PRINTER INCLUDING MEANS FOR ADVANCING ADDITIONAL INK RIBBON AS REQUIRED

Abstract: The present invention relates to a printer for printing data on a print medium, of the type comprising a print head (21) mounted for movement relative to a print medium (21), a supply of ink ribbon (25), including a span of ink ribbon (27) which is interposed between the print head and the print medium, means (23, 24) for moving the print head and for simultaneously advancing ink ribbon from the supply into the span as the print head is moved in at least one direction, and control means (20) for directing print data to the print head and controlling movement of the print head. A printer according to the invention is characterised in that the control means includes means for detecting when the printing density exceeds a predetermined level, and means, responsive to the detection of the predetermined level being exceeded, to move the print head an amount greater than that necessary to position the print head at a succeeding print position so as thereby to cause additional ink ribbon to be advanced from the supply and to avoid superimposing additional print impressions on portions of the ink ribbon where a high density of print impressions has already occurred.

Description: This invention relates to printers for printing data on a print medium, and to apparatus for advancing additional ink ribbon in such a printer as required. Ribbon ink depletion is recognised as a problem for printers of the type in which a print head and an ink ribbon are both moved across a print medium during printing operations and which are capable of printing dense patterns or graphics. The ribbon is typically moved at a slower rate than the print head, and ink depletion occurs when successive dense patterns on a line are printed using the same portion of the ribbon. This repeated use of a portion of the ribbon results in appreciable depletion of the ink in the ribbon and this causes the images of later patterns to be printed lighter. This is undesirable, particularly when printing colour-filled images. A number of approaches has been taken in trying to solve this problem. The most popular solution has employed a motor to drive the ribbon continually independently of the print head, and has been adopted in many small and intermediate printers. However, the use of a separate ribbon feed motor adds significantly to the cost of the printer, and may result in a higher than desirable rate of ribbon consumption. In many printers, particularly the smaller, lower cost printers, the ink ribbon is advanced by a mechanical linkage between the ribbon drive and the print head motor so that movement of the print head by the print head motor will also advance the ribbon. Ribbon ink depletion is especially troublesome in this type of printer. An object of the present invention is to provide means for advancing additional ink ribbon in a printer to avoid ribbon ink depletion, and which is suitable for use in a printer in which movement of the print head controls the advancement of the ink ribbon. The present invention relates to a printer for printing data on a print medium, of the type comprising a print head mounted for movement relative to a print medium, a supply of ink ribbon, including a span of ink ribbon which is interposed between the print head and the print medium, means for moving the print head and for simultaneously advancing ink ribbon from the supply into the span as the print head is moved in at least one direction, and control means for directing print data to the print head and controlling movement of the print head. A printer according to the invention is characterised in that the control means includes means for detecting when the printing density exceeds a predetermined level, and means, responsive to the detection of the predetermined level being exceeded, to move the print head an amount greater than that necessary to position the print head at a succeeding print position so as thereby to cause additional ink ribbon to be advanced from the supply and to avoid superimposing additional print impressions on portions of the ink ribbon where a high density of print impressions has already occurred. The scope of the invention is defined by the appended claims, and how it can be carried into effect is hereinafter particularly described with reference to the accompanying drawings, in which:- Figure 1 is a schematic illustration of a printer according to the invention, Figure 2 is a flow chart illustrating the sequence of operations of the printer illustrated in Figure 1 according to the present invention, Figure 3 illustrates diagramatically the text printed on the print medium and the print impressions on the ink ribbon during a printing operation of the printer illustrated in Figure 1, Figure 4 illustrates diagramatically the amount of movement which is required to advance the ink ribbon of the printer illustrated in Figure 1 for printing text corresponding to successive buffer loads of print data, Figure 5 is a diagram illustrating the movement of the print head of the printer illustrated in Figure 1 during printing of high density characters which can be used for the advancement of additional ink ribbon, and Figure 6 illustrates diagramatically how the density of data in printed characters may be examined to minimise ribbon consumption. A dot matrix printer according to the invention includes a platen 11 (Fig. 1) over which a print medium 12 is moved by means of a pair of tractor devices 13 and 14 in the direction of the arrow A. The print medium is a continuous web of paper having holes 17 parallel to the edges thereof and the tractor devices are in the form of a wheel or belt having protruding pins 16 on the outer surface. The pins 16 of the tractor devices engage the holes 17 formed in the web to provide a positive drive. The two tractor devices 13 and 14 are mounted on a common shaft 18 which may be rotated as required by a motor 19 to advance the print medium over the platen 11. The motor 