Abstract:
A droplet driving control device includes: an output unit which outputs, at droplet ejection timing, a driving waveform for ejecting each droplet at a requested droplet ejection period, the waveform being a reference driving waveform including a plurality of pulse signals which can be set ON or OFF individually; a determination unit which determines whether the droplet ejection period has to be changed or not; an adjustment unit which sets each of the pulse signals of the reference driving waveform ON or OFF selectively based on a determination result of the determination unit to adjust the reference driving waveform to an adjusted driving waveform; and a droplet ejection control unit which ejects each droplet by use of the adjusted driving waveform adjusted by the adjustment unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2015-133319 filed on Jul. 2, 2015. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to a droplet driving control device and an image forming apparatus. 
         [0004]    2. Related Art 
         [0005]    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 
       [0006]    According to an aspect of the invention, there is provided a droplet driving control device comprising: an output unit which outputs, at droplet ejection timing, a driving waveform for ejecting each droplet at a requested droplet ejection period, the waveform being a reference driving waveform including a plurality of pulse signals which can be set ON or OFF individually; a determination unit which determines whether the droplet ejection period has to be changed or not; an adjustment unit which sets each of the pulse signals of the reference driving waveform ON or OFF selectively based on a determination result of the determination unit to adjust the reference driving waveform to an adjusted driving waveform; and a droplet ejection control unit which ejects each droplet by use of the adjusted driving waveform adjusted by the adjustment unit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
           [0008]      FIG. 1  is a schematic configuration diagram showing an example of a main configuration portion of a droplet ejection type recording apparatus according to a first exemplary embodiment; 
           [0009]      FIGS. 2A and 2B  are a plan view of a head according to the first exemplary embodiment and a sectional view showing an internal structure of each droplet ejecting element in the head respectively; 
           [0010]      FIG. 3  is a block diagram of a control portion according to the first exemplary embodiment; 
           [0011]      FIG. 4  is a functional block diagram showing blocked parts of period adjustment control in the control portion according to the first exemplary embodiment; 
           [0012]      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; 
           [0013]      FIGS. 6A, 6B, 6C and 6D  are a reference driving waveform graph and waveform graphs after selection of pulses, a timing chart of adjusted driving periods, a timing chart of each steady driving period, and a timing chart showing a positional relation between the adjusted driving periods and the steady driving period, according to the first exemplary embodiment, respectively; 
           [0014]      FIGS. 7A and 7B  are flow charts showing the flows of droplet ejection period adjustment control routines according to the first exemplary embodiment; 
           [0015]      FIG. 8  is a timing chart showing details of correction of a driving waveform in a step  120  of  FIG. 7 ; 
           [0016]      FIGS. 9A, 9B, 9C and 9D  are a reference driving waveform graph and waveform graphs after selection of pulses, a timing chart of adjusted driving periods, a timing chart of each steady driving period, and a timing chart showing a positional relation between the adjusted driving periods and the steady driving period, according to a second exemplary embodiment, respectively; 
           [0017]      FIGS. 10A, 10B and 10C  are a reference driving waveform graph and waveform graphs after selection of pulses, a timing chart of adjusted driving periods, and a timing chart of each steady driving period (continuous ejection mode), according to a third exemplary embodiment, respectively; and 
           [0018]      FIGS. 11A, 11B and 11C  are a reference driving waveform graph and waveform graphs after selection of pulses, a timing chart of adjusted driving periods, and a timing chart of each steady driving period (continuous ejection mode+adjustment of each continuously ejected droplet landing position), according to a fourth exemplary embodiment, respectively. 
       
    
    
     REFERENCE SIGNS LIST 
       [0000]    
       
         
           
               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  reference driving waveform reading 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  reference driving waveform storage portion 
               88  image formation pattern generating portion 
               90  change necessity information generating portion 
               92  driving waveform correcting portion 
               94  driving instruction portion 
               95  acceptance portion 
               96  pulse selecting portion 
               97  ON/OFF pattern table storage portion 
               98  ejection period adjusting portion 
               99  ejection execution control portion 
           
         
       
     
       DETAILED DESCRIPTION 
     First Exemplary Embodiment 
     Outline of Apparatus 
       [0066]      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 a first exemplary embodiment. 
