Patent Publication Number: US-7216948-B2

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims priority under 35 USC 119 from Japanese Patent Application No. 2003-288510, the disclosure of which is incorporated by reference herein. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image forming apparatus, and in particular, to an image forming apparatus having a recording head in which plural unit recording heads are lined-up in a direction orthogonal to a conveying direction of a recording medium. 
   2. Description of the Related Art 
   Generally, a recording head which is known in an inkjet system has plural nozzles which discharge ink. Ink is discharged from the nozzles by utilizing the vibration of a piezo element or the heat generated by a heat-generating element or the like. Plural recording heads, which discharge inks of the respective colors of yellow, magenta, cyan, black, and the like, are provided. A color image is formed by recording, in a superposed manner, the respective colors of the recording heads. 
   However, currently, inkjet recording devices which are mainly becoming popular are those of a recording method called serial scanning which records line-by-line by moving a recording head reciprocally while conveying a recording sheet. This method is compact and inexpensive, but has the disadvantage that the recording head must be scanned plural times in order to record an image over the entire recording medium and the recording speed is slow. In order to improve the recording speed, the number of scans must be reduced, and the recording head must be made to be long. The technique which has pushed this to the limit is a non-scanning recording method which records over a length which is substantially the same as the recording width. This recording method is an inkjet recording device having a recording head which corresponds to the width of the recording medium and at which a large number of nozzles are lined-up along a length which is substantially the same as the width of the recording medium. Recording is carried out by moving the recording medium with respect to the recording head which is fixed. 
   In addition thereto, there has also been proposed, as such a non-scanning recording method, a structure in which plural recording heads such as those used in serial scanning are lined-up in a staggered manner or the like along the width of the recording medium as unit recording heads. The scanning speed can also be improved with this inkjet recording device. 
   In this way, inkjet recording devices which, while continuously conveying a recording medium, record by non-scanning type recording heads which correspond to the width of the recording medium, have been proposed in order to improve the recording speed and handle application to office use. 
   On the other hand, although a color image is formed by recording, in a superposed manner, the respective colors of plural recording heads as described above, if offset of the recording positions of the recording heads arises, color offset arises and the image quality deteriorates. Thus, techniques such as those disclosed in Japanese Patent Application Laid-Open (JP-A) No. 04-193542 have been proposed. 
   In the technique disclosed in JP-A No. 04-193542, in an inkjet recording device having a full-line-type recording head having plural discharge openings over the entire width of the recording region of a recording medium, subscan offset caused by inclining of the nozzles of the respective colors, and color offset arising due to warping of the nozzles, are detected by reading a pattern on a sheet, and the color offset is corrected by changing the writing timing. 
   As in the invention disclosed in JP-A No. 04-193542, in an image forming device having a full-line-type recording head equipped with plural discharge openings over the entire width of the recording region of the recording medium, the color offset can be corrected merely by changing the writing timing as described above. 
   However, in a recording head equipped with plural unit recording heads, there is the problem that, due to offset in the assembled positions of the respective unit recording heads or the like, the offset of the recording positions cannot be corrected merely by changing the writing timing. 
   SUMMARY OF THE INVENTION 
   The present invention is developed in consideration of the aforementioned, and provides an image forming apparatus which can prevent recording offset between respective unit recording heads in a recording head having plural unit recording heads. 
   An image forming apparatus of a first aspect of the present invention has: a recording head having plural unit recording heads divided in a direction orthogonal to a moving direction of a recording medium; a detecting section detecting at least offset of an image recorded by a vicinity of an end portion, in the direction orthogonal to the moving direction of the recording medium, of the plural unit recording heads; and a correcting section correcting recording offset of the recording head on the basis of results of detection of the detecting section. 
   In accordance with the image forming apparatus of the first aspect of the present invention, the recording head is structured by being divided into plural unit recording heads in the direction orthogonal to the moving direction of the recording medium. Namely, the recording head can carry out recording in the direction orthogonal to the moving direction of the recording medium by the plural unit recording heads. For example, a recording head in which the plural unit recording heads are lined-up in a staggered form along the transverse direction of the recording medium, can be used as the recording head. 
   The detecting section detects at least the offsets of the images recorded between the plural unit recording heads. For example, the offsets of the images recorded between the unit recording heads can be detected by recording predetermined test patterns onto a recording medium or the like at predetermined intervals by using regions between the respective unit recording heads, i.e., vicinities of the both ends of the respective unit recording heads, and by detecting these test patterns. 
   Then, on the basis of the results of detection of the detecting section, the correcting section corrects the recording offset of the recording head. Namely, from the results of detection of the detecting section, the correcting section can correct the recording offsets among the respective unit recording heads. Accordingly, at a recording head having plural unit recording heads, recording offset between the respective unit recording heads can be prevented. 
   An image forming apparatus of a second aspect of the present invention has: a recording head having plural unit recording heads divided in a direction orthogonal to a moving direction of a recording medium; a detecting section detecting at least offset of an image recorded by a vicinity of an end portion, in the direction orthogonal to the moving direction of the recording medium, of the plural unit recording heads; and a correcting section correcting recording offset of the recording head on the basis of results of detection of the detecting section, wherein the recording head has plural blocks, with each block being structured by a predetermined number of the unit recording heads, which can record images of plural colors and which are lined-up along the moving direction of the recording medium, and the plural blocks are lined-up in a staggered form along the direction orthogonal to the moving direction of the recording medium. 
