Patent Publication Number: US-11648768-B2

Title: Liquid ejection apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2020-037168, filed on Mar. 4, 2020, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a liquid ejection apparatus. 
     BACKGROUND 
     A liquid ejection apparatus that supplies a predetermined amount of liquid to a predetermined position is known. Such a liquid ejection apparatus is installed in, for example, an inkjet printer, a 3D printer, a liquid dispensing apparatus, or the like. An inkjet printer ejects ink droplets from an inkjet head to form an image or the like on a surface of a recording medium. A 3D printer ejects droplets of a molding material from a molding material ejection head and the droplets harden to form a three-dimensional modeled object. A liquid dispensing apparatus ejects sample droplets of known volume to supply a predetermined amount of the sample to a plurality of containers or the like. 
     A liquid ejection apparatus has a plurality of channels including nozzles and actuators for forming droplets or dots. The liquid ejection apparatus selects a channel from among the plurality of channels for ejecting a liquid and drive the actuator of the selected channel by applying a drive waveform thereto. When the number of actuators to be driven is large, especially when the actuators are positioned close to each other, the actuators are affected by, for example, concentration of an electric current flowing through a common electrode to which the actuators are commonly connected, or pressure oscillation occurring between the channels. Thus, the amount of liquid ejection may become unstable. 
     Hence, there is a need for a liquid ejection apparatus capable of stable liquid ejection. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    depicts an inkjet printer according to a first embodiment. 
         FIG.  2    depicts an inkjet head in a perspective view according to a first embodiment. 
         FIG.  3    depicts an internal configuration of an inkjet head according to a first embodiment. 
         FIG.  4    depicts an actuator of an inkjet head in a cross-sectional view according to a first embodiment. 
         FIG.  5    is a block configuration diagram of a control system of an inkjet printer according to a first embodiment. 
         FIG.  6    depicts an example actuator drive waveform according to a first embodiment. 
         FIG.  7    depicts an example arrangement of actuators and electrodes according to a first embodiment. 
         FIG.  8    depicts example voltage waveforms according to a first embodiment. 
         FIG.  9    depicts example actuator drive waveforms according to a first embodiment. 
         FIG.  10    depicts an actuator drive circuit according to a first embodiment. 
         FIG.  11    depicts an example arrangement of actuators and electrodes according to a first embodiment. 
         FIG.  12    is a configuration diagram of an actuator drive circuit according to a first embodiment. 
         FIG.  13    depicts a modification example of an actuator drive circuit. 
         FIG.  14    depicts another modification example of an actuator drive circuit. 
         FIG.  15    depicts example actuator drive waveforms. 
         FIG.  16    depicts an actuator drive circuit to which example drive waveforms are applied. 
         FIG.  17    depicts a modification example of an actuator drive circuit to which example drive are applied. 
         FIG.  18    depicts an example delay pattern and delay amount according to a first embodiment. 
         FIG.  19    depicts example actuator drive waveforms according to a second embodiment. 
         FIG.  20    is a configuration diagram of an actuator drive circuit according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     According to an embodiment, a liquid ejection apparatus comprises a liquid ejection unit having a plurality of nozzles and a corresponding plurality of actuators. A drive waveform generation circuit of the apparatus is configured to generate drive waveforms having different drive timings. An actuator drive circuit of the apparatus is configured to apply a first drive waveform to a first actuator in a liquid ejection operation and a second drive waveform to a second actuator in the liquid ejection operation during which the first and second actuators are to be driven at a same nominal time. The first driving waveform and the second drive waveform have different drive timings, and the first actuator is at a position electrically closer along a predetermined direction to a power supply electrode than is the second actuator. 
     Hereinafter, certain embodiments of a liquid ejection apparatus will be described with reference to the accompanying drawings. In the respective drawings, the same components depicted in different drawings will be denoted by the same reference numerals. 
     First Embodiment 
     As an example of an image forming apparatus equipped with a liquid ejection apparatus  1  according to a first embodiment, an inkjet printer  10  for printing an image on a recording medium will be described.  FIG.  1    shows a schematic configuration of the inkjet printer  10 . Inside a housing  11  of the inkjet printer  10 , a cassette  12  that accommodates sheets S, which are an example of a recording medium, an upstream conveyance path  13  for the sheets S, a conveyance belt  14  that conveys each sheet S picked up from the cassette  12 , inkjet heads  100 ,  101 ,  102 , and  103  that eject ink droplets toward a sheet S on the conveyance belt  14 , a downstream conveyance path  15  for the sheets S, a discharge tray  16 , and a controller  17  are disposed. An input operation unit  18 , which is a user interface panel or the like, is disposed on an upper side of the housing  11 . 
     Image data to be printed on the sheet S is generated by, for example, a computer  200 , which is an external device connectable to the inkjet printer  10 . The image data generated by the computer  200  is transmitted to the controller  17  of the inkjet printer  10  through a cable  201  and connectors  202  and  203 . 
     A pick-up roller  204  supplies the sheets S from the cassette  12  and moves the sheets S to the upstream conveyance path  13  one by one. The upstream conveyance path  13  includes feed roller pairs  131  and  132  and sheet guide plates  133  and  134 . Each sheet S is moved to an upper surface of the conveyance belt  14  by the upstream conveyance path  13 . In the drawing, an arrow  104  indicates a conveyance path of the sheets S from the cassette  12  to the conveyance belt  14 . 
