Abstract:
A droplet driving control device includes: a droplet ejection control unit which ejects droplets at requested droplet ejection periods; and an adjustment unit which adjusts control of the droplet ejection control unit using at least continuous two of the droplet ejection periods as one set based on an error of droplet speed with respect to a proper value thereof, so that droplets can be ejected at different droplet ejection periods within a range of the one set, and an average value of the droplet ejection periods within the range of the one set for ejecting the droplets can be equal to each of the requested droplet ejection periods.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2015-133318 flied on Jul. 2, 2015. 
     Background 
     1. Technical Field 
     The present invention relates to a droplet driving control device and an image forming apparatus. 
     2. Related Art 
     In an apparatus which ejects droplets of ink etc. to form an image, such as an inkjet continuous feed printer, a driving frequency for controlling timing of droplet ejection is set in accordance with image formation speed. 
     SUMMARY 
     According to an aspect of the invention, there is provided a droplet driving control device comprising: a droplet ejection control unit which ejects droplets at requested droplet ejection periods; and an adjustment unit which adjusts control of the droplet ejection control unit using at least continuous two of the droplet election periods as one set based on an error of droplet speed with respect to a proper value thereof, so that droplets can be ejected at different droplet ejection periods within a range of the one set, and an average value of the droplet ejection periods within the range of the one set for ejecting the droplets can be equal to each of the requested droplet election periods. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based or the following figures, wherein: 
         FIG. 1  is a schematic configuration diagram showing an example of a main configuration portion of a droplet ejection type recording apparatus according to an exemplary embodiment of the invention; 
         FIGS. 2A and 2B  are plan views showing a head and a sectional view showing an internal structure of each droplet ejecting element in the head according to the exemplary embodiment; 
         FIG. 3  is a block diagram of a control portion according to the exemplary embodiment; 
         FIG. 4  is a functional block diagram showing blocked parts of period adjustment control in the control portion according to the exemplary embodiment; 
         FIGS. 5A and 5B  are a droplet ejection driving frequency to droplet speed fluctuation amount characteristic graph and a droplet ejection period to droplet speed fluctuation amount characteristic graph respectively; 
         FIGS. 6A and 6B  are timing charts of driving waveforms for ejecting droplets according to the exemplary embodiment and a comparative example respectively; 
         FIG. 7  is a flow chart showing the flow of a droplet ejection period adjustment control routine according to the exemplary embodiment; 
         FIG. 8  is a timing chart showing details of correction of a driving waveform in a step  124  of  FIG. 7 ; and 
         FIGS. 9A, 9B, 9C and 9D  are a droplet election period to liquid speed fluctuation amount characteristic graph according to a modification, a timing chart of a driving waveform for ejecting droplets according to a comparative example, a timing chart of a driving waveform for ejecting droplets according to a modification (continuous ejection pattern 1), and a timing chart of a driving waveform for ejecting droplets according to a modification (continuous ejection pattern 2) respectively. 
     
    
    
     REFERENCE SIGNS LIST 
     
         
           10  droplet ejection type recording apparatus 
           12  ( 12 A,  12 B) image forming portion 
           14  control portion 
           16  paper supplying roll 
           18  discharging roll 
           20  feeding roller 
           22  ( 22 A,  22 B) head driving portion 
           24  ( 24 A,  24 B) head 
           26  ( 26 A,  26 B) drying device 
           24 AC,  24 AM,  24 AY,  24 AK head 
           24 BC,  24 BM,  24 BY,  24 BK head 
           30  droplet ejecting member 
           32  nozzle 
           34  pressure chamber 
           36  supply port 
           38  common passage 
           40  diaphragm 
           42  piezoelectric element 
           40 A common electrode 
           42 A individual electrode 
           50  CPU 
           52  RAM 
           54  ROM 
           56  I/O 
           58  bus 
           60  microcomputer 
           62  user interface (UI) 
           64  hard disk (HDD) 
           66  communication I/F 
           70  image formation instruction information accepting portion 
           72  image information importing portion 
           74  designated image formation speed information extracting portion 
           76  image formation pattern generating portion 
           78  droplet ejection period calculating portion 
           80  determination portion 
           82  image formation speed setting range storage portion 
           84  droplet ejection period to droplet speed characteristic table storage portion 
           86  ejection control selecting portion 
           88  adjusted ejection period generating portion 
           90  steady ejection period generating portion 
           92  driving waveform correcting portion 
           94  driving instruction portion 
       
    
     DETAILED DESCRIPTION 
     (Outline of Apparatus) 
       FIG. 1  is a schematic configuration diagram showing a main configuration portion of a droplet ejection type recording apparatus  10  as an example of an image forming apparatus according to an exemplary embodiment of the invention. 
