference for the previous blocks is minus 1. Where a block printed has an intermediate density of heat-intensive bits, (45% to less than 67% black) the cumulative reference is decreased by 1 unless it is already decreased by 2. Where a block printed has a high density of heat-intensive bits (67% to 100% black), two pass printing is specified and the immediately adjacent block is not printed. Where a block printed has a low density of heat-intensive bits (0% to less than 45% black), the cumulative reference is increased by 1, but never above its initial level. A block printed after the cumulative reference is at minus 1 is printed at a reduced print-power level from initial print level; typically about a 5% to 10% reduction from initial print power. A block printed on the second pass or immediately after skipping of a block is printed at initial power. When printing is resumed in a first pass after skipping a block, another block is not skipped unless print density requires it in accordance with the same constraints as just described. More specifically, and with specific reference to this preferred embodiment, processor 17 starts with the first data in the line, which will appear in the far left of the typical printed line, and examines the density codes of the block of line data consisting of the first six characters where the data is in character form or data representing the first 1,52 cm long portion of printed matter where the data is in all points addressable (APA) graphics form. For character data the density codes associated with each character in font memory 13 are added. A total of 42 and less than 60 is acted upon to mark the data of the next following block for reduction in print power. Similarly, for the APA mode, total 1's are summed and divided by total pels, and a result establishing 45% and less than 67% is acted upon to mark the data of the next following block for reduction in print power. When the density codes for character data total less than 42 or the proportions of 1's for APA graphics data total less than 45%, the data is marked for an increase to initial print power for the next following block unless this occurs where the print power has not been reduced, in which event print power is not changed. Finally, when the density codes for character data total more than 60 or the proportion of 1's for APA graphics total 67% ore more, the data is marked to initiate a two pass printing mode. The entire data for each line is examined similarly in blocks or segments of six characters or of 1,52 cm, depending on whether the data is in character form or in APA form. Each of the blocks are contiguous and in sequence along the line to be printed. Where the two pass mode is specified by the density codes totaling 60 or the APA having 67 percent heat-intensive pels, the next block is marked for printing on the second pass. Blocks not to be printed on the second pass are examined as described for the initial block. Where the density codes for character data total 42 and less than 60 or where the bit density of APA is between 45% and less than 67%, either the data is marked to initiate a second pass for the following block or, if print power for the current block is not reduced, then the data is marked for decrease in print power for the next following block to be printed. To further reduce temperature stress in this preferred embodiment, in the APA mode the full 40 electrodes are not used. Instead, 32 contiguous ones of the 40 electrodes are used, and the line feed is adjusted to be approximately 0.34 cm (2/15 inch) so as to just equal the vertical height of 32 electrodes. This reduces the total overall heating. Computation of bit density is based on the full forty electrodes. The blocks may be larger than six characters or 1,52 cm where the overall heat-response characteristics of the system permits. Similarly, they may be smaller where this is convenient. Heat density is approximated for character codes, and where convenient, these approximations may be less precise than those of this preferred embodiment. Since approximations are adequate, the differences from different pitches and the like (wide, narrow or proportionally spaced characters), are ignored in this embodiment. Printing in the second pass could advantageously be in the opposite direction from that of the first pass when the printer is capable of bidirectional printing. Figure 3 shows the printing resulting from the printing of a line, all of which is more that 67% black in bit density (Segment of density of more than 67% black are illustrated as solid black in Figures 3 and 4). The top line illustrates the line to be printed, with the cumulative reference applicable to the printing of each segment or block shown prior to that segment. Because this line is all heavily black, each segment examined results in accumulating a cumulative reference minus two and therefore the next block is skipped. In Figures 3 and 4 the first pass is labeled 1 and shown immediately under the illustration of the full line being printed. The second pass is labeled 2 and is at the bottom. In the Figures 3 illustration, the first block and alternating blocks after the first block are printed in the first pass, while the remaining blocks are printed on a second pass. Figure 4 shows the result of the printing of a line (shown at the top) having a first block of intermediate bit density (intermediate is illustrated by crosshatching), a second block of low bit density (low is illustrated as all white), a third block