Patent Publication Number: US-10780693-B2

Title: Inkjet head

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of U.S. patent application Ser. No. 15/928,816, filed Mar. 22, 2018, which is based upon and claims the benefit of priority from Japanese Patent Application No. 2017-058661, filed on Mar. 24, 2017 and Japanese Patent Application No. 2017-058662, filed on Mar. 24, 2017, the entire contents of each of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to an inkjet head. 
     BACKGROUND 
     In an inkjet head that can discharge multiple droplets from a single nozzle, the number of droplets discharged is adjusted when gradation-type printing is being performed. In the multi-drop printing method in the related art, a drive waveform for discharging a single ink droplet from the nozzle is repeated as many times as necessary to provide the desired total number of droplets. Therefore, as the number of ink droplets is increased, the number of operations of an actuator is also increased, and, as a result, power consumption is increased. In addition, in general, since an operating time increases in direct proportion to the number of ink droplets that are discharged, there is a problem in that it is difficult to increase a drive frequency. 
     For this reason, there is a demand for an inkjet head providing reduced power consumption when discharging multiple ink droplets from a nozzle in a multi-drop printing method, while still being capable of providing high-speed operation. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view of an inkjet head. 
         FIG. 2  is a partial enlarged perspective view of one of piezoelectric members arranged in two rows on a substrate of an inkjet head. 
         FIG. 3  is a partial enlarged cross-sectional view of an inkjet head taken along arrow line F 3 -F 3  in  FIG. 1  in a longitudinal direction. 
         FIG. 4  is a partial enlarged top plan view of one of the piezoelectric members of in inkjet head. 
         FIG. 5  is a cross-sectional view of the inkjet head taken along arrow line F 5 -F 5  in  FIG. 4 . 
         FIG. 6  is a cross-sectional view of the inkjet head taken along arrow line F 6 -F 6  in  FIG. 4 . 
         FIG. 7  is a block diagram of a drive circuit of an inkjet head. 
         FIG. 8  depicts a drive voltage of a 1-drop waveform applied to an actuator of an inkjet head. 
         FIG. 9  depicts a drive voltage of a 1-drop waveform and simulated values of an ink pressure waveform and an ink flow velocity waveform under an application of the 1-drop waveform to an actuator of an inkjet head. 
         FIG. 10  depicts a drive voltage a 2-drop waveform applied to an actuator of an inkjet head. 
         FIG. 11  depicts a drive voltage of a 2-drop waveform and simulated values of an ink pressure waveform and an ink flow velocity waveform under an application of the 2-drop waveform to an actuator of an inkjet head. 
         FIG. 12  depicts a drive voltage of a 3-drop waveform applied to an actuator of an inkjet head. 
         FIG. 13  depicts a drive voltage of a 3-drop waveform and simulated values of an ink pressure waveform and an ink flow velocity waveform under an application of the 3-drop waveform to an actuator of an inkjet head. 
         FIG. 14  depicts a drive voltage of modified 2-drop waveform applied to an actuator of an inkjet head. 
         FIG. 15  depicts a drive voltage of a modified 2-drop waveform and simulated values of an ink pressure waveform and an ink flow velocity waveform under an application of the modified 2-drop waveform to an actuator of an inkjet head. 
         FIG. 16  is a first waveform chart for explaining a method of determining a trailing edge of a contraction pulse and a trailing edge of a weak contraction pulse of the 2-drop waveform. 
         FIG. 17  depicts a circuit diagram having parameters used for the simulated values in  FIGS. 9, 11, 13, 15, 16, 18, 19, 24, 25, 26, 27, and 28 . 
         FIG. 18  is a second waveform chart for explaining the method of determining the trailing edge of the contraction pulse and the trailing edge of the weak contraction pulse of the 2-drop waveform. 
         FIG. 19  is a third waveform chart for explaining the method of determining the trailing edge of the contraction pulse and the trailing edge of the weak contraction pulse of the 2-drop waveform. 
         FIG. 20  depicts a first example of a combination of drive waveform units. 
         FIG. 21  depicts a second example of a combination of drive waveform units. 
         FIGS. 22A, 22B, and 22C  depict waveform examples according to the first example illustrated in  FIG. 20 . 
         FIGS. 23A, 23B, and 23C  depict waveform examples according to the second example illustrated in  FIG. 21 . 
         FIG. 24  depicts a drive voltage of a 2-drop waveform and simulated results of an ink pressure and an ink flow velocity when the time point of the leading edge of the contraction pulse of the 2-drop waveform illustrated in  FIG. 10  is advanced. 
         FIG. 25  depicts a drive voltage of a 2-drop waveform and simulated results of an ink pressure and an ink flow velocity when a contraction pulse of the 2-drop waveform is applied at an earlier timing than in  FIG. 10 . 
         FIG. 26  depicts a drive voltage of a 3-drop waveform and simulated values of an ink pressure and an ink flow velocity when a second contraction pulse of the 3-drop waveform is applied at an earlier timing than in  FIG. 12 . 
         FIG. 27  depicts a drive voltage of a 3-drop waveform and simulated values of an ink pressure and an ink flow velocity when a first contraction pulse of the 3-drop waveform is applied at an earlier timing than in  FIG. 12 . 
         FIG. 28  depicts a drive voltage of a 2-drop waveform and simulated results of an ink pressure and an ink flow velocity when a contraction percentage of the weak contraction pulse of the 2-drop waveform illustrated in  FIG. 10  is changed. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, an inkjet head includes a pressure chamber connected to a nozzle, an actuator corresponding to the pressure chamber and configured to change a volume of the pressure chamber, and a drive circuit configured to drive the actuator causing two or more ink droplets to be consecutively discharged from the nozzle. The drive circuit applies in sequence a first drive waveform for expanding the pressure chamber, a second drive waveform having a first pulse width, a third drive waveform for releasing the pressure chamber from an expanded state, a fourth drive waveform having a second pulse width, and a fifth drive waveform for contracting the pressure chamber. 
     Hereinafter, embodiments of inkjet head that can reduce power consumption and increase an operation speed by discharging multiple ink droplets will be described with reference to the drawings. 
     First, the configuration of an inkjet head  1  will be described with reference to  FIGS. 1 to 6 . 
       FIG. 1  is an exploded perspective view of the inkjet head  1 . For example, the inkjet head  1  is an on-demand type inkjet head using a share mode method. For example, the inkjet head is mounted in an inkjet printer and discharges ink to a recording medium. 
     The inkjet head  1  has a substrate  100 , a frame  200 , a nozzle plate  300 , and a casing  400 . Further, the inkjet head has upstream and downstream side ink manifolds (not specifically illustrated), a drive circuit  40 , and the like in the casing  400 . The drive circuit  40  operates the inkjet head  1 . The upstream and downstream side ink manifolds are connected to upstream and downstream side ink tanks (not specifically illustrated) outside the head  1 . 
     The substrate  100  is a rectangular shaped plate, and one surface of the substrate  100  is a mounting surface  121 . The inkjet head  1  has two lines of piezoelectric members  118 , which extend in the longitudinal direction of the substrate  100  and are arranged in two rows in a central portion of the mounting surface  121 . Each of the piezoelectric members  118  has a trapezoidal cross section in a transverse direction, and the piezoelectric members  118  are disposed in parallel and spaced apart from each other. The substrate  100  includes a multiple supply ports  125  and multiple discharge ports  126  arranged in the longitudinal direction of the piezoelectric members  118 . 
     The supply ports  125  are arranged between the two piezoelectric members  118  in the longitudinal direction of the substrate  100  along the central portion of the substrate  100 . Each of the supply ports  125  penetrates the substrate  100  and is in fluid communication with an upstream side ink manifold, and an end of the supply port  125  is connected to the upstream side ink tank. In other words, the ink, which is supplied to the inkjet head  1  from the upstream side ink tank through the upstream side ink manifold and the supply ports  125 , flows into an ink chamber  116  (see  FIGS. 5 and 6 ). The discharge ports  126  are arranged in two rows outside of the two piezoelectric members  118  with the supply ports  125  interposed therebetween. Each of the discharge ports  126  penetrates the substrate  100  and is in fluid communication with a downstream side ink manifold, and an end of the discharge port  126  is connected to the downstream side ink tank. The ink in the ink chamber  116  is discharged to the downstream side ink tank via the respective discharge ports  126  and the downstream side ink manifold. The ink in the downstream side ink tank disposed outside the head  1  returns back to the upstream side ink tank by a pump (not specifically illustrated). Therefore, the ink is circulated between the respective ink tanks and the ink chamber  116  via the supply ports  125  and the discharge ports  126 . 
     The nozzle plate  300  is a rectangular plate shape, and has multiple nozzles  301  for discharging ink droplets. The nozzles  301  penetrate the nozzle plate  300  and are arranged in two rows in the longitudinal direction of the nozzle plate  300 . An ink repellent film is formed on a surface  302  of the nozzle plate  300  on a side from which the ink droplets are discharged from the nozzles  301 . For example, the ink repellent film is made of a silicon-based liquid repellent material or a fluorine-containing organic material that has liquid repellency. 
     The nozzle plate  300  is disposed to face the mounting surface  121  of the substrate  100  via the frame  200 . With this arrangement, the inkjet head  1  forms the ink chamber  116  surrounded by the substrate  100 , the frame  200 , and the nozzle plate  300 . 
     The frame  200  is disposed between the mounting surface  121  of the substrate  100  and the nozzle plate  300 . The frame  200  has a size that surrounds the two piezoelectric members  118  and surrounds all of the nozzles  301 . 
     The piezoelectric members  118  are formed of lead zirconate titanate (PZT). The piezoelectric members  118  are formed by sticking two plate-shaped piezoelectric bodies together such that polarization directions thereof are opposite to each other. In the example embodiment described herein, the piezoelectric members  118  are bar-shaped extending in the longitudinal direction. Further, the piezoelectric material is not limited to lead zirconate titanate (PZT), and for example, various types of piezoelectric materials such as PTO (PbTiO 3 : lead titanate), PMNT (Pb(Mg 1/3 Nb 2/3 )O 3 —PbTiO 3 ), PZNT (Pb(Zn 1/3 Nb 2/3 )O 3 —PbTiO 3 ), ZnO, and AlN may be used. 
     The piezoelectric members  118  are attached to the mounting surface  121  of the substrate  100 . For example, a thermosetting epoxy-based adhesive is used as an adhesive. 
       FIG. 2  is a partially enlarged perspective view one of piezoelectric members  118  arranged in two rows on the substrate  100 . A portion of the nozzle plate  300  is not illustrated in  FIG. 2  to show an internal structure of the piezoelectric member  118 . 
