BACKGROUND OF THE INVENTION 
     The present invention relates to a method for driving a recording head that is composed of a plurality ink-jet heads. 
     An example of a printer that uses a recording head composed of a plurality of ink-jet heads is an ink-jet color printer, which is equipped with ink-jet heads each for a respective one of four colors of ink (yellow, cyan, magenta, and black) and drives these heads to provide full-color printing. Such a recording head achieves color printing by superimposing each of colored dots of yellow, cyan, magenta, and black on top of the other and therefore has a requirement for strict control of the distance between each head ink nozzle. 
     However, for printing with high resolution, it is difficult to set precisely the distance between each head ink nozzle to an integral multiple of the pixel pitch. For example, in the case of printing resolutions of 300 to 600 dpi (dots per inch), the distance between printed dots is of the order of 40 to 80 μm. In particular, there arises a considerable degradation in image quality, which is due to periodical unevenness of printing resulting from misalignment of dots due to mechanical displacements at the time of mounting each head. This could be solved by increasing the mechanical precision in mounting each head. To this end, great precision would be required of head mounting members and moreover complex and great precision would also be required for mounting adjustment, making the manufacture difficult and increasing the manufacturing cost. 
     A method is also known which solves the misalignment of printed dots by displacing the printing timing according to the pitch corresponding to a mounting error between each ink jet head. For example, the misalignment of printed dots in the sub-scanning direction in which a printing medium travels can be corrected by providing a plurality of clocks that permit the printing initiation timing to be selected for each ink jet head and selecting the printing initiation timing according to the displacements of the printing pitch. For example, the provision of a clock four times the normal printing period allows for electrical correction of up to ¼ pitch of displacement in the sub-scanning direction. 
     Moreover, a method is known which corrects printing positions by providing a delay circuit for a position displacement for each printing line and displacing the printing timing through the delay circuit. Furthermore, as described in Japanese Unexamined Patent Publication No. 7-156452, a method is also known which utilizes a buffer memory to ensure delay control and high picture quality control. 
     In summary, the technique that uses a plurality of clocks that allow the printing initiation timing to be selected for each ink jet head requires the clock speed to be increased, which results in complication of control and an increase in cost. The delay circuit-based technique has a problem that control becomes complicated. The techniques disclosed in the patent publication needs a basic clock of N times the highest response frequency for printing and a buffer memory, which complicates control as a result of the increased driving frequency. 
     BRIEF SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a recording head driving method which permits misalignment between printed dot arrangements in both the sub-scanning and main scanning directions through simple control. 
     According to the present invention, there is provided a method of driving a recording head in which a plurality of ink jet heads each having a large number of ink chambers each provided with an ink nozzle are arranged in parallel with each other in a main scanning direction perpendicular to a direction in which a recording medium moves relative to the heads and are spaced apart from each other by a predetermined distance in a sub-scanning direction in which the recording medium moves, comprising dividing the ink chambers of each of the ink jet heads into N (an integer of two or more) sets each including every N-th ink chamber, placing ink nozzles of N ink chambers each included in a respective one of the N sets in a staggered arrangement, driving the ink chambers on a time division basis for each set, and, when the amount of misalignment between dots recorded by droplets of ink ejected from ink nozzles of a set of ink chambers in a reference head of the heads and dots recorded by droplets of ink ejected from ink nozzles of each set of ink chambers in the other heads than the reference head exceeds one-half of a dot pitch in the sub-scanning direction, changing the order in which the sets of ink chambers are driven on a time division basis to reduce the amount of misalignment of dots below one-half of the dot pitch in the sub-scanning direction. 
     This method allows alignment error between printed dots in the sub-scanning direction to be corrected through simple control. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 is a schematic representation of a print head assembly according to a first embodiment of the present invention; 
     FIG. 2 is a perspective view of the print head assembly shown in FIG. 1; 
     FIG. 3 shows an arrangement of ink nozzles of each ink jet head in the first embodiment; 
     FIGS. 4A,  4 B and  4 C show drive voltage waveforms used in the first embodiment; 
     FIG. 5 shows printed dots by the ink jet heads in the first embodiment; 
     FIGS. 6A and 6B show printed dots in the first embodiment before and after correction of alignment error; 
     FIGS. 7A and 7B show printed dots in the first embodiment before and after correction of alignment error; 
     FIG. 8 shows an ink jet head driver circuit in the first embodiment; 
     FIGS. 9A and 9B are diagrams for use in explanation of methods of measuring the amount L of misalignment between dot arrangements in the first embodiment; 
     FIG. 10 shows an ink jet head driver circuit in a second embodiment of the present invention; 
     FIG. 11 shows an arrangement of printed dot data in the second embodiment; 
     FIGS. 12A through 12F show a relationship between driving timing and printed dot data in the second embodiment; 
     FIGS. 13A through 13F show another relationship between driving timing and printed dot data in the second embodiment; 
     FIG. 14 shows an arrangement of ink nozzles of each ink jet head in a third embodiment of the present invention; 
     FIGS. 15A through 15E show drive voltage waveforms used in the third embodiment; 
     FIGS. 16A and 16B show printed dots in the third embodiment before and after correction of dot alignment error; 
     FIG. 17 is a schematic representation of a print head assembly according to a fourth embodiment of the present invention; 
     FIG. 18 shows an arrangement of ink nozzles of each ink jet head in the fourth embodiment of the present invention; 
     FIGS. 19A and 19B show printed dots in the third embodiment before and after correction of dot alignment error; 
     FIG. 20 is a schematic representation of a print head assembly according to a fifth embodiment of the present invention; 
     FIG. 21 shows an arrangement of ink nozzles of each ink jet head in the fifth embodiment; 
     FIG. 22 is a schematic representation of a print head assembly according to a sixth embodiment of the present invention; 
     FIG. 23 shows an arrangement of ink nozzles of each ink jet head in the sixth embodiment; 
     FIG. 24 is a schematic representation of a print head assembly according to a seventh embodiment of the present invention; 
     FIGS. 25A and 25B show a configuration of the ink jet head used in each of the embodiments of the present invention; 
     FIGS. 26A and 26B show another configuration of the ink jet head used in each of the embodiments of the present invention; 
     FIGS. 27A and 27B show still another configuration of the ink jet head used in each of the embodiments of the present invention; and 
     FIG. 28 shows another example of a staggered arrangement of ink nozzles. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in FIG. 1, ink jet heads  1  and  2 , each with a large number of ink chambers, are fixed with adhesive to both sides of a substrate  3  which are parallel to each other to thereby form one recording head assembly. 