   As described above, in accordance with the present invention, a detecting section, which detects at least offset of images recorded between plural unit recording heads structuring a recording head, and a correcting section correcting the recording offset of the recording head on the basis of results of detection of the detecting section, are provided. Therefore, the present invention has the effect that, in a recording head having plural unit recording heads, it is possible to prevent recording offset among the respective unit recording heads. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention will be described in detail based on the following figures, wherein: 
       FIG. 1  is a drawing showing the basic structure of a recording head in an image forming apparatus according to an embodiment of the present invention; 
       FIG. 2  is a block diagram showing the basic structure of a control system of the image forming apparatus according to the embodiment of the present invention; 
       FIG. 3  is a drawing for explaining recording offset of unit recording heads; 
       FIG. 4  is a drawing for explaining recording offset due to inclining of the unit recording head; 
       FIGS. 5A through 5D  are drawings for explaining correction of recording offset of the unit recording heads; 
       FIGS. 6A through 6C  are drawings for explaining correction of recording offset due to inclining of the unit recording head; 
       FIG. 7  is a drawing showing the schematic structure of the recording head of the image forming apparatus according to the embodiment of the present invention; 
       FIGS. 8A and 8B  are drawings for explaining test patterns for detecting recording offset of the unit recording heads; 
       FIG. 9  is a drawing showing a state in which offset has arisen in the lengthwise and widthwise directions in the test pattern; 
       FIGS. 10A through 10D  are drawings for explaining recording position offset correction between adjacent unit recording heads of the same color; 
       FIGS. 11A through 11D  are drawings for explaining offset correction between unit recording heads of different colors; 
       FIGS. 12A through 12D  are drawings for explaining correction of image inclination (skewing) offset caused by inclining of the unit recording head; 
       FIG. 13  is a drawing showing offset before and after correction when recording by using black and yellow unit recording heads; 
       FIG. 14  is a drawing showing offset before and after correction in an example in which adverse effects due to inclining of a sensor are improved; 
       FIG. 15  is a drawing showing offset before and after correction when recording by using black and yellow unit recording heads in another example of detection and correction of image offset; 
       FIG. 16  is a flowchart showing the flow of processing of detection and correction in the example of  FIG. 15 ; 
       FIG. 17  is a drawing showing offset amounts between the unit recording heads; 
       FIGS. 18A through 18C  are tables showing results of calculation of offset amounts of the unit recording heads; 
       FIG. 19  is a table showing calculated correction amounts; 
       FIG. 20  is a drawing showing an example in which sensors are added to both ends; and 
       FIGS. 21A through 21C  are drawings showing examples in which the unit recording heads are made into a unit. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Hereinafter, examples of embodiments of the present invention will be described in detail with reference to the drawings. 
   First, the basic structure of a recording head of an image forming apparatus according to an embodiment of the present invention will be described. 
     FIG. 1  is a drawing showing the basic structure of the recording head in the image forming apparatus according to the embodiment of the present invention. 
   The image forming apparatus has a recording head  10  extending over substantially the entire width of a recording sheet. The recording head  10  is structured by plural unit recording heads  10 A which are structured by plural nozzles, which discharge ink, being lined-up. In other words, the recording head  10  is structured so as to be divided into the plural unit recording heads  10 A. 
   In further detail, at the recording head  10 , the plural unit recording heads  10 A are disposed along the entire width of the recording sheet in a staggered arrangement of being offset both in the conveying direction of the recording sheet and the transverse direction of the recording sheet (the direction orthogonal to the recording sheet conveying direction). The adjacent unit recording heads  10 A are disposed so as to have regions which overlap one another in the transverse direction of the recording sheet (overlapping regions). Note that a structure in which the overlapping regions are not provided may be used. 
   Sensors  12  for detecting positional offset due to the assembly precision or the like of the respective unit recording heads  10 A are provided at the image forming apparatus. The sensors  12  are disposed so as to correspond to regions between the unit recording heads  10 A. In detail, the sensors  12  are provided at least at positions of detecting the images recorded by the nozzles corresponding to the overlapping regions between the respective unit recording heads  10 A. CCD sensors or the like may be used as the sensors  12 . Further, the sensors  12  may be provided at the recording head  10 . 
   The image forming apparatus records test patterns for correcting the positional offset of the respective unit recording heads  10 A. By detecting the test patterns by the sensors  12 , the positional offset of the unit recording heads  10 A is corrected. 
   The recording heads  10  may be provided for each of respective colors so as to discharge inks of the respective colors of, for example, cyan, magenta, yellow and black. A color image can be formed by providing recording heads of the respective colors. 
     FIG. 2  is a block diagram showing the basic structure of the control system of the image forming apparatus. 
   At the image forming apparatus, the various types of control of the image recording onto a recording sheet are carried out by a controller  14 . The controller  14  is structured by a microcomputer equipped with a CPU, a ROM, a RAM, peripheral devices, and the like. 
   An input device  16  for carrying out various types of settings of the image forming apparatus and various types of instructions such as an instruction to start image recording or the like, a sheet conveying section  18  which conveys the recording sheet, a clock generator  20  which generates a clock for operating the image forming apparatus, a unit recording head recording control section  24  which carries out control for discharging ink from the respective unit recording heads  10 A on the basis of image data  22  inputted to the controller  14 , and the like are connected to the controller  14 . When, for example, the start of image recording is instructed by the input device  16 , recording of an image onto a recording sheet is carried out on the basis of the image data  22  inputted from an external device or a computer or the like. Namely, when an image is to be recorded onto a recording sheet, the controller  14  controls the sheet conveying section  18  to convey the recording sheet to the recording head  10 , and controls the unit recording head recording control section  24  to record images successively by the unit recording heads  10 A. The unit recording head recording control section  24  controls the discharge of ink from the nozzles of the respective unit recording heads  10 A, and controls the timing of the discharge of ink from the respective nozzles (the recording timing). An image is thereby recorded onto the recording sheet. 