         [0067]    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. 
         [0068]    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. 
         [0069]    Similarly, the image forming portion  12 B is provided with a head driving portion  22 B as an example of a droplet ejection control unit. Further, the image forming portion  12 B includes heads  24 B and a drying device  26 B. 
         [0070]    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. 
         [0071]    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. 
         [0072]    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 . 
         [0073]    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. 
         [0074]    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. 
         [0075]    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 the first 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. 
         [0076]    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. 
         [0077]    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. 
         [0078]    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. 
         [0079]    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. 
         [0080]    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. 
         [0081]    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. 
         [0082]    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 . 
         [0083]    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 the first 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. 
         [0084]    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 first exemplary embodiment. When it is mentioned as “ink” or “droplet” simply in the first exemplary embodiment, it may imply “water-based ink” or “water-based ink droplet”. 
       (Head  24 ) 
       [0085]    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. 
         [0086]    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. 
         [0087]    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. 
         [0088]    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 . 
         [0089]    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 . 
         [0090]    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 . 
         [0091]    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. 
         [0092]    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). 
         [0093]    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. The term “image formation speed” which will be referred to simply may include one of the droplet ejection period, the paper feeding speed and the resolution, or all combinations of two or more of the droplet ejection period, the paper feeding speed and the resolution, but do not include any combination incompatible with circumstances. 
         [0094]    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 . 
         [0095]    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. 
         [0096]    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 the first exemplary embodiment) is connected to the I/O  56 . 
         [0097]    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. 
         [0098]    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. 
         [0099]    On the other hand, in the first 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). 
         [0100]    That is, in the first exemplary embodiment, period adjustment control is executed in the following control procedures in the control portion  14  and the head driving portion  22 . 
         [0101]    (Control Procedure 1) When a droplet ejection frequency (droplet ejection period) is determined in accordance with the image formation speed which is set to exceed the upper limit of the setting range, determination is made as to whether residual pressure vibration is less than ±5% or not, based on  FIG. 5A  or  FIG. 5B . 
         [0102]    (Control Procedure 2) As shown in  FIG. 6A , a pulse  1  or a pulse  2  is suitably selected (ON/OFF) in a reference driving waveform of a reference driving waveform period (Tf 0 ) including the pulse  1  and the pulse  2 . Thus, two kinds of driving waveforms are generated. 
         [0103]    As shown in  FIG. 6A , the reference driving waveform has the period Tf 0  (reference driving waveform period). The reference driving waveform is a waveform in which the pulse  1  of a droplet ejection time T 1  is outputted in a rising edge, and the pulse  2  of a droplet ejection time T 3  is then outputted after a lapse of an interval time T 2 . 
         [0104]    Here, there is a case where a pulse signal (see a dotted line in  FIG. 6A ) which is set to have a width of a time T 4  and is convex reversely to the pulse  1  and the pulse  2  may be outputted immediately after the pulse  2 . 
         [0105]    In the reference driving waveform in  FIG. 6A , the pulse signal of the aforementioned dotted line portion is intended to reduce vibration caused by droplet ejection. In other words, since the pulse signal is unnecessary in view of droplet ejection, it is designated by the dotted line in  FIG. 6A . 
         [0106]    Incidentally, although the pulse of the dotted line portion for reducing the vibration is not shown in  FIG. 6B ,  FIG. 6C  and  FIG. 8  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. 
         [0107]    In the first exemplary embodiment, each of the droplet ejection times T 1  and T 3  is equal to a time Tc/2. The interval time T 2  between the pulse  1  and the pulse  2  is a time Tc/4. The time T 4  of the pulse for reducing the vibration is set as a time Tc. As shown in  FIG. 5B , the time Tc is a period of fluctuation with respect to a requested value of the droplet speed so as to be consistent with the reference driving waveform period Tf 0 . 
         [0108]    Here, the pulse  1  (P 1 ) or the pulse  2  (P 2 ) is selected (ON/OFF) in the reference driving waveform in  FIG. 6A . Thus, two kinds of driving waveforms can be generated. 
