Patent Publication Number: US-7722145-B2

Title: Ink jet head driving apparatus and ink jet head driving method

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a technique to control an ink jet head of an ink jet printer, and particularly to a technique to correct, in an ink jet head including many nozzles, variations in the amounts of droplets discharged from the respective nozzles. 
   2. Description of the Related Art 
   An ink jet printer includes an ink jet head. The ink jet head distributes ink supplied from an ink tank into plural pressure chambers, causes pressure to be selectively generated in the respective pressure chambers, and discharges ink droplets from nozzles communicating with the respective pressure chambers. The ink jet printer drives the ink jet head and a recording medium relatively, discharges the ink droplets from the ink jet head, and records an image on the recording medium. 
   With respect to the ink jet head, according to the discharge method of ink droplets, a Piezo system, a thermal system, an electrostatic system or the like is known. 
   In recent years, a printer including an ink jet head in which plural nozzles are provided in a line has been developed. Such a printer has a merit that high speed printing can be performed. On the other hand, when the ink jet head is elongated, it is difficult to keep the amounts of droplets as the discharge characteristics of the respective droplets to be uniform by reason of manufacture or material properties. Thus, there is a problem that uneven density occurs in a recorded image and the picture quality is liable to be degraded. 
   In order to solve the foregoing problem, various techniques are proposed. 
   A switching device is provided which drives actuator elements arranged correspondingly to plural nozzles individually. The waveforms of voltages supplied to the actuator elements are adjusted, so that variations in the actuator elements are eliminated, and the volumes of discharged ink droplets are made uniform with respect to the respective nozzles (JP-A-2003-170588). 
   In an ink jet recording apparatus to form an image based on print data, the discharge pattern of ink discharge is selected from plural waveform patterns and printing is performed (JP-A-2005-153378). 
   Print conditions are previously determined, which include the presence or absence of variation correction of each nozzle, the presence or absence of variation correction of average discharge characteristics of respective groups when plural liquid droplet discharge characteristics are divided into plural groups, and the presence or absence of gradation printing. Further, plural drive waveforms for driving the respective driving elements are determined according to the print conditions. Waveform application means selects the drive waveform according to the print conditions and applies it to the driving element. By this, a reduction in picture quality due to variations in discharge characteristics of droplet nozzles is prevented (JP-A-2006-198902). 
   BRIEF SUMMARY OF THE INVENTION 
   An ink jet head driving apparatus according to a first aspect of the invention is an ink jet head driving apparatus for driving an ink jet head having plural nozzles to discharge supplied ink, and includes actuators provided correspondingly to the respective nozzles and to cause corresponding amounts of ink to be discharged from the nozzles by drive signals, a storage unit configured to store correction data for equalizing the ink discharge amounts from the respective nozzles, a selection unit configured to select one drive signal from the plural drive signals based on the correction data, and a drive unit configured to output the selected drive signal to the actuator at a specified timing, in which the nozzles of the ink jet head are classified into plural groups correspondingly to ink discharge amount characteristics of the nozzles, and the correction data is determined for each of the plural classified groups of the nozzles. 
   An ink jet head driving apparatus according to a second aspect of the invention is an ink jet head driving apparatus for driving an ink jet head having plural nozzles to discharge supplied ink, and includes a nozzle driving device for each of plural blocks obtained by dividing the ink jet head, in which the nozzle driving device includes actuators provided correspondingly to the respective nozzles and to cause corresponding amounts of ink to be discharged from the nozzles by drive signals, a storage unit configured to store correction data for the respective blocks and for equalizing the ink discharge amounts from the respective nozzles, a selection unit configured to select one drive signal from the plural drive signals based on the correction data, and a drive unit configured to output the selected drive signal to the actuator at a specified timing, and in which the nozzles in the block are classified into plural groups correspondingly to ink discharge amount characteristics of the nozzles, and the correction data is determined for each of the plural classified groups of the nozzles. 
   An ink jet head driving method according to a third aspect of the invention is an ink jet head driving method for an ink jet head driving apparatus including an ink jet head having plural nozzles to discharge supplied ink, and actuators provided correspondingly to the respective nozzles and to cause corresponding amounts of ink to be discharged from the nozzles by drive signals, and includes classifying the nozzles of the ink jet head into plural groups correspondingly to ink discharge amount characteristics of the nozzles, determining, for the respective plural groups of the nozzles, correction data for equalizing the ink discharge amounts from the respective nozzles, storing the correction data, selecting one drive signal from the plural drive signals based on the correction data, and outputting the selected drive signal to the actuator at a specified timing. 
   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 SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
       FIG. 1  is a view showing a printing apparatus including a line ink jet head. 
       FIG. 2  is a view showing a structure of an ink jet head driving circuit. 
       FIG. 3  is a view showing a detailed structure of the ink jet head driving circuit. 
       FIG. 4  is a view showing a relation between a print dot diameter and an actuator drive voltage. 
       FIG. 5  is a view showing nozzle positions of a head. 