     The piezoelectric member  118  has an upper surface  118   c  and two inclined surfaces  118   b . The upper surface  118   c  extends in the transverse direction of the substrate  100  in parallel with the mounting surface  121  of the substrate  100 . The two inclined surfaces  118   b  extend toward the mounting surface  121  from either end sides of the upper surface  118   c . Multiple first grooves  131  (hereinafter, also referred to as pressure chambers  131 ) and multiple second grooves  132  (hereinafter, also referred to as dummy chambers  132 ), which extend in the transverse direction of the substrate  100 , are alternately provided on a surface  118   a  of the piezoelectric member  118 . That is, the piezoelectric member  118  has partition walls  133  which separate the first grooves  131  and the second grooves  132 . In other words, each partition wall  133  is a protrusion portion between adjacent first and second grooves  131  and  132 . The opposite ends of the first grooves  131  and the opposite ends of the second grooves  132  are connected to the inclined surfaces  118   b . In the example embodiment described herein, the first grooves  131  and the second grooves  132  are formed in the same shape. However, the shapes of the first grooves  131  and the second grooves  132  may be different from each other in other examples. 
     Wall materials  117  are provided at the both end portions of the second grooves  132 , respectively. The wall materials  117  seal the opposite ends of the second grooves  132 . Each of the wall materials  117  has an upper surface  117   a  provided to be flush with the upper surface  118   c  of the piezoelectric member  118 . The upper surface  118   c  of the piezoelectric member  118  and the upper surfaces  117   a  of the wall materials  117  are attached to the nozzle plate  300 . Therefore, the ink in the ink chamber  116 , is prevented from penetrating into the second grooves  132 . 
       FIG. 3  is a partially enlarged cross-sectional view of the inkjet head  1  illustrated in  FIG. 1  taken along arrow line F 3 -F 3  in the longitudinal direction.  FIG. 4  is a partially enlarged top plan view of the piezoelectric member  118  of the inkjet head  1  illustrated in  FIG. 1 .  FIG. 5  is a cross-sectional view of the inkjet head  1  illustrated in  FIG. 4  taken along arrow line F 5 -F 5 .  FIG. 6  is a cross-sectional view of the inkjet head  1  illustrated in  FIG. 4  taken along arrow line F 6 -F 6 . Hereinafter, a structure of the ink chamber  116  and a method of causing the ink to flow will be described in detail with reference to  FIGS. 3 to 6 . 
     First, as illustrated in  FIG. 3 , the nozzles  301  of the nozzle plate  300  are provided such that one nozzle  301  communicates with one first groove  131 . That is, the nozzle plate  300  has the two rows of nozzles  301  corresponding to the first grooves  131  formed in the two rows of piezoelectric members  118 . There is no nozzle that corresponds to the second grooves  132 . 
     As illustrated in  FIGS. 5 and 6 , the ink chamber  116  is a space surrounded by the mounting surface  121  of the substrate  100 , the nozzle plate  300 , and the frame  200 . The ink chamber  116  includes a first ink chamber  116   a  and second ink chambers  116   b . The first ink chamber  116   a  is a space between the two piezoelectric members  118 . The supply ports  125  communicate with the first ink chamber  116   a . The second ink chambers  116   b  are frame  200  side (outer) spaces of the two piezoelectric members  118 . The discharge ports  126  communicate with the second ink chambers  116   b , respectively. 
     The ink is supplied to the first ink chamber  116   a  via the upstream side ink manifold from the upstream side ink tank outside the head  1 . The ink chamber  116  is slowly filled with the supplied ink. Specifically, the ink flowing into the first ink chamber  116   a  flows toward the two second ink chambers  116   b  outside the first ink chamber  116   a  via the first grooves  131  of the piezoelectric members  118  on the both sides of the first ink chamber  116   a . Therefore, the entire ink chamber  116  surrounded by the frame  200  is filled with the ink. Further, the ink flowing into the second ink chamber  116   b  flows toward the downstream side ink tank in the outside of the head  1  via the downstream side ink manifold through the discharge ports  126 . 
     The both ends of the second grooves  132 , which is alternately disposed between the first grooves  131 , are closed by the wall materials  117 , as illustrated in  FIGS. 4 and 5 . Thus, the ink does not penetrate into the second grooves  132 . As described above, the first grooves  131  serve as a part of a flow path through which the ink is circulated, and the second grooves  132  serve as dummy chambers into which no ink penetrates. 
     Next, electrodes and wires on the substrate  100  and the piezoelectric members  118  will be described. 
     As illustrated in  FIG. 3 , first electrodes  134  are formed in the first grooves  131 , and second electrodes  135  are formed in the second grooves  132 . In the example described in  FIG. 3 , one first electrode  134  is formed in one first groove  131 , and two second electrodes  135  are formed in one second groove  132 . Each first electrode  134  is formed over a pair of the side surfaces  138  and the bottom surface  139  of each first groove  131 . Each second electrode  135  is formed over a side surface  140  and a part of the bottom surface  141  of each second groove  132 . 
     As illustrated in  FIGS. 4 to 6 , first wires  136  extending to the first grooves  131  and second wires  137  extending to the second grooves  132  are provided on the substrate  100  in the second ink chambers  116   b . In detail, one first wire  136  is provided for each first groove  131 , and two second wires  137  are provided for each second groove  132 . One end of the first wire  136  is connected to the first electrode  134  formed in the first groove  131 , and the other end of the first wire  136  is connected to the drive circuit  40  illustrated in  FIG. 1  via a flexible wiring board  40   a . In addition, one end of each of the two second wires  137  is connected to each of the two second electrodes  135  formed in the second groove  132 , and the other end of each of the second wires  137  is connected to the drive circuit  40  via the flexible wiring board  40   a.    
     For example, the first and second electrodes  134  and  135  provided in the first and second grooves  131  and  132  are formed of a nickel thin film. The material of the first and second electrodes  134  and  135  is not limited thereto, and for example, the first and second electrodes  134  and  135  may be formed of a thin film made of Pt (platinum), Al (aluminum), or Ti (titanium). Further, other materials such as Cu (copper), Al (aluminum), Ag (silver), Ti (titanium), W (tungsten), Mo (molybdenum), and Au (gold) may be used as the material of the first and second electrodes  134  and  135 . 
     With the aforementioned configuration, each piezoelectric member  118  may be deformed by a potential difference between the first electrode  134  and the second electrode  135  that faces the first electrode  134  with the piezoelectric member  118  interposed therebetween. That is, an actuator for varying the volume of the first groove  131  is configured with the piezoelectric member  118  and the first and second electrodes  134  and  135  with the piezoelectric member  118  interposed therebetween. Further, one channel for discharging the ink includes the actuator, the first groove  131  filled with the ink, and the nozzle  301  corresponding to the first groove  131 . 
     In the following descriptions, the first groove  131  will be referred to as a pressure chamber  131 , and the second groove  132  will be referred to as a dummy chamber  132 . The drive circuit  40  of the inkjet head  1  will be described with reference to  FIG. 7 . 
       FIG. 7  is a block diagram of a main part of the drive circuit  40  together with a partially enlarged view of the inkjet head  1 . In the inkjet head  1 , the two dummy chambers  132 , which are adjacent to the partition walls  133  of one pressure chamber  131 , are partially illustrated. As described above, the volume of the pressure chamber  131  is changed by the actuator such that the ink can be discharged from the nozzle  301  that communicates with the pressure chamber  131 . The actuator, which is a combination of the partition walls  133 , causes the piezoelectric member  118  to undergo shear deformation by a potential difference between the first electrode  134  in the pressure chamber  131  and the second electrodes  135  in the adjacent dummy chambers  132 , thereby expanding or contracting the volume of the pressure chamber  131 . 
     The drive circuit  40  is a circuit for applying a driving signal of the actuator to the first and second electrodes  134  and  135 . The drive circuit  40  includes a corresponding waveform generating unit  41 , an adjacent waveform generating unit  42 , a printing data setting unit  43 , a waveform selecting unit  44 , a driver unit  45 , and a waveform connection control unit  46 . 
     The waveform generating unit  41  generates a signal S 1  to be applied to the first electrode  134 . The waveform generating unit  42  generates a signal S 2  to be applied to the second electrodes  135  in the two dummy chambers  132  adjacent to the pressure chamber  131 . 
     The printing data setting unit  43  sets external printing data provided from the outside. The waveform selecting unit  44  outputs an ON/OFF selecting signal SL based on the printing data set by the printing data setting unit  43 . An ON time of the selecting signal SL varies depending on a gradation value of the printing data (see  FIGS. 22A to 22C  and  FIGS. 23A to 23C ). 
     The driver unit  45  has a first driver  451  connected to the first electrode  134 , and second drivers  452  connected to the second electrodes  135 . The first driver  451  is interposed between the waveform generating unit  41  and the first electrode  134 . The first driver  451  applies the signal S 1 , which is generated by the waveform generating unit  41 , to the first electrode  134 . Each of the second drivers  452  is interposed between the waveform generating unit  42  and the second electrodes  135 . Each of the second drivers  452  has a floating (high impedance) control input terminal, and the selecting signal SL is input to the floating control input terminal. When the selecting signal SL is ON, the second drivers  452  apply the signal S 2 , which is generated by the waveform generating unit  42 , to the second electrodes  135 . When the selecting signal SL is OFF, the second drivers  452  bring the output into the OFF state, and do not apply the signal S 2 , which is generated by the waveform generating unit  42 , to the second electrodes  135 . 
     The waveform generating unit  41  and the waveform generating unit  42  have a 1-drop waveform setting unit  411  and  421 , a 2-drop waveform setting unit  412  and  422 , a 3-drop waveform setting unit  413  and  423 , and a drive waveform generating unit  414  and  424 , respectively. 
     In the waveform generating unit  41 , the 1-drop waveform setting unit  411  sets drive waveform data for the first electrode  134  for discharging one ink droplet from the nozzle  301 . The 2-drop waveform setting unit  412  sets drive waveform data for the first electrode  134  for continuously discharging two ink droplets from the nozzle  301 . The 3-drop waveform setting unit  413  sets drive waveform data for the first electrode  134  for continuously discharging three ink droplets from the nozzle  301 . 
     In the waveform generating unit  42 , the 1-drop waveform setting unit  421  sets drive waveform data for the second electrodes  135  for discharging one ink droplet from the nozzle  301 . The 2-drop waveform setting unit  422  sets drive waveform data for the second electrodes  135  for continuously discharging two ink droplets from the nozzle  301 . The 3-drop waveform setting unit  423  sets drive waveform data for the second electrodes  135  for continuously discharging three ink droplets from the nozzle  301 . 
     Hereinafter, the drive waveform data set by the respective waveform setting units  411 ,  421 ,  412 ,  422 ,  413 , and  423  will be referred to as drive waveform units. 
     In the waveform generating unit  41 , the drive waveform generating unit  414  selects and connects, in the predetermined order, the drive waveform units set by the respective waveform setting units  411 ,  412 , and  413 . Further, the drive waveform generating unit  414  outputs the drive waveform signal S 1  for the first electrode  134 , to which the drive waveform units are connected, to the first driver  451  of the driver unit  45 . 