         [0109]    Incidentally, in the first exemplary embodiment, as an example for generating each of the driving waveforms, the reference driving waveform is outputted to the head driving portion  22  from the control portion  14  regardless of the condition of the control procedure 1, and then, the pulse  1  or the pulse  2  is selected to be ON/OFF in the head driving portion  22  based on the condition of the control procedure 1. 
         [0110]    (Control Procedure 3 “not Less than Range of ±5%”) 
         [0111]    A driving waveform in which the pulse  1  is set OFF and the pulse  2  is set ON in the reference driving waveform and a driving waveform in which the pulse  1  is set ON and the pulse  2  is set OFF in the reference driving waveform are generated and outputted alternately. Thus, a period Tf 1  shorter by (Tc/4)×n than the droplet ejection period Tf 0  and a period Tf 2  longer by (Tc/4)×n than the designated droplet ejection period Tf 0  are repeated (see  FIG. 68 ). Incidentally, Tc is the period for the residual pressure vibration in  FIG. 5B  so as to be consistent with Tf 0 . In addition, n is an odd number among integers. In the first exemplary embodiment, the relation n=3 is established (that is, ±3Tc/4). 
         [0112]    As a result, the periods are shifted from the designated period Tf 0  by ±3Tc/4 respectively. Accordingly, the period for the residual pressure vibration is secured to be less than ±5% and the designated period Tf 0  is secured in the entire period (see  FIG. 6D ). 
         [0113]    (Control Procedure 4 “Less than Range of ±5%”) 
         [0114]    A single driving waveform in which the pulse  1  is set OFF and the pulse  2  is set ON in the reference driving waveform is generated and outputted. Thus, the droplet ejection period Tf 0  is maintained (see  FIG. 6C ). 
         [0115]      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 . 
         [0116]    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 . 
         [0117]    The image information importing portion  72  imports image information from the communication I/F  66  or the HDD  64  (see  FIG. 3 ) based on the image information importing instruction received from the image formation instruction information accepting portion  70 , and sends the imported image information to a reference driving waveform reading portion  76 . 
         [0118]    A reference driving waveform storage portion  86  is connected to the reference driving waveform reading portion  76 . Upon acceptance of the image information from the image information importing portion  72 , the reference driving waveform reading portion  76  reads a reference driving waveform from the reference driving waveform storage portion  86  and sends the read reference driving waveform to an image formation pattern generating portion  88 . 
         [0119]    By the image formation pattern generating portion  88 , an image formation pattern (presence/absence of droplet ejection based on main scanning and sub-scanning) is generated based on the image information and an ejection period, and sent to a driving instruction portion  94 . The driving instruction portion  94  serves as an example of an output unit. 
         [0120]    On the other hand, designated image formation speed (which may include 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 . The determination portion  80  serves as an example of a determination unit. 
         [0121]    By the droplet ejection 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 . 
         [0122]    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 . 
         [0123]    (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) 
         [0124]    (Determination 2) Determination is made as to whether fluctuation in droplet speed is within a permissible range or not (for example, less than ±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. 
         [0125]    The determination result made by the determination portion  80  is sent to a change necessity information generating portion  90 . Adjustment necessity information required for selection of the pulse  1  and the pulse  2  included in the reference driving waveform is generated by the change necessity information generating portion  90 . 
         [0126]    The change necessity information generating portion  90  is connected to a driving waveform correcting portion  92 . 
         [0127]    The driving waveform correcting portion  92  executes correction of each landing position on a paper sheet P. The correction is an event occurring in the case where determination has been made that the ejection period has to be adjusted and the ejection period has been adjusted. More specifically, as shown in  FIG. 8 , the driving waveform is corrected to thereby change the droplet speed for ejecting a droplet from each nozzle  32  (see  FIG. 2B ). 
         [0128]    The driving waveform correcting portion  92  is connected to the driving instruction portion  94 . 
         [0129]    The image formation pattern generated by the image formation pattern generating portion  88  and the adjustment necessity information (including correction information added if necessary) are sent to the head driving portion  22  (see  FIG. 1 ) by the driving instruction portion  94 . 
         [0130]    The image formation pattern and the adjustment necessity information (including the correction information of the droplet speed added if necessary) are accepted by an acceptance portion  95  of the head driving portion  22 . 