       FIG. 6  is a view for explaining a creation method of correction data. 
       FIG. 7  is a view showing correction data in a case where an ink droplet amount varies smoothly. 
       FIG. 8  is a view showing correction data in a case where an ink droplet amount varies abruptly. 
       FIG. 9  is a view for explaining a creation method of correction data. 
       FIG. 10  is a view showing a printing apparatus in which printing results are photographed with a CCD camera and dot diameters are measured. 
       FIG. 11  is a view for explaining a creation method of correction data. 
       FIG. 12  is a view showing a structure of an ink jet head driving circuit of a print control unit. 
       FIG. 13  is a view showing a detailed structure of the ink jet head driving circuit. 
       FIG. 14  is a view for explaining a creation method of correction data. 
       FIG. 15  is a view for explaining a creation method of correction data. 
       FIG. 16  is a view showing correction data D in a case where variations in the amounts of discharged ink droplets are small. 
       FIG. 17  is a view for explaining a method of detecting local heat generation. 
       FIG. 18  is a view for explaining the occurrence of minute density difference. 
       FIG. 19  is a view for explaining a method of eliminating minute density difference occurring in a changed portion of actuator drive voltage. 
       FIG. 20  is a view for explaining a method of driving a long ink jet head. 
       FIG. 21  is a view showing a structure of an ink jet head driving circuit. 
       FIG. 22  is a view showing readout timing. 
       FIG. 23  is a view showing a structure of an ink jet head driving circuit. 
       FIG. 24  is a view showing a detailed structure of the ink jet head driving circuit. 
       FIG. 25  is a view showing a relation between a print dot diameter and an actuator drive pulse width. 
       FIG. 26  is a view for explaining a creation method of correction data. 
       FIG. 27  is a view showing a printing apparatus in which an image of printing result is captured by a scanner and dot diameters are measured. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   FIRST EMBODIMENT 
   A first embodiment of the invention will be described with reference to the drawings. 
     FIG. 1  shows a printing apparatus including a line ink jet head. 
   The printing apparatus includes an ejection unit  10 , an ink jet head  11 , a print control unit  12 , an ink supply system  13 , a transport belt  14 , a drive roller  15 , a charging roller  17 , a paper feed unit  18  and a paper feed roller  19 . 
   The print control unit  12  controls the print operation of the ink jet head  11 . The charging roller  17  charges the transport belt  14  to cause a recording medium  16  to be adsorbed to the transport belt  14 . The paper feed roller  19  sends out the recording medium  16  from the paper feed unit  18 . 
   The recording medium  16  is taken out by the paper feed roller  19  of the paper feed unit  18 , is adsorbed to the transport belt  14 , and then is transported by the transport belt  14 . At this time, previously created print data is transferred to the ink jet head  11 . The ink jet head  11  controls the print operation based on the print data, and records an image on the recording medium  16 . The recorded recording medium  16  is ejected to the ejection unit  10 . 
     FIG. 2  is a view showing a structure of an ink jet head driving circuit of the print control unit  12 . 
   The ink jet head  11  includes actuators  21 . The actuators  21  are provided correspondingly to the respective nozzles of the ink jet head  11 , and the number thereof is N equal to the number N of the nozzles. The actuators  21  are driven so that the amounts of droplets discharged from the nozzles are controlled. 
   The print control unit  12  includes a driving circuit  22 , a selection circuit  23 , and a correction data storage unit  24 . The driving circuit  22  drives the actuator  21  of the ink jet head  11 . The correction data storage unit  24  stores correction data D for the respective actuators. 
   Various signals for controlling the printing are inputted to the respective units of the print control unit  12 . Drive voltages  26  are inputted to the selection circuit  23 . Print data  25  and a print pulse signal  27  are inputted to the driving circuit  22 . 
   The drive voltages  26  are m kinds (V 1 -Vm) of drive voltages for driving the actuators  21  of the ink jet head  11 . The print data  25  is the data for driving the actuators  21  of the ink jet head  11  to discharge ink. The print pulse signal  27  is the signal for adjusting the print timing, and the actuator  21  of the ink jet head  11  is driven in accordance with this print pulse signal  27 . 
   The correction data D stored in the correction data storage unit  24  and corresponding to the respective actuators  21  of the ink jet head  11  are read out and are supplied to the selection circuit  23 . In the selection circuit  23 , in accordance with the correction data D of the respective actuators  21 , the one drive voltage  26  of the voltages of V 1 -Vm of the drive voltages  26  is selected. The selected drive voltage is supplied to the actuator  21  at the timing of the print pulse signal  27 . 
     FIG. 3  shows a detailed structure of the ink jet head driving circuit. The n actuators  21 , the n driving circuits  22 , the n selection circuits  23 , and the n correction data storage units  24  are prepared, where n is equal to the number of the actuators.  FIG. 3  shows the circuit portion for one actuator. 