     In the waveform generating unit  42 , the drive waveform generating unit  424  selects and connects, in the predetermined order, the drive waveform units set by the respective waveform setting units  421 ,  422 , and  423 . Further, the drive waveform generating unit  424  outputs the drive waveform signal S 2  for the second electrode  135 , to which the drive waveform units are connected, to the second driver  452  of the driver unit  45 . 
     The order in which the drive waveform generating units  414  and  424  select the drive waveform units is controlled by the waveform connection control unit  46 . That is, the waveform connection control unit  46  sets the order for connecting the waveform setting units  411 ,  421 ,  412 ,  422 ,  413 , and  423 , and controls the drive waveform generating units  414  and  424  such that waveform units are connected based on the setting. 
     Here, the drive waveform unit selected by the drive waveform generating unit  414  corresponds to the drive waveform unit simultaneously selected by the drive waveform generating unit  424 . That is, when the drive waveform generating unit  414  selects the drive waveform unit for the 1-drop waveform setting unit  411 , the drive waveform generating unit  424  also selects the drive waveform unit for the 1-drop waveform setting unit  421 . When the drive waveform generating unit  414  selects the drive waveform unit for the 2-drop waveform setting unit  412 , the drive waveform generating unit  424  also selects the drive waveform unit for the 2-drop waveform setting unit  422 . When the drive waveform generating unit  414  selects the drive waveform unit for the 3-drop waveform setting unit  413 , the drive waveform generating unit  424  also selects the drive waveform unit for the 3-drop waveform setting unit  423 . The connection order may be programmable. 
     As described above, while the selecting signal SL is ON, the drive waveform signal S 1  is applied to the first electrode  134 , and the drive waveform signal S 2  is applied to the second electrodes  135 . As such, the actuator is operated by differential voltage between the drive waveform signal S 1  and the drive waveform signal S 2 . While the selecting signal SL is OFF, the drive waveform signal S 1  is applied to the first electrode  134 , but the drive waveform signal S 2  is not applied to the second electrodes  135 , and the second electrodes  135  are brought into a floating state. Therefore, electric potential of the second electrodes  135  follows the electric potential of the first electrode  134  which is induced as the capacitance of the actuator. As a result, no potential difference occurs between the first electrode  134  and the second electrodes  135  such that the actuator is not operated. 
     Next, the drive waveform units providing a 1-drop waveform, a 2-drop waveform, and a 3-drop waveform will be described with reference to  FIGS. 8 to 13 . 
       FIG. 8  depicts a drive voltage of a 1-drop waveform to be applied to the actuator. The drive voltage of the 1-drop waveform is a differential voltage between the drive waveform unit set to the 1-drop waveform setting unit  411  of the waveform generating unit  41  and the drive waveform unit set to the 1-drop waveform setting unit  421  of the waveform generating unit  42 . That is, the drive waveform units for generating the differential voltage illustrated in  FIG. 8  are set to the 1-drop waveform setting unit  411  and the 1-drop waveform setting unit  421 , respectively. As the drive voltage is applied to the actuator, one ink droplet is discharged from the nozzle  301 . This drive voltage waveform will be referred to as a 1-drop waveform. 
       FIG. 9  depicts the drive voltage of the 1-drop waveform and simulated values of an ink pressure and an ink flow velocity under an application of the 1-drop waveform to the actuator, using the equivalent circuit illustrated in  FIG. 17 . The values R, C, and L illustrated in  FIG. 9  correspond to the values R, C, and L of the equivalent circuit illustrated in  FIG. 17 . An electrical current flow in the equivalent circuit in  FIG. 17  corresponds to an ink flow velocity in the vicinity of the pressure chamber  131  of the inkjet head  1 . A voltage across the inductor L in the equivalent circuit in  FIG. 17  corresponds to an ink pressure in the pressure chamber  131  in the vicinity of the nozzle  301 . This correspondence to the equivalent circuit in  FIG. 17  also applies to  FIGS. 11, 13, 15, 16, 18, 19, 24, 25, 26, 27, and 28 . In  FIG. 9 , the drive voltage waveform is indicated by the solid line, the ink pressure waveform is indicated by the dot dashed line, and the ink flow velocity waveform is indicated by the dashed line. In addition, values on a vertical axis are arbitrarily normalized values. 
     As illustrated in  FIG. 8 , the 1-drop waveform includes first to seventh waveform elements e 11  to e 17 . The first waveform element e 11  expands the volume of the pressure chamber  131  and provides negative pressure to the pressure chamber  131  at time t 11 . The second waveform element e 12  generates a first standby time (t 12 -t 11 ) that starts after the first waveform element e 11 . The third waveform element e 13  returns the volume of the pressure chamber  131  to an original state and provides positive pressure to the pressure chamber  131  at time t 12  after the first standby time elapses. The fourth waveform element e 14  generates a second standby time (t 13 -t 12 ) that starts after the third waveform element e 13 . The fifth waveform element e 15  contracts the volume of the pressure chamber  131  and provides positive pressure to the pressure chamber  131  at time t 13  after the second standby time elapses. The sixth waveform element e 16  generates a third standby time (t 14 -t 13 ) that starts after the fifth waveform element e 15 . The seventh waveform element e 17  returns the volume of the pressure chamber  131  to the original state at time t 14  after the third standby time elapses. 
     A combination of the first waveform element e 11 , the second waveform element e 12 , and the third waveform element e 13  forms an expansion pulse P 11  that returns the volume of the pressure chamber  131  to the original state after expanding the volume of the pressure chamber  131 . That is, the first waveform element e 11  corresponds to a leading edge of the expansion pulse P 11 , the second waveform element e 12  corresponds to a pulse width of the expansion pulse P 11 , and the third waveform element e 13  corresponds to a trailing edge of the expansion pulse P 11 . A combination of the fifth waveform element e 15 , the sixth waveform element e 16 , and the seventh waveform element e 17  forms a contraction pulse P 12  that returns the volume of the pressure chamber  131  to the original state after contracting the volume of the pressure chamber  131 . That is, the fifth waveform element e 15  corresponds to a leading edge of the contraction pulse P 12 , the sixth waveform element e 16  corresponds to a pulse width of the contraction pulse P 12 , and the seventh waveform element e 17  corresponds to a trailing edge of the contraction pulse P 12 . 
     At time t 11  when the waveform element e 11  is applied, that is at the leading edge of the expansion pulse P 11 , the partition walls  133  on the both sides are displaced to expand the volume of the pressure chamber  131 . With this displacement, negative pressure is instantaneously applied to the ink in the pressure chamber  131 , as illustrated in  FIG. 9 . As a result, a meniscus of the ink in the nozzle  301  is retracted. 
     Thereafter, the ink pressure is changed from negative to positive in accordance with natural pressure vibration of the ink in the pressure chamber. Further, when the first standby time, during which the waveform element e 12  is applied, has elapsed at time t 12 , that is at the trailing edge of the expansion pulse P 11  when the waveform element e 13  is applied, the volume of the pressure chamber  131  returns to the original state. As illustrated in  FIG. 9 , positive pressure is instantaneously applied to the ink. As described above, when positive pressure is instantaneously applied to the ink by a pulse change in a state in which the ink pressure is positive pressure equal to or higher than a threshold value, the meniscus begins to be advanced and one ink droplet is discharged from the nozzle  301 . That is, the first standby time is a time for waiting until the ink pressure increases from negative pressure at the leading edge of the expansion pulse P 11  to the threshold value. The threshold value is a threshold pressure at which one ink droplet can be discharged by the instantaneous application of positive pressure to the ink at the trailing edge of the expansion pulse P 11 . For most efficient ink discharge, the first standby time, that is the duration of the waveform element e 12 , is set to be ½ of a natural pressure vibration period of the ink in the pressure chamber. 
     Thereafter, the ink pressure is changed from positive to negative in accordance with natural pressure vibration of the ink in the pressure chamber. When the ink pressure is changed to negative, the meniscus is retracted following the ink pressure change. Further, when the second standby time, during which the waveform element e 14  is applied, has elapsed at time t 13 , that is at the leading edge of the contraction pulse P 12  when the waveform element e 15  is applied, the partition walls  133  on the both sides are displaced to contract the volume of the pressure chamber  131 . With this displacement, positive pressure is instantaneously applied to the ink. However, no ink droplet is discharged from the nozzle  301  because the ink pressure is negative at time t 13  at which positive pressure is applied. 
     In a state in which the volume of the pressure chamber  131  is contracted, when the third standby time, during which the waveform element e 16  is applied, has elapsed at time t 14 , that is at the trailing edge of the contraction pulse P 12  when the waveform element e 17  is applied, the volume of the pressure chamber  131  returns to the original state. At this time t 14 , a magnitude of amplitude of pressure vibration of the ink is equal to negative pressure instantaneously applied to the ink at the trailing edge of the contraction pulse P 12 , and the ink flow velocity is zero. Therefore, residual vibration in the pressure chamber  131  is cancelled thereafter. That is, the second standby time and the third standby time are timed such that the residual vibration in the pressure chamber  131  is cancelled at the trailing edge of the contraction pulse P 12 . 
     As described above, as the drive voltage of the 1-drop waveform illustrated in  FIG. 8  is applied to the actuator, the pressure chamber  131  is operated in the order of expansion, return, contraction, and return. Further, with the operations of expansion and return, one ink droplet is discharged from the nozzle  301  that communicates with the pressure chamber  131 . In addition, with the subsequent operations of contraction and return, residual vibration is cancelled after the ink droplet is discharged. 
       FIG. 10  depicts a drive voltage of a 2-drop waveform to be applied to the actuator. The drive voltage of the 2-drop waveform is a differential voltage between the drive waveform unit set to the 2-drop waveform setting unit  412  of the waveform generating unit  41  and the drive waveform unit set to the 2-drop waveform setting unit  422  of the waveform generating unit  42 . That is, the drive waveform units for generating the differential voltage illustrated in  FIG. 10  are set to the 2-drop waveform setting unit  412  and the 2-drop waveform setting unit  422 , respectively. The differential voltage is the drive voltage of the actuator. As the drive voltage is applied to the actuator, two ink droplets are consecutively discharged from the nozzle  301 . This drive voltage waveform will be referred to as a 2-drop waveform. 
       FIG. 11  depicts the drive voltage of the 2-drop waveform and simulated values of an ink pressure and an ink flow velocity under an application of the 2-drop waveform to the actuator. In  FIG. 11 , the drive voltage waveform is indicated by the solid line, the ink pressure waveform is indicated by the dot dashed line, and the ink flow velocity waveform is indicated by the dashed line. In addition, values on a vertical axis are arbitrarily normalized. 