     For example, the droplet ejection type recording apparatus  10  is provided with two image forming portions  12 A and  12 B, a control portion  14 , a paper supplying roll  16 , a discharging roll  18 , and a plurality of feeding rollers  20 . The two image forming portions  12 A and  12 B can form images on opposite surfaces of a paper sheet P in one feeding. 
     In addition, the image forming portion  12 A is provided with a head driving portion  22 A as an example of a droplet ejection control unit. Further, the image forming portion  12 A includes heads  24 A and a drying device  26 A. 
     Similarly, the image forming portion  12 B is provided with a head driving portion  22 B as an example of a droplet election control unit. Further, the image forming portion  12 B includes heads  24 B and a drying device  26 B. 
     Incidentally, there is a case where indication of a suffix “A” and a suffix “B” at the ends of signs may be omitted below when it is not necessary to distinguish between the image forming portion  12 A and the image forming portion  12 B and between common members included in the image forming portion  12 A and the image forming portion  12 B. 
     The control portion  14  drives a not-shown paper feeding motor to control rotation of the feeding rollers  20  which are, for example, connected to the paper feeding motor through a mechanism of gears etc. 
     A long paper sheet P is wound as a recording medium around the paper supplying roll  16 . The paper sheet P is fed in a direction of an arrow A (paper feeding direction) in  FIG. 1  in accordance with rotation of the feeding rollers  20 . 
     Upon acceptance of image information, the control portion  14  controls the image forming portion  12 A based on color information for each pixel of an image contained in the image information. Thus, the image corresponding to the image information is formed on one image formation surface of the paper sheet P. 
     Specifically, the control portion  14  controls the head driving portion  22 A. The head driving portion  22 A drives the heads  24 A connected to the head driving portion  22 A in accordance with droplet ejection timings instructed from the control portion  14 , so as to eject droplets as an example of droplets from the heads  24 A and form the image corresponding to the image information on the one image formation surface of the fed paper sheet P. 
     Incidentally, the color information for each pixel of the image included in the image information includes information expressing the color of the pixel uniquely. In this exemplary embodiment, assume that the color information for each pixel of the image is represented by respective concentrations of yellow (Y), magenta (M), cyan (C), or black (K). Another representation method for expressing the colors of the image uniquely may be used. 
     The heads  24 A include four heads  24 AC,  24 AM,  24 AY and  24 AK corresponding to the four colors, i.e. the Y color, the M color, the C color and the K color, respectively. Droplets of the corresponding colors are ejected from the respective heads  24 A. 
     The control portion  14  controls the drying device  26 A to dry the droplets of the image formed on the paper sheet P to thereby fix the image to the paper sheet P. 
     Then, the paper sheet P is fed to a position opposing to the image forming portion  12 B in accordance with rotation of the feeding rollers  20 . On this occasion, the paper sheet P is turned inside out and fed so that the other image formation surface different from the image formation surface on which the image has been formed by the image forming portion  12 A can face the image forming portion  12 B. 
     The control portion  14  also executes, on the image forming portion  12 B, similar control to the aforementioned control on the image forming portion  12 A. Thus, an image corresponding to the image information can be formed on the other image formation surface of the paper sheet P. 