     The ink jet heads  1  and  2  and the substrate  3  are of such configuration as show in FIG.  2 . 
     That is, the ink jet heads  1  and  2  each with a large number of ink chambers are fixed to the respective sides of the substrate  3 . 
     Ink supply tubes  4  and  5  are connected to the ink jet heads  1  and  2 , respectively. 
     To portions of the ink jet heads  1  and  2  are respectively mounted connectors  6  and  7  with which cables  8  and  9  are connected respectively. Through these cables the ink jet heads  1  and  2  are supplied with drive voltages. P denotes a sheet of paper as a recording medium. 
     As shown in FIG. 3, the ink jet heads  1  and  2  are formed with ink nozzles  4   1  to  4   9 , . . . and  5   1  to  5   9 , . . . each corresponding to a respective one of the ink chambers. These ink nozzles are placed in a staggered arrangement for every three nozzles. That is, these ink nozzles  4   1  to  4   9 , . . . of the ink jet head  1  are arranged at regular intervals of pitch 2P in the main scanning direction perpendicular to the direction of movement of the recording medium indicated by an arrow. 
     In the ink jet head  1 , the ink chambers are divided into three sets each including every third head. That is, the ink nozzles  4   1 ,  4   4 ,  4   7 , etc. form a first set. The nozzles  4   2 ,  4   5 ,  4   8 , etc. form a second set, and the nozzles  4   3 ,  4   6 ,  4   9 , etc. form a third set. The ink nozzles  4   1 ,  4   4 ,  4   7 , etc. in the first set to which reference is made are arranged on a line  1   a . The ink nozzles  4   2 ,  4   5 ,  4   8 , etc. in the second set are arranged on a line  1   b  which is offset from the line  1   a  by given pitch D in the sub-scanning direction in which the recording medium travels. The ink nozzles  4   3 ,  4   6 ,  4   9 , etc. in the third set are arranged on a line  1   c  which is offset from the line  1   b  by the pitch D in the sub-scanning direction. 
     In the ink jet head  2  as well, the ink chambers are divided into three sets each including every third head. That is, the ink nozzles  5   1 ,  5   4 ,  5   7 , etc. form a first set. The nozzles  5   2 ,  5   5 ,  5   8 , etc. form a second set, and the nozzles  5   3 ,  5   6 ,  5   9 , etc. form a third set. 
     Each of the nozzles  5   1  to  5   9 , etc. in the ink jet header  2 , which are arranged at regular intervals of pitch 2P in the main scanning direction, is offset from a corresponding one of the nozzles  4   1  to  4   9 , etc. by pitch P in the main scanning direction. The ink nozzles  5   1 ,  5   4 ,  5   7 , etc. in the first set are arranged on a line  2   a  which is spaced apart from the reference line  1   a  by distance d in the sub-scanning direction. The ink nozzles  5   1 ,  5   4 ,  5   7 , etc. in the second set are arranged on a line  2   b  which is offset from the line  2   a  by given pitch D in the sub-scanning direction. The ink nozzles  5   3 ,  5   6 ,  5   9 , etc. in the third set are arranged on a line  2   c  which is offset from the line  2   b  by the pitch D in the sub-scanning direction. 
     The ink jet heads  1  and  2  are each arranged to provide drive voltage waveforms to their respective ink chambers at the times indicated in FIGS. 4A,  4 B and  4 C, thus performing three-phase driving. That is, the ink jet head  1  allows the ink nozzles  4   1 ,  4   4 ,  4   7 , etc. on the line  1   a  to project droplets of ink at the time shown in FIG. 4A, the nozzles  4   2 ,  4   5 ,  4   8 , etc. on the line  1   b  to project droplets of ink at the time shown in FIG. 4B, and the nozzles  4   3 ,  4   6 ,  4   9 , etc. on the line  1   c  to project droplets of ink at the time shown in FIG.  4 C. The ink jet head  2  allows the ink nozzles  5   1 ,  5   4 ,  5   7 , etc. on the line  2   a  to project droplets of ink at the time shown in FIG. 4A, the nozzles  5   2 ,  5   5 ,  5   8 , etc. on the line  2   b  to project droplets of ink at the time shown in FIG. 4B, and the nozzles  5   3 ,  5   6 ,  5   9 , etc. on the line  2   c  to project droplets of ink at the time shown in FIG.  4 C. 
     Thus, when a line of printing is made with the recording head, the ink jet head  1  first prints dots n 1 , n 2 , n 3 , n 4 , . . . shown in FIG. 5 trough the three-phase driving and the ink jet head  2  then prints dots m 1 , m 2 , m 3 , m 4 , . . . through the three-phase driving, so that one line can be printed at dot pitch P in the main scanning direction. That is, the ink jet heads  1  and  2 , while each having a dot pitch of 2P in the main scanning direction, can make printing at twice the resolution determined by that dot pitch. 
     If the spacing d between the lines  1   a  and  2   a  of the heads  1  and  2  is set such that d=n×3D (n is a positive integer), then a dot line printed by the head  1  can be superimposed upon a dot line by the head  2  because the timing of application of the drive voltages shown in FIGS. 4A,  4 B and  4 C are determined based on the pitch D in the sub-scanning direction. In this case, there is no problem. However, in practice misalignment occurs between dot lines by the heads  1  and  2  due to irregularities in the thickness of the substrate  3  and/or the adhesive layer. 