     As illustrated in  FIG. 10 , the 2-drop waveform includes first to ninth waveform elements e 21  to e 29 . The first waveform element e 21  expands the volume of the pressure chamber  131  and provides negative pressure to the pressure chamber  131  at time t 21 . The second waveform element e 22  generates a first standby time (t 22 -t 21 ) that starts after the first waveform element e 21 . The third waveform element e 23  returns the volume of the pressure chamber  131  to an original state and provides positive pressure to the pressure chamber  131  at time t 22  after the first standby time elapses. The fourth waveform element e 24  generates a second standby time (t 23 -t 22 ) that starts after the third waveform element e 23 . The fifth waveform element e 25  contracts the volume of the pressure chamber  131  and provides positive pressure to the pressure chamber  131  at time t 23  after the second standby time elapses. The sixth waveform element e 26  generates a third standby time (t 24 -t 23 ) that starts after the fifth waveform element e 25 . The seventh waveform element e 27  slightly returns the volume of the pressure chamber  131  at time t 24  after the third standby time elapses. In the example illustrated in  FIG. 11 , assuming that a contraction percentage by the waveform element e 25  is 100%, the volume of the pressure chamber  131  returns such that a contraction percentage becomes 50%. The eighth waveform element e 28  generates a fourth standby time (t 25 -t 24 ) that starts after the seventh waveform element e 27 . The ninth waveform element e 29  returns the volume of the pressure chamber  131  to the original state at time t 25  after the fourth standby time elapses. 
     A combination of the first waveform element e 21 , the second waveform element e 22 , and the third waveform element e 23  forms an expansion pulse P 21  that returns the volume of the pressure chamber  131  to the original state after expanding the volume of the pressure chamber  131 . That is, the first waveform element e 21  corresponds to a leading edge of the expansion pulse P 21 , the second waveform element e 22  corresponds to a pulse width of the expansion pulse P 21 , and the third waveform element e 23  is a trailing edge of the expansion pulse P 21 . A combination of the fifth waveform element e 25 , the sixth waveform element e 26 , and the seventh waveform element e 27  forms a contraction pulse P 22  that partially returns the volume of the pressure chamber  131  after the contracting of the volume of the pressure chamber  131 , thereby bringing the pressure chamber  131  into a weak contraction state in which the pressure chamber  131  is contracted less than in the contraction state maintained by the sixth waveform element e 26 . That is, the fifth waveform element e 25  corresponds to a leading edge of the contraction pulse P 22 , the sixth waveform element e 26  corresponds to a pulse width of the contraction pulse P 22 , and the seventh waveform element e 27  corresponds to a trailing edge of the contraction pulse P 22 . A combination of the eighth waveform element e 28  and the ninth waveform element e 29  forms a weak contraction pulse P 23  that returns the pressure chamber  131  to the original state after maintaining the weak contraction state for a predetermined time. That is, the eighth waveform element e 28  corresponds to a pulse width of the weak contraction pulse P 23 , and the ninth waveform element e 29  corresponds to a trailing edge of the weak contraction pulse P 23 . 
     At time t 21  when the waveform element e 21  is applied, that is at the leading edge of the expansion pulse P 21 , the partition walls  133  on the both sides are displaced to expand the volume of the pressure chamber  131 . With this displacement, negative pressure is applied to the ink in the pressure chamber  131 , as illustrated in  FIG. 11 . As a result, a meniscus of the ink in the nozzle  301  is retracted. 
     Thereafter, the ink pressure is changed from negative to positive in accordance with natural pressure vibration of the ink in the pressure chamber. Further, when the first standby time, during which the waveform element e 22  is applied, has elapsed at time t 22 , that is at the trailing edge of the expansion pulse P 21  when the waveform element e 23  is applied, the volume of the pressure chamber  131  returns to the original state. As illustrated in  FIG. 11 , positive pressure is instantaneously applied to the ink. As described above, when positive pressure is instantaneously applied to the ink by a pulse change in a state in which the ink pressure is positive pressure equal to or higher than a threshold value, the meniscus begins to be advanced and a first ink droplet is discharged from the nozzle  301 . That is, the first standby time is a time for waiting until the ink pressure increases from negative pressure at the leading edge of the expansion pulse P 21  to the threshold value. The threshold value is a threshold pressure at which one ink droplet can be discharged by the instantaneous application of positive pressure to the ink at the trailing edge of the expansion pulse P 21 . In the example illustrated in  FIG. 11 , the first standby time is ½ of the natural pressure vibration period of the ink in the pressure chamber. 
     Thereafter, the ink pressure is changed from positive to negative in accordance with natural pressure vibration of the ink in the pressure chamber. When the ink pressure is changed to negative, the meniscus is retracted following the ink pressure change. Thereafter, the ink pressure is changed back to positive pressure. Further, when the second standby time, during which the waveform element e 24  is applied, has elapsed at time t 23 , that is at the leading edge of the contraction pulse P 22  when the waveform element e 25  is applied, the partition walls  133  on the both sides are displaced to contract the volume of the pressure chamber  131 . With this displacement, positive pressure is instantaneously applied to the ink. Here, time t 23  is a time at which the ink pressure becomes substantially the same value as that at time t 22 . Therefore, as positive pressure is instantaneously applied to the ink by a pulse change in a state in which the ink pressure is positive pressure equal to or higher than a threshold value, the meniscus begins to be advanced and a second ink droplet is discharged from the nozzle  301 . That is, the second standby time is a time for waiting until the ink pressure increases to a pressure at which the second ink droplet can be discharged by the instantaneous application of positive pressure to the ink at the leading edge of the contraction pulse P 22 . 
     In a state in which the volume of the pressure chamber  131  is contracted, when the third standby time, during which the waveform element e 26  is applied, has elapsed time t 24 , that is at the trailing edge of the contraction pulse P 22  when waveform element e 27  is applied, the partition walls  133  on the both sides are displaced so that the volume of the pressure chamber  131  returns slightly. With this displacement, the pressure chamber  131  is brought into a weak contraction state weaker than the contraction state. The weak contraction state is maintained until the fourth standby time, during which the waveform element e 28  is applied, has elapsed. Further, at time t 25  of the trailing edge of the weak contraction pulse P 23  when the waveform element e 29  is applied, the volume of the pressure chamber  131  returns to the original state. At time t 25 , a magnitude of amplitude of vibration of the ink pressure is equal to negative pressure applied to the ink by the trailing edge of the weak contraction pulse P 23 , and the ink flow velocity is zero. Therefore, residual vibration in the pressure chamber  131  is cancelled thereafter. That is, the third standby time and the fourth standby time are timed such that the residual vibration in the pressure chamber  131  is cancelled by the trailing edge of the weak contraction pulse P 23 . 
     As described above, as the drive voltage of the 2-drop waveform illustrated in  FIG. 10  is applied to the actuator, the pressure chamber  131  is operated in the order of expansion, return, contraction, weak contraction, and return. Further, with the first operations of expansion and return, a first ink droplet is discharged from the nozzle  301  that communicates with the pressure chamber  131 . In addition, with the subsequent operation of contraction, a second ink droplet is discharged from the nozzle  301 . Further, with the subsequent operations of weak contraction and return, residual vibration is cancelled after the second ink droplet is discharged. 
       FIG. 12  depicts a drive voltage of a 3-drop waveform to be applied to the actuator. The drive voltage of the 3-drop waveform is a voltage between the drive waveform unit set to the 3-drop waveform setting unit  413  of the waveform generating unit  41  and the drive waveform unit set to the 3-drop waveform setting unit  423  of the waveform generating unit  42 . That is, the drive waveform units for generating differential voltage illustrated in  FIG. 12  are set to the 3-drop waveform setting unit  413  and the 3-drop waveform setting unit  423 , respectively. The differential voltage is the drive voltage of the actuator. As the drive voltage is applied to the actuator, three ink droplets are consecutively discharged by from the nozzle  301 . This drive voltage waveform will be referred to as a 3-drop waveform. 
       FIG. 13  depicts the drive voltage of the 3-drop waveform and simulated values of an ink pressure and an ink flow velocity under an application of the 3-drop waveform to the actuator. In  FIG. 13 , the drive voltage waveform is indicated by the solid line, the ink pressure waveform is indicated by the dot dashed line, and the ink flow velocity waveform is indicated by the dashed line. In addition, values on a vertical axis are arbitrarily normalized. 
     As illustrated in  FIG. 12 , the 3-drop waveform includes first to thirteenth waveform elements e 31  to e 43 . The first waveform element e 31  expands the volume of the pressure chamber  131  and provides negative pressure to the pressure chamber  131  at time t 31 . The second waveform element e 32  generates a first standby time (t 32 -t 31 ) that starts after the first waveform element e 31 . The third waveform element e 33  returns the volume of the pressure chamber  131  to an original state and provides positive pressure to the pressure chamber at time t 32  after the first standby time elapses. The fourth waveform element e 34  generates a second standby time (t 33 -t 32 ) that starts after the third waveform element e 33 . The fifth waveform element e 35  contracts the volume of the pressure chamber  131  and provides positive pressure to the pressure chamber  131  at time t 33  after the second standby time elapses. The sixth waveform element e 36  generates a third standby time (t 34 -t 33 ) that starts after the fifth waveform element e 35 . The seventh waveform element e 37  returns the volume of the pressure chamber  131  slightly at time t 34  after the third standby time elapses. In an example illustrated in  FIG. 13 , assuming that a contraction percentage by the waveform element e 35  is 100%, the volume of the pressure chamber  131  returns such that a contraction percentage becomes 50%. The eighth waveform element e 38  generates a fourth standby time (t 35 -t 34 ) that starts after the seventh waveform element e 37 . The ninth waveform element e 39  contracts the volume of the pressure chamber  131  again and provides positive pressure to the pressure chamber  131  at time t 35  after the fourth standby time elapses. In the example illustrated in  FIG. 13 , assuming that a contraction percentage by the waveform element e 35  is 100%, the volume of the pressure chamber  131  is contracted so as to have the equal contraction percentage. The tenth waveform element e 40  generates a fifth standby time (t 36 -t 35 ) that starts after the ninth waveform element e 39 . The eleventh waveform element e 41  returns the volume of the pressure chamber  131  slightly at time t 36  after the fifth standby time elapses. In the example illustrated in  FIG. 13 , assuming that a contraction percentage by the waveform element e 39  is 100%, the volume of the pressure chamber  131  returns such that a contraction percentage becomes 50%. The twelfth waveform element e 42  generates a sixth standby time (t 37 -t 36 ) that starts after the eleventh waveform element e 41 . The thirteenth waveform element e 43  returns the volume of the pressure chamber  131  to the original state at time t 37  after the sixth standby time elapses. 