   The aforementioned sensors  12  are connected to the controller  14 , and the results of detection of the sensors  12  are inputted to the controller  14 . On the basis of the results of detection of the sensors  12 , the controller  14  corrects the positional offset of the image by controlling the recording timings of the respective unit recording heads  10 A, the positions of the nozzles which are used of the respective unit recording heads  10 A, and the like. 
   Here, description will be given of recording position offset in a case in which an image is recorded by the recording head  10  which is structured as described above. 
   As described above, the recording head  10  is divided in the direction orthogonal to the recording sheet conveying direction, and is divided in the recording sheet conveying direction. The recording positions are determined with there being errors from the time of manufacturing and assembly, between the unit recording heads  10 A structuring the recording head  10 . The dashed lines in  FIG. 3  show the ideal positions of the unit recording heads  10 A, and the solid lines show the positions of the unit recording heads  10 A which are mounted with errors. If recording processing is carried out at a given recording timing or by using given image data without considering this error, the recording positions will differ between the respective unit recording heads  10 A as shown in  FIG. 3 , and jagged edges of the image, streaks in density, white streaks caused by gaps in recording, and the like will arise. Further, as shown in  FIG. 4 , also when the unit recording head  10 A is assembled at an incline, in the same way as described above, jagged edges of the image, streaks in density, white streaks caused by gaps in recording, and the like will arise. Moreover, if the recording heads  10  are provided for the respective colors, when the recording heads  10  are assembled with errors at the respective colors, the recording positions will differ at the respective colors, and color offset (hereinafter called “color registration offset”) will arise. 
   Thus, in the image forming apparatus according to the present embodiment, test patterns are recorded, and by detecting the positions of the test patterns, the amounts of offset of the recording positions of the images are detected, and the recording positions are corrected on the basis of the detected positional offset amounts. 
   Next, description will be given of the detection of recording position offset between the unit recording heads  10 A in the image forming apparatus which is structured as described above. 
   As shown in  FIG. 1 , L-shaped test patterns  30  are recorded by the respective unit recording heads  10 A. The recording positions are positions at the junctures of the unit recording heads  10 A, and the test patterns  30  are recorded at predetermined intervals. 
   The recording positions of the test patterns  30  are detected due to the sensors  12  detecting the recorded test patterns  30 . Here, there is no problem provided that the absolute positional offset from the ideal position can be detected. However, due to the mounting accuracy and the detection timing of the sensors  12  and fluctuations in the moving speeds of the test patterns  30  and the like, there are cases in which it is difficult to learn of the absolute position as the errors in precision are large. Thus, by using a predetermined unit recording head  10 A as a reference, the relative offset with respect to this unit recording head  10 A which is the reference is detected. For example, the times (distances) of lines, which extend in the direction perpendicular to the recording sheet conveying direction, of the reference unit recording head  10 A and another unit recording head  10 A are detected, and the amount of offset from the ideal interval is used as the offset, in the recording sheet conveying direction, with respect to the reference unit recording head  10 A. Further, with respect to the offset in the transverse direction of the recording sheet, similarly, the offset of a line segment extending in the recording sheet conveying direction is detected, and is used as the offset, in the recording sheet transverse direction, with respect to the reference unit recording head  10 A. 
   Note that, also in a case in which the recording heads  10  are provided for the respective colors, similarly, the relative positional offset can be detected by detecting the offset with respect to the unit recording head  10 A which is the reference. 
   Next, correction of the positional offset which is detected as described above will be described. 
     FIG. 5A  is a drawing showing the positions of the unit recording heads  10 A and the image printing regions before correction.  FIG. 5B  is a drawing showing image output timing and image printing region correction.  FIG. 5C  is a drawing showing image positions before correction, and  FIG. 5D  is a drawing showing image positions after correction. 
   For example, a case is considered in which, with the unit recording head  10 A that is positioned topmost in  FIG. 1  being the reference unit recording head  10 A, the test patterns  30  are recorded as described above and the image positional offset amounts with respect to the reference unit recording head  10 A are detected. In this case, as shown in  FIG. 5A , if another unit recording head  10 A is offset from the reference unit recording head, when recording is carried out without correcting the image position, positional offset of the images which corresponds to the offset between the unit recording heads  10 A arises as shown in  FIG. 5C . Note that the dashed lines in  FIG. 5A  show the ideal positions of the unit recording heads  10 A. 
   In this case, the image positional offset in the direction orthogonal to the recording sheet conveying direction (i.e., the image positional offset in the widthwise direction) is corrected by changing the image output regions (the nozzles) of the respective unit recording heads  10 A. Namely, as shown in  FIG. 5B , by changing the printing regions of the unit recording heads  10 A, overlapping of images and gaps between images arising among the unit recording heads can be eliminated. 
   Further, the image positional offset in the recording sheet conveying direction (the lengthwise direction) is corrected by changing the image output timings of the unit recording heads  10 A. Namely, as shown in  FIG. 5B , by recording images by changing the recording timings of the unit recording heads  10 A, lengthwise direction nonalignment of the images between the unit recording heads  10 A can be eliminated. 
   Namely, by changing the printing regions of the unit recording heads  10 A in accordance with the image position offsets detected by the sensors  12  as described above, the offset of the images in the direction orthogonal to the recording sheet conveying direction can be corrected. By changing the recording timings of the unit recording heads  10 A, the offset of the images in the recording sheet conveying direction can be corrected. Note that the image positions after correction are shown in  FIG. 5D . 