19 is controlled by a printer control unit 20. The printer also includes a traversing print head 21 which is mounted for bidirectional movement, as illustrated by the double headed arrow B, laterally across the print medium 12 on the platen 11 upon a support 22. Support 22 extends over and is spaced from the platen 11 so that the print medium 12 passes between the platen 11 and the print head 21. The print head 21 is moved along the support 22 by means of a rotatable threaded shaft 23 which is coupled to the print head 21 and is driven by a motor 24 controlled by the printer control unit 20. The printer control unit 20 controls the data processing and mechanical functions of the printer and coordinates their respective operations, and may further communicate with a host computer to receive the print data relating to the characters which are to be printed. To this end, the printer control unit 20 includes a print medium controller for providing signals to the motor 19 to control movement of the print medium 12 over the platen and past the print head; a print head controller means providing signals to motor 24 to control movement of the print head; and print head actuator means providing signals to actuate the individual printing elements of the print head 21. The control unit 20 further coordinates the flow of data to the print head with the physical movement of the print head. The printer control unit 20 also includes data buffer means into which print data is read when received and from which it is directed to the print head. The printer also includes a replaceable ink ribbon supply unit 25. The ink ribbon may be in the form of a cloth ribbon or Mylar (Registered Trade Mark) film, or in any other suitable form. As illustrated, the ribbon is housed in an ink ribbon cartridge 26 and includes a span 27 of ink ribbon which is interposed between the print head 21 and the print medium 12. A mechanical linkage 28 interconnects the rotatable threaded shaft 23 and an ink ribbon drive in the ribbon cartridge 26 so that actuation of the print head drive motor 24 to move the print head 21 will also cause used ribbon to be drawn from the span 27 into the cartridge 26 so that fresh ribbon is advanced from cartridge 26 into the span 27. Thus, as the print head 21 moves back and forth during printing, fresh ribbon will be continually advanced into the span 27. Depending upon the arrangement of the linkage, ribbon may be advanced either during movement of the print head in a single direction or during movement of the print head in both directions. The linkage may take the form of gears, pulleys or other suitable means. By way of example, one known linkage means for driving an ink ribbon during bidirectional movement of the print head is shown in IBM Technical Disclosure Bulletin Volume 15, No. 7, December 1972, page 2312. The linkage 28 connecting the ink ribbon drive to the print head drive causes the ribbon to be advanced at a rate slower than the rate of print head movement. As a result, the impressions made by the print head on the ribbon are likely to be superimposed upon one another. During the printing of conventional text characters, the print density is normally low enough for the ink ribbon to be able to provide adequate ink for printing the characters on the print medium even with a certain amount of superimposition of print impressions. However, during the printing of dense patterns, such as may occur for example in printing graphics of high printing density, backgrounds, or certain text fonts, the movement of the ribbon is such that ink depletion may occur when the impressions resulting from the printing of successive dense patterns on a line are superimposed on a section of the ribbon. The printer illustrated overcomes this problem by detecting when the printing density exceeds a predetermined threshold level and, in these circumstances, causing additional ink ribbon to be advanced from the supply by causing the print head to move an amount greater than that necessary to position the print head at the succeeding print position. This may be accomplished in various ways, depending upon the particular printer environment. Before providing an illustration of one suitable way in which these operations may be carried out, it may be helpful to first review certain operations of the particular printer illustrated herein. The printer control unit 20 includes a print data buffer which receives and temporarily stores the print data until it can be directed to the print head. After the print data in the print data buffer has been used for a printing operation, the buffer receives more print data and the cycle is repeated. When printing text, the buffer may contain enough print data to print an entire line or several entire lines of text. However, when printing certain modes of graphics which involve large volumes of binary data, several successive buffer loads of data may be required to print across the entire width of the print medium. Under these circumstances, the print head must be repositioned after printing each buffer load of data so as to be at the proper print position for continuing printing with the next buffer load of data. This repositioning phase is necessary due to the time required to accelerate the print head to constant speed for printing and to stop the head upon completion of printing. The repositioning may occur several times during the printing of a line. When extra print head movement is required for additional ribbon advancement as described above, the extra movement is incorporated in the print head repositioning phase. A flow chart representing the sequence of operations leading to the additional ribbon advancement is shown in Figure 2, and will be explained as follows. Initially print data is read into the print data buffer in step 30. Then, during each printing operation using a buffer load of print data, the data in the buffer is examined in step 32 and a determination made in step 34 as to whether the print density has exceeded a