Liquid discharge head

Abstract

To provide a liquid discharge head capable of stably discharging liquid.SOLUTION: A liquid discharge head of an embodiment includes a piezoelectric actuator, a drive circuit, a switch element, and a parallel diode. The piezoelectric actuator includes a piezoelectric body, one terminal that applies a drive waveform, and the other terminal, respectively. The drive circuit applies the drive waveform to the piezoelectric actuators. The switch element connects the respective other terminals of the respective piezoelectric actuators in common, and switches whether to connect a connection point to a ground (GND) or a predetermined potential to apply a common potential or to block the common potential. The parallel diode is a parallel diode of the switch element in which a direction of charging the piezoelectric actuator is a forward direction.SELECTED DRAWING: Figure 23

Description

[Technical field]

[0001] An embodiment of the present invention relates to a liquid ejection head. [Background technology]

[0002] Liquid ejection heads that supply a predetermined amount of liquid to a predetermined position are known. Liquid ejection heads are mounted on, for example, inkjet printers, 3D printers, and dispensing devices. Inkjet printers eject droplets of ink from an inkjet head to form an image or the like on the surface of a recording medium. 3D printers eject droplets of modeling material from a modeling material ejection head, harden the droplets, and form a three-dimensional object. Dispensing devices eject droplets of a sample and supply a predetermined amount to a plurality of containers, etc.

[0003] A liquid ejection head has multiple channels that eject liquid. Each channel has a nozzle that ejects liquid, a pressure chamber that communicates with the nozzle, and an actuator that changes the volume of the pressure chamber. The liquid ejection head selects a channel from the multiple channels that ejects liquid, and drives the actuator by providing a drive signal. When the actuator is driven, the volume of the pressure chamber filled with liquid changes, and liquid is ejected from the nozzle.

[0004] A piezoelectric actuator, which is driven by utilizing the piezoelectric effect of a piezoelectric material, polarizes the piezoelectric material in a polarization direction that realizes the function of changing the volume of a pressure chamber. However, the polarization degradation of the piezoelectric material may occur, for example, during the manufacturing process of a liquid ejection head or during normal use of the liquid ejection head. When the polarization degradation occurs, the liquid ejection performance of the liquid ejection head may deteriorate. A piezoelectric actuator that has been polarized and deteriorated can be repolarized by applying a high voltage in the same direction as the driving direction for a certain period of time. However, the insertion loss due to the switching circuit that connects the piezoelectric actuator to the polarization power source may affect the ejection of liquid, which is the normal operation of the liquid ejection head. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2002-160372 A [Patent Document 2] JP 2010-155418 A [Patent Document 3] JP 2009-83276 A [Patent Document 4] JP 2016-55644 A [Patent Document 5] JP 2005-254211 A [Patent Document 6] JP 2023-132399 A [Patent Document 7] Patent No. 3637246 Summary of the Invention [Problem to be solved by the invention]

[0006] An object of the present invention is to provide a liquid ejection head capable of ejecting liquid stably. [Means for solving the problem]

[0007] A liquid ejection head according to an embodiment of the present invention includes a piezoelectric actuator, a drive circuit, a switch element, and a parallel diode. Each of the piezoelectric actuators includes a piezoelectric body, one terminal for applying a drive waveform, and the other terminal. The drive circuit applies the drive waveform to each of the piezoelectric actuators. The switch element commonly connects the other terminals of each of the piezoelectric actuators, and connects the connection point to ground (GND) or a predetermined potential to switch between applying the common potential or blocking it. The parallel diode is a parallel diode of the switch element whose forward direction is the direction in which the piezoelectric actuator is charged. [Brief description of the drawings]

[0008] [Figure 1]1 is an overall configuration diagram of an inkjet printer equipped with an inkjet head according to a first embodiment. [Diagram 2] FIG. 2 is a perspective view of the inkjet head. [Diagram 3] FIG. 2 is a partially enlarged cross-sectional view of a head portion of the inkjet head. [Figure 4] FIG. 2 is a partially enlarged cross-sectional view of a head portion of the inkjet head. [Diagram 5] FIG. 2 is a partially enlarged plan view of a head portion of the inkjet head. [Figure 6] FIG. 4 is a circuit diagram of a control system for the inkjet head. [Figure 7] 13 is a modified example of the circuit diagram of the control system of the inkjet head. [Figure 8] 2 is an equivalent circuit of the control system of the inkjet head. [Figure 9] The results of a simulation using the above equivalent circuit are shown below. [Figure 10] The results of a simulation using the above equivalent circuit are shown below. [Figure 11] The results of a simulation using the above equivalent circuit are shown below. [Figure 12] The results of a simulation using the above equivalent circuit are shown below. [Figure 13] The results of a simulation using the above equivalent circuit are shown below. [Figure 14] The results of a simulation using the above equivalent circuit are shown below. [Figure 15] 10 is an equivalent circuit of a control system circuit of an inkjet head according to a second embodiment. [Figure 16] The results of a simulation using the above equivalent circuit are shown below. [Figure 17] FIG. 11 is a circuit diagram of a control system of an inkjet head according to a third embodiment. [Figure 18] 2 is an equivalent circuit of the control system of the inkjet head. [Figure 19] The results of a simulation using the above equivalent circuit are shown below. [Figure 20] The results of a simulation using the above equivalent circuit are shown below. [Figure 21] The results of a simulation using the above equivalent circuit are shown below. [Figure 22] The results of a simulation using the above equivalent circuit are shown below. [Diagram 23] FIG. 13 is a circuit diagram of a control system of an inkjet head according to a fourth embodiment. [Figure 24] 13 is a modified example of a control system circuit of the inkjet head according to the fourth embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, a liquid ejection head according to an embodiment will be described in detail with reference to the accompanying drawings, in which the same components are denoted by the same reference numerals.