   Next, correction of inclined offset of the unit recording head will be described. 
     FIG. 6A  is a drawing showing the position of the unit recording head  10 A and the image printing region before correction.  FIG. 6B  is a drawing showing image output timing correction and image printing region correction.  FIG. 6C  is a drawing showing the image position after correction. 
   For example, a case is considered in which, with the unit recording head  10 A that is positioned topmost in  FIG. 1  being the reference unit recording head  10 A, the test patterns  30  are recorded as described above and the image positional offset amounts (inclinations) with respect to the reference unit recording head  10 A are detected. In this case, if another unit recording head  10 A is inclined as shown in  FIG. 6A  with respect to the reference unit recording head  10 A, the image will be inclined when recording is carried out without correcting the image position. 
   The inclining of the image in this case is corrected by dividing the printing region of each unit recording head  10 A, and changing discretely (i.e., in a step-wise manner) the image recording timing of each divisional region. Namely, as shown in  FIG. 6B , the printing region of the unit recording head  10 A is divided in accordance with the inclination of the image. By discretely changing the image recording timings of the respective divisional regions, the inclining of the image can be corrected as shown in  FIG. 6C . Note that the changing of the image recording timing may be effected by using one pixel as the changing step and changing the number of divisions in accordance with the amount of inclination offset, or by fixing the number of divisions and changing the number of pixels of the changing step. 
   In this way, by changing the image output regions (nozzles) of and controlling the recording timings of the respective unit recording heads  10 A on the basis of the results of detection of the sensors  12 , the relative recording positions of the images recorded by the unit recording heads  10 A can be corrected. 
   Next, the image forming apparatus according to the embodiment of the present invention, which puts into practice a recording head structured as described above, will be explained. 
     FIG. 7  is a drawing showing the structure of a recording head of the image forming apparatus according to the embodiment of the present invention. 
   As shown in  FIG. 7 , a recording head  11  is structured by the plural unit recording heads  10 A. The unit recording heads  10 A include the unit recording heads  10 A (Y 0  and Y 1  in  FIG. 7 ) recording yellow images, the unit recording heads  10 A (M 0  and M 1  in  FIG. 7 ) for recording magenta images, the unit recording heads  10 A (C 0  and C 1  in  FIG. 7 ) for recording cyan images, and the unit recording heads  10 A (K 0  and K 1  in  FIG. 7 ) for recording black images. 
   The unit recording heads  10 A of the four colors are lined-up along the recording sheet conveying direction. The unit recording heads  10 A of the four colors form one block, and the recording head  11  is structured by the blocks being lined-up in a staggered form along the direction orthogonal to the recording sheet conveying direction. The recording head  11  is structured by two columns of blocks which are lined-up in the direction orthogonal to the recording sheet conveying direction. The left side column of blocks in  FIG. 7  will be called group  0 , and the right side column of blocks in  FIG. 7  will be called group  1 . 
   Adjacent blocks are disposed so as to have regions which overlap in the transverse direction of the recording sheet (overlapping regions). Note that it is possible to utilize a structure which does not have overlapping regions. 
   Namely, by, as the recording sheet is being conveyed, controlling the recording timings of the unit recording heads  10 A disposed along the width of the recording sheet, images of the respective colors are superposed such that recording of a color image is possible. 
   In the same way as the above-described basic structure, the recording head  11  has the sensors  12  for detecting positional offset due to the assembly accuracy or the like of the respective unit recording heads  10 A. The sensors  12  are provided so as to correspond to positions between the respective blocks. In further detail, the sensors  12  are provided at least at positions of detecting the images recorded by the nozzles corresponding to the overlapping regions between the unit recording heads  10 A. Note that, as mentioned previously, CCD sensors or the like may be employed as the sensors  12 . 
   In the same way as the above-described basic structure, the image forming apparatus according to the embodiment of the present invention records the test patterns  30  for correcting the positional offset of the unit recording heads  10 A (see  FIG. 8A ). By detecting the test patterns  30  by the sensors  12 , the positional offset of the unit recording heads  10 A is corrected. 
   The control system of the image forming apparatus according to the present embodiment is substantially the same as the basic structure of the above-described control system, and differs only with respect to the point that the unit recording heads  10 A, which are connected to the unit recording head recording control section  24 , are provided for each color. Thus, detailed description will be omitted. 
   Next, the detection of the recording position offset between the respective unit recording heads  10 A will be described. 
   As shown in  FIG. 8A , each of the unit recording heads  10 A records one L-shaped test pattern directed in the lengthwise direction (the recording sheet conveying direction) and one L-shaped test pattern directed in the widthwise direction (the direction orthogonal to the recording sheet conveying direction). The recording positions are such that the test patterns are recorded at positions of the junctures of the respective unit recording heads  10 A. In consideration of the offset amount between the unit recording heads  10 A of each color (e.g., Y 0 - 1  and Y 1 - 1  in  FIG. 8A ) or the offset amount between the unit recording heads  10 A of different colors (e.g., Y 0 - 1  and M 0 - 1  in  FIG. 8A ) (the offset amount can be learned from the manufacturing/mounting tolerance of each unit recording head  10 A), the recording interval is set such that the test patterns are recorded so as not to overlap and so as to be spaced apart by an interval which is greater than or equal to the allowable offset amount. The recorded positions of the respective test patterns are detected by the test patterns  30  being detected by the sensors  12 . 
     FIG. 9  shows a state in which offset in the lengthwise and widthwise directions has arisen at the test pattern  30 . The test pattern  30  at the left side in  FIG. 9  is test pattern Y 0 - 1  recorded by unit recording head Y 0 - 1 , and the test pattern  30  at the right side in  FIG. 9  is test pattern Y 1 - 1  recorded by unit recording head Y 1 - 1 . 
