Patent Publication Number: US-6902254-B2

Title: Printer system which uses a plurality of print heads and which controls the print heads with a simple configuration to achieve high accuracy image printing

Description:
This application claims the benefit of Japanese Application No. 2001-242730 filed in Japan on Aug. 9, 2001, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a printer system that performs image printing using a plurality of print heads having a plurality of nozzles for discharging ink droplets. 
     2. Related Art Statement 
     Conventionally, as a color inkjet printer that uses a plurality of print heads, each print head being assigned one color and having a plurality of nozzles for discharging ink droplets, a head-scan color inkjet printer that performs color printing by causing the plurality of print heads to scan in a direction perpendicular to the paper feeding direction has been available. 
     Meanwhile, in recent years, what is called a head-non-scan one-pass inkjet printer has been proposed. In this printer, print head groups are configured such that a plurality of print heads having rows of nozzles is arranged in the paper width direction. The print head groups are provided so as to correspond to the number of colors to be used, each print head group being assigned one color. With this arrangement, the printer performs high-speed printing by feeding only paper in one direction without causing the head to scan. 
     However, in general, inkjet printers become more difficult to manufacture and more expensive as the number of rows of nozzles increases. Thus, a width that can be printed by a single head used in the head-non-scan one-pass inkjet printer is not much different from a width that can be printed with one scan by a head used in the head-scan inkjet printer. As a result, with the head-non-scan one-pass inkjet printer, when the paper width is increased from A4 size to A3 size (JIS standard) . . . , or the number of ink colors is increased, the number of print heads used is significantly increased. 
     Since the head controllers for the discharge of ink from the print heads in the conventional scan-type inkjet printer only control, for example, four print heads for four colors, they do not have a particularly complicated structure. However, with the non-scan inkjet printer, which uses the plurality of print head groups constituted by the plurality of print heads, the positions of data corresponding to the rows of nozzles which are read from a buffer memory need to be changed so as to correspond to the intervals of the rows of nozzles in a large number of heads. In addition, since the number of print heads is greater than that number of printer heads in the scan-type inkjet printer, the capacity of the buffer memory is increased correspondingly. 
     As a result, there is a need for changing the pointer in the memory for each of the large number of the rows of nozzles (for each print head) by using a large-capacity buffer. Thus, when a single head controller needs to be used for the large number of print heads, there is a problem in that the control becomes significantly complicated. 
     In particular, with the non-scan inkjet printer that uses the large number of print head groups, since the print head groups for individual colors are arranged with a certain distance therebetween with respect to the paper feeding direction, there is a possibility that image-printing positions are displaced between print head groups due to an error in a scale pitch, which is a reference for the discharge timing. Thus, the image-printing accuracy may be adversely affected. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to overcome the above problems, and an object of the present invention is to provide a printer system that uses a plurality of print head groups, allows the control of the print heads with a simple configuration, and performs image printing with high accuracy. 
     The printer system of the present invention is a printer system that performs printing on a recording medium. The printer system includes a plurality of printer elements. Each printer element includes a one-color print-head group constituted by a plurality of print heads for a single color, a head controller for controlling the one-color print-head group, and a host interface circuit for controlling the one-color print-head group. The plurality of printer elements is controlled by a single host computer. 
     The other features and advantages of the present invention will become apparent from the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of printer system according to a first embodiment of the present invention. 
         FIG. 2  is a block diagram of a head controller that is included in the printer system of the first embodiment shown in FIG.  1 . 
         FIG. 3A  is a plan view showing the arrangement of a printer section of the printer system of the first embodiment shown in FIG.  1 . 
         FIG. 3B  is a side view showing the arrangement of the printer section of the printer system of the first embodiment shown in FIG.  1 . 
         FIG. 4  is a flow chart showing image-printing processing in a host computer of the printer system of the first embodiment shown in FIG.  1 . 
         FIG. 5  is a flow chart showing image-printing processing in the head controller of the printer system of the first embodiment shown in FIG.  1 . 
         FIG. 6  is a block diagram of a printer system according to a second embodiment of the present invention. 
         FIG. 7  is a side view, viewed from a direction perpendicular to the paper feeding direction, showing a printer section of the printer system of the second embodiment shown in FIG.  6 . 
