Reciprocating color printing system with specified positioning of printing heads relative to a printing sheet

Abstract: A color printing system for color-printing images or characters on a surface of a writing sheet. The color printing system comprises a plurality of printing heads (A→D) which are successively arranged in parallel to each other and a feeding device for moving the writing sheet in a direction perpendicular to the head-reciprocating directions. Each of the printing heads linearly reciprocates so as to go and return in directions along the surface of the writing sheet for the color printing and has a plurality of ink-discharging nozzles (I→N). The plurality of ink-discharging nozzles are successively arranged with a pitch which is equal to or greater than the twice pitch of lines to be finally printed by the printing heads so as to write the images or characters on the writing sheet. The feeding device moves the writting sheet in a direction perpendicular to the head-reciprocating directions. When each of the printing heads (A→D) performs the going-direction printing operation, the feeding device moves the writting sheet by a distance corresponding to M times of the line-printing pitch where M is a predetermined number, and when the same printing head performs the returning-direction printing operation, the feeding device moves the writting sheet by 2N - M where N is the number of the ink-discharging nozzles (I→N).

Description: BACKGROUND OF THE INVENTION The present invention relates to a reciprocating color printer such as a plane-scanning type color printer. Recently, the plane-scanning color printers as printers having a printing head for printing an image or character on a recording sheet by reciprocation are known, as exemplified by description in JP-A-1 75 255. Conventional plane-scanning type color printing systems will briefly be described hereinbelow with reference to Figs. 1 to 3. In Fig. 1, the plane-scanning type color printer is equipped with four printing heads (yellow, magenta, cyan and black printing heads) illustrated at characters A to D, each of which has N ink-discharging nozzles and is arranged to be mounted on an appropriate carriage, not shown, so as to allow reciprocation. A recording sheet illustrated at character P is fed in a direction indicated by an arrow Y and the printing heads A to D are movable in directions (forward and return directions) indicated by an arrow X which is perpendicular to the direction Y. Each printing head discharges ink in accordance with the movement in the forward direction, i.e., from the left side to the right side in the illustration, so as to color-print N lines as illustrated in Fig. 2. In response to completion of the N-line color printing, the recording sheet is fed by an amount corresponding to the N lines for the return direction color printing. Thereafter, the printing head similarly discharges ink in accordance with the movement in the return direction, i.e., from the right side to the left side in the illustration, so as to color-print N lines as illustrated in Fig. 2. In Fig. 3, another conventional plane-scanning type color printer is similarly equipped with printing heads A to D each of which has N ink-discharging nozzles which are successively arranged at a predetermined interval which is twice the line-printing pitch (half density), where the term "line-printing pitch" refers to the pitch, or distance between, lines to be finally printed by the printing heads for printing images or characters on a surface of a writing sheet. When moving in the forward direction, i.e., from the left side to the right side in the illustration, each printing head performs the color-printing at the location of every other line to be printed, i.e., prints at half density, so as to print N (first to Nth) lines. That is, the first to Nth lines are printed by the first to Nth ink-discharging nozzles of the printing head so as to leave a separation for printing one line therebetween. In response to completion of the forward direction printing, the recording sheet P is moved by a distance corresponding to one print line and the printing head moves in the returning direction so as to newly color-print N additional lines between the first to Nth lines formed by the going-direction printing due to the same printing head. There is a problem which arises with the former, however, in that a considerable tone difference occurs between the forward direction printing and the return direction printing because the order of overlapping inks in the forward direction printing is reverse with respect to the order of overlapping inks in the return direction printing. On the other hand, in the case of the latter, the tone difference problem is substantially resolved because the forward direction printing and the return direction printing are alternately performed on every other line, whereas, because the print lines due to the same ink-discharging nozzle are in close relation to each other, the difference of the ink-discharging amount between the nozzles greatly affects the printing quality. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a color printer which is capable of improving the printing quality concurrently with eliminating the tone difference between the forward direction printing and the return direction printing. In accordance with the present invention, there is provided a color printing system for color-printing images or characters on a surface of a writing sheet, the color printing system comprising: a plurality of printing heads which are successively arranged in parallel to each other and each of which linearly reciprocates bidirectionally so as to go in a forward direction and return in a return direction along the surface of the writing sheet for the color printing, each of the plurality of printing heads having a plurality of ink-discharging nozzles facing the surface of the writing sheet, the plurality of ink-discharging nozzles being successively arranged with a pitch which is equal to or greater than twice the pitch of lines to be finally printed by the plurality of printing heads so as to write the images or characters on the surface of the writing sheet; and writing-sheet feeding means for moving the writing sheet in a direction perpendicular to the forward and return directions, wherein when the printing heads and writing sheet feeding means change performance of a printing operation from the forward direction to the return direction the feeding means operates to move the writing sheet by a distance corresponding to M times the pitch of the printed lines, where M is a predetermined odd number in the range of 1< M < 2N-1 and N is the number of the ink-discharging nozzles of each of the plurality of printing heads. In association with a change in performance of the printing operation from the return direction to the forward direction, the feeding means moves the writing sheet by a distance corresponding to 2N-M times the line-printing pitch. Here, it is also preferable to set the predetermined number M to be substantially equal to the number of nozzles N. In this case, when N is an odd number, M is set to be substantially equal to the number of nozzles N, and the feeding means moves the writing sheet by a distance corresponding to N times the line-printing pitch both in changing from the forward direction printing operation to the return direction printing operation and in changing from the return direction printing operation to the forward direction printing operation. Further, when N is an even number, M is chosen to equal N + 1 or N - 1. When M is set to N + 1, the feeding means moves the writing sheet by a distance corresponding to (N + 1) times the line-printing pitch in changing from the return direction printing operation to the forward direction printing operation and moves the sheet by a distance corresponding to (N-1) times the line-printing pitch in turning from the forward direction printing operation to the return direction printing operation. On the other hand, when M is set to N - 1 the feeding means moves the writing sheet by a distance corresponding to (N - 1) times the line-printing pitch in changing from the return direction printing operation to the forward direction printing operation and moves the sheet by a distance corresponding to (N + 1) times the line-printing pitch in turning from the forward direction printing operation to the return direction printing operation. BRIEF DESCRIPTION OF THE DRAWINGS The object and features of the present invention will become more readily apparent from the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings in which: Fig. 1 is an illustration for describing a printing-head arrangement of a conventional color printer; Fig. 2 is an illustration for describing a first conventional reciprocating color printing system; Fig. 3 is an illustration for describing a second conventional reciprocating color printing system; Fig. 4 shows a printing-head arrangement of a reciprocating color printing system of this invention; Fig. 5 is an illustration for describing a reciprocating color printing system according to a first embodiment of this invention; Figs. 6 to 9 are illustrations for describing a reciprocating color printing system according to a second embodiment of this invention. DETAILED DESCRIPTION OF THE INVENTION Fig. 4 is an illustration for describing a reciprocating color printing system according to an embodiment of the present invention. In Fig. 4, the color printing system is provided with yellow, magenta, cyan and black printing heads A to D each of which is mounted on a carriage so as to be movable in directions indicated by an arrow X (horizontal directions in the illustration). Each of the printing heads A to D has N ink-discharging nozzles each of which discharges yellow, magenta, cyan or black ink toward a writing sheet by the aid of an