     The difference (d−n×3D) is classified into the following three ranges: 
     
       
         − D /6 ≦d−n ×3 D&lt;D /6  (1) 
       
     
     
       
           D /6 ≦d−n ×3 D &lt;3 D /6  (2) 
       
     
     
       
         3 D /6 ≦d−n ×3 D &lt;5 D /6  (3) 
       
     
     The difference (d−n 3D) in the range defined by (1) is within the normal range of error accepted when dots are printed by the ink jet heads alone. In this case, printing can be made at the predetermined times. However, when the difference is in the range defined by (2) or (3), some corrections are required because it is outside the normal range of error. 
     For example, when the difference is in the range defined by (2), misalignment takes place between an arrangement of dots n 1 , n 2 , n 3 , n 4 , . . . printed by the head  1  and an arrangement of dots m 1 , m 2 , m 3 , m 4 , . . . printed by the head  2  as shown in FIG.  6 A. 
     To eliminate this misalignment, the ink projecting operation of the ink nozzles  4   1 ,  4   4 ,  4   7 , etc. on the line  1   a  of the head  1  and the ink nozzles  5   2 ,  5   5 ,  5   8 , etc. on the line  2   b  of the head  2  is performed at the time shown in FIG. 4A, the ink projecting operation of the ink nozzles  4   2 ,  4   5 ,  4   8 , etc. on the line  1   b  of the head  1  and the ink nozzles  5   3 ,  5   6 ,  5   9 , etc. on the line  2   c  of the head  2  is performed at the time shown in FIG. 4B, and the ink projecting operation of the ink nozzles  4   3 ,  4   6 ,  4   9 , etc. on the line  1   c  of the head  1  and the ink nozzles  5   1 ,  5   4 ,  5   7 , etc. on the line  2   a  of the head  2  is performed at the time shown in FIG.  4 C. That is, the timing of projecting of ink from the ink nozzles on the respective lines  2   a ,  2   b  and  2   c  of the ink jet head  2  is changed. By such control, the misalignment between the arrangement of dots n 1 , n 2 , n 3 , n 4 , etc. and the arrangement of dots m 1 , m 2 , m 3 , m 4 , etc. is corrected for as shown in FIG.  6 B. 
     When the difference is in the range defined by (3), greater misalignment takes place between the arrangement of dots n 1 , n 2 , n 3 , n 4 , . . . printed by the head  1  and the arrangement of dots m 1 , m 2 , m 3 , m 4 , . . . printed by the head  2  as shown in FIG.  7 A. 
     To eliminate such misalignment, the ink projecting operation of the ink nozzles  4   1 ,  4   4 ,  4   7 , etc. on the line  1   a  of the head  1  and the ink nozzles  5   3 ,  5   6 ,  5   9 , etc. on the line  2   c  of the head  2  is performed at the time shown in FIG. 4A, the ink projecting operation of the ink nozzles  4   2 ,  4   5 ,  4   8 , etc. on the line  1   b  of the head  1  and the ink nozzles  5   1 ,  5   4 ,  5   7 , etc. on the line  2   a  of the head  2  is performed at the time shown in FIG. 4B, and the ink projecting operation of the ink nozzles  4   3 ,  4   6 ,  4   9 , etc. on the line  1   c  of the head  1  and the ink nozzles  5   2 ,  5   5 ,  5   8 , etc. on the line  2   b  of the head  2  is performed at the time shown in FIG.  4 C. That is, the timing of projecting of ink from the ink nozzles on the respective lines  2   a ,  2   b  and  2   c  of the ink jet head  2  is changed. By such control, the misalignment between the arrangement of dots n 1 , n 2 , n 3 , n 4 , etc. and the arrangement of dots m 1 , m 2 , m 3 , m 4 , etc. is corrected for as shown in FIG.  7 B. 
     A head driving method that allows the timing of the projecting of ink to be changed in the manner described above can be implemented by a drive circuit shown in FIG.  8 . 
     In this drive circuit, electrodes for applying voltages to ink chambers  11   1 ,  11   2 ,  11   3 ,  11   4 ,  11   5 ,  11   6 , . . . in the ink jet head  1  are connected to ground through analog switches  12   1 ,  12   2 ,  12   3    12   4 ,  12   5 ,  12   6 , . . . , respectively. The electrodes for applying voltages to the ink chambers  11   1 ,  11   4 , . . . equipped with the ink nozzles  4   1 ,  4   4 , . . . arranged on the line  1   a  of the ink jet head  1  are connected to a line  31  through analog switches  13   1 ,  13   4 , . . . respectively. The electrodes for applying voltages to the ink chambers  11   2 ,  11   5 , . . . equipped with the ink nozzles  4   2 ,  4   5 , . . . arranged on the line  1   b  of the ink jet head  1  are connected to a line  31  through analog switches  13   2    13   5 , . . . , respectively. The electrodes for applying voltages to the ink chambers  11   3 ,  11   6 , . . . equipped with the ink nozzles  4   3 ,  4   6 , . . . arranged on the line  1   c  of the ink jet head  1  are connected to a line  33  through analog switches  13   3 ,  13   6 , . . . , respectively. 
     Electrodes for applying voltages to ink chambers  21   1 ,  21   2 ,  21   3 ,  21   4 ,  21   5 , . . . in the ink jet head  2  are connected to ground through analog switches  22   1 ,  22   2 ,  22   3 ,  22   4 ,  22   5 , . . . , respectively. The electrodes for applying voltages to the ink chambers  21   1 ,  21   4 , . . . equipped with the ink nozzles  5   1 ,  5   4 , . . . arranged on the line  2   a of the ink jet head  2  are connected to a line  34  through analog switches  23   1 ,  23   4 , . . . , respectively. The electrodes for applying voltages to the ink chambers  21   2 ,  21   5 , . . . equipped with the ink nozzles  5   2 ,  5   5 , . . . arranged on the line  2   b  of the ink jet head  2  are connected to a line  35  through analog switches  23   2 ,  23   5 , respectively. The electrodes for applying voltages to the ink chambers  21   3 , . . . equipped with the ink nozzles  5   3 , . . . arranged on the line  2   c  of the ink jet head  2  are connected to a line  36  through analog switches  23   3 , . . . , respectively. 
