Patent Publication Number: US-2022234353-A1

Title: Inkjet head and inkjet recording device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2021-009664, filed Jan. 25, 2021, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein generally relate to an inkjet head and an inkjet recording device. 
     BACKGROUND 
     An inkjet head that uses an actuator as a partition wall of an ink pressure chamber is known. The actuator deforms according to an applied drive signal to change the volume of the pressure chamber, which causes a pressure vibration in the ink. Due to this pressure vibration, ink droplets are ejected from a nozzle connected to the pressure chamber. 
     In such an inkjet head, so-called satellite droplets may be separated from primary (or main) droplets in some cases and land on a medium (paper or the like). Also, in some cases, an ink mist, somewhat similar to satellite droplets but generally smaller in size may be generated when ink droplets are ejected from a nozzle. These satellite droplet and mist phenomena cause deterioration of inkjet print quality, so that it is desirable to suppress these phenomena. 
     To suppress such phenomena, a timing of the drive signal can possibly be adjusted. For example, it has been proposed to adjust a drive signal such that a plurality of ink droplets are ejected within the ejection cycle of what would nominal otherwise be one ink droplet and the multiple ink droplets are combined in the air before landing on a medium. However, if the timing of the drive signal is adjusted only in consideration of avoidance of satellite and mist phenomena, it is likely that optimum pressure vibration cannot be obtained and that ejection stability and print quality deteriorate. 
     Hence, there is a need for an inkjet head and an inkjet recording device capable of suppressing or mitigating ink droplet separation that causes satellite and mist phenomena while maintaining ink ejection stability for achieving higher-quality printing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a configuration example of an inkjet recording device according to an embodiment. 
         FIG. 2  depicts a configuration example of an inkjet head in a perspective view according to an embodiment. 
         FIG. 3  depicts a configuration example of a head body of an inkjet head in an exploded perspective view according to an embodiment. 
         FIG. 4  depicts a partial configuration example of an inkjet head in a cross-sectional view according to an embodiment. 
         FIG. 5  is a block diagram of a configuration example of a control system of an inkjet recording device according to an embodiment. 
         FIG. 6  shows an example of states of a pressure chamber of an inkjet head according to an embodiment. 
         FIG. 7  shows an example of a pressure fluctuation simulation result of a medium-viscosity ink using a drive signal in related art. 
         FIG. 8  shows an example of a pressure fluctuation simulation result of a low-viscosity ink using a drive signal in related art. 
         FIG. 9  shows an example of a waveform of a drive signal used in an inkjet head according to an embodiment. 
         FIG. 10  shows an example of a flying state of ink droplets when a drive signal in related art is used. 
         FIG. 11  shows an example of a flying state of ink droplets when a drive signal according to an embodiment is used. 
         FIG. 12  shows an example of a dot separation suppressing effect due to a pulse width of an auxiliary pulse according to an embodiment. 
         FIG. 13  shows an example of a measurement result of a dot-to-dot distance according to a pulse width of an auxiliary pulse according to an embodiment. 
         FIG. 14  shows an example of a dot separation suppressing effect due to a pulse width of a contraction pulse according to an embodiment. 
         FIG. 15  shows an example of a measurement result of a dot-to-dot distance according to a pulse width of a contraction pulse according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     According to one or more embodiments, an inkjet head includes an actuator and a drive circuit. The actuator deforms in response to a drive signal and changes a volume of a pressure chamber connected to a nozzle so as to eject ink contained in the pressure chamber from the nozzle. The drive circuit applies the drive signal to the actuator. The drive signal includes a main interval during which the ink is ejected from the nozzle and an auxiliary interval during which the ink is not ejected from the nozzle. The main interval includes a first pulse by which a first voltage is applied to the actuator, a first period in which the actuator is maintained at a reference potential, and a second pulse by which a second voltage having a polarity opposite to that of the first voltage is applied to the actuator. The auxiliary interval is prior to the main interval and includes a third pulse in which a third voltage having the same polarity as the first voltage is applied to the actuator and a second period in which the actuator is maintained at the reference potential. 
     Hereinafter, certain example embodiments will be described with reference to the accompanying drawings. The same or substantially similar elements, components, and the like will be denoted by the same reference numerals and duplicate description may be omitted for subsequent instances. If a plurality of the same or substantially similar elements are depicted, a common reference numeral may be used to describe each element in the plurality. 
     An inkjet recording device according to an embodiment forms an image on a medium, such as paper, by using an inkjet head. For example, the inkjet recording device ejects ink from a pressure chamber included in the inkjet head as ink droplets to form an image on a medium. Examples of the inkjet recording device include, but are not limited to, an office inkjet recording device, a barcode inkjet recording device, a Point-Of-Sale (POS) inkjet recording device, an industrial inkjet recording device, a 3D inkjet recording device, and the like. A medium on which the inkjet recording device forms an image is not limited to a specific configuration. 
     As illustrated in  FIG. 1 , the inkjet recording device  1  according to the present embodiment forms an image on an image forming medium S or the like by using a recording material such as ink. As an example, the inkjet recording device  1  includes a plurality of ink ejection units  2 , a head support mechanism  3 , and a medium support mechanism (or a support unit)  4 . The head support mechanism  3  movably supports the ink ejection units  2 . The support mechanism  4  movably supports the image forming medium S. The image forming medium S is, for example, a sheet made of paper, cloth, resin, or the like. 