electric field established between electrodes, for example. The writing sheet illustrated at character P is arranged to be movable in a direction indicated by an arrow Y which is perpendicular to the printing head moving directions X. If the number of printing lines per 1mm (printing density) is taken as d (dots/mm), although in a conventional color printing system the interval between the ink-discharging nozzles is ℓ (= 1/d) which is equal to the line-printing pitch, according to the color printing system of this embodiment, the interval between the first to Nth ink-discharging nozzles is set to be 2ℓ (= 2/d) which is twice the line-printing pitch. That is, the first to Nth ink-discharging nozzles of each of the printing heads A to D are successively arranged at an interval of 2ℓ in directions perpendicular to the head-moving directions X. Here, it is also appropriate to use a printing head arrangement in which the printing density is set to be d , the distance between the nozzles is set to be greater than 2/d and the axes of the printing heads are arranged to be inclined with respect to the printing head moving directions so that the distance between the nozzles assumes 2/d, that is, the line pitch assumes 2ℓ. That is, such a printing head arrangement can be employed for this embodiment under the condition that the inclination of each of the printing heads is adapted to be changeable so that the printing density is d/2. A description will be made hereinbelow in terms of the ink-discharging operation of the printing heads A to D illustrated in Fig. 4. Here, the printing heads A to D move from the left side to the right side in the illustration in the forward-direction printing operation and, on the other hand, move from the right side to the left side in the return-direction printing operation. In the forward-direction printing operation, the first to Nth ink-discharging nozzles (illustrated at 1 to N in the Figure) of each of the printing heads A to D print first to Nth color lines on every other line (with a separation for one print line therebetween) as illustrated in Fig. 5, and in shifting from the forward-direction printing operation to the return-direction printing operation, the writing sheet P is moved in the direction Y by a predetermined amount which corresponds to M times the line-printing pitch (1 < M < 2N-1 and M = an odd number) and then the same printing head moves toward the left side so that the first to Nth ink-discharging nozzles thereof newly print first to Nth color lines on every other line. Here, the first color print line formed by the first ink-discharging nozzle 1 thereof is positioned between the print lines due to the [(M+1)/2]th ink-discharging nozzle and the [(M+3)/2]th ink-discharging nozzles in the forward-direction printing operation. Even if the value of M is taken to be 3 which is a minimum value, (M+1)/2 becomes 2. Thus, the print lines formed by the same ink-discharging nozzle are arranged so as not to be adjacent to each other. In response to completion of the return-direction printing operation, the recording sheet is moved by a distance corresponding to (2N - M) times the line-printing pitch so that the printing head again performs the forward-direction printing operation for the first to Nth lines which are successively arranged in parallel to each other on every other line. In this forward-direction printing operation, the first ink-discharging nozzle 1 of the printing head is positioned between the print lines formed by the [(2N-M+1)/2]th ink-discharging nozzle and the [(2N-M+3)/2]th nozzle. Thereafter, the printing head similarly performs the return-direction printing operation. As a result, the print lines formed by the same ink-discharging nozzle are not adjacent to each other, thereby providing a high-quality color print. A second embodiment of this invention will be described hereinbelow with reference to Fig. 6. In a plane-scanning type color printing system of this embodiment, in shifting from the forward-direction printing operation to the return-direction printing operation, the moved distance of the recording sheet P is set to M times the print line pitch (M is an odd number and determined to be substantially equal to N). In the case of performing the forward and return-direction printing operation, it is generally required to store discharge control signals (print signal) in an appropriate line buffer memory, and the discharge control signals are successively taken from the line buffer memory along the forward direction in the forward-direction printing operation and are successively taken therefrom along the opposite or backward direction in the return-direction printing operation. In practice, it is preferable that the necessary capacity of the line buffer memory is as small as possible. In the case that M is substantially equal to N, not only the memory capacity can be reduced but also the print line due to the first ink-discharging nozzle