     The conveyance belt  14  is a mesh-like endless belt having a large number of through holes formed on the surface thereof. Three rollers including a driving roller  141  and driven rollers  142  and  143  rotatably support the conveyance belt  14 . A motor  205  rotates the conveyance belt  14  by rotating the driving roller  141 . The motor  205  is an example of a driving device. In the drawing, arrow  105  indicates a rotation direction of the conveyance belt  14 . A negative pressure container  206  is disposed on a back side of the conveyance belt  14 . The negative pressure container  206  is connected to a pressure reducing fan  207 . The inside of the negative pressure container  206  becomes a negative pressure due to an air current generated by the fan  207 , and thus the sheet S is held on the upper surface of the conveyance belt  14  by an air pressure difference force (vacuum). In the drawing, arrow  106  indicates a flow direction of an air current. 
     The inkjet heads  100  to  103  are disposed so as to face the sheet S on the conveyance belt  14  at a narrow gap of, for example, 1 mm between the sheet S and the lowermost portion of the inkjet heads  100  to  103 . The inkjet heads  100  to  103  individually eject ink droplets toward the sheet S. An image is formed on the sheet S when the sheet S passes below all of the inkjet heads  100  to  103 . The inkjet heads  100  to  103  each have the same structure except that colors of ink to be ejected therefrom are different. The colors of the ink are, for example, cyan, magenta, yellow, and black. 
     The inkjet heads  100  to  103  are respectively connected to ink tanks  315 ,  316 ,  317 , and  318  and ink supply pressure adjustment devices  321 ,  322 ,  323 , and  324  through ink flow paths  311 ,  312 ,  313 , and  314 . When an image is being formed, the ink in the ink tanks  315  to  318  is supplied to the inkjet heads  100  to  103  by the ink supply pressure adjustment devices  321  to  324 , respectively. 
     After the image is formed, the sheet S is transmitted from the conveyance belt  14  to the downstream conveyance path  15 . The downstream conveyance path  15  includes feed roller pairs  151 ,  152 ,  153 , and  154 , and sheet guide plates  155  and  156  that form a conveyance path of the sheet S. The sheet S is ejected from a discharge port  157  to the discharge tray  16  from the downstream conveyance path  15 . In the drawing, arrow  107  indicates a conveyance path of the sheet S when on the downstream conveyance path  15 . 
     Next, the configuration of each of the inkjet heads  100  to  103  will be described. Since the inkjet heads  101  to  103  have the same structure as the structure of the inkjet head  100 , the inkjet head  100  will be described as representative by reference to  FIGS.  2  to  4   . 
     As shown in  FIGS.  2  to  4   , the inkjet head  100  includes a nozzle head unit  2 , which is an example of a liquid ejection unit, a flexible printed wiring board  3 , which is an example of a film carrier package, and a drive circuit board  4 . The nozzle head unit  2  includes a nozzle plate  21 , an actuator substrate  22  providing a plurality of actuators, a frame member  23  that forms a common ink chamber  26 , and an ink supply unit  24  that supplies ink to the common ink chamber  26 . 
     The nozzle plate  21  is a rectangular plate that can be made of resin, such as polyimide, or metal, such as stainless steel. A plurality of nozzles  25  that eject ink are formed on a surface of the nozzle plate  21 . The nozzle density of the nozzle plate  21  is set to be in a range of, for example, 150 to 1200 dpi. The actuator substrate  22  is, for example, a rectangular substrate made of insulating ceramics. 
     The frame member  23  surrounds a lower part of the actuator substrate  22 . An opening of a lower surface of the frame member  23  is sealed by the nozzle plate  21 . A space partitioned by the frame member  23 , the actuator substrate  22  and the nozzle plate  21  forms the common ink chamber  26 . The common ink chamber  26  comprises common ink chamber portions  261  and  262  with the actuator substrate  22  interposed therebetween. One common ink chamber portion  261  communicates with an ink supply port  27  and functions as an ink supply path that supplies ink to a plurality of pressure chambers  5 . The ink supply port  27  is connected to the ink supply pressure adjustment device  321  (see  FIG.  1   ) through an ink supply tube  28 . The common ink chamber portion  262  communicates with an ink drain port connected to ink drain tube  29  in a manner similar to ink supply port  27  and ink supply tube  28 . The common chamber portion  262  functions as an ink drain path by which supplied ink is removed from the plurality of pressure chambers  5 . The ink drain port is connected via ink drain tube  29  to the ink supply pressure adjustment device  321  to circulate ink through the inkjet head  100 . 
     As shown in  FIGS.  3  and  4   , a plurality of pressure chambers  5 , which form the ink ejection channels together with the nozzles  25 , and a plurality of air chambers  51 , which form dummy channels, are formed on a surface of the actuator substrate  22  positioned in the common ink chamber  26 . The pressure chambers  5  and the air chambers  51  are separated by a piezoelectric member  6  that forms a side wall. The pressure chamber  5  and the air chamber  51  are formed by grooves formed by cutting into the two piezoelectric members  61  and  62  forming the piezoelectric member  6  which is laminated on the surface of the actuator substrate  22 . The grooves are formed in a rectangular shape along the width direction of the substrate. The two piezoelectric members  61  and  62  are laminated together with their polarization directions being opposite to each other (for example, a facing direction). Each pressure chamber  5  communicates with a nozzle  25  on a one-to-one basis. The air chambers  51  are arranged to be positioned on both sides of a pressure chamber  5 . 