     The ink ejection units  2  are supported by the head support mechanism  3  in parallel in a predetermined direction. The head support mechanism  3  is attached to an endless belt  34  hung on rollers  33 . By rotating the rollers  33 , the inkjet recording device  1  moves the head support mechanism  3  in a main scanning direction A intersecting a conveyance direction of the image forming medium S. Each ink ejection unit  2  integrally includes an inkjet head  10  and a circulation device  20 . The ink ejection unit  2  performs an operation of ejecting ink I from the inkjet head  10 . The inkjet recording device  1  uses, for example, a scanning method in which an image is formed on an image forming media S by performing the ink ejection operation while reciprocating the head support mechanism  3  in the main scanning direction A. Alternatively, the inkjet recording device  1  may be configured as a single-pass system in which the ink ejection operation is performed without moving the head support mechanism  3 . In the latter case, it is not necessary to provide the roller  33  and the endless belt  34 . In this case, the head support mechanism  3  can be fixed to, for example, a housing or the like of the inkjet recording device  1 . 
     The ink ejection units  2  respectively eject, for example, four different color inks corresponding to CMYK (cyan, magenta, yellow, and key/black), that is, cyan ink, magenta ink, yellow ink, and black ink. 
     The inkjet head  10  according to the present embodiment is of a shear-mode shared wall type and a circulation type having a side shooter design. The inkjet heads  10  may be of another type in other embodiments. 
       FIG. 2  is a perspective view illustrating an example of the configuration of the inkjet head  10 .  FIG. 3  is an exploded perspective view illustrating an example of the configuration of the inkjet head  10 .  FIG. 4  is a cross-sectional view taken along the line F-F of  FIG. 2 . 
     As illustrated in  FIG. 2 , the inkjet head  10  is mounted on the inkjet recording device  1  and is connected to an ink tank via a component, such as a tube. The inkjet head  10  includes a head body  11 , a unit portion  12 , and a pair of circuit boards  13 . 
     The head body  11  is a device for ejecting ink. The head body  11  is attached to the unit portion  12 . The unit portion  12  includes: a manifold that forms part of a path between the head body  11  and the ink tank; and a member for mounting inside the inkjet recording device  1 . The pair of circuit boards  13  are attached to the head body  11 . 
     As further illustrated in  FIGS. 3 and 4 , the head body  11  includes a base plate  15 , a nozzle plate  16 , a frame member  17 , and a pair of drive elements  18 . As illustrated in  FIG. 4  in a cross-sectional view taken along the line F-F of  FIG. 2 , an ink chamber  19  to which ink is supplied is formed inside the head body  11 . 
     As illustrated in  FIG. 3 , the base plate  15  has a rectangular plate shape made of ceramics, such as alumina. The base plate  15  has a flat mounting (or installation) surface  21 . The base plate  15  has a plurality of supply holes  22  and a plurality of discharge holes  23  open on the mounting surface  21 . 
     The supply holes  22  are provided in the longitudinal direction of the base plate  15  at the central portion of the base plate  15 . Each supply hole  22  communicates with an ink supply unit  121  of the manifold of the unit portion  12 . The supply hole  22  is connected to the ink tank in the circulation device  20  via the ink supply unit  121 . The ink in the ink tank is supplied to the ink chamber  19  through the ink supply unit  121  and the supply hole  22 . 
     The discharge holes  23  are provided in two rows interposing the supply hole  22  therebetween. Each discharge hole  23  communicates with an ink discharge unit  122  of the manifold of the unit portion  12 . The discharge hole  23  is connected to the ink tank in the circulation device  20  via the ink discharge unit  122 . The ink in the ink chamber  19  is collected in the ink tank through the ink discharge unit  122  and the discharge hole  23 . In this manner, the ink circulates between the ink tank and the ink chamber  19 . 
     The nozzle plate  16  is formed of, for example, a rectangular shaped film made of polyimide having a liquid-repellent function on the surface. The nozzle plate  16  faces the mounting surface  21  of the base plate  15 . The nozzle plate  16  is provided with a plurality of nozzles  25 . The plurality of nozzles  25  are aligned in two rows along the longitudinal direction of the nozzle plate  16 . 
     The frame member  17  is formed of, for example, a nickel alloy in a rectangular frame shape. The frame member  17  is interposed between the mounting surface  21  of the base plate  15  and the nozzle plate  16 . The frame member  17  is adhered to the mounting surface  21  and the nozzle plate  16 . The nozzle plate  16  is attached to the base plate  15  with the frame member  17  interposed therebetween. As illustrated in  FIG. 4 , the ink chamber  19  is surrounded by the base plate  15 , the nozzle plate  16 , and the frame member  17 . 
     Each drive element  18  comprises, for example, two plate-shaped piezoelectric bodies formed of lead zirconate titanate (PZT). The two piezoelectric bodies are bonded together so that the polarization directions are opposite to each other in the thickness direction. 
     The pair of drive elements  18  are adhered to the mounting surface  21  of the base plate  15 . The pair of drive elements  18  are arranged in parallel in the ink chamber  19  corresponding to the nozzles  25  arranged in two rows. The drive element  18  is formed in a trapezoidal cross section. The top of the drive element  18  is adhered to the nozzle plate  16 . 
     The drive element  18  is provided with a plurality of grooves  27 . The grooves  27  extend in a direction intersecting the longitudinal direction of the drive element  18 , and the grooves are aligned in the longitudinal direction of the drive element  18 . The plurality of grooves  27  face the plurality of nozzles  25  of the nozzle plate  16 . The drive element  18  of the present embodiment has a plurality of pressure chambers  50  each filled with ink, which are arranged in the groove  27 . 
     Electrodes  28  are provided in the plurality of grooves  27 , respectively. Each electrode  28  is formed, for example, by photoresist patterning and etching process on a nickel thin film. The electrode  28  covers an inner surface of the groove  27 . 