   Hereinafter, the test pattern recorded by the unit recording head  10 A (Y 0 - 1 ) is called test pattern Y 0 - 1 , and the test pattern recorded by unit recording head  10 A (Y 1 - 1 ) is called test pattern Y 1 - 1 . Further, in the following explanation, when the respective unit recording heads  10 A are to be differentiated as individual unit recording heads  10 A, they will be termed, for example, unit recording head Y 0 - 1  as shown in  FIG. 8A . In this reference symbol for each unit recording head, the letter represents the color (Y: yellow, M: magenta, C: cyan, K: black), the number which follows expresses whether that unit recording head is positioned in group  0  or group  1 , and the number after the hyphen expresses which row that unit recording head is positioned in from the end of the recording head  10 . 
   The dashed lines in  FIG. 9  show the ideal state of test pattern Y 1 - 1 . Test pattern Y 1 - 1  is recorded with offset by the lengthwise direction offset amount and the widthwise direction offset amount shown in  FIG. 9 . Here, there is no problem provided that the absolute positional offset from the ideal position can be detected, but precisely learning of the absolute positional offset may be difficult because of the mounting accuracy and the detection timing of the sensors  12 , fluctuations in the moving speed of the test pattern, and the like. Thus, the relative offset is detected by using test pattern Y 0 - 1  as the reference. For example, the offset between test pattern Y 0 - 1  and test pattern Y 1 - 1 , i.e., the lengthwise direction offset between unit recording head Y 0 - 1  and unit recording head Y 1 - 1 , is used. Further, similarly, for the widthwise direction offset, the offset of the line segment extending in the lengthwise direction is measured and is used as the widthwise direction offset between the unit recording heads  10 A. 
   Explanation has been given above of detecting the offset amount between unit recording heads  10 A of the same color (the offset amount between test pattern Y 0 - 1  and test pattern Y 1 - 1 ). However, the detection of positional offset is similarly possible for unit recording heads  10 A other than yellow, such as between unit recording head M 0 - 1  and unit recording head M 1 - 1 , unit recording head C 0 - 1  and unit recording head C 1 - 1 , and unit recording head K 0 - 1  and unit recording head K 1 - 1 . Further, the offset amount between different colors as well can be detected if the relative positional offsets between test pattern Y 0 - 1  and test pattern M 0 - 1 , test pattern Y 0 - 1  and test pattern C 0 - 1 , and test pattern Y 0 - 1  and test pattern K 0 - 1  are learned of. Here, test pattern Y 0 - 1  is used as the reference, but test pattern Y 1 - 1  may be used as the reference, and the offset of test pattern Y 0 - 1  may be detected. The test pattern of another color other than yellow may be used as the reference to detect relative positional offset. The detection of relative positional offset between the unit recording heads  10 A has been described, but the absolute positional offsets of the test patterns of the respective unit recording heads  10 A with respect to the sensors  12  may be detected. If the absolute positional offsets of the respective unit recording heads  10 A are detected and these offsets can be corrected, the image registration offset with respect to the recording sheet can be lessened. 
   Further, explanation has been given of detecting the offset between the unit recording heads  10 A of the same color, i.e., the unit recording heads Y 0 - 1  and Y 1 - 1 , or of detecting the offset between unit recording heads  10 A of different colors (the offset between unit recording head Y 0 - 1  and unit recording head M 0 - 1 ), which offsets are at the widthwise direction end portion (the end portion in the direction orthogonal to the recording sheet conveying direction). However, the positional offset between images of the unit recording heads  10 A can be detected by, in the case of offset between unit recording heads  10 A of the same color, detecting the offset between unit recording head Y 1 - 1  and unit recording head Y 0 - 2  and the offset between unit recording head Y 0 - 2  and unit recording head Y 1 - 2  (and thereafter, detecting positional offset similarly between adjacent unit recording heads  10 A of the same color), or, in the case of offset between unit recording heads  10 A of different colors, detecting the offset between unit recording head Y 1 - 1  and unit recording head M 1 - 1  and the offset between unit recording head Y 0 - 2  and unit recording head M 0 - 2  (and thereafter, detecting offset similarly with respect to cyan and black as well). 
   Note that, as shown in  FIG. 8B , positional offset between the unit recording heads  10 A can be detected also by using another test pattern  31 . Only the configuration of the test pattern is different, but detecting the offset amounts of the test patterns in the lengthwise direction and the widthwise direction is the same. In this case, the recording of test pattern Y 0 - 1  and test pattern Y 1 - 1  (the same holds as well for the other colors), is carried out at the same position in the lengthwise direction. Thus, when there are fluctuations in the recording position in the lengthwise direction, more accurate detection than with the test patterns  30  shown in  FIG. 8A  is possible. Further, by forming a constant gap at the overlap region of the unit recording heads  10 A of the same color (Y 0  and Y 1 ) and measuring the length of this gap, the offset between adjacent unit recording heads  10 A of the same color can be detected with high accuracy. 
     FIGS. 8A and 8B  each show an example in which one test pattern  30 ,  31  is recorded by each of the unit recording heads  10 A. However, in a case in which there are periodic fluctuations or in a case in which the detection error in the detecting of one test pattern is large, plural test patterns may be recorded by each of the unit recording heads  10 A and the detection values can be averaged, so as to improve the accuracy of detection. 