     The line  31  is connected to a waveform generator  37  for generating the drive voltage waveform shown in FIG.  4 A and to each of first terminals  40   a ,  41   a  and  42   a  of selectors  40 ,  41  and  42 . The line  32  is connected to a waveform generator  38  for generating the drive voltage waveform shown in FIG.  4 B and to each of second terminals  40   b ,  41   b  and  42   b  of the selectors  40 ,  41  and  42 . The line  33  is connected to a waveform generator  39  for generating the drive voltage waveform shown in FIG.  4 C and to each of third terminals  40   c ,  41   c  and  42   c  of the selectors  40 ,  41  and  42 . 
     The line  34  is connected to the common terminal of the selector  40 . The line  35  is connected to the common terminal of the selector  41 . The line  36  is connected to the common terminal of the selector  42 . 
     Based on data to be printed from a printing data output circuit  43 , the head drive circuit selectively turns on the analog switches  12   1  to  12   6  or  22   1  to  22   5  to thereby connect the electrodes of the ink chambers associated with the selectively turned-on analog switches to ground. Alternatively, the head drive circuit selectively turns on the analog switches  13   1  to  13   6  or  23   1  to  23   5  to thereby selectively apply the drive voltage waveform shown in FIG. 4A,  4 B or  4 C to each of the electrodes of the ink chambers associated with the selectively turned-on analog switches. When the analog switches  13   1  to  13   5  or  23   1  to  23   5  are selectively driven, droplets of ink are projected from the nozzles of the corresponding ink chambers. 
     The selective application of the drive voltage waveform in FIG. 4A,  4 B or  4 C to each of the ink chamber electrodes is performed through the selectors  40 ,  41 , and  42 . That is, when the difference (d−n 3D) is in the range defined by (1), the selector  40  selects the drive voltage waveform from the waveform generator  37  and outputs it onto the line  34 . The selector  41  selects the drive voltage waveform from the waveform generator  38  and outputs it onto the line  35 . The selector  42  selects the drive voltage waveform from the waveform generator  39  and outputs it onto the line  36 . 
     When the difference (d−n 3D) is in the range defined by (2), the selector  40  selects the drive voltage waveform from the waveform generator  38  and outputs it onto the line  34 . The selector  41  selects the drive voltage waveform from the waveform generator  39  and outputs it onto the line  35 . The selector  42  selects the drive voltage waveform from the waveform generator  37  and outputs it onto the line  36 . 
     When the difference (d−n 3D) is in the range defined by (3), the selector  40  selects the drive voltage waveform from the waveform generator  39  and outputs it onto the line  34 . The selector  41  selects the drive voltage waveform from the waveform generator  37  and outputs it onto the line  35 . The selector  42  selects the drive voltage waveform from the waveform generator  38  and outputs it onto the line  36 . 
     Such control allows the alignment error between dot arrangements produced by the ink jet heads  1  and  2  in the sub-scanning direction to be corrected. That is, the dot misalignment in the sub-scanning direction is minimized, allowing high-resolution printing of good quality. 
     A method of detecting the difference (d−n 3D) involves making printing in a specific pattern for testing as shown in FIG. 9A, observing the result of printing with a microscope, and measuring the spacing L between a line of dots n 1 , n 2 , . . . printed by the ink jet head  1  and a line of dots m 1 , m 2 , . . . printed by the ink jet head  2 . 
     Alternatively, as shown in FIG. 9B, a testing specific pattern having a constant spacing may be printed and the spacing L between a line of dots m 1 , m 2 , . . . and the specific pattern may be determined. 
     Next, a second embodiment of the present invention will be described with reference to FIGS. 10 through 13. In these figures, like reference numerals are used to denote corresponding parts to those in the first embodiment and only different parts will be described. 
     In the second embodiment, as shown in FIG. 10, the waveform generators  37 ,  38  and  39  and the selectors  40 ,  41  and  42  in the first embodiment are replaced with programmable waveform generators  51  to  56 . The programmable waveform generator  51  supplies its drive voltage waveform to analog switches  13   1 ,  13   4 , etc. The programmable waveform generator  52  supplies its drive voltage waveform to the analog switches  13   2 ,  13   5 , etc. The programmable waveform generator  53  supplies its drive voltage waveform to the analog switches  13   3 ,  13   6 , etc. The programmable waveform generator  54  supplies its drive voltage waveform to the analog switches  23   1 ,  23   4 , etc. The programmable waveform generator  55  supplies its drive voltage waveform to the analog switches  23   2 ,  23   5 , etc. The programmable waveform generator  56  supplies its drive voltage waveform to the analog switches  23   3 , etc. 
     In this embodiment, the programmable waveform generators  51  to  56  are set in advance to vary the timing of their respective drive voltage waveform according to the differences (d−n 3D) defined by (1), (2) and (3). 
     The measurement of the differences is made in the same manner as described in connection with FIGS. 9A and 9B. 
     FIG. 11 shows an arrangement of printed dot data, in which d 11 , d 21 , d 31 , d 41 , d 51 , . . . , d 13 , d 23 , d 33 , d 43 , d 53 , . . . , d 15 , d 25 , d 35 , d 45 , d 55 , . . . are printed by the ink jet head  1 , and d 12 , d 22 , d 32 , d 42 , d 52 , . . . , d 14 , d 24 , d 34 , d 44 , d 54 , . . . , d 16 , d 26 , d 36 , d 46 , d 56 , . . . are printed by the ink jet head  2 . 
     In this printing, when the difference (d−n 3D) between the dot arrangement by the ink jet head  1  and the dot arrangement by the ink jet head  2  in the sub-scanning direction is related by (2), the head  1  drives the ink chambers  11   1 ,  11   4 , . . . with a drive voltage waveform shown in FIG. 12A, which results in printed dot data d 11 , d 21 , d 31 , . . . as shown in FIG.  12 D. The ink chambers  11   2 ,  11   5 , . . . are driven with a drive voltage waveform shown in FIG. 12B, resulting in printed dot data d 13 , d 23 , . . . as shown in FIG.  12 E. The ink chambers  11   3 ,  11   6 , . . . are driven with a drive voltage waveform shown in FIG. 12C, resulting in printed dot data d 15 , d 25 , . . . as shown in FIG.  12 F. 