     Here, the first waveform element e 31 , the second waveform element e 32 , and the third waveform element e 33  form an expansion pulse P 31  that returns the volume of the pressure chamber  131  to the original state after expanding the volume of the pressure chamber  131 . That is, the first waveform element e 31  is a leading edge of the expansion pulse P 31 , the second waveform element e 32  has a pulse width of the expansion pulse P 31 , and the third waveform element e 33  is a trailing edge of the expansion pulse P 31 . A combination of the fifth waveform element e 35 , the sixth waveform element e 36 , and the seventh waveform element e 37  forms a first contraction pulse P 32  that slightly returns the volume of the pressure chamber  131  after contracting the volume of the pressure chamber  131  so as to bring the pressure chamber  131  into a contraction state (weak contraction state) weaker than the contraction state maintained by the sixth waveform element e 36 . That is, the fifth waveform element e 35  is a leading edge of the first contraction pulse P 32 , the sixth waveform element e 36  is a pulse width of the first contraction pulse P 32 , and the seventh waveform element e 37  is a trailing edge of the first contraction pulse P 32 . The eighth waveform element e 38  forms a first weak contraction pulse P 33  for maintaining the weak contraction state of the pressure chamber  131  formed by the first contraction pulse P 32  for a predetermined time. That is, the eighth waveform element e 38  is a pulse width of the first weak contraction pulse P 33 . A combination of the ninth waveform element e 39 , the tenth waveform element e 40 , and the eleventh waveform element e 41  forms a second contraction pulse P 34  that slightly returns the volume of the pressure chamber  131  after contracting the volume of the pressure chamber  131  so as to bring the pressure chamber  131  into the weak contraction state. That is, the ninth waveform element e 39  is a leading edge of the second contraction pulse P 34 , the tenth waveform element e 40  is a pulse width of the second contraction pulse P 34 , and the eleventh waveform element e 41  is a trailing edge of the second contraction pulse P 34 . A combination of the twelfth waveform element e 42  and the thirteenth waveform element e 43  forms a second weak contraction pulse P 35  that returns the weak contraction state of the pressure chamber  131  to an original state after maintaining the weak contraction state of the pressure chamber  131  for a predetermined time. That is, the twelfth waveform element e 42  is a pulse width of the second weak contraction pulse P 35 , and the thirteenth waveform element e 43  is a trailing edge of the second weak contraction pulse P 35 . 
     At time t 31  when the waveform element e 31  is applied, that is at the leading edge of the expansion pulse P 31 , the partition walls  133  on the both sides are displaced to expand the volume of the pressure chamber  131 . With this displacement, negative pressure is instantaneously applied to the ink in the pressure chamber  131 , as illustrated in  FIG. 13 . As a result, a meniscus of the ink in the nozzle  301  is retracted. 
     Thereafter, the ink pressure is changed from negative pressure to positive pressure in accordance with a natural pressure vibration period of the ink in the pressure chamber. Further, when the first standby time, during which the waveform element e 32  is applied, has elapsed at time t 32 , that is at the trailing edge of the first expansion pulse P 31  when the waveform element e 33  is applied, the volume of the pressure chamber  131  returns to the original state. As illustrated in  FIG. 13 , positive pressure is instantaneously applied to the ink. As described above, when positive pressure is instantaneously applied to the ink by a pulse change in a state in which the ink pressure is positive pressure equal to or higher than a threshold value, the meniscus begins to be advanced and a first ink droplet is discharged from the nozzle  301 . That is, the first standby time is a time for waiting until the ink pressure increases from negative pressure at the leading edge of the expansion pulse P 31  to the threshold value. The threshold value is a threshold pressure at which one ink droplet can be discharged by the instantaneous application of positive pressure to the ink at the trailing edge of the expansion pulse P 31 . 
     Thereafter, the ink pressure is changed from positive pressure to negative pressure in accordance with natural pressure vibration of the ink in the pressure chamber. Further, in the state in which the ink pressure is positive, when the second standby time, during which the waveform element e 34  is applied, has elapsed at time t 33 , that is at the leading edge of the first contraction pulse P 32  when the waveform element e 35  is applied, the partition walls  133  on the both sides are displaced to contract the volume of the pressure chamber  131 . With this displacement, positive pressure is instantaneously applied to the ink. Here, time t 33  is a time at which the ink pressure becomes substantially the same value as that at time t 32 . Therefore, as positive pressure is instantaneously applied to the ink by a pulse change in the state in which the ink pressure is positive pressure equal to or higher than a threshold value, the meniscus begins to be advanced and a second ink droplet is discharged from the nozzle  301 . That is, the second standby time is a time for waiting until the ink pressure increases to a pressure at which the second ink droplet can be discharged by the instantaneous application of positive pressure to the ink at the leading edge of the first contraction pulse P 32 . 
     The ink pressure is changed to negative pressure after the volume of the pressure chamber  131  is contracted. Further, when the third standby time, during which the waveform element e 36  is applied, has elapsed at time t 34 , that is at the trailing edge of the contraction pulse P 32  when the waveform element e 37  is applied, the partition walls  133  on the both sides are displaced to return the volume of the pressure chamber  131  slightly. With this displacement, the pressure chamber  131  is brought into the weak contraction state weaker than the contraction state, so that the meniscus is retracted. Here, time t 34  is included in a time period in which the ink pressure is being negative pressure and is a time at which negative ink pressure is maximized in the example illustrated in  FIG. 13 . At this time t 34 , the pressure chamber  131  is brought into the weak contraction state, and as a result, the amplitude of vibration of the ink pressure is increased. 
     The weak contraction state is maintained until the fourth standby time, during which the waveform element e 38  is applied and the ink pressure is changed to the positive pressure, has elapsed. Further, at time t 35 , that is at the trailing edge of the weak contraction pulse P 33  when the waveform element e 39  is applied, the partition walls  133  on the both sides are displaced to contract the volume of the pressure chamber  131  again. With this displacement, positive pressure is instantaneously applied to the ink. Further, the meniscus is advanced again. Here, time t 35  is set to be later than a time at which the ink pressure is substantially the same as that at the times t 32  and t 33 . A magnitude of the waveform element e 39 , which provides positive pressure to discharge a third ink droplet, is only a half of a magnitude of the waveform element e 33  for discharging a first ink droplet and a magnitude of the waveform element e 35  for discharging a second ink droplet. Therefore, since it is necessary to wait until the ink pressure becomes higher than those in the case of discharging the first ink droplet and the second ink droplet, the timing of the waveform element  39  is delayed. Further, the ink pressure after performing the operation with the waveform element e 39  at time t 35  is substantially the same value as the ink pressure immediately after times t 32  and t 33 . Therefore, since positive pressure is instantaneously applied to the ink by a pulse change in the state in which the ink pressure is positive pressure equal to or higher than a threshold value, a third ink droplet is discharged from the nozzle  301 . That is, the fourth standby time is a time for waiting until the ink pressure increases to a pressure at which the third ink droplet can be discharged by the instantaneous application of positive pressure to the ink at the leading edge of the second contraction pulse P 34 . 
     In the state in which the volume of the pressure chamber  131  is contracted, when the fifth standby time, during which the waveform element e 40  is applied, has elapsed at time t 36 , that is at the trailing edge of the second contraction pulse P 34  when the waveform element e 41  is applied, the partition walls  133  on the both sides are displaced such that the volume of the pressure chamber  131  returns slightly. With this displacement, the pressure chamber  131  is brought into a weak contraction state weaker than the contraction state. The weak contraction state is maintained until the sixth standby time, during which the waveform element e 42  is applied, has elapsed. Further, at time t 37 , that is at the trailing edge of the second weak contraction pulse P 35  when the waveform element e 43  is applied, the volume of the pressure chamber  131  returns to the original state. At time t 37 , a magnitude of amplitude of vibration of the ink pressure is equal to negative pressure instantaneously applied to the ink by the trailing edge of the second weak contraction pulse P 35 , and the ink flow velocity is zero. Therefore, residual vibration in the pressure chamber  131  is cancelled thereafter. That is, the fifth standby time and the sixth standby time are timed such that the residual vibration in the pressure chamber  131  is cancelled by the trailing edge of the second weak contraction pulse P 35 . 
     As described above, when the drive voltage of the 3-drop waveform illustrated in  FIG. 12  is applied to the actuator, the pressure chamber  131  is operated in the order of expansion, return, contraction, weak contraction, contraction, weak contraction, and return. Further, with the first operations of expansion and return, a first ink droplet is discharged from the nozzle  301  that communicates with the pressure chamber  131 . In addition, with the subsequent operation of contraction, a second ink is discharged from the nozzle  301 . Further, with the subsequent operations of weak contraction and contraction, a third ink droplet is discharged from the nozzle  301 . Further, with the subsequent operations of weak contraction and return, residual vibration is cancelled after the third ink droplet is discharged. 
     By the way, in the aforementioned 2-drop waveform, the weak contraction pulse P 23  is formed at the trailing edge of the contraction pulse P 22 , such that residual vibration is cancelled at the trailing edge of the weak contraction pulse P 23 . The same applies to the case of the 3-drop waveform. However, in a case in which damping of pressure vibration of the ink in the pressure chamber  131  is comparatively low, residual vibration may be cancelled at the trailing edge of the contraction pulse P 22  in the 2-drop waveform or the 3-drop waveform, similar to the 1-drop waveform. 
     In the following, another 2-drop waveform, which cancels residual vibration at the trailing edge of the contraction pulse P 22 , will be described with reference to  FIGS. 14 and 15 . 
       FIG. 14  depicts a drive voltage of a modified 2-drop waveform.  FIG. 15  depicts the drive voltage of the modified 2-drop waveform and simulated values of an ink pressure and an ink flow velocity under the application of the modified 2-drop waveform to the actuator. In  FIG. 15 , the drive voltage waveform is indicated by the solid line, the ink pressure waveform is indicated by the dot dashed line, and the ink flow velocity waveform is indicated by the dashed line. In addition, values on a vertical axis are arbitrarily normalized. 
     As illustrated in  FIG. 14 , the modified 2-drop waveform includes first to seventh waveform elements e 41  to e 47 . The first waveform element e 41  expands the volume of the pressure chamber  131  and provides negative pressure to the pressure chamber  131  at time t 41 . The second waveform element e 42  generates a first standby time (t 42 -t 41 ) that starts after the first waveform element e 41 . The third waveform element e 43  returns the volume of the pressure chamber  131  to an original state and provides positive pressure to the pressure chamber  131  at time t 42  after the first standby time elapses. The fourth waveform element e 44  generates a second standby time (t 43 -t 42 ) that starts after the third waveform element e 43 . The fifth waveform element e 45  contracts the volume of the pressure chamber  131  and provides positive pressure to the pressure chamber  131  at time t 43  after the second standby time elapses. The sixth waveform element e 46  generates a third standby time (t 44 -t 43 ) that starts after the fifth waveform element e 45 . The seventh waveform element e 47  returns the volume of the pressure chamber  131  to the original state at time t 44  after the third standby time elapses. 