   Moreover, in a case in which there are periodic fluctuations, the period of the test pattern and the recorded length of the test pattern must be considered in order to be able to learn of the registration fluctuation in at least a length of the period. For example, if there is a periodic fluctuation of 150 mm, the test pattern recording length should be at least one period (150 mm) or an integer multiple of that period (e.g., 300 mm or 450 mm or the like). If many test patterns (at least two) can be recorded within the range of one fluctuation period, a test pattern period in which an integer number of test patterns (an even number of test patterns) can be recorded should be used (a 75 mm period if there are two test patterns), and if it is not possible to record at least two test patterns within one fluctuation period, a period which is not synchronous with the fluctuation period (225 mm which is offset by half a period from the fluctuation period, or a period which is offset by 1/integer period, or the like) and which is at least two period fluctuations (when offset by 1/integer period, a period equal to the integer multiple of the fluctuation period) should be used. 
   Next, correction of offset between unit recording heads  10 A of the same color will be described.  FIG. 10  is a drawing for explaining correction of recording position offset between adjacent unit recording heads  10 A of the same color. 
   Image recording positions in a state in which unit recording heads K 0 - 1 , K 1 - 1  and K 0 - 2  are offset as shown in  FIG. 10A , are shown in  FIG. 10C . Reference letter c in  FIG. 10C  indicates the image position offset in the widthwise direction (the direction orthogonal to the recording sheet conveying direction), and d indicates the offset in the lengthwise direction (the recording sheet conveying direction). 
   The image position offset c in the widthwise direction is corrected by changing the image output regions (the nozzles) of the unit recording heads  10 A. By changing the image recording regions, which are the image recording regions before correction shown in  FIG. 10A , to the image recording regions after correction shown in  FIG. 10B , the overlapping of the images of the unit recording heads K 0 - 1  and K 1 - 1 , and the gap between the images of the unit recording heads K 1 - 1  and K 0 - 2  can be eliminated. 
   The image position offset d in the lengthwise direction is corrected by changing the image output timings of the unit recording heads  10 A. By correcting from the image output timings before correction in  FIG. 10A  by the image output timing amounts b shown in  FIG. 10B  and making the timings be the image output timings after correction shown in  FIG. 10B , non-alignment in the lengthwise direction between the unit recording heads  10 A can be eliminated. 
     FIG. 10D  shows the image output positions after the offsets in the widthwise direction and the lengthwise direction have been corrected. In this example, the recording region of the reference unit recording head K 0 - 1  is not changed. However, correction may be carried out by dispersing, between the unit recording heads K 0 - 1  and K 1 - 1 , the correction amount of the offset between the unit recording heads K 0 - 1  and K 1 - 1 . In this case, it suffices for the amount of change of the recording region of the unit recording head K 1 - 1  toward the unit recording head K 0 - 1  side to be half. 
   Further, in the example of  FIG. 10 , the output nozzles of each of the unit recording heads  10 A are lined-up in two dimensions as m×n. However, the detection and correction of positional offset of images can be carried out not only in such an arrangement, but also similarly in a one-dimensional arrangement of m×1. 
     FIG. 11  is a drawing for explaining offset correction between unit recording heads  10 A of different colors. The method of correction is similar to the above-described correction of offset between unit recording heads  10 A of the same color. The widthwise direction offset can be corrected by changing the image output regions, and the lengthwise direction offset can be corrected by changing the image output timings. 
   Next, correction of inclination offset of the unit recording heads  10 A will be described.  FIG. 12  is a drawing for explaining correction of image inclination (skewing) offset caused by inclining of the unit recording head  10 A. 
   As shown in  FIG. 12A , when magenta unit recording head M 0 - 1  is inclined in the lengthwise direction with respect to unit recording head K 0 - 1  of black which is the reference color, the image output position is as shown in  FIG. 12C . Reference letter c in  FIG. 12C  is the skew of the image (lengthwise direction offset caused by inclining of the unit recording head  10 A also arises). Correction is carried out by dividing the unit recording head  10 A into plural sections (four sections in  FIG. 12 ), and, as shown in  FIG. 12B , changing the image output timings discretely (i.e., in a stepwise manner). The image output positions after changing the image output timings are shown in  FIG. 12D . The changing of the image output timings may be carried out by using one pixel as the changing step and changing the number of divisions in accordance with the amount of inclination offset, or by fixing the number of divisions and changing the number of pixels of the changing step. In order to lessen jaggedness of the edges at the same unit recording head  10 A, it will be more effective to fix the changing step to one pixel and to change the number of divisions in accordance with the amount of inclined offset. 
   The above-described changing of the image output timings may be carried out by outputting while changing the data storage positions read-out at the time of output without changing the data in the image memory, or may be carried out by outputting the image by changing the image data in the image memory (a buffer memory may be provided) without changing the read-out data positions. With these methods, it is possible to control the changing of the output timing of specific image data. 
   Next, image offset detection and correction processings when offset between the same color, offset between different colors, and inclined offset are combined, will be described. 
     FIG. 13  shows offsets before and after correction when recording by using black and yellow unit recording heads  10 A. The plural sensors  12  are disposed at the positions where the respective unit recording heads  10 A are adjacent. (In  FIG. 13 , the unit recording heads  10 A are divided into twelve in the widthwise direction, and there are therefore eleven sensors  12 .) Correction is carried out so as to eliminate offset between adjacent portions of the single reference color (black in this example) as described above, and the color which is the object of correction at the same position in the lengthwise direction (yellow in this example) is corrected such that the output images of the respective unit recording heads  10 A match the reference color. 
   However, in this example, as shown by the image after correction of  FIG. 13 , there are cases in which image skewing arises in the image after correction due to inclining of the unit recording head K 0 - 1  of the reference color K. Such image skewing arises due to the inability to learn of the absolute offset of the inclining of the unit recording head K 0 - 1  of the reference color K due to inclining of the sensors  12  themselves which is caused by dispersion in the mounting of the individual sensors  12  or the like. 