     The heads  24 B include four heads  24 BC,  24 BM,  24 BY, and  24 BK corresponding to the four colors, i.e. the Y color, the M color, the C color and the K color, respectively. Droplets of the corresponding colors are ejected from the respective heads  24 B. 
     The control portion  14  controls the drying device  26 B to dry the droplets of the image formed on the paper sheet P to thereby fix the image to the paper sheet P. 
     Then, the paper sheet P is fed to the discharging roll  18  and wound around the discharging roll  18  in accordance with rotation of the feeding rollers  20 . 
     Incidentally, the configuration of the apparatus for forming images on front and back surfaces of a paper sheet P in one feeding starting at the paper supplying roll  16  and ending at the discharging roll  18  has been described as the droplet ejection type recording apparatus  10  according to this exemplary embodiment. It is however a matter of course that the droplet ejection type recording apparatus  10  may be a droplet ejection type recording apparatus for forming an image on a single surface. 
     In addition, ink as an example of a droplet includes water-based ink, oil-based ink serving as ink containing a solvent which can be evaporated, ultraviolet-curable type ink, etc. However, assume that water-based ink is used in the this exemplary embodiment. When it is mentioned as “ink” or “droplet” simply in this exemplary embodiment, it may imply “water-based ink” or “water-based ink droplet”. 
     (Head  24 ) 
     As shown in  FIG. 2A , each of the heads  24  applied to the image forming portion  12  has droplet ejecting members  30  which are arranged in a longitudinal direction of the head. Incidentally, the longitudinal direction of the head is a direction intersecting with a feeding direction of the paper sheet P (a direction of an arrow A in  FIG. 2A ), and may be referred to as main scanning direction. In addition, the feeding direction of the paper sheet P (the direction of the arrow A in  FIG. 2A ) may be referred to as sub-scanning direction. 
     The layout of the droplet ejecting members  30  is not limited to a single array line in the main scanning direction. In some dot pitch (resolution), a plurality of array lines of droplet ejecting members  30  provided in the sub-scanning direction may be arrayed two-dimensionally in accordance with predetermined rules so that ejection timing in each array line can be controlled in accordance with the array line pitch and feeding speed of the paper sheet P. 
     As shown in  FIG. 2B , the droplet ejecting members  30  are provided with nozzles  32  and pressure chambers  34  corresponding to the nozzles  32  respectively. 
     A supply port  36  is provided in each of the pressure chambers  34 . The pressure chambers  34  are connected to a common passage (common passage  38 ) through the supply ports  36 . 
     The common passage  38  has a role of receiving supply of ink from an ink supply tank (not shown) as an ink supply source and distributing the received supply of the ink to the respective pressure chambers  34 . 
     A diaphragm  40  is attached to an upper surface of a ceiling portion of the pressure chamber  34  in each droplet ejecting member  30 . In addition, a piezoelectric element  42  is attached to the upper surface of the ceiling portion of the pressure chamber. The diaphragm  40  is provided with a common electrode  40 A. The piezoelectric element  42  is provided with an individual electrode  42 A. When a voltage is selectively applied between the individual electrode  42 A of the piezoelectric element  42  and the common electrode  40 A, the selected piezoelectric element  42  is deformed so that a droplet can be ejected from the nozzle  32  and new ink can be supplied from the common passage  38  to the pressure chamber  34 . 
     Each of the head driving portions  22  ( 22 A and  22 B) is controlled by the control portion  14  (see  FIG. 1 ) based on the image information to generate a driving signal for applying a voltage to each of the individual electrodes  42 A of the piezoelectric elements  42  independently. 
     To eject each droplet, image formation speed (droplet ejection period) which can guarantee designated image quality can be set in a predetermined setting range (particularly with a maximum image formation speed Vmax as an upper limit). 
     Incidentally, a lower limit of the setting range is not particularly limited. Theoretically, it will go well as long as the lower limit of the setting range is a positive number (a number larger than 0). In addition, the setting may include one or both of paper feeding speed and the resolution in addition to the image formation speed. 