         FIG. 8  is a side view, viewed from a direction perpendicular to the paper feeding direction, showing the printer section of the modification of the printer system of the second embodiment shown in FIG.  6 . 
         FIG. 9  is an enlarged view showing an arrangement of K-color print dots and C-color print dots when the printer system performs image printing. 
         FIG. 10  is a side view, viewed from a direction perpendicular to the paper feeding direction, of the printer section of the modification of the printer system of the second embodiment shown in FIG.  6 . 
     
    
    
     DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described below with reference to the accompanying drawings. 
       FIG. 1  is a block diagram showing a printer system of a first embodiment of the present invention.  FIG. 2  is a block diagram showing a head controller of the printer system of this embodiment.  FIG. 3A  is a plan view showing the arrangement of a printer section of the printer system of this embodiment, and  FIG. 3B  is a side view along arrow A in FIG.  3 A. 
     A printer system  1  of this embodiment is a type of inkjet printer that performs printing in one pass. Specifically, the printer system  1  includes a plurality of groups of print heads, each print head having a row of nozzles for discharging ink droplets, arranged over the entire width of the paper, and, during printing, feeds paper, which is a recording medium, in the feed direction without causing the print heads to scan in the horizontal direction (in the width direction and the feeding direction of the paper). 
     As shown in  FIG. 1 , the printer system  1  includes a host computer  2  that is responsible for the overall control of the printer system  1 , and printer elements  11 ,  12 ,  13 , and  14  for four colors, i.e., K (black), C (cyan), M (magenta), and Y (yellow). The printer system  1  further includes a paper feed controller  61 , a paper feed mechanism  63 , a head cleaning controller  62 , and a head cleaning mechanism  64 . 
     The printer elements  11 ,  12 ,  13 , and  14  include K, C, M, and Y head controllers  15 ,  16 ,  17 , and  18 , respectively, which are connected in a SCSI (small computer system interface) interface daisy chain to the host computer  2  via a SCSI host adapter ( 1 )  3 . Meanwhile, the paper feed controller  61  and the head cleaning controller  62  are connected to the host computer  2  via corresponding RS-232C interfaces. 
     As shown in  FIG. 1 , the K printer element  11  includes the K head controller  15  for black and a K print head group  20 . The C printer element  12  includes the C head controller  16  for cyan and a C print head group  27 . The M printer element  13  includes the M head controller  17  for magenta and an M print head group  34 . The Y printer element  14  includes the Y head controller  18  for yellow and a Y print head group  41 . 
     As shown in  FIG. 2 , the K head controller  15  includes a CPU  51  for controlling the controller, an interface protocol controller circuit  52  connected to the host computer  2  via a SCSI interface bus, and a buffer memory  53  for storing black print data captured through the interface protocol controller circuit  52 . The K head controller  15  further includes a head drive circuit  54  for driving the K print head group  20 , and a ROM  55  and a RAM  56  which store data related to printing control. With this arrangement, the controller  15  does not have a complicated configuration since it controls only up to six heads for the same color. 
     In accordance with an encoder output signal from the paper feed mechanism  63  and black image-print data, the head drive circuit  54  controls the drive timing, pulse voltage, pulse width, and the like for each of the print heads constituting the K print head group  20 , thereby controlling the timing and the amount of ink to be discharged. 
     The K print head group  20  is constituted by six heads, i.e., a K print head ( 0 )  21 , a K print head ( 1 )  22 , a K print head ( 2 )  23 , a K print head ( 3 )  24 , a K print head ( 4 )  25 , and a K print head ( 5 )  26 . Each K print head has an ink discharge surface, on which a row of nozzles  48  ( FIG. 3A ) is arranged along the Y direction for discharging ink. 
     The print heads  21  to  26  of the K print head group  20  are arranged in two rows of three heads along two oblique lines EA and EB at predetermined positions above the upper surface of a slide region of a paper feeding stage  72 . In this arrangement, the rows of nozzles of the print heads are provided over the entire print width of the paper. For example, when the print head  23  is assumed to be a reference head of the print heads  21  to  26 , the print heads  21 ,  22 ,  24 ,  25 ,  26  are arranged at predetermined distances relative to the reference print head  23  in the paper feeding direction. This means that image-print data for each print head contains blank data that depends upon the corresponding predetermined distance, i.e., data for instructing non-discharge of ink. 