     FIG. 14  shows an improvement example to address the above problem. The divided sensors  12  as shown in  FIG. 13  are not used, and a full-line-type sensor  13 , which can detect all of the output images of the unit recording heads  10 A in the widthwise direction, is used. It is thereby possible to correct the inclinations of the respective unit recording heads  10 A by using the same reference, and the image skewing offset shown in  FIG. 13  can be suppressed to a certain extent as shown by image  1  after correction in  FIG. 14 . 
   Even when the plural sensors  12  are used as shown in  FIG. 13 , the inclinations of the respective unit recording heads  10 A can be detected if the offset of each of the sensors  12  which has arisen at the time of the manufacturing and mounting thereof can be known. The following methods are examples of methods of detecting the offsets of the sensors  12 . 
   (A) A mark recorded in advance on the recording sheet or on a belt for conveying the recording sheet or the like (e.g., a straight line running along the direction in which the sensors  12  are lined-up, or the like), is detected by the sensors  12 . The offsets of the respective sensors  12  are detected, and are corrected at the same time at the time of correcting the offsets of the unit recording heads  10 A. 
   (B) An end portion of the recording sheet is detected by the sensors  12 , and in the same way as mentioned above, the offsets of the sensors  12  are corrected at the same time at the time of correcting the offsets of the unit recording heads  10 A. 
   (C) A test pattern  30  recorded on the recording sheet is detected by the sensors  12 , and the same test pattern is measured by an external measuring device. The errors therebetween are corrected at the same time at the time of correcting the offsets of the unit recording heads  10 A. 
   By applying such a method, the recording offset can be corrected such as in image  2  after correction of  FIG. 14 , and offset in detection which is caused by the mounting tolerance of the sensors  12  themselves can be corrected. Further, also when the full-line-type sensor  13  is used, by applying a method such as described above, offset in detection which is caused by the mounting tolerance of the sensor  13  itself can be corrected, and the recording offset of the image can be corrected highly precisely. 
   Next, another example of the detection and correction of image offset when offset between the same color, offset between different colors, and inclined offset are combined, will be described. 
     FIG. 15  is a drawing showing offset before and after correction when recording is carried out by the black and yellow unit recording heads  10 A in another example of image offset detection and correction. In this example, the edges of adjacent unit recording heads  10 A are not matched, but rather, by using the black unit recording head K 0 - 1  which is furthest toward the edge in  FIG. 15  as a reference, the positions of the output images of the other black unit recording heads  10 A and the yellow (the same holds for magenta and yellow as well) unit recording heads  10 A are matched. 
     FIG. 16  is a flowchart showing the flow of detection and correction processings in the example of  FIG. 15 . 
   First, when an instruction for registration adjustment is inputted to the image forming apparatus from the input device  16  or an external computer or the like, in step  100 , the recording sheet or the like is conveyed to the position of the recording head  10 , and the test patterns  30  are recorded successively thereon. 
   In step  102 , the recorded test patterns  30  are successively detected by the sensors  12  which are provided at the image forming apparatus.  FIG. 17  is a drawing showing the amounts of offset between the unit recording heads  10 A at this time. The detected offset amounts are the relative offset amounts of the output patterns of the unit recording heads  10 A adjacent to the reference color black (E-K 11 , E-K 12 , and the like in  FIG. 17 ), and the yellow offset which includes the offset with respect to the output pattern of the adjacent reference color unit recording head  10 A, and the relative offset with respect to the output pattern of the same reference color unit recording head  10 A in the lengthwise direction (E-YK 11 A, E-YK 11 B, and the like in  FIG. 17 ). 
   Then, in step  104 , on the basis of the positions of the test patterns  30  detected by the sensors  12 , the offsets within the reference color (the same color) are computed as described above. In step  106 , correction values for offset within the reference color (the same color) are computed. In step  108 , correction values for offset between different colors are computed. 
   For example, as shown in  FIG. 18 , the offsets of the output patterns of the respective black and yellow unit recording heads  10 A when the unit recording head K 0 - 1  at the end of the reference color is used as the reference (the reference does not have to be the black unit recording head  10 A at the edge; another color can be used as the reference, or another unit recording head  10 A at the center or the like can be used as the reference), are computed (in  FIG. 17 , E-K 111 , E-K 112 , and E-YK 111 A, E-YK 111 B, and the like). The offset amounts of the respective unit recording heads  10 A are shown in  FIG. 18 . Note that  FIG. 18A  shows offsets when, among the adjacent unit recording heads  10 A, the unit recording head  10 A which is close to the unit recording head K 0 - 1  of the reference color black is used as the reference.  FIG. 18B  shows offsets when the unit recording head K 0 - 1  of the reference color black is used as the reference for all. Further, the skew offset amounts of yellow with respect to the reference color black are computed as shown in  FIG. 18C . In this example, the correction amounts are the positive/negative reverses of the computed offset amounts, and are computed as shown in  FIG. 19 . 
   Next, in step  110 , correction processing is carried out. Namely, the image printing regions and the output timings are changed as described above in accordance with the respective computed correction values. In this way, the offsets of the respective unit recording heads  10 A can be corrected as is shown by the image after correction of  FIG. 15 . 
   In the above description, image output position offset caused by offset of the positions of the unit recording heads  10 A includes initial offset at the time of assembling the device (hereinafter called “shipping time offset”), and offset arising due to changes at the time when the device is transported or warping of the floor surface on which the device is set or the like (hereinafter called “setting time offset”), and the like. 