     Two cover plates  67  that each forma side wall on the opposite short sides of the air chamber  51  are respectively provided on both outer facing surfaces of the actuator substrate  22 . The ends of the air chambers  51  are blocked off from the common ink chamber  26  (more particularly, one end is blocked off from common ink chamber portion  261  and the other end is blocked off from common ink chamber portion  262 ) by the cover plates  67 . Each cover plate  67  is formed of, for example, a zirconia plate having a thickness of about 50 μm. In the cover plate  67 , groove-shaped openings  68  corresponding to the shape and positions of the pressure chambers  5  are formed so that the pressure chambers  5  are open to both the common ink chamber portions  261  and  262  and ink can flow through the pressure chambers  5  from the common ink chamber portion  261  to the common ink chamber portion  262 . That is, so the common ink chamber portions  261  and  262  can communicate with each other. The opening  68  of the cover plate  67  on the common ink chamber portion  261  side can be referred to as an ink supply port, the opening  68  of the cover plate  67  on the common ink chamber portion  262  side can be referred to as an ink drain port. Ink is supplied to, and flows from, the pressure chambers  5  through these ink supply and drain ports. 
     As shown in  FIG.  4   , an electrode  63  is integrally formed on an upper surface and side surfaces of each of the pressure chambers  5 . Furthermore, electrically separated electrodes  64  are respectively formed on each side surface (left side and right side surfaces in the drawing) of each of the air chambers  51 . The electrodes  63  are each connected to a common electrode  65 . The electrodes  64  are each connected to and individual electrodes  66 . The common electrode  65  and the individual electrodes  64  may be referred to as wiring electrodes. A contact point between the electrode  63  of a pressure chamber  5  and the common electrode  65  is one terminal of an actuator  8 , and a contact point between an electrode  64  of an adjacent air chamber  51  and the corresponding individual electrode  66  is the other terminal of the actuator  8 . The electrodes  63  and  64 , the common electrode  65 , and the individual electrodes  66  are formed of, for example, a thin nickel film. The common electrode  65  and the individual electrodes  66  on the actuator substrate  22  are insulated by, for example, an insulating layer (not separately depicted). For example, the common electrode  65  is grounded. The individual electrodes  66  apply a drive voltage to the actuator  8  of each channel. With this configuration, an electric field is applied in a direction intersecting (for example, orthogonally intersecting) with a polarization axis of the piezoelectric member  6  (more particularly, the piezoelectric portions  61  and  62 ), and the piezoelectric member  6  on both sides of the pressure chamber  5  is shear-mode deformed. Thereby, inside of the pressure chamber  5  is compressed, and ink is ejected from the nozzle  25 . This forms a capacitance type actuator  8  of a shear mode type. 
     Referring back to  FIG.  2   , the common electrode  65  and the individual electrodes  66  are electrically connected to the flexible printed wiring board  3 , and the flexible printed wiring board  3  is electrically connected to the drive circuit board  4 . The flexible printed wiring board  3  includes an integrated circuit (IC)  31  for driving particular electrodes corresponding to particular nozzles  25 . The drive circuit board  4  temporarily stores print data received from the controller  17  ( FIG.  1   ) of the inkjet printer  10  and applies a drive voltage to the actuators  8  so as to eject ink at a predetermined timing. 
       FIG.  5    is a block configuration diagram of a control system of the inkjet printer  10 . The controller  17  includes a CPU  170 , a ROM  171 , a RAM  172 , an I/O port  173 , and an image memory  174 . The CPU  170  controls the motor  205 , the ink supply pressure adjustment devices  321  to  324 , the operation unit  18 , and various sensors with signals through the I/O port  173 . The image data from the computer  200 , which is an external device communicably connected to the inkjet printer  10 , is transmitted to the controller  17  through the I/O port  173  and stored in the image memory  174 . The CPU  170  transmits the image data stored in the image memory  174  to a drive circuit  7  in the appropriate order for image forming or printing. The drive circuit  7  comprises the flexible printed wiring board  3  and the drive circuit board  4 . 
     The drive circuit  7  includes a print data buffer  71 , which is a channel data supply unit, a decoder  72 , and a driver  73 . The print data buffer  71  stores the image data in time series for each channel. The decoder  72  controls the driver  73  for each channel based on the image data stored in the print data buffer  71 . The driver  73  applies a drive waveform to each actuator  8  of each channel based on the control of the decoder  72 . 
     Next, referring to  FIG.  6   , the drive waveform for the actuator  8  will be described.  FIG.  6    shows, as an example of the drive waveform, a multi-drop drive waveform in which ink is dispensed four times (four droplets) in one drive cycle to form dots on the recording medium (e.g., sheet S). This drive waveform is a so-called “pull drive waveform.” The drive waveform is not limited to the waveform in which four droplets are dispensed and, in general, any number of droplets of one or more can be adopted. The drive waveform is not limited to the pull drive waveform. For example, a push drive waveform or a push-pull drive waveform may be used. 
     The drive waveform applies a bias voltage to the capacitance type actuator  8  until time t 1 , which is the start of the ink discharge operation. Next, after a discharge from time t 1  to time t 2 , a charge voltage is applied from time t 2  to time t 3 , thereby performing the first ink droplet ejection. After a discharge from time t 3  to time t 4 , a charge voltage is applied from time t 4  to time t 5 , thereby performing the second ink droplet ejection. After a discharge from time t 5  to time t 6 , a charge voltage is applied from time t 6  to time t 7 , thereby performing the third ink droplet ejection. After a discharge from time t 7  to time t 8 , a charge voltage is applied from time t 8  to time  9 , thereby performing the fourth ink droplet ejection. The bias voltage is again applied at time t 9  after the completion of the last droplet ejection to attenuate residual oscillation in the pressure chamber  5 . 