   In the correction data storage unit  24 , the correction data D for the actuator  21  is stored in the form of information of, for example, 2 bits. The 2-bit information is read out from the correction data storage unit  24 , is supplied to the selection circuit  23 , and is inputted to the decoder. Only one of four selection signals S 11 -S 14  outputted from the decoder becomes “H” in accordance with the correction data D. 
   The print data  25  serially transmitted from the outside is decomposed into print data for the respective actuators  21  by a shift register (not shown) of the driving circuit  22 . The print pulse signal  27  and the print data  26  decomposed into the data for the respective actuators  21  generate a drive pulse P 1  through an AND circuit in the driving circuit  22 . The drive pulse P 1  and the selection signals S 11 -S 14  are connected to a switching element through AND circuits of the selection circuit  23 . This switching element is connected so as to select one of the drive voltages (V 1 -V 4 )  26  supplied from the outside of the selection circuit  23 . 
   As a result, the drive voltage  26  selected by the decoder based on the correction data D is supplied to the drive circuit  22  at the timing of the drive pulse P 1 . The supplied voltage is directly supplied to the actuator  21  and is also supplied to the discharge circuit at the same time. In this way, according to the ink jet head driving circuit, the voltage can be selectively changed in accordance with the correction data D. 
   Incidentally, the pulse width of the print pulse signal  27  has only to be set so that the largest amount of ink droplet is discharged when ink is discharged. 
   Next, a creation method of the correction data D will be described. 
   The correction data D is obtained correspondingly to the ink discharge amount at the time when the actuator  21  of the ink jet head  11  is driven. As a method of obtaining the discharge amount of ink, there is a method of using a print dot diameter at the time when an image is formed with single dots on the recording medium  16 , a line width of an image at the time when the image is formed with continuous dots by scanning the recording medium  16  and the ink jet head  11  relatively, volume calculation by an image pickup and image processing of an ink droplet discharged from the ink jet head  11 , or the like. Hereinafter, as an example, a method of obtaining a correction amount in accordance with a dot diameter will be described. 
     FIG. 4  shows a relation between a print dot diameter and an actuator drive voltage. As the drive voltage becomes high, the amount of discharged ink droplet becomes large, and as a result, the print dot diameter becomes large. 
     FIG. 5  shows nozzle positions of the head. In the drawing, N nozzles are provided. The nozzle positions are denoted by # 1 , . . . , #N. 
     FIG. 6(A)  shows measurement results of print dot diameters corresponding to the nozzle positions of the ink jet head  11 . This drawing shows an example in which all the actuators  21  of the ink jet head  11  are driven to form an image and the dot diameters are measured. 
   Next, the maximum value and the minimum value of the dot diameter actual measurement results are obtained and they are divided into groups. With respect to the number of the divided groups at this time, since the correction data D explained in  FIG. 3  has 2 bits in the example, a description will be given to an example in which division into four parts is performed. 
   In  FIG. 6(B) , the relation between the print dot diameter and the actuator drive voltage explained in  FIG. 4  is arranged next to  FIG. 6(A)  and is made to correspond thereto. An actuator drive voltage VH of a portion where the print dot diameter becomes the maximum value and an actuator drive voltage VL of a portion where the print dot diameter becomes the minimum value are obtained. The voltage of (VH-VL) is divided into four equal parts and is divided into four groups. The center voltages of the respective groups are set to V 1 , V 2 , V 3  and V 4  in ascending order. 
   Based on the measurement results of the print dot diameters, an actuator drive voltage is determined from the grouped voltages V 1 -V 4 . A portion belonging to a group in which the print dot diameter is large is given the actuator drive voltage V 1 , and a portion belonging to a group in which the print dot diameter is small is given the actuator drive voltage V 4 .  FIG. 6(C)  shows the relation between the nozzle position and the actuator drive voltage. 
   As a result, the correction data D is stored in the correction data storage unit  24  such that it is “00”B when the actuator drive voltage is V 1 , “01”B when the actuator drive voltage is V 2 , “10”B when the actuator drive voltage is V 3 , and “11”B when the actuator drive voltage is V 4 , and the actuator is driven in accordance with this. 
   Incidentally, as another method, a method is also easily conceivable in which all actuators are divided into plural parts, and the correction data D is created in this divided range. In the application of this method, there is a point to which attention is to be paid. Although this method can deal with the ink jet head  11  having relatively gentle variations in ink droplet amounts, in the case where the change is abrupt, the correction accuracy becomes poor.  FIG. 7  shows correction data in the case where the ink droplet amount varies gently, and  FIG. 8  shows correction data in the case where the ink droplet amount varies abruptly. 
   Further, another method will be described. In the above description, with respect to the division of the actuator drive voltages V 1 -V 4 , the range between the voltage of the portion where the print dot becomes maximum and the voltage of the portion where the print dot becomes minimum is divided into four equal parts and the correction data D is created.  FIG. 9  is a view for explaining the another method. 