     When there is a change in the setting of the image formation speed, frequency control (droplet ejection period control) is executed on each of the heads  24  by the head driving portion  22 . 
     As shown in  FIG. 3 , the control portion  14  is equipped with a microcomputer  60 . The microcomputer  60  is provided with a CPU  50 , an RAM  52 , an ROM  54 , an I/O  56 , and a bus  58 . The bus  58  such as a data bus or a control bus connects the CPU  50 , the RAM  52 , the ROM  54  and the I/O  56  to each other. 
     A user interface (UI)  62 , a hard disk (HDD)  64 , and a communication I/F  66  which is performed by radio (or cable) are connected to the I/O  56 . In addition, a device I/F  68  which serves as a connection terminal to any of external devices (the head driving portions  22  and the drying devices  26  in this exemplary embodiment) is connected to the I/O  56 . 
     Here, in a specific high-frequency band exceeding the upper limit (Vmax) which can guarantee the image quality, droplet speed or a droplet amount fluctuates in accordance with residual pressure vibration (see a frequency band fm in  FIG. 5A  and a period range width Tm in  FIG. 5B ) of each piezoelectric element  42 . Therefore, the image formation speed is limited to the setting range (upper limit) which is not affected by the pressure vibration. 
     In other words, at an image formation speed exceeding a frequency corresponding to the maximum image formation speed Vmax serving as the upper limit, a landing position of the droplet on the paper sheet P or the size of the landed droplet varies to thereby lower the image quality. 
     On the other hand, in this exemplary embodiment, control for suppressing the fluctuation in the droplet speed or the droplet amount is constructed in the frequency band in which the droplet speed or the ink droplet amount fluctuates (the specific high-frequency band exceeding the frequency corresponding to the maximum speed Vmax). 
     That is, in this exemplary embodiment, period adjustment control is executed in the following control procedures in the control portion  14 . 
     (Control Procedure 1) When a droplet ejection frequency (droplet ejection period) is determined in accordance with image formation speed, determination is made as to whether residual pressure vibration is less than ±5% or not, based on  FIG. 5A  or  FIG. 5B . 
     (Control Procedure 2) When the residual pressure vibration is in a range of not less than ±5%, a period Tf1 and a period Tf2 are generated as shown in  FIG. 6A . The period Tf1 is shorter by Tc/4 than a designated droplet ejection period Tf0. The period Tf2 is longer Tc/4 than the designated droplet ejection period Tf0. Incidentally, Tc is a period of the residual pressure vibration in  FIG. 5B  so as to be consistent with Tf0. 
     (Control Procedure 3) The periods Tf1 and Tf2 generated thus are repeated as one set. 
     As a result, the periods Tf1 and Tf2 are shifted from the designated period Tc by ±Tc/4 respectively. Accordingly, the residual pressure vibration is secured to be less than ±5%, and the designated period Tf0 is secured in the entire period. 
       FIG. 4  is a functional block diagram showing blocked parts of period adjustment control in the control portion  14  for suppressing fluctuation in the droplet speed or the droplet amount in control concerned with ejection control of a droplet from each droplet ejecting member  30 . Incidentally, the respective blocked parts of the functional block diagram of  FIG. 4  do not limit the hardware configuration of the control portion  14 . 
     An image formation instruction is accepted from the UI 62  (see  FIG. 3 ) by an image formation instruction information accepting portion  70 . The image formation instruction information accepting portion  70  is connected to an image information importing portion  72  and a designated image formation speed information extracting portion  74 . 
     The image information importing portion  72  imports image information from the communication I/F  66  or the HDD  64  (see  FIG. 3 ) based on an image information importing instruction received from the image formation instruction information accepting portion  70 , and sends the imported image information to an image formation pattern generating portion  76 . 
     On the other hand, designated image formation speed (paper feeding speed and/or resolution) is extracted from the image formation instruction information by the designated image formation speed information extracting portion  74 . The extracted image formation speed is sent to a droplet ejection period calculating portion  78  and a determination portion  80 . 