     The voltage applied at the time of ink ejection is smaller than the bias voltage, and a voltage value is determined based on, for example, an attenuation rate of pressure oscillation in the pressure chamber  5 . A time period between time t 1  and time t 2 , a time period between time t 2  and time t 3 , a time period between time t 3  and time t 4 , a time period between time t 4  and time t 5 , a time period between time t 5  and time t 6 , a time period between time t 6  and time t 7 , a time period between time t 7  and time t 8 , and a time period between time t 8  and time t 9  are respectively set to a half cycle of an oscillation cycle λ of an inherent pressure oscillation that is determined by, for example, characteristics of ink being ejected and an internal structure dimensions of the inkjet head. The half cycle of the inherent oscillation cycle λ is also referred to as an acoustic length (AL). For example, when the oscillation cycle λ is 4 μs, the half cycle is 2 μs. 
       FIG.  7    schematically shows an example arrangement of the actuators  8  (#1, #2, #3 . . . #n) on the actuator substrate  22  and the wiring of the common electrodes  65  and the individual electrodes  66 . For convenience of drawing, the structure of each actuator  8  is simplified. One terminal of the actuator  8  is connected to the common electrode  65 . The other terminal of the actuator  8  is connected to an individual electrode  66 . In this case, when a large number of actuators  8  are driven at the same time, a large current flows in the common electrode  65  and a voltage drop occurs on the common electrode  65 . This may deform the voltage waveform being applied to the actuators  8  located at a position far away from voltage supply units (which are at left and right ends in the drawing), that is, for example, a position near the center, and the ink may not be ejected in a desired or expected manner. 
     Comparing the case of driving four actuators  8  at the same time and the case of driving  656  actuators  8  at the same time by using the inkjet head  100  equipped with 1312 actuators  8 , the voltage waveform deforms as shown in  FIG.  8   . This indicates that when the number of actuators  8  that are driven at the same time is small, the charge of the actuators  8  starts immediately after the start of energization. On the other hand, when the number of actuators  8  that are driven at the same time is large, at the initial stage of the actuator charge, the ground (Gnd) potential rises and the charging current does not flow, thereby causing the waveform to rise steeply at the beginning. Thereafter, since the actuator charge is performed through a resistance of the common electrode  65 , the rising of the waveform becomes gentle. As a result, the net voltage applied to the actuator  8  decreases, and the ink ejection speed decreases. 
     In order to alleviate the current concentration in the common electrode  65 , as shown in  FIG.  9   , a drive waveform A and a drive waveform B, whose drive timings are mutually shifted are selectively applied to the actuators  8 . The drive timing of the drive waveform B is delayed with respect to the drive waveform A by a half cycle (for example, 2 μs) of the oscillation cycle λ of pressure oscillation. By delaying the drive timing in this manner, the drive waveform B has an opposite phase with respect to the drive waveform A between time t 2  to time t 8 . 
       FIG.  10    shows an example of an actuator drive circuit that selectively applies the drive waveform A and the drive waveform B to the actuators  8  according to the first embodiment. The actuator drive circuit is formed on the driver  73  of the drive circuit  7  ( FIG.  5   ), for example. The individual electrodes  66  of each actuator  8  connect a drive transistor  82  to a switch  83 . The actuators  8  of odd-numbered channels (#1, #3 are connected to a waveform A generation unit  85 . The actuators  8  of even-numbered channels (#2, #4 are connected to a waveform B generation unit  86 . The application points for the drive waveform A and the drive waveform B are thus alternately allocated such that #1=A, #2=B, #3=A, #4=B, #5=A, #6=B, #7=A, #8=B . . . . The waveform A generation unit  85  and the waveform B generation unit  86  are each examples of a drive waveform generation circuit, but in some examples these units may be combined into one circuit. The print data buffer  71  applies a signal for appropriately turning on the switches  83  to the channels for ejecting ink corresponding to the print data. The predetermined drive waveform A or B is applied, through the drive transistor  82 , to the channels for which the respective switch  83  has been turned on. 
     In the present first embodiment, an actuator drive circuit or the like applies the drive waveform A or B to the channels that are located at an electrically closest position on the common electrode  65 . The electrically closest position on the common electrode  65  is one example of “a close position in a predetermined condition direction” in the present embodiment. Since the channels are arranged at equal intervals along the common electrode  65  extending in the X direction in the example arrangement shown in  FIG.  10   , the electrically close direction on the common electrode  65  is along the X direction. In an alternative instance, the arrangement direction of the channels is not limited to the X direction, and the channels may be arranged diagonally in the XY directions as shown in  FIG.  11   . In another instance, in the arrangement of  FIG.  10  or  11   , the position of the nozzle  5  in the Y direction may be finely adjusted by the delay of the drive timing. Therefore, depending on the wiring direction of the common electrode  65  and the arrangement of the channels, the electrically closest direction may not be the X direction. Also, the electrically closest channels on the common electrode  65  may not necessarily all be adjacent channels to each other. Furthermore, although it is desirable that the position is the electrically “closest” position on the common electrode  65 , the electrically close position need not necessarily strictly be the electrically “closest” position as long as cancellation of the current can be still realized. 