   Since  FIG. 9(A)  is the same as  FIG. 6(A) , its description will be omitted. Next, the maximum value and the minimum value of dot diameter actual measurement results are obtained and they are divided into groups. With respect to the number of the divided groups at this time, since the correction data D explained in  FIG. 3  has 2 bits in the example, a description will be given to an example in which division into four parts is performed. 
   In  FIG. 9(B) , the relation between the print dot diameter and the actuator drive voltage explained in  FIG. 4  is arranged next to  FIG. 9(A)  and is made to correspond thereto. An actuator drive voltage VH of a portion where the print dot diameter becomes the maximum value and an actuator drive voltage VL of a portion where the print dot diameter becomes minimum value are obtained. 
   The range between the maximum diameter and the minimum diameter of the print dot diameter is divided into four equal parts and is divided into four groups. The center voltages of the respective groups are set to be V 1 , V 2 , V 3  and V 4  in ascending order. 
   Based on the measurement result of the print dot diameter, an actuator drive voltage is determined from the grouped voltages V 1 -V 4 . A portion belonging to a group in which the print dot diameter is large is given the actuator drive voltage V 1 , and a portion belonging to a group in which the print dot diameter is small is given the actuator drive voltage V 4 .  FIG. 9(C)  shows the relation between the nozzle position and the actuator drive voltage. 
   As a result, the correction data D is stored in the correction data storage unit  24  such that it is “00”B when the actuator drive voltage is V 1 , “01”B when the actuator drive voltage is V 2 , “10”B when the actuator drive voltage is V 3 , and “11B” when the actuator drive voltage is V 4 , and the actuator is driven in accordance with this. 
   Incidentally, with respect to the division number in the above, since the correction data of  FIG. 3  has 2 bits, the division into four parts is performed, however, when the correction data D has 3 bits, division into eight parts may be performed. However, when the division number is made large, the amount of the correction data D is also increased in accordance with that, and therefore, it is desirable that the data has approximately 2 bits or 3 bits. 
   Besides, in the above description, the example has been described in which the variations in the ink droplet amounts are corrected by using the print dot diameter. However, no limitation is made to this example, and it can be similarly performed by measuring the line width of a straight line formed with continuous ink droplets discharged from the respective actuators. 
   Further, the ink droplet directly discharged from the ink jet head  11  is stroboscopically photographed to obtain the volume of the ink droplet amount, and the variations in the ink droplet amounts may be obtained based on this measurement. 
   Further, it is also possible to adjust the correction data D while printed dots are measured. 
     FIG. 10  shows a printing apparatus in which a print result is photographed by a CCD camera, and dot diameters thereof are measured. An image read by the CCD is supplied to an image processing unit. The image processing unit performs a correction such as a shading correction and performs binarization. Based on this image processing result, the image processing unit measures the diameters of dots formed by ink droplets discharged from respective actuators. The dot diameters of the measurement results are supplied to a print control unit  12 . The print control unit  12  creates correction data D and adjusts actuator drive voltages. 
     FIG. 11(A)  shows the measurement result of print dot diameters corresponding to nozzle positions of an ink jet head  11 . 
   Two kinds of variations exist in a curved line of the measurement result. The first variation is a variation occurring in a nozzle array direction of a line head and having a relatively low frequency component. A second variation is a variation caused by the variation of adjacent actuators and having a relatively high frequency component. 
   Because of the second variation, there is a case where when correction is performed for each actuator, unevenness in discharge volume becomes noticeable by contraries. In  FIG. 11(B) , a part of  FIG. 11(A) , that is, a portion encircled by a circle is enlarged and shown. At a boundary portion produced when grouping is performed, voltages to drive adjacent actuators are alternately changed. When this portion performs image formation, a noticeable result is obtained. Then, the print dot diameters are movement-averaged in the nozzle position direction, and the correction data D is created based on this result. By performing the processing in this way, as shown in  FIG. 11(C) , the curved line is smoothed, and unevenness of an image can be made unnoticeable. 
   SECOND EMBODIMENT 
   In a second embodiment, the same portions as those of the first embodiment are denoted by the same symbols and their description will be omitted. 
     FIG. 12  is a view showing a structure of an ink jet head driving circuit of a print control unit  12  according to the second embodiment. 
   The second embodiment is different from the first embodiment in that a print control unit  12  further includes a D/A converter  71 . Since the structure of the other portions is the same as that of  FIG. 2 , their detailed description will be omitted. 
   The D/A converter  71  generates plural kinds of actuator drive voltages V 1 -Vm. The type of the actuator drive voltage generated by the D/A converter  71  is outputted from a correction data storage unit  24 . 
     FIG. 13  shows a detailed structure of the ink jet head driving circuit according to the second embodiment. A 3-bit designation line for designating the kind of the actuator drive voltage to be generated is provided between the correction data storage unit  24  and the D/A converter  71 . 
   Designation data S of 3-bit information for the D/A converter  71  is stored in the correction data storage unit  24 . The 3-bit information is read out from the correction data storage unit  24  and is supplied to the D/A converter  71 . In accordance with the designation data S, the D/A converter  71  generates the drive voltage designated by 3 bits. The D/A converter  71  generates four kinds of actuator drive voltages. An actuator drive voltage group selected by the correction data D is selected, and the selected actuator drive voltage group is supplied to a selection circuit  23 . 