     The C, M, and Y head controllers  16 ,  17 , and  18  have the same configurations as the K head controller  15 , and control the timing and the amount of ink to be discharged from the C print head group  27 , the M print head group  34 , and the Y print head group  41 , respectively. 
     The C print head group  27  is also constituted by six C print heads ( 0 )  28  to ( 5 )  33  each having a row of nozzles. The M print head group  34  is constituted by six M print heads ( 0 )  35  to ( 5 )  40  each having a row of nozzles. The Y print head group  41  is also constituted by six Y print heads ( 0 )  42  to ( 5 )  47  each having a row of nozzles. 
     Similarly, with regard to the print heads ( 0 )  28  to ( 5 )  33  of the C print head group  27 , the print heads ( 0 )  35  to ( 5 )  40  of the M print head group  34 , and the print heads ( 0 )  42  to ( 5 )  47  of the Y print head group  41 , the print heads of each print head group are arranged along two oblique lines in two rows along the Y direction, which is the width direction of the paper, at predetermined positions over a region in which the paper feeding stage  72  slides. 
     The K, C, M, and Y print head groups  20 ,  27 ,  34 , and  41  are positioned in the X direction (the paper feeding direction) with an accuracy corresponding to the resolution such that the reference print heads  23 ,  30 ,  37 , and  44  are arranged with predetermined distances L 1 , L 2 , and L 3  therebetween. In addition, each print head of the K, C, M, and Y print head groups  20 ,  27 ,  34 , and  41  are also positioned with reference to the print heads  23 ,  30 ,  37 , and  44  with an accuracy at which the relative position in the X and Y directions of each print head corresponds to the resolution. When it is difficult to position each print head with an accuracy corresponding to a resolution, the discharge timing may be controlled by the CPU  51  and the head drive circuit  54  with a sub-pixel accuracy. 
     As shown in  FIGS. 3A and 3B , the paper feed mechanism  63  has a base plate  71  and the paper feeding stage  72 . The paper feeding stage  72  is provided on the base plate  71  and is driven to slide by a linear driving section in the feed direction (X direction). The sliding movement thereof is controlled by the paper feed controller  61 . 
     A scale  74  for an encoder is provided on a side surface of the base plate  71  along the X direction. The pitch of the scale  74  is, for example, 0.070 mm, which corresponds to the resolution of the printer system. 
     The feeding stage  72  is driven by a feed driving section, not shown, in the X direction, and is equipped with one encoder (sensor)  73  at a position facing the scale  74  of the base plate  71 . The encoder (sensor)  73  detects the amount of movement of the feeding stage  72  to determine the discharge timing of ink from the print heads. The encoder (sensor)  73  is formed of, for example, a photo-interrupter. The encoder  73  outputs encoded pulse signals, which are scale-pattern detection pulses output in response to movement relative to the scale  74 , to the K, C, M, and Y head controllers  15  to  18 . Each controller senses the amount of movement of the feeding stage  72  in accordance with the encoded pulse signal and internally generates the ink discharge timing. 
     The K, C, M, and Y head controllers  15 ,  16 ,  17 , and  18  are assigned SCSI-ID (identification) numbers that are different from one another. In this embodiment, the K head controller  15  is assigned SCSI-ID #0, the C head controller  16  is assigned SCSI-ID #1, the M head controller  17  is assigned SCSI-ID #2, and the Y head controller  18  is assigned SCSI-ID #3. For this case, the LUN (logical unit number) is 0. 
     Each piece of data output from the host computer  2  is identified with an ID number and is then captured by the corresponding head controller. Instead of the SCSI interfaces, IEEE-1394 interfaces may be used. In such a case, the K, C, M, and Y head controller  15 ,  16 ,  17 , and  18  are each assigned a different bus ID and a different node ID. 