         [0131]    The adjustment necessity information is extracted and sent to a pulse selecting portion  96  by the acceptance portion  95 . 
         [0132]    An ON/OFF pattern table storage portion  97  is connected to the pulse selecting portion  96 . 
         [0133]    As shown in  FIG. 4 , a table indicating the relation between adjustment necessity and ON/OFF patterns of the pulse  1  and the pulse  2  is stored in the ON/OFF pattern table storage portion  97 . 
         [0134]    By the pulse selecting portion  96 , the pulse  1  and/or the pulse  2  included in the reference driving waveform are/is selected based on the ON/OFF pattern table, and sent to an ejection period adjusting portion  98 . The ejection period adjusting portion  98  serves as an example of an adjustment unit. 
         [0135]    A reference driving waveform which is an image formation pattern is extracted from the acceptance portion  95  by the ejection period adjusting portion  98 . 
         [0136]    Therefore, when adjustment is determined to be unnecessary as an adjustment necessity determination result, a single driving waveform with a steady ejection period Tf 0  in which the pulse  1  is set ON and the pulse  2  is set OFF is generated by the ejection period adjusting portion  98 . 
         [0137]    On the other hand, when adjustment is determined to be necessary as an adjustment necessity determination result, a driving waveform with an adjusted ejection period Tf 1  in which the pulse  1  is set OFF and the pulse  2  is set ON and a driving waveform with an adjusted ejection period Tf 2  in which the pulse  1  is set ON and the pulse  2  is set OFF are generated by the ejection period adjusting portion  98 . 
         [0138]    An ejection execution control portion  99  serving as an example of a droplet ejection control unit is connected to the ejection period adjusting portion  98  to thereby execute ejection of each droplet based on an ejection period set as either the steady ejection period or one of the adjusted ejection periods. 
         [0139]    An effect of the first exemplary embodiment will be described below in accordance with flow charts of  FIGS. 7A and 7B . 
         [0140]      FIG. 7A  is the flow chart showing the flow of period adjustment control performed by the control portion  14  for suppressing fluctuation in droplet speed or droplet amount in control concerned with ejection control of a droplet from each droplet ejecting member  30 .  FIG. 7B  is the flow chart showing the flow of period adjustment control performed by the head driving portion  22  for suppressing fluctuation in droplet speed or droplet amount in control concerned with ejection control of a droplet from the droplet ejecting member  30 . 
       (Control on Control Portion  14  Side) 
       [0141]    As shown in  FIG. 7A , determination is made as to whether there is an image formation instruction or not in a step  100 . When the determination results in NO, the routine is terminated. In addition, when the determination results in YES in the step  100 , the routine goes to a step  102  in which image information is imported by the image information importing portion  72 . Then, the routine goes to a step  104  in which an image formation pattern is generated. Then, the routine goes to a step  106 . 
         [0142]    In the step  106 , designated image formation speed information is extracted. Then, the routine goes to a step  108 . 
         [0143]    In the step  108 , each droplet ejection period is calculated based on the image formation speed. Next, in a step  110 , image formation speed setting range information (table) is read from the image formation speed setting range storage portion  82 . The routine goes to a step  112  in which determination is made as to whether the image formation speed is within the setting range or not. 
         [0144]    When the determination results in YES in the step  112 , the routine goes to a step  116 . 
         [0145]    In addition, when the determination results in NO in the step  112 , conclusion is made that the image formation speed is out of the setting range. Then, the routine goes to a step  114  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 the step  116 . 
         [0146]    In the step  116 , adjustment necessity information of the droplet ejection period depending on the image formation speed is generated. 
         [0147]    That is, when the image formation speed is within the setting range, adjustment of the droplet ejection period is unnecessary (adjustment is unnecessary). When the image formation speed is out of the setting range and an error of the residual vibration is not less than ±5%, information indicating that the adjustment is necessary (adjustment is necessary) is generated. 
         [0148]    In a next step  118 , determination is made as to whether correction of the driving waveform is necessary or not. That is, when determination is made that adjustment of the droplet ejection period is unnecessary, correction of the driving waveform is unnecessary. On the other hand, when determination is made that adjustment of the droplet period is necessary, it is necessary to correct the driving waveform using the droplet speed correspondingly to a deviation in the ejection timing. 