     In contrast, the head  2  drives the ink chambers  21   2 ,  21   5 , . . . with a drive voltage waveform shown in FIG. 13A, which results in printed dot data d 14 , d 24 , d 34 , . . . as shown in FIG.  13 E. The ink chambers  21   3 , . . . are driven with a drive voltage waveform shown in FIG. 13B, resulting in printed dot data d 26 , d 36 , . . . as shown in FIG.  13 F. The ink chambers  21   1 ,  21   4 , . . . are driven with a drive voltage waveform shown in FIG. 13C, resulting in printed dot data d 12 , d 22 , d 32 , . . . as shown in FIG.  13 D. 
     Such control allows corrections when the difference (d−n 3D) is in the range defined by (2). In a similar manner, corrections can be made when the difference is in the range defined by (3). Thus, as is the case with the first embodiment, the second embodiment also allows misalignment in the sub-scanning direction between dot arrangements produced by the heads  1  and  2  to be corrected. That is, the dot misalignment in the sub-scanning direction is minimized, allowing high-resolution printing of good quality. 
     A third embodiment of the present invention will be described next with reference to FIGS. 14,  15  and  16 . 
     As shown in FIG. 14, ink jet heads  61  and  62  each with a large number of ink chambers are attached to both sides of a substrate  63  with adhesive, thus forming one print head assembly. 
     The ink jet heads  61  and  62  are provided with ink nozzles  64   1  to  64   10 , . . . ,  65   1  to  65   9 , . . . each of which is associated with a respective one of the ink chambers. These nozzles are placed in a staggered arrangement for every five nozzles. 
     That is, in the ink jet head  61 , the ink nozzles  64   1  to  64   10  are arranged at regular intervals of pitch 2P in the main scanning direction perpendicular to the direction indicated by an arrow in which a recording medium moves. The ink chambers are divided into five sets each including every fifth chamber. That is, the ink nozzles  64   1 ,  64   6 , etc. form a first set. The nozzles  64   2 ,  64   7 , etc. form a second set. The nozzles  64   3 ,  64   8 , etc. form a third set. The nozzles  64   4 ,  64   9 , etc. form a fourth set. The nozzles  64   5 ,  64   10 , etc. form a fifth set. The ink nozzles  64   1 ,  64   6 , etc. in the first set to which reference is made are arranged on a line  1   d . The ink nozzles  64   2 ,  64   7 , etc. in the second set are arranged on a line  1   e  which is offset from the line  1   d  by given pitch D in the sub-scanning direction in which the recording medium travels. The ink nozzles  64   3 ,  64   8 , etc. in the third set are arranged on a line if offset from the line  1   e  by the pitch D in the sub-scanning direction. The ink nozzles  64   4 ,  64   9 , etc. in the fourth set are arranged on a line  1   g  offset from the line  1   f  by the pitch D in the sub-scanning direction. The ink nozzles  64   5 ,  64   10 , etc. in the fifth set are arranged on a line  1   h  offset from the line  1   g  by the pitch D in the sub-scanning direction. 
     In the ink jet head  62 , each of the nozzles  65   1  to  65   9 , etc., which are arranged at regular intervals of pitch 2P in the main scanning direction, is offset from a corresponding one of the nozzles  64   1  to  64   9 , etc. by pitch P in the main scanning direction. The ink chambers are divided into five sets each including every fifth chamber. That is, the ink nozzles  65   1 ,  66   6 , etc. form a first set. The nozzles  65   2 ,  65   7 , etc. form a second set. The nozzles  65   3 ,  65   8 , etc. form a third set. The nozzles  65   4 ,  65   9 , etc. form a fourth set. The nozzles  65   5 , etc. form a fifth set. The ink nozzles  65   1 ,  65   6 , etc. in the first set to which reference is made are arranged on a line  2   d  offset from the reference line  1   d  by distance d in the sub-scanning direction. The ink nozzles  65   2 ,  65   7 , etc. in the second set are arranged on a line  2   e  offset from the line  2   d  by given pitch D in the sub-scanning direction in which the recording medium travels. The ink nozzles  65   3 ,  65   8 , etc. in the third set are arranged on a line  2   f  offset from the line  2   e  by the pitch D in the sub-scanning direction. The ink nozzles  65   4 ,  65   9 , etc. in the fourth set are arranged on a line  2   g  offset from the line  2   f  by the pitch D in the sub-scanning direction. The ink nozzles  65   5 , etc. in the fifth set are arranged on a line  2   h  offset from the line  2   g  by the pitch D in the sub-scanning direction. The ink jet heads  61  and  62  are each arranged to provide drive voltage waveforms to their respective ink chambers at the times indicated in FIGS. 15A to  15 F, thus performing five-phase driving. That is, the ink nozzles  64   1 ,  64   6 , etc. on the line  1   d  of the head  61  and the ink nozzles  65   1 ,  65   6 , etc. on the line  2   d  of the head  62  are allowed to project droplets of ink at the times indicated in FIG.  15 A. The ink nozzles  64   2 ,  64   7 , etc. on the line  1   e  of the head  61  and the ink nozzles  65   2 ,  65   7 , etc. on the line  2   e  of the head  62  project droplets of ink at the times indicated in FIG.  15 B. The ink nozzles  64   3 ,  64   8 , etc. on the line if of the head  61  and the ink nozzles  65   3 ,  65   8 , etc. on the line  2   f  of the head  62  project droplets of ink at the times indicated in FIG.  15 C. The ink nozzles  64   4 ,  64   9 , etc. on the line  1   g  of the head  61  and the ink nozzles  65   4 ,  65   9 , etc. on the line  2   g  of the head  62  project droplets of ink at the times indicated in FIG.  15 D. The ink nozzles  64   5 ,  64   10 , etc. on the line  1   h  of the head  61  and the ink nozzles  65   5 , etc. on the line  2   h  of the head  62  project droplets of ink at the times indicated in FIG.  15 E. 