     When printing an image, the printer system  1  configured as described above according to this embodiment first captures image data into the host computer. The host computer  2  then transfers image-print range data to the K, C, M, and Y head controllers  15 ,  16 ,  17 , and  18  via the SCSI host adapter  3 . The host computer  2  extracts pieces of image data for individual colors from the captured image data and also transfers the extracted pieces of image data to the corresponding K, C, M, and Y head controllers  15 ,  16 ,  17 , and  18  via the SCSI host adapter  3 . The pieces of image data for individual colors include K data, which is image data for the color K, C data, which is image data for the color C, M data, which is image data for the color M, and Y data, which is image data for the color Y. These pieces of image data are written into the corresponding buffer memory  53  and so on as image-print data. In this case, each piece of data transferred via the SCSI adapter ( 1 )  3  is identified with a SCSI-ID and is forwarded to the corresponding K, C, M, or Y head controller  15 ,  16 ,  17 , or  18 . 
     Subsequently, the paper feed controller  61  start moving of the paper feeding stage  72  of the paper feed mechanism  63  from a predetermined start position, thereby starting the feeding of the paper  75 . When a count of the number of pulses output from the encoder  73  becomes a value corresponding to a print start position P 72 A, the CPU  51  of the K head controller  15  instructs the head drive circuit  54  to start printing an image. 
     In accordance with the K-color image-print data stored in the buffer memory  53  and the pulse data output from the encoder  73 , the head drive circuit  54  then begins to control ink discharged from each print head  21  to  26  of the K print head group  20 . That is, the ink discharge timing is controlled while being synchronized with a pulse signal output from the encoder  73 , and, simultaneously, in accordance with the K-color image-print data, the amount of ink discharged is controlled with a voltage for driving each print head. 
     Further, when the count of the number of pulses output from the encoder  73  reaches a value corresponding to an image-print start position P 72 B (a position at a distance of L 1  from the print start position P 72 A in the +X direction) for the C print head group  27 , the CPU of the C head controller  16  instructs the head drive circuit to print an image. 
     Likewise, when the number of pulses output from the encoder  73  reaches a count number corresponding to a position P 72 C located at a distance L 1 +L 2  in the +X direction from the print start position P 72 A, or a position P 72 D located at a distance L 1 +L 2 +L 3  in the +X direction from the print start position P 72 A, the corresponding M print head group  34  or Y print head group  41  starts printing an image. 
     When each of the print head groups  20 ,  27 ,  34 , and  41  completes printing of a corresponding predetermined number of dots, each CPU issues a status message indicating the completion of the printing command. When the host computer  2  recognizes the message, the operation for image printing ends. 
     The operation for image-printing (printing) in the printer system  1  of this embodiment will now be described with reference to the flow charts in  FIGS. 4 and 5 . 
       FIG. 4  is a flow chart showing the processing in the host computer  2  for printing an image, and  FIG. 5  is a flow chart showing the processing in each head controller, controlled by the host computer  2 , for printing an image. 
     First, in the host computer  2 , host-side image-control is initiated, as shown in the flow chart in FIG.  4 . That is, in steps S 11  and S 12 , a command for setting an image-print start position and a command for setting an image-print width are issued to the K head controller  15  via the SCSI-ID #0. 
     Next, in steps S 13  and S 14 , a command for setting an image-print start position and a command for setting an image-print width are issued to the C head controller  16  via the SCSI-ID #1. 
     Further, in steps S 15  and S 16 , a command for setting an image-print start position and a command for setting an image-print width are issued to the M head controller  17  via the SCSI-ID #2. 
     Further, in steps S 17  and S 18 , a command for setting an image-print start position and a command for setting an image-print width are issued to the Y head controller  18  via the SCSI-ID #3. 
     In steps S 19  to S 22 , K data, C data, M data, and Y data, which are image-print data for the corresponding individual colors, are forwarded to the K, C, M, and Y head controllers  15 ,  16 ,  17 , and  18  via the SCSI-ID #0 to #3. 
     In steps S 23  to S 26 , print commands are issued to the K, C, M, Y head controllers  15   16 ,  17 , and  18  via the SCSI-ID #0 to #3. 
     Subsequently, in step S 27 , driving control for feeding paper is started to prepare the driving of the paper feed mechanism, thereby making the printer system ready for image printing. 
     In step S 28 , a printing completion command sent from the K head controller  15  via the SCSI-ID #0 is recognized, and in steps S 29  to S 31 , similarly, the print completion commands sent from the C, M, and Y head controllers  16 ,  17 , and  18  are recognized via the SCSI-ID #1, #2, and #3, respectively. The process then proceeds to step S 32 , in which the host computer  2  finishes driving of feeding the paper, and completes the image-printing control. 