     By the droplet election period calculating portion  78 , a droplet period (droplet ejection period) is calculated based on the image formation speed accepted from the designated image formation speed information extracting portion  74 , and sent to the determination portion  80 . Incidentally, although the calculation result may be a droplet ejection frequency (a reciprocal number of the period), it is assumed here that the period is calculated in conformity with  FIG. 5B . 
     An image formation speed setting range storage portion  82  and a droplet ejection period to droplet speed characteristic data table storage portion  84  are connected to the determination portion  80 . Determination about the following two conditions is made by the determination portion  80 . 
     (Determination 1) Determination is made as to whether the designated image formation speed is within a setting range or not (particularly exceeds a maximum speed Vmax as an upper limit or not) 
     (Determination 2) Determination is made as to whether fluctuation in droplet speed is within a permissible range or not (for example, ±5% shown in  FIGS. 5A and 5B  or not). Incidentally, the determination 2 may be made when the designated image formation speed exceeds the setting range in the determination 1. 
     The determination result made by the determination portion  80  is sent to an ejection control selecting portion  86 . When the designated image formation speed exceeds the setting range in the determination 1 and the fluctuation in droplet speed exceeds the permissible range in the determination 2 (determination that adjustment is necessary), the ejection control selecting portion  86  issues an instruction to an adjusted election period generating portion  88  to generate droplet ejection periods (Tf1, Tf2). The adjusted ejection period generating portion  88  serves as an example of an adjustment unit. 
     On the other hand, when the designated image formation speed does not exceed the setting range in the determination 1, or when the designated image formation speed exceeds the setting range in the determination 1 but the fluctuation in droplet speed does not exceed the permissible range in the determination 2 (determination that adjustment is not necessary), the ejection control selecting portion  86  issues an instruction to a steady ejection period generating portion  90  to generate a droplet ejection period (Tf0). 
     The adjusted ejection period generating portion  88  executes adjustment to suppress the fluctuation in droplet speed caused by residual pressure vibration in order to make the droplet speed consistent with the steady ejection period Tf0. More specifically, the adjusted ejection period generating portion  88  generates the period Tf1 and the period Tf2, as shown in  FIG. 6A . The period Tf1 is shorter by Tc/4 (see  FIG. 6A ) than the steady ejection period Tf0. The period Tf2 is longer by Tc/4 (see  FIG. 6A ) than the steady ejection period Tf0. The two periods Tf1 and Tf2 are used as one set and repeated in units of one set of the periods. Thus, deviations of the two periods Tf1 and Tf2 can be cancelled with each other so that the period as a whole can correspond to the original designated period Tf0. Incidentally, Tc is a period of the fluctuation in droplet speed, which is the same as the period Tf0 (see  FIG. 5B ). 
     Incidentally, vibration caused by droplet ejection in each dotted line portion is reduced in driving waveforms in  FIGS. 6A and 6B . Although a pulse of the dotted line portion for reducing the vibration is not shown in  FIG. 8  and  FIGS. 9A to 9D  which will be described later, it is preferable that practical driving waveforms are used as driving waveforms including the pulses of the dotted line portions. 
     The adjusted ejection period generating portion  88  and the steady ejection period generating portion  90  are connected to the image formation pattern generating portion  76  respectively. 
     The image information is imported from the image information importing portion  72  to the image formation pattern generating portion  76  which generates an image formation pattern based on the image information and the ejection period or periods. The image formation pattern generated by the image formation pattern generating portion  76  is sent to a driving waveform correcting portion  92 . 
     The driving waveform correcting portion  92  executes correction of landing position of droplets on a paper sheet P. The correction is an event occurring when ejection timings have been adjusted by the adjusted ejection periods. More specifically, as shown in  FIG. 8 , a driving waveform is corrected to change the speed of each droplet ejected from each nozzle  32  (see  FIG. 2B ). 
     The driving waveform correcting portion  92  is connected to a driving instruction portion  94 . The driving instruction portion  94  sends a driving signal to the head driving portion  22  (see  FIG. 1 ) based on the image formation pattern in which the droplet speed has been corrected by the driving waveform correcting portion  92  if necessary. 