     A combination of the first waveform element e 41 , the second waveform element e 42 , and the third waveform element e 43  forms an expansion pulse P 41  that returns the volume of the pressure chamber  131  to the original state after expanding the volume of the pressure chamber  131 . That is, the first waveform element e 41  is a leading edge of the expansion pulse P 41 , the second waveform element e 42  is a pulse width of the expansion pulse P 41 , and the third waveform element e 43  is a trailing edge of the expansion pulse P 41 . A combination of the fifth waveform element e 45 , the sixth waveforms element e 46 , and the seventh waveform element e 47  forms a contraction pulse P 42  that returns the volume of the pressure chamber  131  to the original state after contracting the volume of the pressure chamber  131 . That is, the fifth waveform element e 45  is a leading edge of the contraction pulse P 42 , the sixth waveform element e 46  is a pulse width of the contraction pulse P 42 , and the seventh waveform element e 47  is a trailing edge of the contraction pulse P 42 . 
     At time t 41 , that is at the leading edge of the expansion pulse P 41  when the waveform element e 41  is applied, the partition walls  133  on the both sides are displaced to expand the volume of the pressure chamber  131 . With this displacement, negative pressure is instantaneously applied to the ink in the pressure chamber  131 , as illustrated in  FIG. 15 . As a result, a meniscus of the ink in the nozzle  301  is retracted. 
     Thereafter, the ink pressure is changed from negative pressure to positive pressure in accordance with a natural pressure vibration period of the ink in the pressure chamber. Further, when the first standby time, during which the waveform element e 42  is applied, has elapsed at time t 42 , that is at the trailing edge of the expansion pulse P 41  when the waveform element e 43  is applied, the volume of the pressure chamber  131  returns to the original state. In this case, as illustrated in  FIG. 15 , positive pressure is instantaneously applied to the ink. As described above, when positive pressure is instantaneously applied to the ink by a pulse change in the state in which the ink pressure is positive pressure equal to or higher than a threshold value, the meniscus begins to be advanced. Further, a first ink droplet is discharged from the nozzle  301 . That is, the first standby time is a time for waiting until the ink pressure increases from negative pressure at the leading edge of the expansion pulse P 41  to a threshold value. The threshold value is a threshold pressure at which one ink droplet can be discharged by the instantaneous application of positive pressure to the ink at the trailing edge of the expansion pulse P 41 . 
     Thereafter, the ink pressure is changed from positive pressure to negative pressure in accordance with natural pressure vibration of the ink in the pressure chamber. When the ink pressure is changed to negative pressure, the meniscus is retracted late. Thereafter, the ink pressure is changed back to positive pressure. Further, when the second standby time, during which the waveform element e 44  has elapsed at time t 43 , that is at the leading edge of the contraction pulse P 42  when the waveform element e 45  is applied, the partition walls  133  on the both sides are displaced to contract the volume of the pressure chamber  131 . With this displacement, positive pressure is instantaneously applied to the ink. Here, time t 43  is a time at which the ink pressure becomes substantially the same value as that at time t 42 . Therefore, as positive pressure is instantaneously applied to the ink by a pulse change in the state in which the ink pressure is positive pressure equal to or higher than a threshold value, the meniscus begins to be advanced and a second ink droplet is discharged from the nozzle  301 . That is, the second standby time is a time for waiting until the ink pressure increases to a pressure at which the second ink droplet can be discharged by the instantaneous application of positive pressure to the ink at the leading edge of the contraction pulse P 42 . 
     In the state in which the volume of the pressure chamber  131  is contracted, when the third standby time, during which the waveform element e 46  is applied, has elapsed at time t 44 , that is at the trailing edge of the contraction pulse P 42  when the waveform element e 47  is applied, the volume of the pressure chamber  131  returns to the original state. At time t 44 , a magnitude of amplitude of vibration of the ink pressure is equal to negative pressure instantaneously applied to the ink by the trailing edge of the contraction pulse P 42 , and the ink flow velocity is zero. Therefore, residual vibration in the pressure chamber  131  is cancelled thereafter. That is, the third standby time is timed such that the residual vibration in the pressure chamber  131  is cancelled by the trailing edge of the contraction pulse P 42 . 
     As described above, as the drive voltage of the modified 2-drop waveform illustrated in  FIG. 14  is applied to the actuator, the pressure chamber  131  is operated in the order of expansion, return, contraction, and return. Further, with the first operations of expansion and return, a first ink droplet is discharged from the nozzle  301  that communicates with the pressure chamber  131 . In addition, with the subsequent operation of contraction, a second ink droplet is discharged from the nozzle  301 . Further, with the subsequent operation of return, residual vibration is cancelled after the ink droplet is discharged. 
     In the modified 2-drop waveform illustrated in  FIG. 14 , the waveform element, which may be used to cancel residual vibration, is limited to the waveform element e 47  that is the trailing edge of the contraction pulse P 42 . Further, since the output timing of the waveform element e 47  is limited to the aforementioned timing, a degree of freedom is small at the time of cancellation. Whether the modified 2-drop waveform illustrated in  FIG. 14  is available depends on a magnitude of damping of residual vibration of the ink. That is, in a case in which the damping of residual vibration of the ink is comparatively high, a pressure change in the waveform element e 47  is too large, and as a result, residual vibration may not be cancelled well in some instances. 
     During an application of the 2-drop waveform or the 3-drop waveform illustrated in  FIG. 10 or 12 , the pressure chamber  131  is in the weak contraction state after the trailing edge of the contraction pulse P 22  or the second contraction pulse P 34 . While the pressure chamber  131  is in the weak contraction state after the trailing edge of the contraction pulse, it is possible to adjust the time t 25  for the waveform element e 29  or the time t 27  for the waveform element e 43  for cancellation. For this reason, the timing for cancellation of the residual vibration may not be uniquely determined. In the following, a method of determining timings of waveform elements for cancellation of the residual vibration will be described using a 2-drop waveform as an example with reference to  FIGS. 16 to 19 . 
       FIG. 16  is a waveform chart for explaining residual vibration after stopping the contraction pulse P 22  at time t 24  and a simulation result of an ink pressure and an ink flow velocity under a hypothetical condition that the weak contraction state of the pressure chamber  131  is continuously maintained without stopping the weak contraction pulse P 23  of the 2-drop waveform at time point t 25 , for the purpose of explaining a method of determining an appropriate time t 25  at which the weak contraction pulse P 23  should be stopped. In FIG.  16 , the drive voltage waveform is indicated by the solid line, the ink pressure waveform is indicated by the dot dashed line, and the ink flow velocity waveform is indicated by the dashed line. In addition, values on a vertical axis are arbitrarily normalized. 
     As illustrated in  FIG. 16 , residual vibration would not be cancelled if the weak contraction state of the pressure chamber  131  is maintained even after time t 25 . The magnitude of the residual vibration depends on a timing of the time t 24  at which the contraction state transitions to the weak contraction state. If the time t 24  at which the contraction state transitions to the weak contraction state is shifted to before or after time t 24 , the ink pressure and the ink flow velocity change at the time t 24 , and thereafter, a magnitude of the residual vibration changes. In an example illustrated in  FIG. 16 , the residual vibration increases if the time t 24  is shifted earlier, and the residual vibration decreases if the time t 24  is shifted later. That is, a value of the ink pressure at the time when the ink velocity is zero can be adjusted by adjusting a timing of the time t 24  earlier or later. Therefore, a condition that an ink pressure amplitude at a time when the ink flow velocity is zero coincides with an ink pressure amplitude after the weak contraction state of the pressure chamber  131  returns to an initial state can be found by a simulation varying timings of the time t 24 . The timing of the time t 24  that satisfies this condition is set as the time t 24 . Further, the time at which the ink flow velocity is zero is set as timing at the trailing edge of the weak contraction pulse P 23 , that is, time t 25 . As such, it is possible to cancel residual vibration, as illustrated in  FIG. 10 . 
     The simulation may be performed using an equivalent circuit illustrated in  FIG. 17 . The equivalent circuit is a circuit in which a series circuit including a resistor R, a capacitor C, and an inductor L is connected to a voltage source V. In the case of the 2-drop waveform illustrated in  FIG. 11 , the resistor R is 0.33Ω, the capacitor C is 0.37 μF, and the inductor L is 0.65 pH. Further, in this case, the first standby time (t 22 -t 21 ) is 1.56 μs, the second standby time (t 23 -t 22 ) is 2.80 μs, the third standby time (t 24 -t 23 ) is 2.94 μs, and the fourth standby time (t 25 -t 24 ) is 0.66 μs. This equivalent circuit is extracted from residual vibration characteristics of the inkjet head  1 , and the values of the resistor R, the capacitor C, and the inductor L are determined based on the residual vibration characteristics. 
     A loss of the pressure chamber  131  is represented by the value of the resistor R of the equivalent circuit. If a loss of the pressure chamber  131  is higher, that is, the value of the resistor R is larger, pressure amplitude of the residual vibration is smaller. In this case, time t 24  at which the contraction state transitions to the weak contraction state should be shifted earlier. In this way, it is possible adjust the pressure amplitude of the residual vibration at when the ink flow velocity is zero, up to the pressure amplitude generated by the change of the state of the pressure chamber  131  from the weak contraction state to the initial state. Then, the time ink flow velocity is zero is set as time t 25  at which the weak contraction state is ended. 
     For example, when the appropriate times t 24  and t 25  are selected by increasing the resistor R to 0.38Ω and performing the simulation, the drive voltage waveform, the ink pressure waveform, and the ink flow velocity waveform are made as illustrated in  FIG. 18 . In  FIG. 18 , the first standby time (t 22 -t 21 ) is 1.56 μs, the second standby time (t 23 -t 22 ) is 2.80 μs, the third standby time (t 24 -t 23 ) is 2.84 μs, and the fourth standby time (t 25 -t 24 ) is 0.86 μs. 
     On the contrary, when a loss of the pressure chamber  131  is lower, that is, the value of the resistor R is smaller, residual vibration is larger. In this case, time t 24  at which the contraction state transitions to the weak contraction state should be shifted later. In this way, it is possible adjust the pressure amplitude of the residual vibration at when the ink flow velocity is zero, down to the pressure amplitude generated by the change of the state of the pressure chamber  131  from the weak contraction state to the initial state. Then, the time ink flow velocity is zero is set as time t 25  at which the weak contraction state is ended. 
     For example, when appropriate times t 24  and t 25  are selected by decreasing the resistor R to 0.28Ω and performing the simulation, the drive voltage waveform, the ink pressure waveform, and the ink flow velocity waveform are made as illustrated in  FIG. 19 . In  FIG. 19 , the first standby time (t 22 -t 21 ) is 1.56 μs, the second standby time (t 23 -t 22 ) is 2.80 μs, the third standby time (t 24 -t 23 ) is 3.14 μs, and the fourth standby time (t 25 -t 24 ) is 0.36 μs. 