becomes adjacent to the print line due to the ink-discharging nozzle disposed at the vicinity of the N/2th ink-discharging nozzle, thereby substantially removing the disadvantage resulting from the difference of the ink-discharging amounts between the nozzles. In Fig. 6, character K is a positive integer, and when N is an odd number, N = 2K - 1 (= n) , and when N is an even number, N = 2K (= n') . In the second embodiment, the recording sheet P is moved by a distance which is obtained by multiplying the line-printing pitch by a value shown by the following table. Moreover, as illustrated in Fig. 6, irrespective of N = an odd number or an even number, the print line adjacent to the print line due to the first ink-discharging nozzle is the print line due to the Kth ink-discharging nozzle. A detailed description will be made hereinbelow with reference to Fig. 7 in the case that the ink-discharging nozzle number N is an odd number ( N = 2K - 1 = n ). A line buffer memory for temporarily storing the print signals at every print line is divided in correspondance with N print lines. In the case of using such a line buffer memory, the moved amount of the recording sheet P assumes a minimum value when M = N . Here, it is possible to use three N-line buffer memories E to G. As illustrated in Fig. 7, in the forward-direction printing operation, the forward-direction print signals are taken from the N-line buffer memories E and F, and at the same time the next N-line print signals are stored in the N-line buffer memory G. In response to completion of the forward-direction printing operation, the recording sheet P is moved so that M = N before the return-direction print signals stored in the memories F and G are taken out along the opposite direction in order to perform the return-direction printing operation and, at the same time, the N-line signals are inputted and stored in the memory E. Further, in response to completion of the return-direction printing operation, the recording sheet P is similarly moved by M = N , before the forward-direction print signals stored in the memories G and E are derived along the forward direction so as to perform the forward-direction printing operation and, at the same time, the next N-line print signals are inputted and stored in the memory F. With the above-mentioned operations being repeatedly performed, the color-printing can be completed. It is possible to effectively perform the reciprocation printing with the three N-line buffer memories being cyclically used. Secondly, a description will be made hereinbelow with reference to Fig. 8 in the case that the nozzle number N is an even number ( N = 2K = n' ). In this case, in shifting from the forward-direction printing operation to the return-direction printing operation, the recording sheet P is moved by a feeding amount corresponding to M = N - 1 (i.e., equal to (N-1) times the line-printing pitch), and in shifting from the return-direction printing operation to the forward-direction printing operation, the recording sheet P is moved by a feeding amount corresponding to 2N - M = N + 1 (i.e., equal to (N+1) times the line-printing pitch). As illustrated in Fig. 8, in the N-line buffer memories E to G, the print signals for the forward-direction printing are stored in the even addresses and the print signals for the return-direction printing are stored in the odd addresses. Here, if required, it is also appropriate for the print signals for the forward-direction printing to be stored in the odd addresses and the print signals for the return-direction printing to be stored in the even addresses. In this case, as illustrated in Fig. 9, when N is an even number, i.e., 2K, the print line due to the first ink-discharging nozzle is positioned between the print lines due to the (K+1)th ink-discharging nozzle and the (K+2)th ink-discharging nozzle. Further, in changing from the forward-direction printing operation to the return-direction printing operation, the recording sheet P is moved by a feeding amount corresponding to M = 2K + 1 = N + 1 . On the other hand, in changing from the return-direction printing operation to the forward-direction printing operation, the recording sheet P is moved by a feeding amount corresponding to 2N - M = N -1 . Since in the second embodiment the print line due to the ink-discharging nozzle disposed at one end of the printing head becomes adjacent to the print line due to the ink-discharging nozzle disposed at the center portion of the printing head, and therefore, it is possible not only to improve the color print quality but also to reduce the number of the line buffer memories. It should be understood that the foregoing relates to only preferred embodiments of the present invention, and that it is intended to cover all changes and modifications of the embodiments of the invention herein used for the purposes of the disclosure, which do not constitute departures from the scope of the invention.