   Incidentally, since the operations of the other circuits are similar to those of the first embodiment, their detailed description will be omitted. 
     FIG. 14  is a view for explaining a creation method of the correction data according to the second embodiment.  FIG. 14(A)  shows measurement results of print dot diameters corresponding to nozzle positions of the ink jet head  11 . This drawing shows an example in which all the actuators  21  of the ink jet head  11  are driven to form an image and the dot diameters are measured. Incidentally, with respect to the results of the measurement of two heads, the result of the first head is represented by a broken line, and the result of the second head is represented by a solid line. As stated above, the maximum value and the minimum value of the dot diameter measurement results are obtained and they are divided into groups. With respect to the number of the divided groups at this time, since the correction data D explained in  FIG. 3  has 2 bits, the division into four parts is performed. 
   In  FIG. 14(B) , the relation between the print dot diameter and the actuator drive voltage explained in  FIG. 4  is arranged next to  FIG. 14(A)  and is made to correspond thereto. 
   An actuator drive voltage VH of a portion where the print dot diameter becomes the maximum value and an actuator drive voltage VL of a portion where the print dot diameter becomes the minimum value are obtained. As shown in  FIG. 14(A) , since the broken line (first head) and the solid line (second head) are different from each other, the actuator drive voltages VL and VH are respectively different from each other. 
   With respect to the broken line (first head), the actuator drive voltage of the portion where the print dot diameter becomes the maximum value is made VH 1 , and the actuator drive voltage of the portion where the print dot diameter becomes the minimum value is made VL 1 . With respect to the solid line (second head), the actuator drive voltage of the portion where the print dot diameter becomes the maximum value is made VH 2 , and the actuator drive voltage of the portion where the print dot diameter becomes the minimum value is made VL 2 . 
   With respect to each of the broken line (first head) and the solid line (second head), the voltage is divided into four equal parts and is divided into four groups. Center voltages of the groups of the broken line (first head) are made V 11 , V 12 , V 13  and V 14  in ascending order. Center voltages of the groups of the solid line (second head) are made V 21 , V 22 , V 23  and V 24  in ascending order. 
   Based on the measurement result of the print dot diameter, an actuator drive voltage is determined from the grouped voltages V 11 -V 14  and V 21 -V 24 . In the broken line (first head), a portion where the print dot diameter is large is given the actuator drive voltage V 11 , and a portion where the print dot diameter is small is given the actuator drive voltage V 14 . In the solid line (second head), a portion where the print dot diameter is large is given the actuator drive voltage V 21 , and a portion where the print dot diameter is small is given the actuator drive voltage V 24 . 
     FIG. 14(C)  shows the relation between the nozzle position and the actuator drive voltage. The actuators are driven by the actuator drive voltages indicated by the broken line (first head) and the solid line (second head). The actuator drive voltages V 11 -V 14  and V 21 -V 24  different from each other at this time are respectively stored in the D/A converter  71 . When correction is performed, the designation data S corresponding to the actuator drive voltage is set in the D/A converter  71 , and V 11 -V 14  and V 21 -V 24  are generated. Incidentally, with respect to the correction of each of the actuators, the readout is performed from the correction data storage unit  24  similarly to the case explained in  FIG. 5  and the driving is performed in accordance with the actuator drive voltage. 
   As described above, since the D/A converter  71  is provided and the actuator drive voltage can be adjusted according to the characteristic of the head, even if the actuator drive voltage varies for each head, the adjustment can be performed. 
   Incidentally, in the invention, although the example has been described in which the actuator drive voltage is divided, a system in which the print dot diameter is divided may be adopted. 
   THIRD EMBODIMENT 
   A third embodiment is different from the first embodiment in a creation method of correction data D. Accordingly, the same portions as those of the first embodiment are denoted by the same symbols and their detailed description will be omitted. 
   In the first embodiment, the correction data D is determined from the actuator drive voltage VH of the portion where the print dot diameter becomes the maximum value and the actuator drive voltage VL of the portion where the print dot diameter becomes the minimum value. However, the correction accuracy varies according to variations in ink jet heads. 
     FIG. 15  is a view for explaining the creation method of the correction data D. 
     FIG. 15(A)  shows measurement results of print dot diameters corresponding to nozzle positions of an ink jet head  11 . This drawing shows an example in which all actuators  21  of the ink jet head  11  are driven to form an image and dot diameters are measured. 
   In  FIG. 15(B) , the relation between the print dot diameter and the actuator drive voltage explained in  FIG. 4  is arranged next to  FIG. 15(A)  and is made to correspond thereto. The actuator drive voltage is previously divided into equal parts. The actuator drive voltage is selected correspondingly to variations in the amounts of ink droplets of the respective actuators  21 . Since the correction data D explained in  FIG. 3  has 2 bits, the actuator drive voltage is previously divided into four or more parts. For example, it is divided into six parts, and respective divided reference voltages are made V 1   a -V 6   a . The actuator drive voltages V 1 -V 4  are selected from V 1   a -V 6   a.    