of high bit density, a fourth block (the bit density of which is not significant), a fifth block of intermediate bit density, a sixth block of intermediate bit density, and a seventh block (the bit density of which is not significant). The first block brings the cumulative reference to minus 1. The first two blocks bring the cumulative reference to zero and therefore do not have cumulative bit density calling for a second pass. Those two blocks, as well as the next block, are therefore printed in the first pass. The minus 2 significance of the high-density content of the third block requires a second pass. The fourth block or segment is therefore not printed in the first pass. The next following block is printed and its intermediate density brings the cumulative reference to minus 1. Since the cumulative reference is only minus 1, the next block, (the sixth block) is printed on the first pass. That block (the sixth block) has an intermediate density, thereby lowering the cumulative reference to minus 2 and requiring a second pass. The fourth and seventh blocks are therefore not printed on the first pass and are printed on the second pass. This invention makes possible efficiencies in ribbon use and speed of printing in systems where the efficiencies can be utilized. One of these is the potential of not feeding ribbon during any skipped block. That would result in essentially the same amount of ribbon being used for two passes as for one. Another is that the second pass may be by return and printing only those blocks not printed in the first pass. This may be as few as one block. In the actual embodiment, overall heating is such that ribbon feed during printing is continued over skipped blocks as this has a cooling effect. Return to only skipped block must be provided for by appropriate memory and computation or logical capability. A major advantage which is inherent in this invention is that the second pass is completely avoided where the data is such that it is unnecessary. Various implementations and potential variations of this invention will be apparent, all within the scope of this invention. In particular, systems permitting a broader range of print power may be implemented by employing more than one lower level of print power which is specified by different reductions to the cumulative reference. Thus, when the cumulative reference is brought to minus two, a second print-power level, lower than the level at minus one of the cumulative reference, would be employed, and the two pass would not be initiated, for example, except when the cumulative reference would be brought to minus 3 by the increment resulting from a block.

Claims: A thermal printing system comprising : a printhead (3) with print elements (5) for thermal printing in a column mounted perpendicular to a line to be printed by said elements (5) by movement of said print head (3) along said line; determination means (13) to determine from data to be printed the existence of a predetermined proportion of data for which said elements are activated; and means responsive (17,19) to said determination means to cause printing by said print elements (5) of alternating segments of data along a line passed across by said printhead (3), the segments not printed in a first pass of said line being printed in a second pass, characterized in that it comprises : examining means to examine data to be printed in said lines in adjoining segments, each segment comprising substantially all data of each column to be printed along the length of said line; accumulating means to accumulate the results provided for by said examining means and to determine at least a low, an intermediate, and a high proportion of data for which said elements are activated; means to activate said print elements either at an initial power or at at least one power level lower than said initial power; and means responsive to said accumulating means to cause printing of alternating ones of said examined segments when said accumulating means have determined a high proportion of data, to cause printing of segments not printed after said alternating is conducted by passing said print elements over said segments not printed a second time, and to cause printing at said lower power level when said accumulating means have determined an intermediate proportion of data. Printing system as in claim 1 in which said segments are in the order of magnitude of 1.50 cm (about 0.6 inch) along the length of said line being printed, said examining means determine for each said segment one status for data having said high proportion of data for which said elements are activated, a second status for data having said intermediate proportion of data for which said elements are activated and a third status for data having said low proportion of data for which said elements are activated and in which said accumulating means accumulate a balance resulting from each status for contiguous segments all of which are printed, with said one status being accumulated as a factor representing approximately twice the proportion as said second status, and said third status representing approximately the negative of said second status. Printing system as in claims 1 or 2 in which segments printed immediately after segments which are not printed are printed at said initial power level.

Inventors: DAMON, WESLEY BRIAN, RAVITZ, PATTERSON CARY, VOSS, JOSEPH CARL
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1987
Main classification: B41J2
IPC codes: B41J2, G06K15, B41J33