     In the case of  FIG.  10   , since the voltage drop of the actuator  8  (#6) and the voltage drop of the actuator  8  (#7) are different only by the voltage drop generated in a short line segment between #6 and #7, it can be said that #6 and #7 are electrically close to each other. For example, if the actuators are configured such that #7 is discharged when #6 is charged, the voltage drop is generated only by a short line segment between #6 and #7 and the voltage drop in other portions of the common electrode  65  is not substantially affected. 
     In the case of  FIG.  11   , when the relationship between the actuator  8  (#9) and the actuator  8  (#8) is considered, a portion that has common impedance is limited to the area on the left of the actuator  8  (#8). The wiring resistance R of the electric path of the common electrode  65  of the actuator  8  (#8) is half the wiring resistance  2 R of the electric path of the common electrode  65  of the actuator  8  (#9). Therefore, a half of the voltage drop that occurs in the electric path of the common electrode  65  up to the actuator  8  (#9) occurs in the portion that has common impedance with the actuator  8  (#8). The portion from the actuator  8  (#8) to the actuator  8  (#9) contributes to the voltage drop of the actuator  8  (#9) but does not contribute to the voltage drop of the actuator  8  (#8). Since this portion also connects with the actuator  8  (#10) and the actuator  8  (#16), the voltage drop of this portion also changes depending on whether or not the actuator (#10) to the actuator  8  (#16) are being driven (charged/discharged). Thus, even when the actuator  8  (#8) and the actuator  8  (#9) have an electrical positional relationship, for example, as long as the actuator  8  (#8) is charged when the actuator  8  (#9) is discharged, charges are transferred between the two, and the effect on voltage drop is small. 
     As for the relationship between the actuator  8  (#9) and the actuator  8  (#10), the common electrode  65  has common impedance in the whole portion excluding the short line segment between the actuator  8  (#9) and the actuator  8  (#10), and the voltage drop that occurs in the electric path of the common electrode  65  reaching each of the actuator  8  (#9) and the actuator  8  (#10) mostly occurs in the portion having the common impedance. For example, the wiring resistance of the electric path of the common electrode  65  reaching each of the actuator  8  (#9) and the actuator  8  (#10) occurs in a portion where most of the electric impedance is the common impedance. Since a difference in the voltage drop between the actuator  8  (#9) and the actuator  8  (#10) is limited to the slight voltage drop, which is caused by driving the actuator  8  (#9) in the short line segment between the actuator  8  (#9) and the actuator  8  (#10), it can be said that the actuator  8  (#9) and the actuator  8  (#10) are electrically close to each other. In a case of such a condition, for example, if the actuator  8  (#10) is discharged when the actuator  8  (#9) is charged, the voltage drop occurs only in this short line segment between #9 and #10, and the voltage drop in other portions of the common electrode  65  is not affected. 
       FIG.  12    shows an example configuration in which the actuator drive circuit shown in  FIG.  10    is applied to the shear mode type actuator  8  shown in  FIG.  4   . In  FIG.  12   , the drive transistor  82  and the switch  83  have been omitted, and the configuration thereof has been simplified by collective representation as an AND gate  87 . 
     In the configuration as shown in  FIG.  12   , as for the actuators  8  driven at the same time, in the portion where the charging timing of the even-numbered actuator  8  (#2, #4 . . . ) matches with the discharging timing of the odd-numbered actuator  8  (#1, #3 . . . ), a current does not flow in the common electrode  65  and a charge is transferred between the even-numbered actuator  8  and the odd-numbered actuator  8 . As a result, the voltage drop on the common electrode  65  is suppressed, the ink ejection is stabilized, and the print quality is improved. For example, when the actuator drive circuit of  FIG.  10    is used, it is further advantageous that the voltage drop when all the channels eject ink can be suppressed. 
     In the present embodiment, the phrase “the actuators  8  driven at the same time” includes not only actuators whose drive timings are exactly the same but also actuators whose drive timings are different but drive cycles (for example, the charging cycles and the discharging cycles of the actuators  8 ) are partially overlapped with each other, in the group of the actuators  8  that eject ink. Further, while one example of the “close position in the predetermined condition direction” is an electrically close position on the common electrode  65 , another example may be a position where a separation distance between the pressure chambers  5  is small such that an effect of pressure oscillation can be alleviated or suppressed. 
       FIG.  13    shows a modification example of the actuator drive circuit that selectively applies the drive waveform A and the drive waveform B to the actuators  8 . In this modification example, the actuators  8  that apply the drive waveform A and the actuators  8  that apply the drive waveform B are not set alternately one-to-one but rather every other two of the actuators  8  in the arrangement depicted in  FIG.  13    are applied with a different waveform. For example, the drive waveform A and the drive waveform B are allocated such that #1=A, #2=A, #3=B, #4=B, #5=A, #6=A, #7=B, #8=B . . . . Also, in this case, in the portion where the charging timing coincides with the discharging timing of the actuators  8  to be driven at the same time, the current does not flow in the common electrode  65  and the voltage drop on the common electrode  65  can be suppressed. For example, when the actuator drive circuit in  FIG.  13    is used, there is a further advantage that the voltage drop can be suppressed when driving only the even-numbered channels or the odd-numbered channels at the same time in a case of printing of halftone or the like. 