         [0149]    Therefore, when determination is made that correction is necessary in the step  118 , the routine goes to a step  120  in which correction information of the driving waveform (correction of the droplet speed) is added to the adjustment necessity information (see  FIG. 8 , and details will be given later). Then, the routine goes to a step S 122 . 
         [0150]    On the contrary, when determination is made that correction is unnecessary in the step  118 , correction information is not added to the adjustment necessity information. Then, the routine goes to the step  122 . 
         [0151]    In the step  122 , the image formation pattern information (the step  104 ), the adjustment necessity information (the step  116 ) and the correction information of the droplet speed if necessary (the step  120 ) are sent as driving instruction information to the head driving portion  22 . Then, the routine is terminated. 
         [0152]    Incidentally, control of the head driving portion  22  which will be described below may be executed in a lump by the control portion  14 . 
       (Control on Head Driving Portion  22  Side) 
       [0153]    As shown in  FIG. 7B , determination is made in a step  150  as to whether a driving instruction has been accepted or not. When the determination results in NO, the routine is terminated. 
         [0154]    In addition, when the determination results in YES in the step  150 , the routine goes to a step  152  in which adjustment necessity information is extracted from the driving instruction information. Then, the routine goes to a step  154 . 
         [0155]    In the step  154 , a pulse ON/OFF pattern table is read from the pulse ON/OFF pattern table storage portion  97 . Next, the routine goes to a step  156 . 
         [0156]    In the step  156 , a period kind (steady ejection period or adjusted ejection period) is determined based on the adjustment necessity information. Then, the routine goes to a step  158 . 
         [0157]    When determination is made in the step  158  that the period kind is adjusted ejection period, the routine goes to a step  160  in which a reference driving waveform is read from the driving instruction information. Next, the routine goes to a step  162  in which an adjusted ejection period Tf 1  and an adjusted ejection period Tf 2  are generated based on the reference driving waveform with reference to the pulse ON/OFF pattern table. Then, the routine goes to a step  168  (see  FIG. 6A  and  FIG. 6B ). 
         [0158]    On the other hand, when determination is made in the step  158  that the period kind is steady ejection period, the routine goes to a step  164  in which a reference driving waveform is read from the driving instruction information. Next, the routine goes to a step  166  in which a steady ejection period Tf 0  is generated based on the reference driving waveform with reference to the pulse ON/OFF pattern table. The routine goes to the step  168  (see  FIG. 6A  and  FIG. 6C ). 
         [0159]    In the step  168 , droplet ejection is executed based on the generated ejection period or periods (the steady ejection period or the adjusted ejection periods). Then, the routine is terminated. 
         [0160]    Here, correction of the driving waveform in the step  120  in  FIG. 7A  will be described in detail. 
         [0161]    As shown in  FIG. 8 , when the adjusted ejection periods Tf 1  and Tf 2  are generated for ejecting droplets, every second droplet is ejected earlier by a period (3Tc/4)×2 (see  FIG. 6D ). When every second droplet is ejected earlier by a period (3Tc/4)×2, droplets ejected at the period Tf 2  can reach a paper sheet P earlier than droplets ejected at the period Tf 1 , 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 . 
         [0162]    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. 
         [0163]    Therefore, correction is performed in such a manner that an ejection speed VTf 2  of the period Tf 2  whose ejection timing is earlier by the period (3Tc/4)×2 with respect to the period Tf 1  is made slower than an ejection speed VTf 1  of the period Tf 1 . The speed correction is set based on a distance (T.D. “Throw Distance”) between the nozzle and the paper sheet. 
         [0164]    Due to the correction, the droplets ejected at the period Tf 2  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. 
         [0165]    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%. 
         [0166]    For example, with reference to an intermediate point, the ejection speed VTf 1  of the period Tf 1  may be made slower by 50% of an amount to be corrected and the ejection speed VTf 2  of the period Tf 2  may be made faster by 50% of the amount to be correct. 
       Second Exemplary Embodiment 
       [0167]    A second exemplary embodiment will be described below. Incidentally, in the second exemplary embodiment, the same portions as those in the first exemplary embodiment will be referred to by the same signs respectively and correspondingly, and description thereof will be omitted. 