     On the other hand, image-print control by the CPUs included in the K, C, M, and Y head controllers  15 ,  16 ,  17 , and  18 , which receive the commands from the host computer  2 , are individually executed in accordance with the flow chart of FIG.  5 . 
     Specifically, under the control of the CPU  51  and the like of each head controller, in step S 41 , a position counter indicating a feed position of paper is pre-set, and in step S 42 , a counter indicating the number of dots for image-print width is pre-set. Further, in step S 43 , a starting address for reading data from the buffer memory  53  is set. 
     In step S 44 , a value for an image-print start counter is set. The value is set in accordance with the image-print start-position data set in step S 11 . 
     In step S 45 , an image-print width is set. This image-print width is set in accordance with the value of image-print width setting forwarded in step S 12 , in which image-printing processing is performed at the host-side. 
     In step S 46 , the movement of the feeding stage  72  is initiated. In response to the movement of the feeding stage  72 , an output from the encoder  73  is captured by the head drive circuit  54  and the like of each head controller  15 ,  16 ,  17 , and  18 . 
     In step S 47 , when it is determined that the feeding stage  72  has reached the image-print start position, the process proceeds to step S 48  and image printing is initiated. It is to be noted that, in step S 47 , the image-print start-position of the feeding stage  72  is determined in accordance with the count of the number of pulse signals output from the encoder  73 . 
     Also in this case, when it is determined, from the count value of the pulse signals output from the encoder  73 , that the C, M, and Y print head groups  27 ,  34 , and  41  reach count values corresponding to the distances L 1 , L 2 , and L 3  from the image-print start position for the K printing head group  20 , the C, M, and Y print head groups  27 ,  34 , and  41  start image-printing. 
     In step S 49 , when it has been recognized that printing a predetermined number of dots is completed, the process proceeds to step S 50 , in which, image printing ends as completion of the print commands. 
     As the printer system of this embodiment, in an inkjet printer system that performs printing in one pass by feeding paper in the feed direction, ink discharging from the head group  20  to  41 , each having a plurality of print heads, is controlled by the corresponding head controllers  15  to  18 . In addition, for controlling the head controllers  15  to  18 , the SCSI interfaces are connected in a daisy chain, and the head controllers  15  to  18  are identified with the SCSI IDs to transfer commands and image-print data, thereby controlling the printing. This can facilitate data transfer and the like from the single host computer  2  to each head controller, without complicated image-printing control. 
     When a need arises for increasing the number of colors to be printed in this printer system, for example, when a need exists for adding LC (light cyan) and LM (light magenta) to the K, C, M, and Y colors, the kinds of colors for image printing can be easily increased by assigning an unused SCSI-ID #4 to an LC head controller and assigning an unused SCSI ID #5 to an LM head controller. 
     When a need arises for increasing the image-print width, which is perpendicular to the paper feeding direction, for example, when a printer capable of printing A4-size paper with six heads per color is converted into a printer capable of printing A3-size paper, a SCSI host adapter ( 2 )  4  is installed in the host computer  2  in addition to the existing SCSI host adapter ( 1 )  3  and a SCSI ID that can be controlled by the additionally-installed host adapter is assigned to an additionally-installed head controller. A printer whose image-print width can be easily increased in this manner can be controlled by the single host computer  2 . 
     In this embodiment, as shown in  FIG. 3A , since heads are arranged along the lines EA and EB in slanting directions rather than in straight lines, it is also possible to arrange the rows of nozzles of the heads such that the rows partially overlap each other in the paper width direction. In this case, the present applicant has disclosed, in Japanese Unexamined Patent Application Publication No. 2000-1609, a technology that is applicable to such an arrangement, though, the publication is directed to a head-scan inkjet printer. In the publication, a plurality of rows of nozzles are arranged in the paper feeding direction such that the rows overlaps each other, and image-print data in overlapping portions are corrected. 
     In the first embodiment, although each K, C, M, and Y of the head controllers  15 ,  16 ,  17 , and  18  is assigned the different ID number, as an alternative thereto, each of the K, C, M, and Y head controllers  15 ,  16 ,  17 , and  18  can be assigned a different LUN number while being assigned the same SCSI ID number. 