       FIG.  14    shows another modification example of the actuator drive circuit which selectively applies the drive waveforms A and B to the actuators  8 . In the examples of  FIGS.  10  and  13   , the drive waveform to be applied to each channel is fixed to be either the drive waveform A or the drive waveform B. However, with the actuator drive circuit shown in  FIG.  14   , which includes a waveform reference selection circuit  9 , either the drive waveform A or the drive waveform B can be selectively applied to most channels. Thus, channels at the electrically closest positions on the common electrode  65  among those actuators  8  to be driven at the same time can selectively receive the drive waveform A or B as appropriate. Alternatively, channels to be driven at the same time at the positions for which the physical distance between the pressure chambers  5  is close can selectively receive the drive waveform A or B as appropriate. 
     The waveform reference selection circuit  9  includes a first AND circuit  91 , a second AND circuit  92 , a NOT circuit  93 , an EXOR circuit (exclusive OR circuit)  94 , a first switch  95  on the waveform A side, and a second switch  96  on the waveform B side. With this configuration, which drive waveform is to be applied to the channel can be determined in advance, starting from, for example, channel #1 at the end portion. In the example shown in  FIG.  14   , the drive waveform A is selected as the waveform to be applied to the first channel (#1) in a fixed manner. However, the second and subsequent channels (from #2 upward) are connected to both the waveform A generation unit  85  and the waveform B generation unit  86 , and the waveform reference selection circuit  9  selects which of the waveforms A and B is to be applied to the second and subsequent channels. 
     For example, when ink is to be ejected from the first (#1), second (#2), third (#3) and fifth (#5) channels at the same time, in the first channel (#1), a signal “1” from the print data buffer  71  is applied to the first switch  95  to turn ON the switch, and the drive waveform A is applied. In the second channel (#2), the signal “1” from the print data buffer  71  is applied to the first AND circuit  91 , the signal “1” from the first channel (#1) is set to “0” by the NOT circuit  93 , and the set signal is applied to the first AND circuit  91 . Thus, the first switch  95  on the waveform A side is turned OFF for the second channel (#2). On the other hand, in the second AND circuit  92 , the signal “1” from the print data buffer  71  and the signal “1” from the first channel (#1) are applied to turn ON the second switch on the waveform B side, and the waveform. B is thus applied to the second channel (#2). In the same manner, the drive waveform A is selected for the third channel (#3). 
     Next, since the fourth channel (#4) is not driven in this example, the signal “0” from the print data buffer  71  is applied to the first AND circuit  91  and the second AND circuit  92 , and both switches  95  and  96  are turned OFF. In the fifth channel (#5), the signal “1” from the print data buffer  71  is applied to the first AND circuit  91 , and the signal “1”, which is output from the EXOR circuit  94  of the fifth channel (#5) in response to both the signal “0” from the fourth channel (#4) and the signal “1” from the EXOR circuit  94  of the fourth channel (#4), is set to “0” by the NOT circuit  93  and applied to the first AND circuit  91 . Thus, the first switch on the waveform A side is turned OFF. In the second AND circuit  92 , the signal “1” from the print data buffer  71  and the signal “1” from the EXOR circuit  94  are applied to turn ON the second switch  96  on the waveform B side, and the drive waveform B is applied. As a result, the drive waveforms are allocated such that #1=A, #2=B, #3=A, #4=Off, and #5=B. In a case where the fourth channel (#4) is also to be driven, as for the fifth channel (#5), by referring to the drive waveform B applied to the fourth channel (#4), a drive waveform A is selected. 
     The actuator drive circuit shown in  FIG.  14    searches for a driven channel positioned on the left side of a to-be-driven channel in an electrically close direction on the common electrode  65  and checks whether the driven channel on the nearest left side is driven by the drive waveform A or the drive waveform B. Alternatively, the actuator drive circuit searches for a driven channel that is positioned to the left side of the to-be-driven channel for which the physical distance between the pressure chambers  5  is close and checks whether the nearest driven channel on the left side is driven by the drive waveform A or the drive waveform B. The actuator drive circuit selects the drive waveform B for the to-be-driven channel when the drive waveform applied to the driven channel on the nearest left side is A, and selects the drive waveform A for the to-be-driven channel when the drive waveform applied to the driven channel on the nearest left side is B. By using this actuator drive circuit, it is possible to alternately drive the channels with the drive waveform A and the drive waveform B regardless of the print pattern, and it is also possible to cancel the current flowing in the common electrode  65  regardless of the drive pattern. According to the present embodiment, the determination of which drive waveform is to be applied to which channel does not necessarily start from the leftmost channel (#1). 
     In the example arrangement shown in  FIG.  14    using only the drive waveform A and the drive waveform B, there may be a case where when attempting to cancel the current flowing in the common electrode  65  using the drive waveform B, the current is not canceled at the beginning part (time t 1 ) and the end part (time t 9 ) of the waveform. In order to alleviate the current concentration at the beginning part (time t 1 ) and the end part (time t 9 ) of the waveform, a shorter time delay may be added to the current cancellation of the adjacent channel. As an example, drive waveforms A to H (delay 0 to 7) shown in  FIG.  15    can be used. The drive waveform C delays the drive timing with respect to the drive waveform A by one half of the half cycle of the pressure oscillation (delay 2). The drive waveform D delays the drive timing with respect to the drive waveform C by a half cycle of the pressure oscillation (delay 6). The drive waveform E delays the drive timing with respect to the drive waveform A by a quarter of the half cycle of the pressure oscillation (delay 1). The drive waveform F delays the drive timing with respect to the drive waveform E by a half cycle of the pressure oscillation (delay 5). The drive waveform G delays the drive timing with respect to the drive waveform A by three fourths of the half cycle of the pressure oscillation (delay 3). The drive waveform H delays the drive timing with respect to the drive waveform G by a half cycle of the pressure oscillation (delay 7). 