         [0168]    The second exemplary embodiment is characterized in the following point. That is, a period (Tf 1 ) shorter by 5Tc/4 than a steady ejection period Tf 0  (i.e. corresponding to a fluctuation period Tc) used as a reference and a period (Tf 2 ) longer by 5Tc/4 than the designated droplet ejection period Tf 0  are set as adjusted ejection periods Tf 1  and Tf 2 . 
         [0169]    In the second exemplary embodiment, period adjustment control is executed in the following control procedures in a control portion  14 . 
         [0170]    (Control Procedure 1) When a droplet ejection frequency (droplet ejection period) is determined in accordance with an image formation speed which is set to exceed an upper limit of a setting range, determination is made as to whether residual pressure vibration is less than ±5% or not, based on  FIG. 5A  or  FIG. 5B . 
         [0171]    (Control Procedure 2) As shown in  FIG. 9A , a pulse  1  or a pulse  2  is suitably selected (ON/OFF) in a reference driving waveform of a reference driving waveform period (Tf 0 ) including the pulse  1  and the pulse  2 . Thus, two kinds of driving waveforms are generated. 
         [0172]    As shown in  FIG. 9A , the reference driving waveform has the period Tf 0  (reference driving waveform period). The reference driving waveform is a waveform in which the pulse  1  of a droplet ejection time T 1  is outputted in a rising edge, and the pulse  2  of a droplet ejection time T 3  is then outputted after a lapse of an interval time T 2 . 
         [0173]    Here, there is a case where a pulse signal (see a dotted line in  FIG. 9A ) which is set to have a width of a time T 4  and is convex reversely to the pulse  1  and the pulse  2  may be outputted immediately after the pulse  2 . 
         [0174]    In the reference driving waveform in  FIG. 9A , the pulse signal of the aforementioned dotted line portion is intended to reduce vibration caused by droplet ejection. In other words, since the pulse signal is unnecessary in view of droplet ejection, it is designated by the dotted line in  FIG. 9A . 
         [0175]    Incidentally, although the pulse of the dotted line portion for reducing the vibration is not shown in  FIG. 9B  and  FIG. 9C , it is preferable that practical driving waveforms are used as driving waveforms including the pulses of the dotted line portions. 
         [0176]    In the second exemplary embodiment, each of the droplet ejection times T 1  and T 3  is equal to a time Tc/2. The interval time T 2  between the pulse  1  and the pulse  2  is a time 3Tc/4. The time T 4  of the pulse for reducing the vibration is set as a time Tc. As shown in  FIG. 5B , the time Tc is a period of fluctuation with respect to a requested value of a droplet speed so as to be consistent with the reference driving waveform period Tf 0 . 
         [0177]    Here, the pulse  1  (P 1 ) or the pulse  2  (P 2 ) is selected (ON/OFF) in the reference driving waveform in  FIG. 9A . Thus, two kinds of driving waveforms can be generated. 
         [0178]    Incidentally, in the second exemplary embodiment, as an example for generating each of the driving waveforms, the reference driving waveform is outputted to one of head driving portions  22  from the control portion  14  regardless of the condition of the control procedure 1, and then, the pulse  1  or the pulse  2  is selected to be ON/OFF in the head driving portion  22  based on the condition of the control procedure 1. 
         [0179]    (Control Procedure 3 “not Less than Range of ±5%”) 
         [0180]    A driving waveform in which the pulse  1  is set OFF and the pulse  2  is set ON in the reference driving waveform and a driving waveform in which the pulse  1  is set ON and the pulse  2  is set OFF in the reference driving waveform are generated and outputted alternately. Thus, a period Tf 1  shorter by (Tc/4)×n than the droplet ejection period Tf 0  and a period Tf 2  longer by (Tc/4)×n than the designated droplet ejection period Tf 0  are repeated (see  FIG. 9B ). Incidentally, Tc is the period for the residual pressure vibration in  FIG. 5B  so as to be consistent with Tf 0 . In addition, n is an odd number among integers. In the second exemplary embodiment, the relation n=5 is established (that is, ±5Tc/4). 