     FIG. 15(C)  shows the relation between the nozzle position and the actuator drive voltage. The actuator drive voltage is selected according to the print dot diameter actual measurement result and from the previously divided voltages. From this drawing, V 2   a  is selected for V 1 , V 3   a  is selected for V 2 , V 4   a  is selected for V 3 , and V 5   a  is selected for V 4 . When correction is performed, the selection data D corresponding to this actuator drive voltage is set in a D/A converter  71 , and V 1 -V 4  (V 2   a -V 5   a ) are generated. 
   As a result, the correction data D is stored in the correction data storage unit  24  such that it is “00”B when the actuator drive voltage is V 1 , “01”B when the actuator drive voltage is V 2 , “10”B when the actuator drive voltage is V 3 , and “11”B when the actuator drive voltage is V 4 . The actuator is driven in accordance with the correction data D. 
     FIG. 16  shows the correction data D in a case where variations in the amounts of discharged ink droplets are small. In this case, the actuator drive voltage comes to have one kind, and even if correction is performed, an improvement is not made. Accordingly, in this case, the correction is not performed. 
   Incidentally, in the foregoing description, although the system of dividing the actuator drive voltage has been described, no limitation is made to this mode, and a system of dividing the print dot diameter may be adopted. 
   FOURTH EMBODIMENT 
   In the fourth embodiment, the same portions as those of the first embodiment are denoted by the same symbols and their description will be omitted. 
   In the method described in the first embodiment, when the respective actuator drive voltages are adjusted in accordance with the correction data D and the ink droplet amounts are corrected, in the case printing is continuously performed at the same position of the recording medium  16 , the actuator at the portion generates heat and the ink droplet amount becomes large. As a result, local unevenness occurs in the corrected ink droplet amount. In the fourth embodiment, a method of correcting the local unevenness generated by such heat generation will be described. 
   The local heat generation portion is detected, and correction data D is rewritten correspondingly to the portion. As a method of detecting the local heat generation portion, for example, when image formation is performed, a portion of the actuator driven so as to continuously discharge ink has only to be detected. 
     FIG. 17  is a view for explaining the method of detecting the local heat generation. 
   A print control unit  12  is newly provided with a line memory. Print data  25  is inputted to a driving circuit  22  and is also inputted to the line memory. The print data  25  for the past n lines is stored in the line memory. With respect to the print data  25  for the n lines, n data corresponding to each actuator position are subjected to an AND operation. The operation result is made a temperature correction signal for correcting influence due to temperature. The correction data D of each actuator is adjusted based on the temperature correction signal. For example, when the temperature correction signal becomes ON, since printing is continuously performed for the n lines, an adjustment is made so that the correction data D becomes a voltage lower by one level. 
   In the foregoing description, a portion where image data is continuous is detected, and the correction voltage of the portion is adjusted, however, the invention is not limited to this embodiment. As shown in  FIG. 10 , an increase in ink droplet due to a temperature rise may be detected from an image photographed by the CCD camera attached to the printing apparatus. 
   FIFTH EMBODIMENT 
   In a fifth embodiment, the same portions as those of the first embodiment are denoted by the same symbols and their description will be omitted. 
   In the method as described in the first embodiment, when the ink droplet amount is corrected by adjusting the respective actuator drive voltages in accordance with the correction data D, a minute density difference occurs in a changing portion of the corrected actuator drive voltage. 
     FIG. 18  is a view for explaining the occurrence of the minute density difference. Since  FIG. 18(A) ,  FIG. 18(B)  and  FIG. 18(C)  are the same as  FIG. 5 , their description will be omitted. 
     FIG. 18(D)  shows diameters of print dots printed as a result of image formation in which actuator drive voltages V 1 -V 4  of  FIG. 18(C)  are used and ink droplets are made to fly. As compared with  FIG. 18(A) , variations in the nozzles can be suppressed to be small with respect to the whole head. However, in the voltage changing portion, the ink droplet correction is not satisfactory, and a stepped portion (density difference) occurs. In  FIG. 19 , a circle portion of  FIG. 18(D)  is enlarged and shown. 
     FIG. 19  is a view for explaining a method of eliminating the minute density difference occurring in the changing portion of the actuator drive voltage. 
   The boundary position of actuators is moved horizontally so that the actuator of the boundary portion is not corrected continuously with the same correction voltage, that is, the boundary portion does not become continuous. As stated above, the correction data D is changed each time one line is printed, and the continuity at the boundary portion where the actuator drive voltage is changed is eliminated, so that the density difference does not become noticeable, and the correction accuracy can be improved. 
   Incidentally, in an edge portion of an image, a phenomenon in which the end becomes dense occurs due to the occurrence of cross-talk, not due to the change of the actuator drive voltage. Also in this case, when the edge portion is moved horizontally, the density difference can be made not noticeable. 