     A plurality of wiring patterns  35  are provided on the base plate  15 , extending from the mounting surface  21  to and over the drive element  18 . The wiring patterns  35  are formed, for example, by photoresist patterning and etching on a nickel thin film. 
     The wiring patterns  35  exist on both sides of the longitudinal row of the supply holes  22  at positions corresponding to the pair of the drive elements  18  and extend from one side-end portion  211  and another side-end portion  212  of the mounting surface  21  in the width direction of the base plate  15 . Each of the side-end portions  211  and  212  includes not only an edge of the mounting surface  21  but also a peripheral region of the edge. Therefore, the wiring patterns  35  may extend from either the edge or the edge peripheral region of the mounting surface  21 . 
     The wiring pattern  35  that extends from the side-end portion  211  is shown in  FIG. 4 . The configuration of the wiring pattern  35  of the other side-end portion  212  is the same or substantially the same as that of the wiring pattern  35  of the side end portion  211 . 
     The wiring line pattern  35  has a first portion  351  and a second portion  352 . The first portion  351  extends in a linear shape from the side end portion  211  of the mounting surface  21  toward the drive element  18 . The neighboring first portions  351  extend parallel to each other (see  FIG. 3 ). The second portion  352  extends from one end portion of the first portion  351  to and over the electrode  28 . The second portion  352  is electrically connected to the electrode  28 . 
     For the drive element  18 , there are electrodes  28  among the plurality of electrodes  28  designated as a first electrode group  31  and other electrodes  28  among the plurality of electrodes  28  are designated as a second electrode group  32 . 
     The first electrode group  31  and the second electrode group  32  are separated from each other by a central portion of the drive element  18  in the longitudinal direction. That is the central portion of the drive element  18  can be considered as a border dividing the first electrode group  31  from the second electrode group  32 . The second electrode group  32  is adjacent to the first electrode group  31  across the central portion of the drive element  18 . Each of the first and second electrode groups  31  and  32  includes, for example, one-hundred and fifty-nine (159) electrodes  28 . The number of the electrodes  28  is not limited thereto. 
     Referring back to  FIG. 2 , each of the circuit boards  13  has a board body  44  and a pair of film carrier packages (FCP)  45 . The FCP can also be referred to as a tape carrier package (TCP) in some instances. 
     The board body  44  is a rigid printed wiring board (printed circuit board) formed in a rectangular shape. Various electronic components and connectors can be mounted on the board body  44 . The pair of FCPs  45  are attached to the board body  44 . 
     Each of the FCPs  45  has a resin film  46  on which a plurality of wirings are formed. The resin film  46  has flexibility. Each FCP  45  also has a head drive circuit  47  connected to the plurality of wirings. The film  46  is a tape automated bonding (TAB) element or the like. The head drive circuit  47  is an integrated circuit (IC) for applying voltages to the electrodes  28 . The head drive circuit  47  is fixed to the film  46  by a resin. 
     One end portion of the FCP  45  is thermocompression bonded to the first portion  351  of the wiring pattern  35  by an anisotropic conductive film (ACF)  48 . By doing so, the plurality of wirings of the FCP  45  are electrically connected to the wiring patterns  35 . 
     By connecting the FCP  45  to the wiring patterns  35 , the head drive circuit  47  is electrically connected to the electrodes  28  via the wirings of the FCP  45 . The head drive circuit  47  applies a voltage to the electrodes  28  via the wirings of the film  46 . 
     The voltage application deforms each of the drive elements  18  in shear mode such that the volume of each of the pressure chambers  50  in which the electrode  28  is provided increases or decreases. By doing so, the pressure of the ink in the pressure chamber  50  changes, and the ink is ejected from the nozzle  25 . In this manner, the drive element  18  that separates the pressure chamber  50  serves as an actuator for applying the pressure vibration to the inside of the pressure chamber  50 . 
     The circulation device  20  illustrated in  FIG. 1  is integrally connected to an upper portion of the inkjet head  10  by a connecting component made of a metal or the like. The circulation device  20  includes a predetermined circulation path configured to allow ink to circulate through the ink tank and the inkjet head  10 . The circulation device  20  includes a pump for circulating the ink. The ink is supplied from the circulation device  20  into the inkjet head  10  through the ink supply unit  121  by an action of the pump, passes through a predetermined flow path, and then is sent from the inside of the inkjet head  10  to the circulation device  20  through the ink discharge unit  122 . 
     Further, the circulation device  20  supplies the ink to the circulation path from a cartridge provided as a supply tank outside the circulation path. 
     An example of a circuit configuration of the inkjet recording device  1  according to the present embodiment is illustrated in FIG.  5 . 
     The inkjet recording device  1  includes a processor  101 , a ROM  102 , a RAM  103 , a communication interface  104 , a display unit  105 , an operation unit  106 , a head interface  107 , a bus  108 , and the inkjet head  10 . 
     The processor  101  corresponds to a central portion of a computer that performs processes and control required for operation of the inkjet recording device  1 . The processor  101  controls each unit to realize various functions of the inkjet recording device  1  based on a program or programs, such as system software, application software, or firmware, stored in the ROM  102 . The processor  101  is, for example, a central processing unit (CPU), a micro processing unit (MPU), a system on a chip (SoC), a digital signal processor (DSP), a graphics processing unit (GPU), or the like. Alternatively, the processor  101  is a combination of these components. 
     The ROM  102  is a non-volatile memory used exclusively for reading data, which corresponds to a main memory portion of the computer in which the processor  101  is used as a central portion. The ROM  102  stores the program. The ROM  102  also stores data or various set values used by the processor  101  to perform various processes. 