     Thus, when a line of printing is made with the recording head, the ink jet head  61  prints dots n 1 , n 2 , n 3 , n 4 , . . . as in FIG. 5 trough the three-phase driving and the ink jet head  62  prints dots m 1 , m 2 , m 3 , m 4 , . . . through the three-phase driving, so that one line can be printed at dot pitch P in the main scanning direction. That is, the ink jet heads  61  and  62 , while each having a dot pitch of 2P in the main scanning direction, can make printing at twice the resolution determined by that dot pitch. 
     If the spacing d between the lines  1   d  and  2   d  of the heads  61  and  62  is set such that d=n×5D (n≧1), then a dot line printed by the head  61  can be superimposed upon a dot line by the head  62  because the timing of application of the drive voltage waveforms shown in FIGS. 15A to  15 E are determined based on the pitch D in the sub-scanning direction. In this case, there is no problem. In practice, however, misalignment will occur between the dot lines printed by the heads  61  and  62  due to irregularities in the thickness of the substrate  63  and/or the adhesive layer. 
     When the amount of misalignment between the line of dots n 1 , n 2 , n 3 , . . . printed by the head  61  and the line of dots m 1 , m 2 , m 3 , . . . printed by the head  62  is in excess of one-half of the dot pitch in the sub-scanning direction, similar control to that in the above-described embodiments allows the amount of misalignment to be reduced below one-half of the dot pitch in the sub-scanning direction. 
     In this embodiment, since one dot pitch in the sub-scanning direction is further subdivided the accuracy of printing position can be increased in comparison with the above-described embodiments in which the ink nozzles are staggered for every three nozzles. However, in the five-phase driving for one line of printing, the printing speed becomes slower than in the three-phase driving. 
     Thus, the choice of the three-phase driving or the five-phase driving depends on tradeoff between the printing speed and the accuracy of printing position. 
     Next, a fourth embodiment of the present invention will be described with reference to FIGS. 17,  18  and  19 . 
     In FIG. 17,  71 ,  72  and  73  denote ink jet heads for projecting cyan ink, magenta ink, and yellow ink, respectively. The cyan ink jet head  71  and the magenta ink jet head  72  are attached to both sides of a substrate  74  with adhesive, and the magenta ink jet head  72  and the yellow ink jet head  73  are attached to both sides of a substrate  75  with adhesive, thereby forming one color print head assembly. 
     As shown in FIG. 18, the ink jet heads  71 ,  72  and  73  are formed with ink nozzles  76   1  to  76   9 , . . . ,  77   1  to  77   9 , . . . , and  78   1  to  78   9 , . . . each of which is associated with a respective one of ink chambers. These ink nozzles are placed in a staggered arrangement for every three nozzles. That is, the ink nozzles  76   1  to  76   9 , . . . of the ink jet head  71  are arranged at regular intervals of pitch P in the main scanning direction perpendicular to the direction of movement of the recording medium indicated by an arrow. 
     In the ink jet head  71 , the ink chambers are divided into three sets each including every third head. That is, the ink nozzles  76   1 ,  76   4 ,  76   7 , etc. form a first set. The nozzles  76   2 ,  76   5 ,  76   8 , etc. form a second set, and the nozzles  76   3 ,  76   6 ,  76   9 , etc. form a third set. The ink nozzles  76   1 ,  76   4 ,  76   7 , etc. in the first set to which reference is made are arranged on a line ca. The ink nozzles  76   2 ,  76   5 ,  76   8 , etc. in the second set are arranged on a line cb which is offset from the line ca by given pitch D in the sub-scanning direction. The ink nozzles  76   3 ,  76   6 ,  76   9 , etc. in the third set are arranged on a line cc which is offset from the line cb by the pitch D in the sub-scanning direction. 
     In the ink jet head  72  as well, the ink chambers are divided into three sets each including every third head. That is, the ink nozzles  77   1 ,  77   4 ,  77   7 , etc. form a first set. The nozzles  77   2 ,  77   5 ,  77   8 , etc. form a second set, and the nozzles  77   3 ,  77   6 ,  77   9 , etc. form a third set. 
     In the ink jet head  73 , each of the nozzles  78   1  to  78   9 , etc., which are arranged at regular intervals of pitch P in the main scanning direction, is aligned with a corresponding one of the nozzles  76   1  to  76   9 , etc. in the head  71  in the sub-scanning direction. The ink nozzles  78   1 ,  78   4 ,  78   7 , etc. in the first set are arranged on a line ya which is spaced apart from the reference line ca by distance d 2  in the sub-scanning direction. The ink nozzles  78   2 ,  78   5 ,  78   8 , etc. in the second set are arranged on a line yb offset from the line ya by given pitch D in the sub-scanning direction. The ink nozzles  78   3 ,  78   6 ,  78   9 , etc. in the third set are arranged on a line yc offset from the line yb by the pitch D in the sub-scanning direction. 
     The ink jet heads  71 ,  72  and  73  are each arranged to provide drive voltage waveforms to their respective ink chambers at the times indicated in FIGS. 4A,  4 B and  4 C as in the first embodiment, thus performing three-phase driving. That is, the ink nozzles  76   1 ,  76   4 ,  76   7 , etc. on the line ca of the head  71 , the ink nozzles  77   1 ,  77   4 ,  77   7 , etc. on the line ma of the head  72  and the ink nozzles  78   1 ,  78   4 ,  78   7 , etc. on the line ya of the head  73  project ink at the times indicated in FIG.  4 A. The ink nozzles  76   2 ,  76   5 ,  76   8 , etc. on the line cb of the head  71 , the ink nozzles  77   1 ,  77   4 ,  77   7 , etc. on the line mb of the head  72  and the ink nozzles  78   2 ,  78   5 ,  78   8 , etc. on the line yb of the head  73  project ink at the times indicated in FIG.  4 B. The ink nozzles  76   3 ,  76   6 ,  76   9 , etc. on the line cc of the head  71 , the ink nozzles  77   3 ,  77   6 ,  77   9 , etc. on the line mc of the head  72  and the ink nozzles  78   3 ,  78   6 ,  78   9 , etc. on the line yc of the head  73  project ink at the times indicated in FIG.  4 C. 