   Besides, according to this system, for example, correction of the amount of ink droplet discharged from the ink jet head  11  using a multi-drop system and capable of performing gradation printing can be performed similarly. 
   SIXTH EMBODIMENT 
   In a sixth embodiment, the same portions as those of the first embodiment are denoted by the same symbols and their detailed description will be omitted. 
   In the sixth embodiment, a driving method of a long ink jet head will be described. In the case where correction is performed over the whole long ink jet head by the method described in the first embodiment, the correction accuracy of the discharge variation is reduced. This is because in the long ink jet head  11 , not only the working accuracy thereof, but also variations in the material itself can not be neglected, and for example, variations in the maximum value and the minimum value of the amounts of discharged ink droplets become large as compared with the short head. 
     FIG. 20  is a view for explaining a method of driving the long ink jet head. 
   The driving range of the long ink jet head is divided into plural parts, and correction is performed so that the ink droplet amounts become uniform in each of the divided ranges. In this case, a combination may be made with the method of the second embodiment in which the D/A converter  71  is included, or the method of the third embodiment in which when the driving range is grouped, the adjustment can be performed in the range wider than the grouping number. The high accuracy correction becomes possible by combining these methods with the long ink jet head. Incidentally, since the details of the correction have been described in the second embodiment and the third embodiment, the duplicate description will be omitted. 
   SEVENTH EMBODIMENT 
   In a seventh embodiment, the same portions as those of the first embodiment are denoted by the same symbols and their description will be omitted. 
   In the seventh embodiment, correction data D is stored in an ink jet head  11 . The correction data D is read out from the ink jet head  11  and is stored in a correction data storage unit  24 . 
     FIG. 21  is a view showing a structure of an ink jet head driving circuit of a print control unit  12  according to the seventh embodiment. 
   In the ink jet head  11 , a broken line portion is a connector and can be detached. The correction data D is written in the ink jet head  11 . For example, a PROM (Programmable Read-Only Memory) is used, and the correction data is written at the time point of manufacture. With respect to the creation of the correction data D, the method described in the first embodiment is used. Besides, the correction data storage unit  24  includes a RAM (Random Access Memory). 
   The readout operation of the correction data D will be described.  FIG. 22  is a view showing readout timing. A correction data readout signal rises at the time of turning on power, and a readout mode occurs. Next, the correction data D is read out from the PROM in synchronization with a clock, and is directly written into the correction data storage unit  24 . Incidentally, necessary data is included in the correction data D correspondingly to the foregoing respective embodiments. For example, in the case where the D/A converter  71  of the second embodiment is used, the designation data S is included in the correction data D. 
   EIGHTH EMBODIMENT 
   An eighth embodiment is different from the first embodiment in that the amount of an ink droplet is controlled with the width of a drive pulse. Accordingly, in the eighth embodiment, the same portions as those of the first embodiment are denoted by the same symbols and their detailed description will be omitted. 
     FIG. 23  is a view showing a structure of an ink jet head driving circuit of a print control unit  12 . 
   An ink jet head  11  includes actuators  21 . The actuators  21  are provided to correspond to respective nozzles of the ink jet head  11 , and the number N thereof is equal to the number N of the nozzles. The actuator  21  is driven so that the amount of a droplet discharged from the nozzle is controlled. 
   The print control unit  12  includes a driving circuit  22 , a selection circuit  23 , and a correction data storage unit  24 . The driving circuit  22  drives the actuator  21  of the ink jet head  11 . The correction data storage unit  24  stores correction data D for the respective actuators. 
   Various signals for controlling the printing are inputted to the respective units of the print control unit  12 . Drive voltages  28  and drive voltage pulses  29  are inputted to the selection circuit  23 . Print data  25  is inputted to the driving circuit  22 . 
   The drive voltages  28  are the voltages for driving the actuators  21  of the ink jet head  11 . The drive voltage pulses  29  are m kinds (P 1 -Pm) of pulse signals for driving the actuators  21  of the ink jet head  11 . The actuators  21  of the ink jet head  11  are driven with the drive voltages  28  having the drive pulse widths. The print data  25  is the data for driving the actuators  21  of the ink jet head  11  to discharge ink. 
   The correction data D corresponding to the respective actuators  21  of the ink jet head  11  stored in the correction data storage unit  24  is read out and is supplied to the selection circuit  23 . In the selection circuit  23 , in accordance with the correction data D of the respective actuators  21 , one pulse signal is selected from P 1 -Pm of the drive pulses  29 . The selected pulse signal is supplied to the actuator  21  at the timing of the print pulse signal  27 . 
     FIG. 24  shows a detailed structure of an ink jet head driving circuit. The n actuators  21 , the n driving circuits  22 , the n selection circuits  23  and the n correction data storage units  24  are prepared, where n is equal to the number of actuators.  FIG. 24  shows a circuit portion for one actuator. 