     The RAM  103  is a memory used for reading and writing data, which corresponds to a main memory portion of the computer in which the processor  101  is used as a central portion. The RAM  103  is used as a so-called work area or the like for temporarily storing data used by the processor  101  to perform various processes. 
     The communication interface  104  is for the inkjet recording device  1  to communicate with a host computer or the like via a network or a communication cable. 
     The display unit  105  displays a screen for notifying an operator of the inkjet recording device  1  of various pieces of information. The display unit  105  is, for example, a display such as a liquid crystal display or an organic electro-luminescence (EL) display. 
     The operation unit  106  accepts an input operation by an operator of the inkjet recording device  1 . The operation unit  106  is, for example, a keyboard, a keypad, a touch pad, a mouse, or the like. Furthermore, as the operation unit  106 , a touch pad superimposed on the display panel of the display unit  105  can also be used. The display panel provided on a touch panel can be used as the display unit  105 , and the touch pad provided on the touch panel can be used as the operation unit  106 . 
     The head interface  107  is provided for the processor  101  to communicate with the inkjet head  10 . The head interface  107  transmits gradation data and the like to the inkjet head  10  under the control of the processor  101 . 
     The bus  108  includes a control bus, an address bus, a data bus, and the like and transmits signals to and from each unit of the inkjet recording device  1 . 
     The inkjet head  10  includes a head driver  100  as a control unit. 
     The head driver  100  is a drive circuit for operating the inkjet head  10 . The head driver  100  includes the head drive circuit  47  and the like. The head driver  100  is, for example, a line driver. The head driver  100  stores one or more waveform data WD. 
     The head driver  100  repeatedly generates a single drive signal based on the waveform data WD. Then, the head driver  100  controls the number of times of ejecting ink to each pixel on the image forming medium S based on the gradation data transmitted from the head interface  107 . Each time the single drive signal is generated and applied to the drive element  18 , one ink droplet (that is one main drop) is ejected from the nozzle  25  of the inkjet head  10 . Therefore, the inkjet recording device  1  expresses shading depending on, for example, how many drops of ink are ejected to each pixel. The more sets of ink are ejected to one pixel, the darker the shade of the corresponding color in the pixel becomes. 
     In one instance, the head driver  100  is provided to an administrator, a user, or the like of the head driver  100  with the waveform data WD stored therein. In another instance, the head driver  100  may be provided to an administrator, a user, or the like without the waveform data WD stored therein. In still another instance, the head driver  100  may be provided to an administrator, a user, or the like with other waveform data are stored. The appropriate waveform data WD may be separately provided to an administrator, a user, or the like and written to the head driver  100  under operation by the administrator, the user, or the like or by a service person or the like. The provision of the waveform data WD may be realized, for example, by recording of data on a non-transitory removable storage medium, such as a magnetic disk, a magneto-optical disk, an optical disk, or a semiconductor memory, or by downloading via a network or the like. 
     Upon the application of the drive signal, the drive element  18  (which is a piezoelectric body) deforms in shear mode. Due to this deformation, the volume of the pressure chamber  50  changes. 
     In this example, it is assumed that the pressure chamber  50  will be in a normal (e.g., not contracted and not expanded) state when the drive signal is not being applied or otherwise the present potential value of the drive signal is 0 V. If the potential of the drive signal is positive, the pressure chamber  50  contracts, and the volume of the pressure chamber  50  decreases as compared with the normal state. If the potential of the drive signal is negative, the pressure chamber  50  expands, and the volume of the pressure chamber  50  increases as compared with the normal state. As the volume of the pressure chamber  50  changes, the pressure on the ink in the pressure chamber  50  changes. The inkjet head  10  ejects ink upon application of a drive signal having a specific waveform. 
     As shown in  FIG. 6 , a pressure chamber  502  that is the same or substantially the same as the pressure chamber  50  of the inkjet head  10  according to the present embodiment changes to a standby state, a “PULL (Half)” state, a “PULL (Full”) state, a “PUSH (Half)” state, and a “PUSH (Full)” state. 
     In the standby state, the pressure chamber  502  is in a normal state. As illustrated in  FIG. 6 , the head driver  100  sets all potentials of an electrode  282  formed in the pressure chamber  502  and electrodes  281  and  283  formed in pressure chambers  501  and  503  on both sides adjacent to the pressure chamber  502  to a reference potential of 0 V (or ground potential GND). The chambers  501  and  503  are the same or substantially the same as the pressure chamber  50 , and the electrodes  281 ,  282 ,  283  are the same or substantially the same as the electrode  28  in the present embodiment. In this standby state, a drive element  181  interposed between the pressure chamber  501  and the pressure chamber  502  and a drive element  182  interposed between the pressure chamber  502  and the pressure chamber  503  do not cause any distortion. The drive elements  181  and  182  are the same or substantially the same as the drive element  18  in the present embodiment. 
     In the PULL (Half) state, the pressure chamber  502  expands. The head driver  100  sets the electrode  282  of the pressure chamber  502  to a potential of 0 V and applies a voltage of +V to the electrodes  281  and  283  of the pressure chambers  501  and  503 . In this state, an electric field of voltage value of 1V acts on each of the drive elements  181  and  182  in a direction intersecting the polarization direction of the drive element  18 . By this action, each of the drive elements  181  and  182  deforms outward to expand the pressure chamber  502 . 