     For printing one line using the head assembly thus arranged, the ink jet head  71  is first three-phase driven to print dots, the head  72  is then three-phase driven to print selectively dots on the printed dots by the head  71 , and the head  73  is finally three-phase driven to print selectively dots on the printed dots by the heads  71  and  72 . Thus, color printing is made by printing three colored dots of cyan, magenta, and yellow independently or superimposed upon each other. 
     If the spacing d 1  between the line ca of the cyan head  71  and the line ma of the magenta head  72  is set such that d 1 =n 3D (n≧1) and the spacing d 2  between the line ca of the cyan head  71  and the line ya of the yellow head  73  is set such that d 2 =n′ 3D (n′≧1), then the printed dots by the heads  72  and  73  can be superimposed accurately upon the printed dots by the head  71  because the timing of application of the drive voltage waveforms shown in FIGS. 4A,  4 B and  4 C is determined based on the pitch D in the sub-scanning direction. In this case, there is no problem. In the presence of irregularities in the thickness of the substrates  74  and  75  and/or the adhesive layer, however, misalignment will occur among the cyan dots, the magenta dots, and the yellow dots. That is, such misalignment of dots as shown in FIG. 19A is produced. 
     The misalignment of dots is measured in the same way as that of the first embodiment described in connection with FIGS. 9A and 9B. 
     In such a case, as with the first embodiment, the position of each dot in the sub-scanning direction is adjusted and the dot misalignment is minimized by varying the order allocated for the lines of the head  72  and/or head  73  to project ink. As a result, the cyan, magenta and yellows dots can become superimposed accurately as shown in FIG. 19B, achieving color printing of good quality. 
     Hereinafter, a fifth embodiment of the present invention will be described with reference to FIGS. 20 and 21. 
     As shown in FIG. 20, n ink jet heads  81   1  to  81   n  each having a large number of ink chambers arranged are staggered on either side of a substrate  82  to form an elongate line print head assembly. This line print head assembly is placed so that its longitudinal line coincides with the main scanning direction perpendicular to the direction in which a recording medium moves and makes printing onto the recording medium on a line by line basis. 
     In each of the ink jet heads  81   1  to  81   n  of the line print head assembly, as in the ink jet heads  1  and  2  in the first embodiment, the ink nozzles are staggered for every three nozzles and the ink chambers are divided into three sets each including every third chamber. In such a line print head, as shown in FIG. 21, each of lines of ink nozzles  83  in the head  81   1  and the corresponding line of ink nozzles  84  of the head  81   3  placed on the same side as the head  81  may be offset from each other by Δd in the sub-scanning direction. 
     In such a case, the line offset can be corrected by making the timing of projecting of ink from each set of ink nozzles in the head  81   3  differ from the timing of projecting of ink from the corresponding set of ink nozzles in the head  81   1 . That is, as described previously in the first embodiment, the position offset in the sub-scanning direction is corrected by changing the order in which the drive voltage waveforms shown in FIGS. 4A,  4 B and  4 C are selected, i.e., by changing the timing of ink projection. The position offset in the sub-scanning direction of the ink nozzles of the heads  81   1  and  81   2  that are placed on the opposed surfaces of the substrate  82  can be corrected through exactly the same control as in the first embodiment. 
     A sixth embodiment of the present invention will be described next with reference to FIGS. 22 and 23. 
     As shown in FIG. 22, n ink jet heads  85   1  to  85   n  each equipped with a large number of ink chambers are arranged side by side on the same side of a substrate  86  and fixed to the substrate with adhesive, thereby forming an elongate line print head assembly. This line print head assembly is placed so that its longitudinal line coincides with the main scanning direction perpendicular to the direction in which a recording medium moves and makes printing onto the recording medium on a line by line basis. 
     In each of the ink jet heads  85   1  to  85   n  of the line print head assembly, as in the ink jet heads  1  and  2  in the first embodiment, the ink nozzles are staggered for every three nozzles and the ink chambers are divided into three sets each including every third chamber. In such a line print head, as shown in FIG. 23, each of lines of ink nozzles  86  in the head  85   1  and the corresponding line of ink nozzles  87  of the head  85   2  may be offset from each other by Δd in the sub-scanning direction. 
     In such a case, the line offset can be corrected by making the timing of projecting of ink from each set of ink nozzles in the head  85   2  differ from the timing of projecting of ink from the corresponding set of ink nozzles in the head  85   1 . That is, as described previously in the first embodiment, the position offset in the sub-scanning direction is corrected by changing the order in which the drive voltage waveforms shown in FIGS. 4A,  4 B and  4 C are selected, i.e., by changing the timing of ink projection. 
     A seventh embodiment of the present invention will be described next with reference to FIG.  24 . 
     As shown in FIG. 24, for example, four ink jet heads  91   1 ,  91   2 ,  91   3 ,  91   4 , each equipped with a large number of ink chambers, are arranged such that they are in parallel with each other in the direction indicated by an arrow in which a recording medium moves, i.e., in the sub-scanning direction and are spaced apart from each other by predetermined distance in the main scanning direction perpendicular to the direction in which the recording medium moves, thereby forming a serial print head assembly. 
     In each of the ink jet heads  91   1  to  91   4  of the serial print head assembly, the ink nozzles are staggered for every three nozzles and the ink chambers are divided into three sets each including every third chamber. When reference is made to a line  1   i  on which a first set of ink nozzles  92  in the leftmost head  91   1  are arranged, a line  2   i  on which a first set of ink nozzles  93  of the head  91   2  are arranged is spaced apart by distance d 1  from the line l i . A line  3   i  on which a first set of ink nozzles  94  of the head  91   3  are arranged is spaced apart by distance d 2  from the line  1   i . A line  4   i  on which a first set of ink nozzles  95  of the head  91   4  are arranged is offset by distance d 3  from the line  1   i.    