   With regard to shipping time offset and setting time offset, it suffices to record the aforementioned test patterns  30  at the time of shipping and the time of setting, and to correct the positions of the output images of the respective unit recording heads  10 A (registration adjustment). Further, in addition thereto, registration adjustment may be carried out when the power of the device is turned on, when the device wakes after having been in an energy-saving mode, when the device recovers after there has been trouble therewith, after maintenance operations on the device have been completed, when modules of the device (including the unit recording heads  10 A, the belts, the sensors, and the like) are replaced, and the like. 
   The above embodiment describes an example in which eleven of the sensors  12  are provided with respect to the twelve divisions in widthwise direction of the unit recording heads  10 A. However, here, the skewing offset at the both widthwise direction ends (Y 0 - 1  and Y 1 - 1 ) cannot be detected. Thus, as shown in  FIG. 20 , a structure may be used in which the sensors  12  are added at the both ends, so that all of the skew offset amounts can be detected. In this way, as shown in  FIG. 20 , the skew offset can be corrected at the unit recording heads  10 A at the both ends of the recording head  10  as well. The image positions before and after correction at this time are shown as the image after correction in  FIG. 20 . Accordingly, when the recording head  10  according to the embodiment of the present invention is divided into the unit recording heads  10 A, if the number of divisions in the direction orthogonal to the moving direction of the recording sheet is N, by providing (N−1) or (N+1) sensors  12 , the recording offsets among the respective unit recording heads  10 A can be corrected. 
   Further, the structure of the unit recording head  10  is not limited to the above-described embodiment. The plural unit recording heads  10 A may be formed into a unit, and this unit may be installed in the device overall. In this case, the unit recording head mounting tolerance within the unit is small as compared with the tolerance for each unit. Further, the tolerance within the recording head unit is substantially that at the time of manufacturing and assembly, and the amount of change thereafter can be made to be small. Accordingly, when the recording head unit is manufactured and shipped out, if the offsets of the unit recording heads  10 A within the unit are measured, these amounts are stored, and the image outputs are always corrected by those amounts after device set-up, the tolerance of the unit recording heads  10 A within the unit can be reduced. Further, in this case, after set-up of the device, it suffices to detect and correct the offset amounts for each of the recording head units. 
   Three examples of forming the unit recording heads as units are shown in  FIG. 21 .  FIG. 21A  shows an example in which the unit recording heads in the widthwise direction, i.e., the unit recording heads of the same color, have been formed into units on a column-by-column basis (e.g., the unit recording heads Y 0 - 1  through Y 0 - 6  are formed into a single unit  50 ).  FIG. 21B  shows an example in which the unit recording heads are formed into units of each of the four colors (e.g., the unit recording heads Y 0 - 1 , M 0 - 1 , C 0 - 1 , and K 0 - 1  form the single unit  50 ).  FIG. 21C  shows an example in which the unit recording heads are made into a unit which includes the widthwise direction columns of the four colors (e.g., the unit recording heads Y 0 - 1  through Y 0 - 6 , M 0 - 1  through M 0 - 6 , C 0 - 1  through C 0 - 6 , and K 0 - 1  through K 0 - 6  are one unit). The arrangements of the sensors  12  for detecting the image positional offsets among the respective units  50  are also shown in these drawings. 
   In  FIG. 21A , in order to detect the offsets among the respective units  50 , a total of four of the sensors  12  are disposed at the both ends. Two of the sensors  12  are disposed at each side in order to accurately detect the offset between the units  50 , and there are cases in which it is sufficient to provide one sensor  12  at each side. Further, the reason why the sensors  12  are disposed at the both sides is in order to detect the inclined offset (skewing) of each of the units  50 . If there is no need to detect the inclined offset for each of these units  50  in this way, it is possible to provide only one of the sensors  12  at the central portion. Further, in  FIG. 21B , in order to detect the offsets among the respective units  50 , the sensors  12  are disposed between the respective units  50 , and the offset per unit  50  can be detected and corrected. Moreover, in  FIG. 21C , because the unit recording heads are made into a unit which includes the widthwise direction columns of the four colors, it suffices to provide two of the sensors  12  for detecting the offsets of the respective units. 
   In the above embodiment, description is given of an example in which correction is carried out by using black as the reference in order to match the other three colors with the black unit recording head  10 A. However, for example, the average values of the maximum and the minimum of the three offset amounts of yellow and black, magenta and black, and cyan and black can be made to coincide. In this way, the correction amount is small. For example, there is the advantage that less buffer memory is used at the time of skewing correction. 
   In the present invention, the correcting section may correct the recording offsets of the respective unit recording heads in the direction orthogonal to the moving direction of the recording medium, or may correct the recording offsets of the respective unit recording heads in the moving direction of the recording medium. 
   The recording head may include unit recording heads which record images of plural colors. 
   The correcting section may correct the recording offsets in the direction orthogonal to the moving direction of the recording medium by changing the recording regions of the unit recording heads. The correcting section may correct the recording offsets in the moving direction of the recording medium by changing the recording timings of the unit recording heads. 
   Further, the correcting section may change the recording timings of the unit recording heads by offsetting the inclining of the image recording positions due to the inclining of the unit recording heads. 
   When the number of divisions of the recording head in the direction orthogonal to the recording medium moving direction is N, (N− 1 ) detecting sections may be provided. Or, when the number of divisions of the recording head in the direction orthogonal to the recording medium moving direction is N, (N+1) detecting sections may be provided. 
   In addition, the recording head may be structured by plural units, with each unit being formed by a predetermined number of unit recording heads.