     A printer system according to a second embodiment of the present invention will now be described. 
       FIG. 6  is a block diagram of a printer system of this embodiment.  FIG. 7  is a side view, viewed from a direction perpendicular to the paper feeding direction, showing a printer section of the printer system of this embodiment, and also showing movement positions of a feeding stage. 
     As in the printer system  1  of the first embodiment, a printer system  70  of the second embodiment is a type of inkjet printer system that includes print head groups, each having a row of nozzles for discharging ink droplets, arranged over the entire width of paper and feeds the paper in the feed direction to perform printing in one pass, without causing the print heads to scan in a horizontal direction (the width direction and the feeding direction of the paper). The printer system  70  is different from the printer system  1  of the first embodiment in that encoders, provided at the feeding stage, for detecting a feed position are arranged so as to correspond to the print head groups. Portions different from those of the printer system of the first embodiment will now be described. In the following description, the same elements, other than the feeding stage, as those of the printer system  1  of the first embodiment are denoted with the same reference numerals. 
     The printer system  1  of the first embodiment, as described above, is configured as follows. The K, C, M, and Y print heads  20 ,  27 ,  34 , and  41  are positioned in the X direction (the paper feeding direction) at the predetermined distance L 1 , L 2 , and L 3  with an accuracy corresponding to the resolution. The feeding stage  72  is provided with only one encoder  73  for the above-described four print head groups. The pulse signals output from the encoder  73  are counted to determine whether the front end of the paper  75  reaches the lower portion of the reference print head of each print head group, thereby starting of printing by the print heads. 
     The scale  74 , which is provided on a side surface of the base plate  71 , inevitably has a pitch error (pitch irregularity). In a normal printing condition, for example, K-color printing dots and C-color printing dots are printed at the same positions, thereby representing a correct color. 
     When, however, the scale  74  has a pitch error, as described above, a printing pitch error occurs because a section of the scale  74  used for printing the color K and a section of the scale  74  used for printing the color C are different from each other. As shown in an enlarged view in  FIG. 9 , in which an arrangement of K-color printing dots and C-color printing dots during image printing are shown, there is a possibility that the C-color printing dots are printed at positions between the K-color printing dots. In this printing condition, it is impossible to represent a correct color. 
     In the case shown in  FIG. 9 , for example, the pitch of a scale region for use in printing the color K is 70 μm, and is different from the pitch of a scale region for use in printing the color C by 0.05% (i.e., 0.035 μm). The color K and the color C are assumed to be printed one on another in a region corresponding to 1,000 dots. In this case, at the beginning (the left side in  FIG. 9 ) of image printing in the region, they are printed one on another in a substantially correct manner, but at the end (the right side in  FIG. 9 ) of the image printing, the C color dot is displaced 35 μm from the correct position as a result of the multiplication of the pitch difference of 0.05% by 1,000 dots. 
     Accordingly, in the printer system  70  of this embodiment, as shown in  FIG. 7 , a feeding stage  81  is provided with four encoders (sensors)  82 ,  83 ,  84 , and  85  corresponding to the four K, C, M, and Y print head group  20 ,  27 ,  34 , and  41 . The four encoders (sensors) each detect a movement position of the feeding stage  81  relative to each print head group to determine the discharge timing of ink from each print head. The scale  74  provided on a side surface of the base  71  is the same as the scale  74  in the first embodiment, and tolerates the pitch error. 
     As in the first embodiment, the reference print heads  23 ,  30 ,  37 , and  44  of the K, C, M, and Y print head groups  20 ,  27 ,  34 , and  41 , respectively, are positioned in the paper feeding direction, i.e., in the X direction, with an accuracy corresponding to the resolution with predetermined distances L 1 , L 2 , and L 3  therebetween. Naturally, the print heads other than the reference print heads of the K, C, M, and Y print head groups  20 ,  27 ,  34 , and  41  are also adjusted at predetermined distances (in the paper feeding direction) relative to the reference print heads described above. 