     This type of serial print head assembly is arranged to move in the main scanning direction with a recording medium stopped to thereby print multiple lines of dots at a time. 
     The print head assembly forms a color serial print head when the heads  91   1 ,  91   2 ,  91   3 ,  91   4 , are used as heads for cyan, magenta, yellow, and black, respectively. 
     When dots printed by the heads  91   1 ,  91   2  and  91   3  are out of register in the main scanning direction, the order in which the drive voltage waveforms shown in FIGS. 4A,  4 B and  4 C are selected can be changed as in the first embodiment, i.e., the timing of ink projection from the ink nozzles can be changed for each set of ink nozzles in the heads  91   1 ,  91   2  and  91   3  to correct the registration error of dots in the main scanning direction. 
     Next, detailed configurations of the ink jet heads in the embodiments thus far described will be described. 
     FIGS. 25A and 25B show an ink jet head of the type that heats ink in the ink chambers and then projects it. Grooves of U-shaped cross section are formed in one surface of a substrate  101  at predetermined pitch. The grooves are covered on top with a board  102  and covered in front with an orifice plate  104  formed with ink nozzles  103 , thus forming a large number of ink chambers  105 . Within each ink chamber are formed a heating element  106  and electrode patterns  107  and  108  for energizing the heating element, which, in turn, are covered with a protective coating  109 . 
     In this type of ink jet head, application of a drive pulse to the heating element  106  through the electrode patterns  107  and  108  rapidly heats ink within the ink chamber, so that it boils and forms a bubble of vapor, thus allowing a droplet of ink to be ejected from the ink nozzle. 
     Constructing a line print head from such ink jet heads would require a considerable amount of electric power when a large number of heating elements are driven at the same time, resulting in an increase in the size of a power supply used. The time-division driving of the ink chambers reduces the number of heating elements that are driven at the same time, allowing the supply amount of electric power to be reduced and hence the size of power supply to be reduced. If, when such time-division driving is performed, the ink jet heads are configured and controlled as in the previously described embodiments, the alignment error of dots can be corrected. This is the case with the time-division driving of a serial print head. 
     FIGS. 26A and 26B show an ink jet head of the type that ejects ink within ink chambers by mechanical vibrations of a piezoelectric material. Grooves of U-shaped cross section are formed in one surface of a substrate  111  made of a piezoelectric material, when an actuator that is polarized in the direction of an arrow is formed in the middle of each groove. Electrode patterns  113  and  114  are formed on the opposed surfaces of the actuator. The grooves are covered on top with a board  115  and covered in front with an orifice plate  117  formed with ink nozzles  116 , thus forming a large number of ink chambers  118 . 
     With this type of ink jet head, application of a drive pulse between the electrode patterns  113  and  114  causes mechanical deformation in the actuator  112 , resulting in a change in the volume of the ink chamber. A change in the volume of the ink chamber involves a change in pressure in the ink chamber, allowing ink to be ejected from the nozzle. 
     Constructing a line print head or a serial print head from such ink jet heads and driving ink chambers on a time-division basis allow alignment error of dots to be corrected as in the previously described embodiments. 
     FIGS. 27A and 27B show an ink jet head of the type that ejects ink by mechanical vibrations of piezoelectric elements. Two polarized piezoelectric elements are glued together with their directions of polarization opposed to each other to form a substrate  121 . U-shaped grooves are formed at predetermined pitch in one surface of the substrate across the two piezoelectric elements. An electrode pattern  122  is formed on the sidewalls and the bottom of each groove. The grooves are covered on top with a board  123  and covered in front with an orifice plate  125  formed with ink nozzles  124 , thus forming a large number of ink chambers  126 . 
     With this type of ink jet head, application of a drive pulse between the electrode pattern  122  of an ink chamber (center ink chamber) and the electrode pattern  122  of each of two ink chambers  126  on both sides of that ink chamber causes mechanical deformation in substrate portions between the two ink chambers, resulting in a change in pressure of the center ink chamber from which ink is to be ejected. The change in pressure allows ink to be ejected from the ink nozzle  124  associated with the center ink chamber. 
     With this type of ink jet head, since substrate portions each between an ink ejecting chamber and an adjacent ink chamber are deformed, it follows that the ink chambers are driven on a time-division basis. Configuring and controlling this type of ink jet head in the manner described in connection with each of the embodiments allows alignment errors of dots to be corrected. 
     Although the preferred embodiments of the present invention have been described in terms of ink jet heads, the present invention is also applicable to thermal heads. 
     The previously described staggered arrangements of ink nozzles may include such an arrangement as shown in FIG.  28 . 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.
In a method of driving a plurality of ink jet heads, each ink jet head comprises a plurality of ink chambers that are each provided with an ink nozzle. The ink nozzles of each of the ink jet heads are divided into N sets each including every N-th ink nozzle, where N is an integer of two or more, and the N sets of the ink nozzles are arranged in parallel longitudinal rows in a staggered arrangement. The respective ink nozzles in corresponding rows of the ink jet heads are spaced apart from each other by a distance equal to an integral multiple of a pitch of the rows of the ink nozzles in each of the ink jet heads in a sub-scanning direction in which a recording medium is moved, and the ink nozzles eject ink as the ink nozzles move relative to the recording medium in a main scanning direction perpendicular to the sub-scanning direction. The ink nozzles of each of the N sets are driven to eject ink on a same line on the recording medium, by controlling a timing at which the ink nozzles of each set are driven and a relative speed between the ink jet heads and the recording medium. When an amount of misalignment between dots recorded by a reference set of the ink nozzles and dots recorded by another set of the ink nozzles exceeds one-half of a dot pitch defined in the sub-scanning direction, an order in which the sets of ink nozzles are driven is changed to reduce the amount of misalignment between dots to below one-half of the dot pitch defined in the sub-scanning direction.