     In the PULL (Full) state, the pressure chamber  502  expands more than PULL (Half). The head driver  100  applies a negative voltage of “−V” to the electrodes  282  of the pressure chamber  502  and applies a voltage of “+V” to the electrodes  281  and  283  of the pressure chambers  501  and  503 . In this state, an electric field having a voltage value of 2 V acts on each of the drive elements  181  and  182  in a direction intersecting the polarization direction of the drive element  18 . By this action, each of the drive elements  181  and  182  deforms outward to further expand the pressure chamber  502  than PULL (Half). 
     In the PUSH (Half) state, the pressure chamber  502  contracts. The head driver  100  sets the electrode  282  of the pressure chamber  502  to a potential of 0 V and applies a voltage of “−V” to the electrodes  281  and  283  of the pressure chambers  501  and  503 . In this state, an electric field of the voltage value 1 V acts on each of the drive elements  181  and  182  in a direction opposite to the drive voltage of PULL (Half) or PULL (Full). By this action, each of the drive elements  181  and  182  deforms inward to contract the pressure chamber  502 . 
     In the PUSH (Full) state, the pressure chamber  502  contracts more than PUSH (Half). As The head driver  100  applies a voltage of “+V” to the electrodes  282  of the pressure chamber  502  and applies a voltage of “−V” to the electrodes  281  and  283  of the pressure chambers  501  and  503 . In this state, an electric field having a voltage value of 2 V acts on each of the drive elements  181  and  182  in a direction opposite to the drive voltage of PULL (Half) or PULL (Full). By this action, each of the drive elements  181  and  182  deforms inward to further contract the pressure chamber  502  than PUSH (Half). 
     When the volume of the pressure chamber  502  expands or contracts, pressure vibration (oscillation) occurs in the pressure chamber  502 . Due to this pressure vibration, the pressure in the pressure chamber  502  increases and the ink droplets are ejected from the nozzle  25  that communicates with the pressure chamber  502 . 
     In this manner, the drive elements  181  and  182  that separate the pressure chambers  501 ,  502 , and  503  from each other serve as actuators for applying the pressure vibration to the inside of the pressure chamber  502  that has the drive elements  181  and  182  as wall surfaces. That is, the pressure chamber  50  expands or contracts according to the operation of the drive element  18 . 
     Each pressure chamber  50  shares the drive element  18  (as a partition wall) with an adjacent pressure chamber  50 . For this reason, the head driver  100  cannot drive each pressure chamber  50  individually. As one example, the present embodiment applies three-division driving in which the head driver  100  divides pressure chambers  50  into three driving sets of every two chambers and drives the heads accordingly. Embodiments of the disclosure are not limited thereto. Four-division driving, five-division driving, or the like may be used. 
     An example of a pressure fluctuation simulation result of a medium-viscosity ink using a drive signal in the related art is shown in  FIG. 7 . Herein, a medium-viscosity ink refers to an ink of 5 centipoise (cps) or more. The simulation was performed by using an LCR equivalent circuit (not separately illustrated) that simulates an inkjet head. In  FIG. 7 , the horizontal axis represents time. The thick solid line “drive voltage” is a waveform representing a voltage change of the drive signal. The drive signal includes a pulse PD and a pulse PP. The pulse PD is a waveform representing application of a negative voltage (−1.0 V) from the reference potential of 0 V to expand the pressure chamber  50  and subsequent application of 0V to contract the pressure chamber  50  to the normal state. In the pulse PD, due to the expansion of the pressure chamber  50  by the application of the negative voltage (−1.0 V) and the subsequent contract of the pressure chamber  50  back to the normal state by the application of the reference potential 0 V, the pressure in the pressure chamber  50  rises so that ink droplets are ejected from the nozzle  25 . The pulse PP is a waveform applied after the pulse PD. The pulse PP is a waveform representing application of a positive voltage (+1.0 V) from the reference potential of 0 V to contract the pressure chamber  50  and subsequent application of 0 V to expand the pressure chamber  50  back to the normal state. The pulse PP is applied after a certain period of time has elapsed after the application of the pulse PD. The coarse broken line “pressure” in  FIG. 7  is a waveform representing a change in the pressure on the ink in the vicinity of the nozzle  25 . The one-dot dashed line “flow rate” in  FIG. 7  is a waveform representing a change in the flow rate of the ink flowing into the nozzle  25 . The thin solid line “meniscus” in  FIG. 7  is a waveform representing a change in the shape of the liquid surface of the ink at the nozzle  25 . The change in the meniscus corresponds to the change in the volume of ink in the vicinity of the nozzle. The fine broken line “propulsive force” in  FIG. 7  is a waveform representing a change in the force pushing out the ink. The propulsive force is proportional to both pressure and meniscus. In the interval between the pulse PD and the pulse PP, the potential of the drive signal is maintained at 0 V, but the pressure still fluctuates during this interval, and the flow rate, the meniscus, and the propulsive force also fluctuate greatly. After the pulse PP, the potential of the drive signal is maintained at 0 V again, but residual vibration still occurs in the pressure, the flow rate, the meniscus, and the propulsive force. 
     An example of a pressure fluctuation simulation result of a low-viscosity ink using the same drive signal as that used for the simulation result in  FIG. 7  is shown in  FIG. 8 . Herein, a low-viscosity ink refers to an ink of less than 5 cps. Each waveform in  FIG. 8  corresponds to each waveform described with respect to  FIG. 7 . 