       FIG.  16    shows an example of the actuator drive circuit which selectively applies the delays 0 to 7 (that is, drive waveforms A to H) to the actuators  8 . The seven drive waveforms A to H from the waveform generation unit  89  are allocated to the first channel (#1) to the eighth channel (#8) in the order of delays 0 to 7. The same is applied to the ninth channel (#9) and the subsequent channels. Each switch  83  can be selectively turned ON by the signal from the print data buffer  71 . The print data buffer  71  turns ON the switches  83  of the channels to be driven at the same time. Thus, each channel is driven by the drive waveforms A to H allocated to the respective channels. When the actuator drive circuit in  FIG.  16    is used, the charging current and the discharging current of the actuators  8  of the channels #1 and #2, #3 and #4, #5 and #6, and #7 and #8 mutually cancel the current flowing in the common electrode  65 , and at the beginning timing (time t 1 ) and the end timing (time t 9 ) for the waveform that cannot be canceled, the current is dispersed to suppress the voltage drop of the common electrode  65 . As a result, ink ejection stabilizes, and printing quality improves. 
     The actuator drive circuit that applies a plurality of drive waveforms to the actuators  8  may be configured in a programmable manner.  FIG.  17    shows an example of an actuator drive circuit  300  capable of generating the plurality of drive waveforms corresponding to the drive waveforms A to H by allocating a delay time to each actuator in a programmable manner using the drive waveform shown in  FIG.  6    as a common drive waveform. By the actuator drive circuit  300 , it is possible to determine to which channels the drive waveforms A to H are allocated at which drive timings among the drive timings (delays 0 to 7), and to start generating the drive waveforms A to H at the allocated drive timings. 
     The actuator drive circuit  300  includes a waveform generation circuit  301  and a waveform allocation circuit  302 . The waveform generation circuit  301  includes a plurality of delay circuits  303 , a delay time setting memory  304 , a plurality of drive waveform generation circuits  305 , and a drive waveform setting memory  306 . The plurality of delay circuits  303  and the plurality of drive waveform generation circuits  305  are connected in series, respectively. There are eleven pairs of the delay circuits  303  and the drive waveform generation circuits  305 , for example. 
     In the drive waveform setting memory  306 , common drive waveform information is stored. In this example, the drive waveform shown in  FIG.  5    is a common drive waveform. In the delay time setting memory  304 , the set values of the delay amounts for delay 0 to delay 7 are stored. For the drive waveforms A to H, the set values are delay 0 (0.00 μs), delay 1 (0.50 μs), delay 2 (1.00 μs), delay 3 (1.50 μs), delay 4 (2.00 μs), delay 5 (2.50 μs), delay 6 (3.00 μs), and delay 7 (3.50 μs), for example. 
     The waveform allocation circuit  302  includes a selector  307  and a drive waveform selection memory  308 . In the drive waveform selection memory  308 , one or more “allocation patterns” that set which of the delay amounts 0 to 7 are to be allocated to which of the channels are stored.  FIG.  18    shows example allocation patterns. As shown in  FIG.  18   , in for different allocation patterns (left page portions of  FIG.  18   ), delays selected from among the eight different kinds of delays (delay 0 to 7) are allocated to a matrix with 4 columns and 8 rows. In the table shown in  FIG.  18   , the vertical and horizontal axes do not necessarily represent the structural row and column positions of the actuators  8 , but the delay in the row n, column m position of a table corresponds to the delay for the (n+(m−1)×8)th channel.  FIG.  18    also shows (right page portions) the delay times allocated to each channel using the corresponding allocation pattern. For convenience of drawing, the 13th and subsequent rows are omitted from the depiction in  FIG.  18    (right page portions), but the 13th and subsequent rows are similarly allocated with delay times according to the respective allocation patterns. 
     The selector  307  is, for example, a selector for the “11 to 1” portion of the 32 channels (ch). The selector  307  is connected to each of an output end of each drive waveform generation circuit  305 . Further, output ends of the 32 chs connected the selector  307  are connected to the channels through the switches  309 , respectively. 
     With respect to the channels, eight channels form one set, and four sets of channels (for a total of 32 channels in a channel group) constitute one region. For example, seven regions (not at all separately depicted) are provided in total. Furthermore, in some examples, a plurality of channels can share the same channel (ch) among the seven regions so that the channel 1 of the region 1 and the channel 33 of the region 2 are the same channel (ch). Each switch  309  selectively controls whether to apply the drive signal from the selector  307  to each of the channels. The print data buffer  71  turns ON the switches  309  of the channels that are to be driven at the same time. 
     In the drive circuit  300  according to the present first embodiment, when a print trigger is applied to the delay time setting memory  304 , each of the delay circuits  303  waits for the respective delay time (0.00 μs to 3.50 μs) to elapse and then activates each of the drive waveform generation circuits  305 . The drive waveform generation circuits  305  output the drive waveforms stored in the drive waveform setting memory  306 . Therefore, the generation start timings of the drive waveforms differ from each other by the difference of the respective delay amounts. 