   The correction data D of 2-bit information of the actuator  21  is stored in the correction data storage unit  24 . The 2-bit information is read out from the correction data storage unit  24 , is supplied to the selection circuit  23  and is inputted to the decoder. Only one of four selection signals S 11 -S 14  outputted from the decoder becomes “H” in accordance with the correction data D. 
   The actuator drive pulses  29  and the four selection signals S 11 -S 14  outputted from the decoder are inputted to an AND circuit. Thus, with respect to the actuator drive pulses  29 , only one pulse width is selected therefrom. 
   The print data  25  serially transmitted from the outside is decomposed into print data of the respective actuators  21  by a shift register (not shown) of the driving circuit  22 . The selected drive pulse  29  and the print data  25  decomposed into the data of the respective actuators  21  are connected to a switching element through an AND circuit in the selection circuit  23 . This switching element is connected so as to select the drive voltage  28  supplied from the outside of the selection circuit  23 . 
   As a result, the drive voltage  28  having the width of the drive pulse selected with the correction data D and by the decoder is supplied to the driving circuit  22 . The supplied voltage is directly supplied to the actuator  21  and is also simultaneously supplied to a discharge circuit. In this way, according to the ink jet head driving circuit, the drive pulse width can be selectively changed in accordance with the correction data D. 
   Next, a creation method of the correction data D will be described. 
   The correction data D is obtained correspondingly to the ink discharge amount at the time when the actuator  21  of the ink jet head  11  is driven. As a method of obtaining the ink discharge amount, there is a method using a print dot diameter at the time when an image is formed with single dots on a recording medium  16 , a line width of an image at the time when the image is formed with continuous dots by scanning the recording medium  16  and the ink jet head  11  relatively, volume calculation by an image pickup and image processing of an ink droplet discharged from the ink jet head  11  or the like. Hereinafter, the method of obtaining the correction amount in accordance with the dot diameter will be described as an example. 
     FIG. 25  shows a relation between a print dot diameter and an actuator drive pulse width. As the pulse width becomes wide, the amount of a discharged ink droplet becomes large, and as a result, the print dot diameter becomes large. 
     FIG. 26(A)  shows measurement results of print dot diameters corresponding to nozzle positions of the ink jet head  11 . This drawing shows an example in which all the actuators  21  of the ink jet head  11  are driven to form an image and the dot diameters are measured. 
   Next, the maximum value and the minimum value of the dot diameter actual measurement results are obtained, and they are divided into groups. With respect to the number of the groups at this time, since the correction data D explained in  FIG. 3  has 2 bits, the division into four parts is performed. 
   In  FIG. 26(B) , the relation between the print dot diameter and the actuator drive pulse width explained in  FIG. 25  is arranged next to  FIG. 26(A)  and is made to correspond thereto. An actuator drive pulse width PH of a portion where the print dot diameter becomes the maximum value and an actuator drive pulse width PL of a portion where the print dot diameter becomes the minimum value are obtained. The range of (PH-PL) is divided into four equal parts and is divided into four groups. The center pulse widths of the respective groups are set to be P 1 , P 2 , P 3  and P 4  in ascending order. 
   Based on the measurement result of the print dot diameter, an actuator drive pulse width is determined from the grouped pulse widths P 1 -P 4 . A portion belonging to a group in which the print dot diameter is large is given the actuator drive pulse width P 1 , and a portion belonging to a group where the print dot diameter is small is given the actuator drive pulse width P 4 .  FIG. 26(C)  shows the relation between the nozzle position and the actuator drive pulse width. 
   As a result, the correction data D is stored in the correction data storage unit  24  such that it is “00”B when the actuator drive pulse width is P 1 , “01”B when the actuator drive pulse width is P 2 , “10”B when the actuator drive pulse width is P 3 , and “11”B when the actuator drive pulse width is P 4 , and the actuator  21  is driven in accordance with this. 
   NINTH EMBODIMENT 
   In a ninth embodiment, the same portions as those of the first embodiment are denoted by the same symbols and their description will be omitted. 
     FIG. 27  shows a printing apparatus in which an image of a print result is captured by a scanner and its dot diameter is measured. The printing apparatus includes a reading device for reading an image. 
   The user causes a specified pattern for generating correction data D to be recorded on a recording medium  16 . The recorded recording medium  16  is set on the reading device, and an operation of reading the printed pattern is performed by the scanner. The image read by the reading device is supplied to an image processing unit. The image processing unit performs a correction such as a shading correction, and binarizes the image. Based on the image processing result, the image processing unit measures dot diameters formed by ink droplets discharged from the respective actuators. The dot diameters of the measurement results are supplied to a print control unit  12 . The print control unit  12  creates correction data D and adjusts the actuator drive voltage. 
   Incidentally, the specified pattern to be printed is not limited to one expressing the dot diameter, but may be one expressing the line width of a straight line, or may be a combination of these. 
   Although the respective embodiments of the invention have been described, the system of the ink jet head may be any of a Piezo system, a thermal system, and an electrostatic system. 
   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.