     In comparison with  FIGS. 7 and 8 , ejection of the low-viscosity ink causes much more residual vibration with respect to the pressure, the flow rate, the meniscus, and the propulsive force after the pulse PP than the ejection of the medium-viscosity ink. Such residual vibration results in dot separation and dispersal during the ink ejection and deteriorates print quality. With the drive signal in the related art, while the residual vibration can be suppressed or mitigated to some extent in the case of the medium-viscosity, the residual vibration cannot be suppressed if low-viscosity ink is used, and the print quality will be deteriorated. For this reason, the medium-viscosity ink is generally recommended for high quality printing in the related art. 
     In the inkjet head  10 , when a single ink droplet is ejected the ejected ink droplet may become separated during flight. This phenomenon is called dot separation. The separation of the ink droplet can occur in various shapes, but generally separation produces a main drop, a forward drop, and a backward drop occurs. For convenience of description, “main drop” is considered to refer to the largest of the ink droplets formed during flight. The “forward drop” is considered to refer to an ink droplet separated to the image forming medium S side from the main drop. The “backward drop” is considered to refer to an ink droplet separated to the nozzle side from the main drop. Separated drops may land at different positions on the image forming medium S when either the inkjet head  10  or the image forming medium S move during ejections, and if the degree of separation is large, the print quality can be deteriorated. In this context, “dispersal” refers to an erroneous ejection in which the main drop does not eject in the first place or the main drop does not form from the ejected ink. The lower the viscosity of the ink, the more likely dot separation and dispersal will occur. In general, it is expected that printing quality can be improved by suppressing dot separation and dispersal. 
       FIG. 9  depicts an example of a waveform of a drive signal used in the inkjet head  10  according to an embodiment. For the simplicity of description, it will be assumed that the inkjet head  10  operates in a single drop mode by which one printed dot is to be formed on a medium using one ink droplet, and thus a drive signal of a cycle of ejecting one ink droplet (referred to as “single cycle”) will be described. The head driver  100  ejects a predetermined amount of ink droplets from the nozzle  25  every cycle by applying the drive signal illustrated in  FIG. 9  to the drive element  18 . 
     In the example, the drive signal includes an auxiliary interval TA and a main interval TM within each cycle T. The main interval TM is an interval during which ink droplets are ejected from the nozzle  25 . The main interval TM includes an expansion pulse (“Draw”), a retention period (“Release”), and a contraction pulse (“Push”). 
     The expansion pulse (Draw) is a first type pulse in the main interval TM and applies a first voltage V d  to the drive element  18 . In the example, the first voltage V d  is a negative voltage (for example, −1.0V). When the expansion pulse (Draw) is applied, the drive element  18  deforms in shear mode to expand the volume of the pressure chamber  50 . 
     In the example, the pulse width W d  of the expansion pulse (Draw) corresponds to the time width starting from the reference potential of 0 V, passing through −0.5 V, reaching −1.0 V, passing through −0.5 V again, and returning to the reference potential of 0 V. The pulse width W d  of the expansion pulse (Draw) is, for example, 1.52 μs. The time when the voltage is maintained at an intermediate voltage (−0.5 V) during the falling edge and the rising edge of the pulse is about 0.2 μs. The application of the intermediate voltage is provided by taking into consideration the power efficiency, but such a stepwise pulse is not necessarily used in the present embodiment. Once the expansion pulse (Draw) returns to 0 V, the pressure in the pressure chamber  50  rises, and the ink is ejected from the nozzle  25 . The expansion pulse (Draw) is also called the ejection pulse. 
     The retention period (Release) is a period after the expansion pulse (Draw) during which the drive element  18  is maintained at the reference potential (for example, 0 V) that does not cause deformation of the drive element  18 . Similarly to those illustrated in  FIGS. 7 and 8 , pressure fluctuations (oscillations) occur during the retention period (Release). 
     The contraction pulse (Push) is a second type pulse in the main interval TM and is after the retention period (Release). The contraction pulse (PUSH) applies a second voltage V p  having a polarity opposite to that of the first voltage V d  to the drive element  18 . In the example, the second voltage V p  is a positive voltage (for example, +1.0V). When the contraction pulse (Push) is applied, the drive element  18  deforms in shear mode to contract the volume of the pressure chamber  50 . The contraction pulse (Push) is also called a cancel pulse and dampens or offsets the pressure vibration occurring by the expansion pulse (Draw). 
     In the example, the pulse width W p  of the contraction pulse (Push) corresponds to the time width starting from the reference potential of 0 V, passing through +0.5 V, reaching +1.0 V, passing through +0.5 V again, and returning to the reference potential of 0 V. Half the time of the natural vibration cycle 2AL of the pressure chamber  50  is defined as one AL (acoustic length). The pulse width W p  of the contraction pulse (Push) has a maximum time width of about one AL. The pulse width W p  is, for example, 1.20 μs. The time when the voltage is maintained at +0.5 V during the rising edge and the falling edge of the pulse is about 0.2 μs. The stepwise pulse takes power efficiency into consideration but is not necessarily used in the present embodiment. 
     The length of the retention period (Release) is set so that the distance between the center of the pulse width W d  of the expansion pulse (Draw) and the center of the pulse width W p  of the contraction pulse (Push) is maintained to be 2AL. That is, the length of the retention period (Release) is equal to the natural vibration cycle (2AL) of the pressure chamber  50  (more particularly, the pressure chamber  50  with an ink/liquid therein). The length of the retention period (Release) is determined after the pulse width W p  of the contraction pulse (Push) is set. The length of the retention period (Release) is, for example, 1.68 μs. In this example, the natural vibration cycle (2AL) 3.04 μs. 
     The auxiliary interval TA is provided in each cycle T before the main interval TM within the same cycle T. The auxiliary interval TA is an interval during which ink droplets are not ejected from the nozzle  25 . The auxiliary interval TA includes an auxiliary pulse (“deBst”) and a rest period (Rest). 