     The encoders  82 ,  83 ,  84 , and  85  are associated with the reference print heads  23 ,  30 ,  37 , and  44  of the K, C, M, and Y print head groups  20 ,  27 ,  34 , and  41 , respectively, which are sequentially arranged in the +X direction, and are sequentially mounted on the feeding stage  81  in the opposite direction, i.e., in the −X direction. That is, the encoders  82 ,  83 ,  84 , and  85  are positioned in the −X direction (in the direction opposite to the paper feeding direction) with an accuracy corresponding to the resolution so as to match the distances L 1 , L 2 , and L 3  of the heads described above. ( FIG. 7 ) 
     As shown in  FIG. 6 , the pulse signals output from the encoders  82 ,  83 ,  84 , and  85  are individually captured by the K, C, M, and Y head controllers  15 ,  16 ,  17 , and  18 , respectively. Each head controller counts the pulse signals output from the corresponding encoder to determine whether the count reaches a count value corresponding to the image-print start position for each color. 
     In the printer system  70  of this embodiment, under the control of the host computer  2 , the image-print range data, image-print data, image-print start command, and the like are transferred to each head controller  15 ,  16 ,  17 , or  18  via the corresponding SCSI interface, thereby executing image printing, as in the printer system  1  of the first embodiment. 
     Detection of a feeding state of the paper  75  during image printing and operation of starting image printing will now be described with reference to  FIG. 7 , etc. First, under the control of the CPU  51  included in each head controller, the feeding stage  81  is started from a predetermined start position. When the encoder  82  reaches a reference position S 0  ( FIG. 7 ) for detecting the edge of paper on the base plate  71  (i.e. when the feeding stage  81  reaches a position P 81 A) and the count number of pulse signals output from the encoder  82  reaches a count number corresponding to an image-print start position for color K, the image-printing of the color K is started. 
     Next, when the encoder  83  reaches the reference position S 0  ( FIG. 7 ) for detecting the edge of the paper (i.e., when the feeding stage  81  reaches a position P 81 B) and the count of the number of the pulse signals output from the encoder  83  reaches a predetermined value, the C print head group  27  starts image-printing. 
     As in the case for the color K, the image printing by the C print head group  27  is controlled by the amount of movement from the common reference position S 0 . 
     Similarly, the M print head group  34  and the Y print head group  41  start image-printing when the corresponding encoders  84  and  85  reach the reference position S 0 , which is common to the case for the K print head group  20 , (i.e., reach positions P 81 C and P 81 D of the feeding stage  81 ). 
     As described above, the feeding position of the paper  75  for all the print head groups is detected with reference to the common reference position S 0  for detecting the edge of paper and all the print head groups perform image-printing in the same region on the scale  74 . Thus, even if there is a pitch error in the scale  74  on the base plate  71 , as described above, displacement, as shown in  FIG. 9 , of the print dots between the print head groups does not occur. 
     The printer system  70  of the embodiment described above offers the same advantage as the printer system of the first embodiment. Additionally, since the printer system  70  uses the plurality of encoders corresponding to the print head groups, it can perform high-quality printing without being affected by a pitch error of the scale  74  on the base plate  71 . 
     In the printer system  70  of this embodiment, the encoders for detecting the movement position of the feeding stage are provided so as to correspond to the reference print heads of the print head groups. Alternatively, the encoders may be provided so as to correspond to the rows of the print heads that are arranged along the paper width, i.e., so as to correspond to the print heads arranged in the paper feeding direction of the print heads. This arrangement allows the control of ink discharge timing with still higher accuracy. Thus, this arrangement is the most advantageous in a printer system in which a plurality of long heads, each head being able to perform printing along the entire paper width, is arranged in the paper feeding direction. 
     While the printer system of each embodiment described above can perform printing in four colors, the substance of the present invention is not limited thereto and thus can be applied to a printer system with a single color or five or more colors. 
     While the scale  74  is provided over the movement region of the feeding stage  81  in the printer system  70  of the embodiment described above, for example, a scale  74   a  having a short length may alternatively be provided over a region from the reference position S 0  to the right, i.e., +L 1 , +L 2 , and +L 3 , as shown in FIG.  8 . 
     In this case, the feeding stage  81  is fed from the left to the right in the figure. When the encoder  82  reaches the reference position S 0  (i.e., the position P 81 A), the encoder  82  detects output pulse signals for the first time. When the encoder  82  detects the output pulses, image printing of the color K is started. 