     The drive waveforms from the respective drive waveform generation circuits  305  are applied to the selector  307 . The selector  307  distributes the drive waveforms (which have different generation start times) to the channels according to the allocation pattern (having 8 rows and 4 columns) stored in the drive waveform selection memory  308 . Then, the allocation pattern is shifted in the +X direction and repeatedly applied to allocate the drive waveforms to all the channels that are two-dimensionally arranged (see  FIG.  18   ). Each drive waveform allocated by the selector  307  is applied to the actuator  8  of the channel whose switch  309  is turned ON. 
     Second Embodiment 
     Next, an inkjet head  400  according to a second embodiment will be described with reference to  FIGS.  19  and  20   . The inkjet head  400  of the second embodiment has the same or substantially the same configuration as that of the first embodiment except that drive waveforms having completely opposite phases are generated and applied to the actuators  8  at the same drive timing, for example. Thus, the same configuration elements, components, or the like will be denoted by the same reference numerals as those of the first embodiment, and the detailed description thereof will be omitted. 
       FIG.  19    shows drive waveforms I and J that form dots by dispensing ink once in one drive cycle, as an example of the drive waveforms of completely opposite phases. In the drive waveform I, a negative voltage is applied to the actuator  8  as a bias voltage from time t 1  to time t 2 . Then, voltage V 0  (=0 V) is applied from time t 2  (that is when the ink ejection operation is started) to time t 3 . Then, the ink is dispensed by applying a positive voltage from time t 3  to time t 4 . 
     In the drive waveform J, a positive voltage is applied to the actuator  8  as a bias voltage from time t 1  to time t 2 . Then, voltage V 0  (=0 V) is applied from time t 2  to time t 3 . Then, the ink is dispensed by applying a negative voltage from time t 3  to time t 4 . The drive waveform I and the drive waveform J are thus inverted from each other. 
     As shown in  FIG.  20   , for the even-numbered actuators  8  (#2, #4 . . . ), the electrode  63  of a pressure chamber  5  is grounded to the ground (Gnd) through the common electrode  65 , and a drive waveform is applied to the electrode  64  of the air chamber  51  through an individual electrode  66  (similar to  FIG.  12   ). The drive waveform to be applied is the drive waveform J, for example. For the odd-numbered actuators  8  (#1, #3 . . . ), the electrode  64  of the air chamber  51  is grounded to the ground (Gnd) through the common electrode  65 , and a drive waveform is applied to the electrode  63  of the pressure chamber  5  through an individual electrode  66 . The drive waveform to be applied is, for example, the drive waveform I. That is, the even-numbered actuators  8  (#2, #4 constitute a first group of actuators  8  that pressurize the pressure chambers  5  when positive voltages are applied, and the odd-numbered actuators  8  (#1, #3 constitute a second group of actuators  8  that pressurize the pressure chambers  5  when negative voltages are applied. 
     In the inkjet head  100  of the first embodiment, the drive waveforms in which the drive timings are shifted are applied to cancel the current of the common electrode  65 . In the inkjet head  400  of the second embodiment, the drive waveform I is applied to some actuators  8  at the same time the drive waveform J is applied to some other actuators  8 . That is, in the same operation, the first group of actuators  8  (even-numbered actuators  8 ) and the second group of actuators  8  (odd-numbered actuators  8 ) receive drive waveforms I and J having completely opposite phases. Thus, drive waveforms I and J can be applied at the same drive timing. Since a period of time in which a positive voltage is applied matches with a period of time in which a negative voltage is applied in the drive waveform I and the drive waveform J, even when the actuators  8  are driven at the same time, the current of the common electrode  65  can be canceled. 
     According to any of the present embodiments, when the number of actuators  8  to be driven is large, particularly when some of the actuators to be driven are disposed at electrically close positions, current concentration on the common electrode  65  can be suppressed. As a result, it is possible to stabilize liquid ejection parameters such as the ejection speed and the ejection amount. For example, in a sequential supply type process, when a voltage drop might occur in the common electrode  65 , a difference in the actuator drive voltage actually applied to some of the actuators  8  may be different from some others or the intended drive voltage. As a result, liquid ejection characteristics may be uneven across the plurality of actuators  8 , which may cause uneven density of dispensed ink droplet on the printing surface. However, according to the present embodiments, it is possible to suppress the voltage drop that might otherwise occur on the common electrode  65  that is connected to the plurality of actuators  8 , thereby uneven printing density can be avoided or reduced. Alternatively, by applying the present embodiments in such a manner that the drive waveforms with different drive timings are applied to the actuators  8  at the positions in which the physical distance between the pressure chambers  5  is close, an influence of pressure oscillation between the channels can be alleviated, and thus the liquid ejection can be stabilized. 
     The inkjet head  100  is not limited to the shear mode type actuator  8  in which the ejection channels and the dummy channels are alternately arranged. For example, the plurality of nozzles  25  and the plurality of actuators  8  may be arranged on the surface of the nozzle plate  5 . Other droplet-on-demand type piezoelectric actuators may be used as the actuators  8 . 
     In the present embodiments, an inkjet head  100  (or  400 ) of an inkjet printer  10  has been described as an example of a liquid ejection apparatus  1 . In other embodiments, the liquid ejection apparatus  1  may be a molding material ejection head of a 3D printer or a sample ejection head of a liquid dispensing apparatus. 
     While certain embodiments have been described, these embodiments have been presented by way of example only and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.