     The auxiliary pulse (deBst) is a third type pulse within the cycle T, and a third voltage V a  having the same polarity as the first voltage V d  of the expansion pulse (Draw) is applied to the drive element  18 . In the example, the amplitude of the auxiliary pulse (that is a voltage applied by the auxiliary pulse) is one-half (½) of the amplitude of the expansion pulse (that is a voltage applied by the expansion pulse (Draw)). For example, the voltage applied by the auxiliary pulse is −0.5 V. The pulse width W a  of the auxiliary pulse (deBst) has a time width of AL×⅓ at the maximum. That is, the pulse width W a  of the auxiliary pulse (deBst) is one-sixth (⅙) or less of the natural vibration cycle of the pressure chamber  50 . The pulse width W a  of the auxiliary pulse (deBst) is, for example, 0.5 μs. 
     The rest period (Rest) maintains the drive element  18  at the reference potential after the auxiliary pulse (deBst). The rest period (Rest) is held for a length of 2AL. That is, the length of the rest period (Rest) is equal to the natural vibration cycle of the pressure chamber  50 . 
     In the auxiliary interval TA, the auxiliary pulse (deBst) expands the pressure chamber  50  by applying a negative voltage to the drive element  18 . That is, the head driver  100  changes the pressure chamber  50  from the standby state to a PULL (Half) state. When the pressure chamber  50  expands, the pressure in the pressure chamber  50  decreases, and as a result, ink will be filled into the pressure chamber  50  from the common ink chamber  5 . During the rest period (Rest), by keeping drive element  18  at the reference potential, the pressure chamber  50  returns from the PULL (Half) to the standby state. When the pressure chamber  50  returns to the standby state, the pressure chamber  50  contracts from the previously expanded state, and the pressure in the pressure chamber  50  rises, but this pressure change is set so as not to eject the ink droplets from the nozzle. That is, in the auxiliary interval TA, the pressure chamber  50  expands and relaxes, but ink droplets are not ejected. 
     Then, in the main interval TM, the expansion pulse (Draw) causes the pressure chamber  50  to re-expand by applying a negative voltage to the drive element  18  again. That is, the head driver  100  changes the state of the pressure chamber  50  from the standby state to the PULL (Full) state (though passing through PULL (Half) state as an intermediate state). Thus, the pressure chamber  50  expands again, and the pressure in the pressure chamber  50  decreases. Since the expansion pulse (Draw) utilizes a voltage twice that of the auxiliary pulse (deBst), the pressure chamber  50  is expanded further than with application of the auxiliary pulse (deBst). 
     During the retention period (Release), by maintaining the drive element  18  at the reference potential, the pressure chamber  50  returns again to the standby state (via PULL (Half)state). Since the voltage change applied to the drive element  18  is greater than the voltage change in the auxiliary interval TA, greater pressure change occurs in the ink contained in the pressure chamber  50 . 
     The contraction pulse (Push) contracts the pressure chamber  50  by applying a positive voltage to the drive element  18 . That is, the head driver  100  changes the state of the pressure chamber  50  from the standby state to the PUSH (Full) state (via PUSH (Half)). 
     Accordingly, in the main interval TM, the pressure chamber  50  expands, relaxes, contracts, and relaxes in sequence. In this process, as the pressure in the pressure chamber  50  rises, the speed of the meniscus in the nozzle  25  exceeds a threshold value for ejecting ink droplets. When the speed of the meniscus exceeds the ejection threshold value, ink droplets are ejected from the nozzle  25  connected to the pressure chamber  50 . 
     The specific voltage values illustrated in  FIG. 9  represent only one example, and other values may be used. Similarly, each time length described in the disclosure is only one example and may be appropriately determined according to specific operating conditions, usage environment, structural parameters, and the like to be utilized. 
     According to the present embodiment, by providing the auxiliary interval TA prior to or in front of the main interval TM and expanding the pressure chamber  50  without ejecting ink, the residual pressure vibration caused by the previous cycle can be more effectively suppressed. By doing so, stable ink ejection can be performed after suppression of the previously induced vibration, and print quality can be improved. Furthermore, changing the pulse width W a  of the auxiliary pulse (deBst) changes the degree of separation of the forward drop, and changing the pulse width W p  of the contraction pulse (Push) changes the degree of separation of the backward drop. Therefore, the print quality can be further improved by selecting the appropriate values of the pulse widths W a  and W p  according to the usage environment, structural parameters, operating conditions, or the like. 
       FIG. 10  is an example of a flying state of ink droplets when the drive signal of the related art (as illustrated in  FIG. 7 ) is used. In  FIG. 10 , the horizontal axis represents the distance (GAP) from nozzle surface (GAP=0.0 mm, 0.5 mm, and 1.0 mm are specifically labeled), flight time (time) increases from the uppermost stage (pa) downward through to the stages (pb), (pc), (pd) and (pe). In the example, the ink droplets are dot-separated immediately after ejection (stage (pa)), and the degree of separation (that is the distance between the ink droplets) increases as time elapses and the distance from the nozzle surface increases. 
       FIG. 11  is an example of a flying state of ink droplets when the drive signal illustrated in  FIG. 9  is used. In  FIG. 11 , the same conditions as those in  FIG. 10  are used except the drive signal is different. In  FIG. 11 , the horizontal axis again represents the distance from the nozzle surface, and flight time increases from the uppermost stage (a) downward through the stages (b), (c), (d) and (e). In the example, the ink droplets are dot-separated immediately after ejection (stage (a)), but it is observed that the ink droplets initially separated during the flying are subsequently combined into one droplet, and thus, substantially no droplet separation is observed in stages (b) to (e). 