predetermined threshold level. In the printing of graphics characters, this determination may be carried out by counting the average number of dots per unit area to be printed and comparing the dot density value thus obtained with a predetermined dot density threshold. A similar approach may also be employed in the printing of text characters by determining whether any of the stored characters in the buffer are such as to produce a density above a predetermined threshold level. Alternatively, the determination of whether the printing density threshold level has been reached may be based upon whether the printer is printing in a particular print mode or font likely to produce high density printing. If the density threshold level is exceeded, then a mark is made in step 36. In step 38, it is determined whether examination of the data in the buffer has been completed. If not, the data in the buffer continues to be examined by returning to step 32. If so, it is determined in step 40 whether the print density threshold level has been exceeded by checking for one or more marks (step 36). If one or more marks have been made during the examination, the print head will be moved an extra distance in order to advance additional fresh ribbon so as to avoid superimposing print impressions on areas of the ribbon which have already been used previously for high density printing. In step 42, the additional extra travel which is required by the print head in order to advance the fresh ribbon is calculated. The calculation of this distance is best understood from the following discussion which derives an exemplary equation for print head movement. In this example, the parameters are as follows: B n = actual length of the text just printed using a print data buffer load. B n+1 actual length of the text to be printed using the next print data buffer load, D = distance moved by print head during deceleration. A = distance moved by print head during acceleration. R = ratio of print head movement to ribbon movement. S = distance moved by print head without ribbon movement which occurs at a change of direction of the print head due to gear slop. Because the ribbon moves slower than the print head by a factor of R, the length of the portion of the ink ribbon occupied by the print impressions resulting from the print data in buffer B n is (B n - B n /R). This is illustrated in Figure 3 where the upper block 46 represents the length B n on the print medium 12 of the text corresponding to the print data in the buffer, and the lower block represents at 48 the length (B n -B n /R) of the portion of the ink ribbon occupied by the print impressions corresponding to that text and at 50 the distance (B n /R) moved by the ink ribbon during the printing operation. The ends 52 and 54 of the upper block represent the positions of the print head before and after printing respectively. After printing text represented by the print data in the buffer B n , the ribbon needs to move a distance of (B n - B n /R) to clear the portion of the ribbon just used in printing out the current data in the buffer and an additional distance of (B n+1 - B n+1 /R) to provide fresh ribbon in position for receiving the impressions the next load of data in the buffer when printing is continued in the same direction. This is illustrated in Figure 4 where the uppermost block illustrates the condition of the ribbon upon completion of printing the current buffer load with a fresh ribbon portion 56, an immediately used portion 58 and previously used portion 60. The arrow 62 indicates the next print position. The middle block illustrates the appearance of the ribbon after it has been advanced to the right a distance of (B n - B n /R) to clear the portion of the ribbon used in printing the current buffer load together with a fresh portion 64 including portion 56. The lowermost block illustrates the appearance of the ribbon as it is ready to receive the next buffer load with an additional fresh portion 68, a fresh portion 66 including portion 64 and the used portion 58. At this point the ribbon has been advanced a distance of (B n - (B n /R) + B n+1 - (B n+1 /R)). In order for this amount of ribbon to be advanced, the print head needs to be moved a total distance of R (B n - (B n /R) + B n+1 - (B n+1 /R)). The extra distance is moved through by the print head carrier during its printing/repositioning phase as shown in Figure 5. Referring to this figure, the numbered points are as follows: 1 Initial starting position of the print head. 1-2 Acceleration of the print head. 2-3 Printing phase of buffer load to produce text of length B n . 3-9 Compensation distance to advance ribbon. After 9 Printing phase of buffer load to produce text of length B n+1 . If the total printing/repositioning movement of the print head is to provide for all the required ink ribbon movement, then, assuming that the ribbon is driven in the forward direction during both directions of print head movement: R (B n - (B n /R) + B n+1 - (B n+1 /R))= 2X + 2(A+D-S). (1) where X is the distance the print head moves at a constant speed, and A and D are fixed for a given speed. Solving for X from equation (1): X = ((R-1)/2) (B n + B n+1 ) - (A + D - S) (2) Assuming that the ribbon is driven in the forward direction only during forward print head movement then the formula becomes: X = (R-1) (B n + B n+1 ) - (A + D - S). (3) If one cycle of printing/repositioning of the print head is insufficient to advance the ink ribbon by the amount required, the cycle can be repeated as many times as is necessary to effectively increase the value of X. The foregoing calculation of the required amount of print head movement assumes the worst case of ink depletion, i.e. that the print impressions resulting from a buffer load of data deplete the entire section of ribbon used. In reality, however, there are many cases where the major ink depletion occurs past the beginning of the printing of a buffer load. By introducing an additional variable, fired pattern density, the print