     Next, when the encoder  83  reaches the reference position S 0  (i.e., the position P 81 B), similarly, the encoder  83  detects output pulse signals for the first time. When the encoder  83  detects the output pulses, image printing of the color C is started. 
     Likewise, when the encoders  84  and  85  each reach the reference position S 0  (i.e., the positions P 81 C and P 81 D, respectively), the encoders  84  and  85  each detect output pulse signals for the first time. When the encoders  84  and  85  detect the output pulses, image printing of the colors M and Y are started, respectively. 
     In this manner, setting the scale  74   a  to be shorter than the movement region of the feeding stage  81  and setting the edge of the scale  74   a  at the reference position S 0  at which image printing is started allows control of image-print start timing for each color. In addition, displacement of image printing position due to a scale pitch error becomes the same for each color, thus, displacement of dots does not occur. 
     While the printer systems  1  and  70  of the embodiments described above use the linear-driven feeding stage  72  and  81  as paper feed mechanisms, respectively, the substance of the present invention is not limited thereto and is also applicable to a printer system using a paper feed mechanism employing a belt driven mechanism. In such a case, a scale pattern is printed on the belt, and a single encoder or a plurality of encoders corresponding to the number of print head groups is used as an encoder for detecting a paper feeding position and is fixed to a main portion supporting the belt driving mechanism or is supported so that the position can be adjusted. 
     In addition, a rotating drum mechanism can be used as the paper feed mechanism. A modification, in which such a rotating drum mechanism is used, of the printer section for the printer system  70  of the second embodiment will now be described with reference to FIG.  10 . 
     As shown in the side view in  FIG. 10 , the printer section in this modification is rotatably mounted on the cylindrical fulcrum  96 , and has a rotating drum  91  that can suck and hold the paper  75 . The K print head  21 , the C print head  28 , the M print head  35 , and the Y print head  42  are fixed and supported in the outer circumference direction of the rotating drum  91 . The cylindrical fulcrum  96  is provided with a scale  97  for circumferential encoders. Meanwhile, encoders (sensors)  92 ,  93 ,  94 , and  95  are fixed to the rotating drum  91  at positions corresponding to the K print head  21 , the C print head  28 , the M print head  35 , and the Y print head  42 , respectively. 
     The encoders (sensors)  92 ,  93 ,  94 , and  95  read the scale  97  of the cylindrical fulcrum  96  as in the case for the second embodiment, and count the encoder pulse signals to thereby determine whether or not the rotating drum  91  reaches a corresponding image-print start position for each print head. The encoders (sensors)  92 ,  93 ,  94 , and  95  are positioned and fixed so as to have the same intervals as the K print head  21 , the C print head  28 , the M print head  35 , and the Y print head  42 , respectively. 
     In the printer section of the modification having the configuration described above, when starting image-printing, the rotating drum on which the paper  75  is held is rotated in the +X direction, and when the encoder  92  detects that the K print head  21  reaches the corresponding image-print start position, the K print head  21  starts image-printing in the color K. Likewise, when the encoders  93 ,  94 , and  95  detect the image-print start position for the C print head  28 , the M print head  35 , and the Y print head  42 , respectively, the print heads  28 ,  35 , and  42  perform image printing in the corresponding colors. 
     When the printer section of this modification is used, such a printer system can perform the same control processing as the printer system  70  of the second embodiment and can provide the same advantage. In particular, the modification can reduce the space occupied by devices in the paper feeding direction and thus is advantageous in placing the printer device in a small place. 
     As described above, according to the printer system of the present invention, when the printer elements are connected to the host computer, they are connected via the common host interface. This can facilitate the connection of the printer elements, and can easily deal with a case in which the number of printer elements is increased or the print paper width is increased. In addition, this can facilitate the assignment of the printer elements by the host interface, and also can facilitate increasing the print width. Additionally, since the plurality of encoders is provided so as to correspond to the rows of the print heads or print head groups, the discharge timing of ink from the print heads can be controlled accurately, thereby allowing high-quality printing. In addition, since the plurality of encoders is provided so as to correspond to the rows of the print heads, the discharge timing of ink from different print heads can be controlled accurately, thereby allowing high-quality printing.