     An example of determining the optimum value of the pulse width in the inkjet head  10  according to the present embodiment will be described. 
     First, an example of determining the optimum value with respect to the pulse width W a  of the auxiliary pulse (deBst) will be described with reference to  FIGS. 12 and 13 . 
       FIG. 12  shows the dot separation suppressing effect due to the pulse width W a  of the auxiliary pulse (deBst). In the test, the value of the pulse width W a  (deBst) of the drive signal of  FIG. 9  was set to various values as shown in  FIG. 12  (deBst=0.2 μs, 0.3 μs, 0.4 μs, and 0.5 μs), and ink was ejected from the nozzle  25  of the inkjet head  10  at each setting. The conditions/settings other than the pulse width W a  were kept constant. The flying state of the ink was imaged at a position of GAP=0.5 mm from the nozzle, and the evaluation was performed by measuring the distance between the main drop MD and the forward drop FD. 
     Separation between the main drop MD and the forward drop FD was observed at deBst=0.2 μs, but substantially no separation was observed at deBst=0.5 μs. The backward drop BD did not change much even as the deBst value was changed. 
       FIG. 13  shows the measurement results of dot-to-dot distance according to changes in the pulse width W a  of the auxiliary pulse (deBst). The numerical values in the “dot-to-dot distance” sub-columns correspond to respective drop positions on the distance scale as indicated in  FIG. 12 , with the position value “5” in  FIG. 13  for the main drop column indicating GAP=0.5 mm. The distance value (difference Δ) “2.6” between the main drop value and the forward drop value indicates Δ=2.6×10 −1  mm. The “stability” column entry is a three-stage evaluation based on a visual determination. “Stability=◯” denotes that there is no erroneous ejection such as bending or dispersal. “Stability=x” denotes that there is erroneous ejection such as bending or dispersal. “Stability=Δ” denotes marginal case between no erroneous ejection and erroneous ejection. 
     At pulse width W a =0.2 μs, difference Δ=2.6×10 −1  mm. When pulse width W a  was increased, Δ became smaller, and when pulse width W a =0.5 μs, Δ=0. At pulse width W a =0.6 μs, no separation of the forward drop was observed, but the stability was reduced. Therefore, in this example, an optimum pulse width W a =0.5 μs for the auxiliary pulse (deBst) was obtained. 
     Next, an example of determining the optimum value for the pulse width W p  of the contraction pulse (Push) will be described with reference to  FIGS. 14 and 15 . 
       FIG. 14  shows one example of the dot separation suppressing effect due to the pulse width W p  of the contraction pulse (Push). In the test, the pulse width W p  (Push) of the drive signal of  FIG. 9  was set to different values as shown in  FIG. 14  (Push=0.9 μs, 1.0 μs, 1.1 μs, and 1.2 μs), and ink was ejected from the nozzle  25  of the inkjet head  10 . The conditions other than the pulse width W p  were kept be constant. Similarly to  FIG. 12 , the flying state of the ink was imaged at a position of a distance GAP=0.5 mm from the nozzle, and an evaluation was performed by measuring the distance between the main drop MD and the backward drop BD (rather than the forward drop FD in  FIG. 12 ). 
     Separation between the main drop MD and the backward drop BD was observed at Push=0.9 μs, but almost no separation was observed at Push=1.1 μs. 
       FIG. 15  shows the measurement results for the dot-to-dot distance for different values of the pulse width W p  of the contraction pulse (Push). Similarly to  FIG. 13 , the numerical value in the “dot-to-dot distance” column corresponds to the scale position in  FIG. 14 , and the listed position value “5” of the main drop indicates GAP=0.5 mm. Therefore, the distance (difference Δ) “0.5” between the main drop and the backward drop indicates Δ=0.5×10 −1  mm. The “stability” is again a three-stage evaluation by visual determination performed in a similar manner that described with respect to  FIG. 13 . 
     At pulse width W p =0.5 μs, A=0.5×10 −1  mm. At pulse width W p =0.7 μs, the difference spreads to A=1×10 −1  mm, but at pulse width W p =1.1 μs and pulse width W p =1.2 μs, A=0 was obtained. When pulse width W p  was further increased, the stability decreased at pulse width W p =1.3 μs, and a phenomenon similar to dispersal occurred and the difference Δ expanded at pulse width W p =1.52 μs. Therefore, in this example, the optimum pulse width W p =1.1 μs or 1.2 μs for the contraction pulse (Push) was obtained. 
     In this manner, the separation of the forward drop can be suppressed by providing an auxiliary pulse (deBst) that reduces the pressure vibration and the Rest period that pauses fora certain period of time prior to the expansion pulse (Draw). Furthermore, the separation of the backward drop can be suppressed by a contraction pulse (Push) that reduces the pressure vibration generated by the expansion pulse (Draw). By appropriately selecting the pulse widths of both the auxiliary pulse (deBst) and the contraction pulse (Push), the dot separation suppressing effect can be further improved. Such a separation suppressing effect can also be obtained even if a low-viscosity ink (less than 5 cps) is used. 
     The inkjet head  10  and the inkjet recording device  1  provided with the inkjet head  10  according to the present embodiment can realize the ejection of ink droplets without dot separation by applying a drive signal as described above to the drive element  18  (an actuator). Accordingly, it is possible to provide an inkjet head  10  and an inkjet recording device  1  capable of effectively suppressing the dot separation and dispersal of ink while maintaining ejection stability and performing high-quality printing. 
     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 disclosure. 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 disclosure. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the disclosure.