     Since the step of bringing the pressure chamber into the weak contraction state is provided at the trailing edge of the contraction pulse as described above, it is possible to adjust the waveform element e 29  or the waveform element e 43  for cancellation in accordance with a magnitude of damping of residual vibration of the ink, and as a result, the degree of freedom is widened at the time of cancellation. 
     Next, an operation of the drive circuit  40  will be described with reference to  FIG. 20  to  FIGS. 23A to 23C . 
       FIG. 20  depicts a first example of a combination of drive waveform units. In  FIG. 20 , the drive waveform generating units  414  and  424  select the 1-drop waveform setting units  411  and  421  twice, subsequently select the 2-drop waveform setting units  412  and  422  twice, and then generate a drive waveform signal by connecting the drive waveform units. In  FIG. 20 , the waveform signal S 1  is a drive waveform signal S 1  which is generated by the drive waveform generating unit  414  and applied to the first electrode  134  of the pressure chamber  131  via the first driver  451 . The waveform signal S 2  is a drive waveform signal S 2  which is generated by the drive waveform generating unit  424  and applied to the second electrodes  135  of the two adjacent dummy chambers  132  via the second drivers  452 . A waveform signal ΔV indicates differential voltage between the drive waveform signal S 1  and the drive waveform signal S 2 . In addition, a first unit U 1  indicates waveforms of the drive waveform units selected for the first time by the drive waveform generating units  414  and  424 , and differential voltage thereof. A second unit U 2  indicates waveforms of the drive waveform units selected for the second time by the drive waveform generating units  414  and  424 , and differential voltage thereof. Likewise, third and fourth units U 3  and U 4  indicate waveforms of the drive waveform units selected for the third or fourth time, and differential voltage thereof. 
     In the first example illustrated in  FIG. 20 , when the waveform of the first unit U 1  or the second unit U 2  is applied to the actuator of the pressure chamber  131 , one ink droplet is discharged from the nozzle  301 . When the waveform of the third unit U 3  or the fourth unit U 4  is applied to the actuator of the pressure chamber  131 , two ink droplets are consecutively discharged from the nozzle  301 . 
     The waveform selecting unit  44  outputs a selecting signal that validates a period of the first unit U 1  when a gradation value of printing data is 1. When the gradation value is 2, the waveform selecting unit  44  outputs a selecting signal that validates a period of the first unit U 1  and a period of the second unit U 2 . When the gradation value is 3, the waveform selecting unit  44  outputs a selecting signal that validates periods of the 2nd and 3rd units U 2 , U 3 . When the gradation value is 4, the waveform selecting unit  44  outputs a selecting signal that validates periods of the first to third units U 1  to U 3 . When the gradation value is 5, the waveform selecting unit  44  outputs a selecting signal that validates periods of the 2nd to 4th units U 2 , U 3 , U 4 . When the gradation value is 6, the waveform selecting unit  44  outputs a selecting signal that validates periods of the first to fourth units U 1  to U 4 . 
       FIG. 22A  illustrates a waveform example in which the waveform selecting unit  44  outputs the selecting signal SL that validates the period of the first unit U 1 . For the period of the first unit U 1  in which the selecting signal SL is ON, the drive waveform signal S 1  is applied to the first electrode  134 , and the drive waveform signal S 2  is applied to the second electrode  135 . As a result, differential voltage ΔV between the drive waveform signal S 1  and the drive waveform signal S 2  is applied to the actuator of the pressure chamber  131 , and as a result, one ink droplet is discharged from the nozzle  301  that communicates with the pressure chamber  131 . For the periods of the second to fourth units U 2 , U 3 , and U 4  in which the selecting signal SL is OFF, the drive waveform signal S 1  is applied to the first electrode  134 , but the drive waveform signal S 2  is not applied to the second electrode  135 , and the second electrode  135  comes into a floating state. For this reason, electric potential of the second electrode  135  depends on electric potential of the first electrode  134 . As a result, the differential voltage ΔV becomes zero, and as a result, no ink droplet is discharged. As such, one ink droplet is discharged during one printing cycle. 
       FIG. 22B  illustrates a waveform example in which the waveform selecting unit  44  outputs the selecting signal SL that validates the periods of the first to third units U 1 , U 2 , and U 3 . For the periods of the first to third units U 1 , U 2 , and U 3  in which the selecting signal SL is ON, the drive waveform signal S 1  is applied to the first electrode  134 , and the drive waveform signal S 2  is applied to the second electrode  135 . As a result, the differential voltage ΔV between the drive waveform signal S 1  and the drive waveform signal S 2  is applied to the actuator of the pressure chamber  131 , and as a result, four ink droplets are consecutively discharged from the nozzle  301  that communicates with the pressure chamber  131 . That is, one ink droplet is discharged for the period of the first unit U 1 , and one ink droplet is also discharged for the period of the second unit U 2 . In addition, two ink droplets are sequentially discharged for the period of the third unit U 3 . For the period of the fourth unit U 4  in which the selecting signal SL is OFF, the drive waveform signal S 1  is applied to the first electrode  134 , but the drive waveform signal S 2  is not applied to the second electrode  135 , and the second electrode  135  comes into a floating state. For this reason, electric potential of the second electrode  135  depends on electric potential of the first electrode  134 . As a result, the differential voltage ΔV becomes zero, and as a result, no ink droplet is discharged. As such, four ink droplets are discharged in one printing cycle. 
       FIG. 22C  illustrates a waveform example in which the waveform selecting unit  44  outputs the selecting signal SL that validates the periods of the first to fourth units U 1 , U 2 , U 3 , and U 4 . For the periods of the first to fourth units U 1 , U 2 , U 3 , and U 4  in which the selecting signal SL is ON, the drive waveform signal S 1  is applied to the first electrode  134 , and the drive waveform signal S 2  is applied to the second electrode  135 . As a result, the differential voltage ΔV between the drive waveform signal S 1  and the drive waveform signal S 2  is applied to the actuator of the pressure chamber  131 , and as a result, six ink droplets are consecutively discharged from the nozzle  301  that communicates with the pressure chamber  131 . That is, one ink droplet is discharged for the period of the first unit U 1 , and one ink droplet is also discharged for the period of the second unit U 2 . In addition, two ink droplets are sequentially discharged for the period of the third unit U 3 , and two ink droplets are also continuously discharged for the period of the fourth unit U 4 . As such, six ink droplets are discharged in one printing cycle. 
     Although not illustrated, when the waveform selecting unit  44  outputs the selecting signal SL that validates the period of the first unit U 1  and the period of the second unit U 2 , two ink droplets are continuously discharged in one printing cycle. 
     Therefore, the ink droplets are selectively discharged by one ink droplet, two ink droplets, four ink droplets, or six ink droplets in accordance with printing data, thereby realizing a multi-drop method of performing gradation printing. 
     Although not illustrated, when the waveform selecting unit  44  outputs the selecting signal SL that validates the period of the second unit U 2  and the period of the third unit U 3 , three ink droplets are continuously discharged in one printing cycle. 
     Although not illustrated, when the waveform selecting unit  44  outputs the selecting signal SL that validates the period of the second unit U 2  and the periods of the third and fourth units U 3  and U 4 , five ink droplets are continuously discharged in one printing cycle. 
     When it is programmable which period the waveform selecting unit  44  validates in relation to a predetermined gradation value, zero to six ink droplets may be discharged with any combination of the units U 1  to U 4  in relation to the gradation value. 
       FIG. 21  depicts a second example of a combination of drive waveform units. In  FIG. 21 , the drive waveform generating units  414  and  424  the 1-drop waveform setting units  411  and  421  twice, subsequently select the 2-drop waveform setting units  412  and  422  once, further select the 3-drop waveform setting units  413  and  423  once, and then generate a drive waveform signal. In  FIG. 21 , the symbols S 1 , S 2 , ΔV, U 1 , U 2 , U 3 , and U 4  are the same as those illustrated in  FIG. 20 . 
     In an example illustrated in  FIG. 21 , when the waveform of the first unit U 1  or the second unit U 2  is applied to the actuator of the pressure chamber  131 , one ink droplet is discharged from the nozzle  301 . When the waveform of the third unit U 3  is applied to the actuator of the pressure chamber  131 , two ink droplets are consecutively discharged from the nozzle  301 . When the waveform of the fourth unit U 4  is applied to the actuator of the pressure chamber  131 , three ink droplets are consecutively discharged from the nozzle  301 . 
     The waveform selecting unit  44  outputs a selecting signal that validates a period of the first unit U 1  when a gradation value of printing data is 1. When the gradation value is 2, the waveform selecting unit  44  outputs a selecting signal that validates a period of the first unit U 1  and a period of the second unit U 2 . When the gradation value is 3, the waveform selecting unit  44  outputs a selecting signal that validates periods of the 2nd and 3rd units U 2 , U 3 . When the gradation value is 4, the waveform selecting unit  44  outputs a selecting signal that validates periods of the first to third units U 1  to U 3 . When the gradation value is 5, the waveform selecting unit  44  outputs a selecting signal that validates periods of the 3rd and 4th units U 3 , U 4 . When the gradation value is 6, the waveform selecting unit  44  outputs a selecting signal that validates periods of the 2nd to 4th units U 2 , U 3 , U 4 . When the gradation value is 7, the waveform selecting unit  44  outputs a selecting signal that validates periods of the first to fourth units U 1  to U 4 . 
       FIG. 23A  illustrates a waveform example in which the waveform selecting unit  44  outputs the selecting signal SL that validates the period of the first unit U 1 . In addition,  FIG. 23B  illustrates a waveform example in which the waveform selecting unit  44  outputs the selecting signal SL that validates the periods of the first to third units U 1 , U 2 , and U 3 . Because these examples are identical to the examples described with reference to  FIGS. 22A and 22B , a description thereof will be omitted. 
       FIG. 23C  illustrates a waveform example in which the waveform selecting unit  44  outputs the selecting signal SL that validates the periods of the first to fourth units U 1 , U 2 , U 3 , and U 4 . For the periods of the first to fourth units U 1 , U 2 , U 3 , and U 4  in which the selecting signal SL is ON, the drive waveform signal S 1  is applied to the first electrode  134 , and the drive waveform signal S 2  is applied to the second electrode  135 . As a result, the differential voltage ΔV between the drive waveform signal S 1  and the drive waveform signal S 2  is applied to the actuator of the pressure chamber  131 , and as a result, seven ink droplets are consecutively discharged from the nozzle  301  that communicates with the pressure chamber  131 . That is, one ink droplet is discharged for the period of the first unit U 1 , and one ink droplet is also discharged for the period of the second unit U 2 . In addition, two ink droplets are sequentially discharged for the period of the third unit U 3 , and three ink droplets are continuously discharged for the period of the fourth unit U 4 . As such, seven ink droplets are discharged in one printing cycle. 
     Although not illustrated, when the waveform selecting unit  44  outputs the selecting signal SL that validates the period of the first unit U 1  and the period of the second unit U 2 , two ink droplets are consecutively discharged in one printing cycle. 