head movement can be minimised. The density of the fired pattern (pattern produced by firing the individual dot printing element) is the main factor causing ink depletion. Therefore, pattern density can be tracked in order to minimise print head movement. There is an experimental threshold level of pattern density below which the ribbon can safely accept new patterns before recycling. This threshold level is ribbon and application dependent. To determine the point on the ribbon which crosses the threshold level, the dot distribution is successively calculated for small incremental portions of the buffer load processing. For example, after every graphic character, (12 dots), the dot distribution density is calculated and a test is made whether this dot distribution density exceeds the critical threshold level. When the dot distribution density exceeds this value, the location of the occurrence is marked. Thus, as illustrated in Figure 6, the initial pattern 70 of dots indicates they are relatively sparsely distributed, while the patterns 72 of dots indicate a high density of dots. The first occurrence of the high density dots at 72 is indicated at X and represents the point where threshold level was exceeded. The portion d of the ribbon to the left of this threshold level crossing X, where the density is below the threshold level, is the portion of the ribbon which can be reused for printing out the next buffer load, while the remaining portion D of ribbon to the right of the point of threshold level crossing X is considered as being fully used and depleted. The locations of subsequent threshold level crossings can also be marked (e.g. the last crossing Y as well as the first crossing X) so as thereby to determine depleted portions of the ribbon, and suitable logic can be applied to maximise reuse of undepleted portions of the ribbon while avoiding overprinting on the thus identified depleted portions. Thus, for example, as indicated in Figure 6, the last occurrence of a high density pattern 72 is indicated at Y, and the portion between X and Y represents a depleted portion of the ribbon which cannot be reused, while the portion to the right of the point Y could be reused in appropriate circumstances.

Claims: 1. A printer for printing data on a print medium, comprising a print head (21) mounted for movement relative to a print medium (12), a supply of ink ribbon (25), including a span of ink ribbon (27) which is interposed between said print head and said print medium, means (23, 24) for moving said print head and for simultaneously advancing ink ribbon from said supply into said span as said print head is moved in at least one direction, and control means (20) for directing print data to said print head and controlling movement of said print head, characterised in that said control means includes means for detecting when the printing density exceeds a predetermined level, and means, responsive to the detection of said predetermined level being exceeded, to move said print head an amount greater than that necessary to position said print head at a succeeding print position so as thereby to cause additional ink ribbon to be advanced from said supply and to avoid superimposing additional print impressions on portions of said ink ribbon where a high density of print impression has already occurred. 2. A printer according to claim 1, characterised in that said control means (20) also includes means for computing the amount of additional movement of said print head which is required to advance said additional ink ribbon. 3. A printer according to claim 1 or 2, characterised in that said means for detecting when the printing density exceeds a predetermined level comprises means for discriminating between several printing modes of different density. 4. A printer according to claim 1, 2 or 3, characterised in that said control means for directing print data to said print head includes a data buffer for storing print data to be directed to said print head, and in that said means for detecting when the printing density exceeds a predetermined level includes means for examining the print data in said data buffer. 5. A printer according to claim 4, characterised in that said means for examining the print data in said data buffer includes means for examining the density of said print data in successive incremental loads in said data buffer and means for recording the location of the load containing the first occurrence when the density of the print data exceeds said predetermined level. 6. A printer according to claim 4, characterised in that said means for examining the print data in said data buffer includes means for examining the density of the print data in successive incremental loads in said data buffer and means for recording the locations of the loads where the density of the print data exceeds said predetermined value thereby to determine the ink depleted regions of said ribbon. 7. A printer according to claim 4, 5 or 6, characterised in that said control means includes means for examining the content of the current buffer load of print data and the content of the next succeeding buffer load of print data and for computing therefrom the amount of additional movement of said print head which is required to advance sufficient ink ribbon into position for performing printing corresponding to the next succeeding buffer load of print data. 8. A printer according to any preceding claim, characterised in that said print head is a dot matrix print head mounted for movement laterally across said print medium. 9. A printer according to any preceding claim, including linkage means coupled to the means for moving the print head, to advance the ink ribbon from the supply into the span in response to movement of the print head in at least one direction.

Inventors: DO, PHUC K., HEDRICK, DENNIS R., KENDRICK, BARNES R., TROUPES, DEMETRIOS, WYKE, CAROL E.
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1986
Main classification: B41J33
IPC codes: B41J31, B41J33, B41J35