     An effect of the exemplary embodiment will be described below. 
       FIG. 7  is a flow chart showing the flow of a droplet ejection period adjustment control routine. 
       FIG. 7  is the flow chart showing the flow of the period adjustment control routine performed by the control portion  14  for suppressing fluctuation in droplet speed or droplet amount in control concerned with control of ejection of a droplet from each droplet ejecting member  30 . 
     Determination is made in a step  100  as to whether there is an image formation instruction or not. When the determination results in NO, the routine is terminated. On the other hand, when the determination results in YES in the step  100 , the routine goes to a step  102  in which designated image formation speed information is extracted. Then, the routine goes to a step  104 . 
     In the step  104 , a droplet ejection period is calculated based on the image formation speed. Next, in a step  106 , image formation speed setting range information (table) is read from the image formation speed setting range storage portion  82 . Then, the routine goes to a step  108  in which determination is made as to whether the image formation speed is within a setting range or not. 
     When the determination results in YES in the step  108 , the routine goes to a step  110 . 
     On the other hand, when the determination results in NO in the step  108 , conclusion is made that the image formation speed is out of the setting range. Then, the routine goes to a step  112  in which a “droplet ejection period to droplet speed” characteristic table is read from the “droplet ejection period to droplet speed” characteristic table storage portion  84 . Then, the routine goes to a step  114 . 
     In the step  114 , an error of droplet speed in the droplet ejection period determined based on the image formation speed is determined. 
     That is, when determination is made in the step  114  that the error is within a permissible range, the routine goes to the step  110 . On the other hand, when determination is made in the step  114  that the error is out of a permissible range (fox example, not less than ±5%), the routine goes to a step  116 . 
     In the step  110 , a steady ejection period Tf0 is generated, and the routine then goes to a step  118 . In the step  116 , adjusted ejection periods Tf1 and Tf2 are generated, and the routine then goes to the step  118 . 
     In the step  118 , image information is imported by the image information importing portion  72 . Next, the routine goes to a step  120  in which an image formation pattern is generated. Then, the routine goes to a step  122 . 
     In the step  122 , determination is made as to whether it is necessary to correct a driving waveform or not. That is, when the steady ejection period Tf0 is generated, it is not necessary to perform the correction. On the other hand, when the adjusted ejection periods Tf1 and Tf2 are generated, it is necessary to correct the driving waveform by changing droplet speed correspondingly to deviations of the ejection timings. 
     Therefore, when determination is made in the step  122  that it is necessary to perform the correction (the adjusted ejection periods Tf1 and Tf2 are generated), the routine goes to a step  124  in which the correction of the driving waveform (correction of the droplet speed) is executed (see  FIG. 8  and details will be given later). Then, the routine goes to a step  126 . 
     On the other hand, when determination is made in the step  122  that it is not necessary to perform the correction (the steady ejection period Tf0 is generated), the routine goes to the step  126  without executing the correction. 
     In the step  126 , a driving signal is outputted to the head driving portion  22  ( 22 A,  22 B). Then, the routine is terminated. In the head driving portion  22  ( 22 A,  22 B), the respective heads  24  are controlled based on the inputted driving signal to execute image formation. 
     The correction of the driving waveform in the step  124  of  FIG. 7  will be described here in detail. 
     When the adjusted ejection periods Tf1 and Tf2 are generated for ejecting droplets as shown in  FIG. 8 , every second droplet is ejected at earlier timing by a period (Tc/4)×2. When every second droplet is ejected at earlier timing by the period (Tc/4)×2, each droplet ejected at the period Tf2 can reach the paper sheet P earlier than each droplet ejected at the period Tf1, as designated by dotted line positions in  FIG. 8 . The paper sheet P is fed in a direction of an arrow A in  FIG. 8 . 