Claims: A color printing system for color-printing images or characters on a surface of a writing sheet (P), said color printing system comprising: a plurality of printing heads (A to D) which are successively arranged in parallel to each other and each of said plurality of printing heads linearly reciprocates bidirectionally so as to go in a forward direction and return in a return direction along the surface of said writing sheet for the color printing, each of said plurality of printing heads having a plurality of ink-discharging nozzles (N) facing the surface of said sheet, said plurality of ink-discharging nozzles being successively arranged with a pitch which is equal to or greater than twice a pitch of lines to be finally printed by said plurality of printing heads so as to write the images or characters on the surface of said writing sheet; and writing sheet feeding means for moving said writing sheet in a direction perpendicular to the forward- and return directions, characterised in that in association with a change in performance of a printing operation by said plurality of printing heads and said writing sheet feeding means from the forward direction to the return direction, said feeding means operates to move said writing sheet by a distance corresponding to M times the pitch of the printed lines, where M is a predetermined odd number having a range of 1 < M < 2N - 1 and N is the number of said ink-discharging nozzles of each of said plurality of printing heads, and, in association with a change in performance of the printing operation from the return direction to the forward direction, said feeding means moves said writing sheet by a distance corresponding to 2N-M times the line-printing pitch. A color printing system as claimed in claim 1, wherein the predetermined number M is set to be substantially equal to the number of nozzles N, and when N is an odd number, said feeding means moves said writing sheet by a distance corresponding to N times the line-printing pitch both in changing from the forward direction printing operation to the return direction printing operation, and in changing from the return direction printing operation to the forward direction printing operation. A color printing system as claimed in claim 1, wherein the predetermined number M is set to be substantially equal to the nozzle number N, and when N is an even number, said feeding means moves said writing sheet by a distance corresponding to (N + 1) or (N - 1) times the line-printing pitch in changing from the return direction printing operation to the forward direction printing operation and correspondingly moves said writing sheet by a distance corresponding to (N - 1) or (N - 1) times the line-printing pitch, respectively, in changing from the forward direction printing operation to the return direction printing operation. A method for color-printing images or characters on a surface of a writing sheet in a color printing system including a plurality of printing heads (A to D) which are successively arranged in parallel to each other and each of said plurality of printing heads linearly reciprocates bidirectionally so as to go in a forward direction and return in a return direction along the surface of said writing sheet for the color printing, each of said plurality of printing heads having a plurality of ink-discharging nozzles (N) facing the surface of said sheet, said plurality of ink-discharging nozzles being successively arranged with a pitch which is equal to or greater than twice a pitch of lines to be finally printed by said plurality of printing heads so as to write the images or characters on the surface of said writing sheet; and writing-sheet feeding means for moving said writing sheet in a direction perpendicular to the forward- and return directions, characterised by comprising the steps of, causing said feeding means to move said writing sheet (P) by a distance corresponding to M times the pitch of the printed lines after said plurality of printing heads perform the printing operation in the forward direction to position said writing sheet to be printed by movement of said plurality of printing heads in the return direction, where M is a predetermined odd number having a range of 1 < M < 2N - 1 and N is the number of ink-discharging nozzles of each of said plurality of printing heads, and causing said feeding means to move said writing sheet by a distance corresponding to 2N - M times the line-printing pitch after said plurality of printing heads perform the printing operation in the return direction to position said writing sheet to be printed by movement of said plurality of printing heads in the forward direction. The color printing method as claimed in claim 4, comprising the further step of setting the predetermined number M to be substantially equal to the number of nozzles N, and when N is an odd number, causing said feeding means to move said writing sheet by a distance corresponding to N times the line-printing pitch both in changing from the forward direction printing operation to the return direction printing operation, and in changing from the return direction printing operation to the forward direction printing operation. The color printing method as claimed in claim 4, comprising the further step of setting the predetermined number M to be substantially equal to the number of nozzles N, and when N is an even number, causing said feeding means to move said writing sheet by a distance corresponding to (N + 1) or (N - 1) times the line-printing pitch in changing from the return direction printing operation to the forward direction printing operation and correspondingly, to move said writing sheet by a distance corresponding to (N - 1) or (N + 1) times the line-printing pitch, respectively, in changing from the forward direction printing operation to the return direction printing operation.

Inventors: IWAZAWA TOSHIYUKI, MIURA MASAYOSHI, IWAZAWA, TOSHIYUKI, MIURA, MASAYOSHI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1990
Main classification: B41J2
IPC codes: B41J2, B41J19