         [0181]    (Control Procedure 4 “Less than Range of ±5%”) 
         [0182]    A single driving waveform in which the pulse  1  is set OFF and the pulse  2  is set ON in the reference driving waveform is generated and outputted. Thus, the droplet ejection period Tf 0  is maintained (see  FIG. 9C ). 
         [0183]    As a result, the periods are shifted by ±5Tc/4 from the designated period Tf 0 . Accordingly, the period for the residual pressure vibration is secured to be less than ±5% and the designated period Tf 0  is secured in the entire period (see  FIG. 9D ). 
       Third Exemplary Embodiment 
       [0184]    A third exemplary embodiment will be described below. Incidentally, in the third exemplary embodiment, the same portions as those in the first exemplary embodiment will be referred to by the same signs respectively and correspondingly, and description thereof will be omitted. 
         [0185]    The third exemplary embodiment is characterized in the following point. That is, a driving waveform for continuous ejection driving is used as a modification of the driving waveform for droplet ejection so that the problem (maintenance of quality at an image formation speed out of a permissible range) described in the first exemplary embodiment can be solved even when, for example, two “large droplets” are landed. 
         [0186]    Incidentally, the continuous ejection driving is 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 a paper sheet P is fed). In the aforementioned modification, two “large droplets” are landed on one and the same dot. 
         [0187]    In the third exemplary embodiment, period adjustment control is executed in the following control procedures in a control portion  14 . 
         [0188]    (Control Procedure 1) When a droplet ejection frequency (droplet ejection period) is determined in accordance with an image formation speed which is set to exceed an upper limit of a setting range, determination is made as to whether residual pressure vibration is less than ±5% or not, based on  FIG. 5A  or  FIG. 5B . 
         [0189]    (Control Procedure 2) As shown in  FIG. 10A , a pulse  1 , a pulse  2  or a pulse  3  is suitably selected (ON/OFF) in a reference driving waveform of a reference driving waveform period (Tf 0 ) including the pulse  1 , the pulse  2  and the pulse  3 . Thus, two kinds of driving waveforms are generated. 
         [0190]    As shown in  FIG. 10A , the reference driving waveform has the period Tf 0  (reference driving waveform period). The reference driving waveform is a waveform in which the pulse  1  of a droplet ejection time T 1  is outputted in a rising edge, the pulse  2  of a droplet ejection time T 3  is then outputted after a lapse of an interval time T 2 , and the pulse  3  of a droplet ejection time T 5  is then outputted after a lapse of an interval time T 4 . 
         [0191]    Here, there is a case where a pulse signal (see a dotted line in  FIG. 10A ) which is set to have a width of a time T 6  and is convex reversely to the pulse  1 , the pulse  2  and the pulse  3  may be outputted immediately after the pulse  3 . 
         [0192]    In the reference driving waveform in  FIG. 10A , the pulse signal of the aforementioned dotted line portion is intended to reduce vibration caused by droplet ejection. In other words, since the pulse signal is unnecessary in view of droplet ejection, it is designated by the dotted line in  FIG. 10A . 
         [0193]    Incidentally, although the pulse of the dotted line portion for reducing the vibration is not shown in  FIG. 10B  and  FIG. 10C , it is preferable that practical driving waveforms are used as driving waveforms including the pulses of the dotted line portions. 
         [0194]    In the third exemplary embodiment, each of the droplet ejection times T 1 , T 3  and T 5  is equal to a time Tc/2. The interval time T 2  between the pulse  1  and the pulse  2  is a time Tc/4. The interval time T 4  between the pulse  2  and the pulse  3  is a time Tc/2. The time T 6  of the pulse for reducing the vibration is set as a time Tc. As shown in  FIG. 5B , the time Tc is a period of fluctuation with respect to a requested value of a droplet speed so as to be consistent with the reference driving waveform period Tf 0 . 
         [0195]    Here, the pulse  1  (P 1 ), the pulse  2  (P 2 ) or the pulse  3  (P 3 ) is selected (ON/OFF) in the reference driving waveform in  FIG. 10A . Thus, two kinds of driving waveforms can be generated. In the third exemplary embodiment, a driving waveform having a combination (P 2  and P 3 ) of the pulse  2  and the pulse  3  and a driving waveform having a combination (P 1  and P 3 ) of the pulse  1  and the pulse  3  are generated for “large droplet” use. 