     In this case, unstable fluctuation in ejection timing among droplets can be avoided due to the ejection timing control based on the period adjustment. However, for example, in accordance with some threshold for determining whether the image quality is good or poor, the image quality may be determined to be poor. 
     Therefore, correction is performed in such a manner that an ejection speed VTf2 of the period Tf2 whose ejection timing is earlier by the period (Tc/4)×2 with respect to the period Tf1 is made slower than an ejection speed VTf1 of the period Tf1. The speed correction is set based on a distance (T.D. “Throw Distance”) between the nozzle and the paper sheet. 
     Due to the correction, the droplets ejected at the period Tf2 are displaced to solid line positions from the dotted line positions in  FIG. 8  on the paper sheet P so that an interval between adjacent ones of the droplets can be constant. 
     Incidentally, the invention is not limited to the case where one of the ejection speeds is adjusted to the other ejection speed. To describe in an extreme manner, the two speeds may be corrected so that the sum of added values of correction ratios can reach 100%. 
     For example, with an intermediate point as a reference, ejection speed VTf1 of the period Tf1 may be made slower by 50% (period Tc/4) of an amount to be corrected and ejection speed VTf2 of the period Tf2 may be made faster by 50% (period Tc/4) of the amount to be corrected. 
     (Modifications) 
     In  FIGS. 9A to 9D , driving waveforms for performing continuous ejection driving are used as modifications of the droplet ejection driving waveform, for example, in order to land “large droplets” and “small droplets”. 
     The continuous ejection driving means driving by which a plurality of droplets can be landed in one and the same position (strictly the positions which can be regarded as one and the same dot though not concentric because the paper sheet P is being fed). 
     For example, a single driving waveform is prepared (stored) as the driving waveform in advance. Respective pulses can be set ON/OFF independently by the head driving portion  22 A,  22 B (see  FIG. 1 ) side. 
     When a “large droplet” is formed, both pulses are set ON so that droplets can be ejected in the two pulses respectively (continuous ejection driving). 
     When a “small droplet” is formed, one (front) pulse is set OFF and the other (rear) pulse is set ON so that a droplet can be ejected in the other (rear) pulse. 
     The modifications show that period adjustment according to the exemplary embodiment and speed correction can be performed even in the continuous ejection driving waveforms. 
       FIG. 9A  is the same period characteristic graph as the period characteristic graph showing the influence of pressure vibration in  FIG. 5B .  FIG. 9B  is an output timing chart of a continuous ejection driving waveform as a comparative example, when period adjustment and speed correction are not executed on the driving waveform. 
     In the comparative example of  FIG. 9B , the driving waveform is affected by pressure vibration in the same manner as the driving waveform (single pulse) in the exemplary embodiment, and further, continuous ejection timings may fluctuate irregularly relatively to one another to thereby accelerate lowering of the image quality in the case of the continuous ejection driving. 
       FIGS. 9C and 9D  are output timing charts of continuous ejection driving waveforms of patterns having different combinations of “large droplets” and “small droplets”. 
       FIG. 9C  is a continuous ejection pattern of a “large droplet”→a “large droplet”→a “large droplet”→a “large droplet”, to which both front and rear pulses are applied. In addition, the amplitude of the rear pulse in each driving waveform is corrected for speed correction in  FIG. 9C . 
     In addition,  FIG. 9D  is a continuous election pattern of a “large droplet”→a “small droplet”→a “small droplet”→a “large droplet”, to which only rear pulses are applied to the “small droplets”. In addition, the rear pulse of each driving waveform is inevitably selected and the amplitude of the selected rear pulse is corrected for speed correction in  FIG. 9D . 
     In other words, in any continuous ejection pattern in which “large droplets” and “small droplets” are mixed, including  FIG. 9C  and  FIG. 9D , the same pulse (rear pulses) are selected so that speed correction can be made. 
     Incidentally, although the exemplary embodiment (including the modifications) has a configuration in which two periods are used as one set to maintain a requested period every two periods, three periods or more may be used as one set for generating a driving waveform. 
     The foregoing description of the embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention defined by the following claims and their equivalents.