     Therefore, the ink droplets are selectively discharged as one ink droplet, two ink droplets, four ink droplets, or seven ink droplets in accordance with printing data, thereby realizing a multi-drop method of performing gradation printing. 
     Although not illustrated, when the waveform selecting unit  44  outputs the selecting signal SL that validates the period of the second unit U 2  and the period of the third unit U 3 , three ink droplets are consecutively discharged in one printing cycle. 
     Although not illustrated, when the waveform selecting unit  44  outputs the selecting signal SL that validates the periods of the third and fourth units U 3  and U 4 , five ink droplets are consecutively discharged in one printing cycle. 
     Although not illustrated, when the waveform selecting unit  44  outputs the selecting signal SL that validates the periods of the 2nd to 4th units U 2 , U 3 , and U 4 , six ink droplets are consecutively discharged in one printing cycle. 
     When it is programmable which period the waveform selecting unit  44  validates in relation to a predetermined gradation value, zero to seven droplets may be effectively discharged by validating any combination of the periods of the units U 1  to U 4  in relation to the gradation value. 
     There are multiple combinations of the periods of the units U 1  to U 4  for discharging a predetermined number of ink droplets in one printing cycle. For example, to discharge two ink droplets in one printing cycle, the period of the unit U 3  may be used, or the periods of the units U 1  and U 2  may be used. To discharge three ink droplets in one printing cycle, the periods of the units U 1  and U 3  may be combined, the period of the unit U 4  may be used, or the periods of the units U 2  and U 3  may be used. To discharge five ink droplets in one printing cycle, the periods of the units U 1 , U 2 , and U 4  may be combined, or the periods of the units U 3  and U 4  may be combined. Because timing for discharging ink droplets varies depending on such combinations even for discharging a same number of ink droplets, there may be a difference in printing characteristics. A combination for discharging a predetermined number of ink droplets in one printing cycle may be selected in accordance with desired printing characteristics. 
     The inkjet head  1  according to the example embodiments described above can discharge two ink droplets from the nozzle  301  by using the 2-drop waveform illustrated in  FIG. 10 or 14 . The 2-drop waveform discharges two ink droplets with a sequence of operations of single expansion, return, and contraction. This sequence is identical to those of the 1-drop waveform illustrated in  FIG. 8 . Therefore, it is possible to discharge two ink droplets as the same number of times the charging and discharging as in the 1-drop waveform, and as a result, it is possible to reduce power consumption and heat generation for discharging ink droplets. In addition, no waveform element for cancelling residual vibration is inserted between the first ink droplet and the second ink droplet, and the residual vibration is cancelled by the returning operation after the consecutive discharge of two ink droplets ends, and as a result, time required to discharge two ink droplets is reduced. As a result, a high-speed operation is enabled. 
     The degree of freedom when cancelling residual vibration is higher in the case in which the 2-drop waveform illustrated in  FIG. 10  is used than in the case in which the 2-drop waveform illustrated in  FIG. 14  is used, and as a result, it is possible to appropriately cancel residual vibration. As a result, discharge stability is improved, printing quality is improved, and a higher-speed operation is enabled. 
     The inkjet head  1  according to the example embodiments described above can discharge three ink droplets from the nozzle  301  by using the 3-drop waveform illustrated in  FIG. 12 . The 3-drop waveform discharges three ink droplets with a series of operations of single expansion, return, contraction, weak contraction, and contraction. This series of operations reduces the number of times the charging and discharging must be performed in comparison with the case in which three ink droplets are discharged by using the 1-drop waveform and the 2-drop waveform in combination, and as a result, it is possible to reduce power consumption and heat generation for discharging ink droplets. In addition, time required to discharge all of three ink droplets is shorter, and, as a result, a high-speed operation is enable. Furthermore, in the case in which the 3-drop waveform illustrated in  FIG. 12  is used, it is possible to cancel residual vibration after three ink droplets are consecutively discharged from the nozzle  301 . 
     Hereinafter, modified examples of the present example embodiments described above will be described. 
     In the example embodiments described above, as illustrated in  FIGS. 11, 13, and 15 , the ink pressure at times t 23 , t 33 , and t 43  when the second ink droplet is discharged is set to be substantially the same as the ink pressure at the times t 22 , t 32 , and t 42  at which the first ink droplet is discharged. However, the two ink pressures do not have to be equal to each other. In summary, it is sufficient for the ink pressure to have reached positive pressure such that the ink may be discharged by a pulse change of the waveform elements e 25 , e 35 , and e 45  for discharging the second ink droplet. 
       FIG. 24  depicts a drive voltage of a 2-drop waveform and simulated results of an ink pressure and an ink flow velocity. In  FIG. 24 , time t 23  of the leading edge of the contraction pulse P 22  is advanced from the 2-drop waveform illustrated in  FIG. 10 . In  FIG. 24 , the drive voltage waveform is indicated by the solid line, the ink pressure waveform is indicated by the dot dashed line, and the ink flow velocity waveform is indicated by the dashed line. In addition, values on a vertical axis are arbitrarily normalized. 
     In this example, while a time at which the normalized ink pressure is 0.75 is set as time t 22  of the trailing edge of the expansion pulse P 21 , a time at which the normalized ink pressure is 0.5 is set as time t 23  of the leading edge of the contraction pulse P 22 . In this waveform, the discharge velocity of the second ink droplet is lower than that of the first ink droplet, but even with this 2-drop waveform, it is possible to discharge two ink droplets from the nozzle  301 . 
       FIG. 25  depicts a drive voltage of a 2-drop waveform and simulated values of an ink pressure and an ink flow velocity. In  FIG. 25 , time t 23  of the leading edge of the contraction pulse P 22  is further advanced from the 2-drop waveform illustrated in  FIG. 24 . In  FIG. 25 , the drive voltage waveform is indicated by the solid line, the ink pressure waveform is indicated by the dot dashed line, and the ink flow velocity waveform is indicated by the dashed line. In addition, values on a vertical axis are arbitrarily normalized. 
     In this example, while a time at which the normalized ink pressure is 0.75 is set as time t 22  of the trailing edge of the expansion pulse P 21 , a time at which the normalized ink pressure is changed to positive pressure is set as time t 23  of the leading edge of the contraction pulse P 22 . In this waveform, the discharge velocity of the second ink droplet becomes further lower than that of the first ink droplet, but even with this 2-drop waveform, it is possible to continuously discharge two ink droplets from the nozzle  301 . 
       FIG. 26  depicts a drive voltage of a 3-drop waveform and simulated values of an ink pressure and an ink flow velocity. In  FIG. 26 , time t 35  of the leading edge of the second contraction pulse P 34  is advanced from the 3-drop waveform illustrated in  FIG. 12 . In  FIG. 26 , the drive voltage waveform is indicated by the solid line, the ink pressure waveform is indicated by the dot dashed line, and the ink flow velocity waveform is indicated by the dashed line. In addition, values on a vertical axis are arbitrarily normalized. 
       FIG. 13  illustrates that a time at which the normalized ink pressure is 0.75 is set as time t 32  of the trailing edge of the expansion pulse P 31 , and a time at which the normalized ink pressure is 1.3 is set as time t 35  of the leading edge of the second contraction pulse P 34 . In contrast, in  FIG. 26  illustrating a modified example, a time at which the normalized ink pressure is 0.75 which is equal to that in  FIG. 13  is set as time t 32  of the trailing edge of the expansion pulse P 31 . However, because time t 35  is advanced, a time at which the normalized ink pressure is 1.0 lower than that in  FIG. 13  is set as time t 35  of the leading edge of the second contraction pulse P 34 . Even with this 3-drop waveform, it is possible to continuously discharge three ink droplets from the nozzle  301 . Further, in this 3-drop waveform, the flow velocity of the third ink droplet is decreased. 
       FIG. 27  depicts a drive voltage of a 3-drop waveform and simulated values of an ink pressure and an ink flow velocity. In  FIG. 27 , time t 33  of the leading edge of the first contraction pulse P 32  from the 3-drop waveform illustrated in  FIG. 12 . In  FIG. 27 , the drive voltage waveform is indicated by the solid line, the ink pressure waveform is indicated by the dot dashed line, and the ink flow velocity waveform is indicated by the dashed line. In addition, values on a vertical axis are arbitrarily normalized. 
     In this example, while a time at which the normalized ink pressure is 0.75, which is equal to that in  FIG. 13  is set as time t 32  of the trailing edge of the expansion pulse P 31 , a time at which the normalized ink pressure is 0.5 lower than that in  FIG. 13  is set as time t 33  of the leading edge of the first contraction pulse P 32  by advancing time t 33 . Further, time t 34  of the trailing edge of the first contraction pulse P 32  is delayed, thereby decreasing a peak of negative pressure. Therefore, it is possible to reduce positive pressure applied to the adjacent channels, and prevent bubbles from being formed in the pressure chamber  131  by negative pressure. In this 3-drop waveform, a discharge velocity of the second ink droplet is decreased, but even with this 3-drop waveform, it is possible to continuously discharge two ink droplets from the nozzle  301 . 
     In the example embodiments described herein, as illustrated in  FIGS. 11 and 13 , contraction percentages of the weak contraction pulses P 23 , P 33 , and P 35  are 50% when contraction percentages of the contraction pulses P 22 , P 32 , and P 34  are 100%. If the contraction percentages of the weak contraction pulses P 23 , P 33 , and P 35  are 50%, there is an advantage in that a driving power source is simplified. However, the present disclosure is not limited to the example. 
       FIG. 28  depicts simulated results of an ink pressure and an ink flow velocity when in the 2-drop waveform illustrated in  FIG. 10  a contraction percentage of the weak contraction pulse P 23  is 30% and a contraction percentage of the contraction pulse P 22  is 100%. Even with this 2-drop waveform, one ink droplet is discharged from the nozzle  301  because positive pressure is applied to the ink by a pulse change in the state in which the ink pressure is positive pressure equal to or higher than a threshold value at times t 22  and t 23 . At time t 25 , a magnitude of amplitude of the ink pressure is equal to negative pressure instantaneously applied to the ink by the trailing edge of the weak contraction pulse P 23 , and the ink flow velocity becomes zero. Therefore, residual vibration in the pressure chamber  131  is cancelled. 
     The configuration of the inkjet head  1  is not limited to the configuration described with reference to  FIGS. 1 to 6 . For example, an inkjet head, which has one piezoelectric member for each pressure chamber, may be applied, or an inkjet head in which electric potential of one of a pair of electrodes of a piezoelectric member is fixed and a drive waveform is applied to the other electrode may be applied. Alternatively, there may be applied a shared wall type inkjet head in which all of the first and second grooves  131  and  132  are defined as pressure chambers to be filled with the ink, and three sets of the pressure chambers are separately operated in every second set. 
     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.