         [0196]    Incidentally, in the third exemplary embodiment, as an example for generating each of the driving waveforms, the reference driving waveform is outputted to one of head driving portions  22  from the control portion  14  regardless of the condition of the control procedure 1, and then, the pulse  1 , the pulse  2  or the pulse  3  is selected to be ON/OFF in the head driving portion  22  based on the condition of the control procedure 1. 
         [0197]    (Control Procedure 3 “not Less than Range of ±5%”) 
         [0198]    A driving waveform in which the pulse  1  is set OFF, the pulse  2  is set ON and the pulse  3  is set ON in the reference driving waveform and a driving waveform in which the pulse  1  is set ON, the pulse  2  is set OFF and the pulse  3  is set ON in the reference driving waveform are generated and outputted alternately. Thus, a period Tf 1  shorter by (Tc/4)×n than the droplet ejection period Tf 0  and a period Tf 2  longer by (Tc/4)×n than the designated droplet ejection period Tf 0  are repeated (see  FIG. 10B ). Incidentally, Tc is the period for the residual pressure vibration in  FIG. 5B  so as to be consistent with Tf 0 . In addition, n is an odd number among integers. In the third exemplary embodiment, the relation n=7 is established (that is, ±7Tc/4). 
         [0199]    (Control Procedure 4 “Less than Range of ±5%”) 
         [0200]    A single driving waveform in which the pulse  1  is set OFF, the pulse  2  is set ON and the pulse  3  is set ON in the reference driving waveform is generated and outputted. Thus, the droplet ejection period Tf 0  is maintained (see  FIG. 10C ). 
         [0201]    As a result, the periods are shifted by ±7Tc/4 from the designated period Tf 0 . Accordingly, the period for the residual pressure vibration is secured to be less than ±5% and the designated period Tf 0  is secured in the entire period. 
         [0202]    Incidentally, although two “large droplets” have been shown as an example of continuous ejection in the third exemplary embodiment, the invention may be applied to continuous ejection of two or more droplets including “small droplets” and “intermediate droplets”. 
       Fourth Exemplary Embodiment 
       [0203]    A fourth exemplary embodiment will be described below. Incidentally, in the fourth exemplary embodiment, the same portions as those in the third exemplary embodiment will be referred to by the same signs respectively and correspondingly, and description thereof will be omitted. 
         [0204]    The fourth exemplary embodiment is characterized in the following point. That is, correction of a deviation in landing timing (correction of droplet speed in the first exemplary embodiment) is taken into consideration when each droplet is ejected in an adjusted ejection frequency in continuous ejection driving which has been described in the third exemplary embodiment. 
         [0205]    As shown in  FIG. 11A , a reference driving waveform applied in the fourth exemplary embodiment has the same time widths (T 1  to T 6 ) as the reference driving waveform (see  FIG. 10A ) in the third exemplary embodiment. 
         [0206]    The fourth exemplary embodiment is different from the third exemplary embodiment in the amplitude of a pulse  2  (voltage value). The pulse  2  is smaller in amplitude than a pulse  1  and a pulse  3 . Accordingly, at the pulse  2 , droplet speed is slower and landing timing is later correspondingly. 
         [0207]    The pulse  2  is a pulse which is selected in an adjusted ejection period Tf 1  and not selected in an adjusted ejection period Tf 2 . 
         [0208]    Therefore, when the ejection period does not have to be adjusted, a single driving waveform in which the pulse  1  is not selected is repeated as shown in  FIG. 11C . Accordingly, droplet ejection speed is not affected but all the droplets are outputted at the same droplet speed. 
         [0209]    On the other hand, when the ejection period has to be adjusted and the adjusted ejection period Tf 1  and the adjusted ejection period Tf 2  are outputted alternately, the driving waveform in which the pulse  2  is selected and the driving waveform in which the pulse  2  is not selected are outputted alternately. Accordingly, control consistent with the speed adjustment (see  FIG. 8 ) according to the first exemplary embodiment is performed. As a result, the landing positions can be corrected. 
         [0210]    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.