[0010] (First embodiment) As an example of an image forming apparatus equipped with the liquid ejection head of the first embodiment, an inkjet printer 10 that prints an image on a recording medium will be described. Fig. 1 shows a schematic configuration of the inkjet printer 10. The inkjet printer 10 has a housing 11 and includes a cassette 12 that stores a sheet S, which is an example of a recording medium, an upstream transport path 13 for the sheet S, a transport belt 14 that transports the sheet S taken out of the cassette 12, a plurality of inkjet heads 100-103 that eject ink droplets toward the sheet S on the transport belt 14, a downstream transport path 15 for the sheet S, an ejection tray 16, and a control board 17. An operation unit 18 that is a user interface is located on the upper side of the housing 11.

[0011] Image data to be printed on the sheet S is generated, for example, by a computer 200, which is an externally connected device. The image data generated by the computer 200 is sent to a control board 17 of the inkjet printer 10 via a cable 201 and connectors 202 and 203.

[0012] The pickup roller 204 supplies the sheets S one by one from the cassette 12 to the upstream conveying path 13. The upstream conveying path 13 is composed of a pair of feed rollers 131 and 132 and sheet guide plates 133 and 134. The sheet S is fed to the upper surface of the conveying belt 14 via the upstream conveying path 13. An arrow 104 in the figure indicates the conveying path of the sheet S from the cassette 12 to the conveying belt 14.

[0013] The conveyor belt 14 is a mesh-like endless belt with many through holes formed on its surface. Three rollers, a drive roller 141 and driven rollers 142 and 143, support the conveyor belt 14 so that it can rotate freely. A motor 205 rotates the conveyor belt 14 by rotating the drive roller 141. The motor 205 is an example of a drive device. In the figure, 105 indicates the rotation direction of the conveyor belt 14. A negative pressure container 206 is disposed on the back side of the conveyor belt 14. The negative pressure container 206 is connected to a fan 207 for reducing pressure. The fan 207 creates a negative pressure inside the negative pressure container 206 by forming an airflow, and adsorbs and holds the sheet S on the upper surface of the conveyor belt 14. In the figure, 106 indicates the flow of the airflow.

[0014] Inkjet heads 100-103, which are an example of liquid ejection heads, are arranged to face sheet S, which is attracted to and held on conveyor belt 14, with a small gap of, for example, 1 mm between them. Each of inkjet heads 100-103 ejects ink droplets toward sheet S. When sheet S passes below inkjet heads 100-103, the inkjet heads print images. Each of inkjet heads 100-103 has the same structure, except that they eject different colors of ink. The ink colors are, for example, cyan, magenta, yellow, and black.

[0015] The inkjet heads 100-103 are connected to ink tanks 315-318 and ink supply pressure adjusting devices 321-324 via ink flow paths 311-314, respectively. Each ink tank 315-318 is disposed above each inkjet head 100-103. During standby, each ink supply pressure adjusting device 321-324 adjusts the pressure inside each inkjet head 100-103 to a negative pressure, for example, -1.2 kPa, relative to atmospheric pressure so that ink does not leak from the nozzles 24 (see FIG. 2) of the inkjet heads 100-103. During image formation, ink from each ink tank 315-318 is supplied to each inkjet head 100-103 by the ink supply pressure adjusting devices 321-324.

[0016] After the image is formed, the sheet S is sent from the conveyor belt 14 to a downstream conveying path 15. The downstream conveying path 15 is composed of pairs of feed rollers 151, 152, 153, and 154, and sheet guide plates 155 and 156 that define the conveying path of the sheet S. The sheet S passes through the downstream conveying path 15 and is sent from a discharge port 157 to a discharge tray 16. An arrow 107 in the figure indicates the conveying path of the sheet S.

[0017] Next, a description will be given of the configuration of the inkjet heads 100 to 103. The inkjet head 100 will be described below with reference to Figures 2 to 6, but the inkjet heads 101 to 103 have the same structure as the inkjet head 100.

[0018] As shown in Fig. 2, the inkjet head 100 includes a head unit 2, which is an example of a liquid ejection unit. The head unit 2 is connected to a flexible printed wiring board 21, which is an example of a film wiring board. The flexible printed wiring board 21 is connected to a printed circuit board 22, which is an example of a relay board. The head unit 2 includes a nozzle plate 23, which is an example of a nozzle unit. The head unit 2 is connected to the ink supply pressure adjustment device 321 of Fig. 1 via an ink flow path 311.

[0019] The nozzles 24 of each channel that ejects ink are arranged along a first direction, for example, the X direction, of the nozzle plate 23. The nozzle density is set within a range of 150 to 1200 dpi, for example. The nozzles 24 are not limited to being arranged in a single row, and may be arranged in multiple rows. A detailed configuration of the head unit 2 will be described later.

[0020] The flexible printed wiring board 21 is a flexible printed wiring board using a synthetic resin film such as polyimide. The flexible printed wiring board 21 is equipped with a driving IC (Integrated Circuit) 3, which is a driver chip (hereinafter referred to as a driving IC). The printed circuit board 22 is a hard through-hole board in which glass fiber-reinforced epoxy resin layers and copper wiring layers are laminated in multiple layers. The driving IC 3, which serves as a control unit, temporarily stores print data sent via the printed circuit board 22 from the control board 17, which is equipped with a CPU and serves as the control unit of the inkjet printer 10, and provides driving signals to each channel so that ink is ejected at a predetermined timing.

[0021] 3 to 5 are partial cross-sectional views of the head unit 2. The nozzle plate 23 is bonded to one surface of the pressure chamber substrate 4. The nozzle plate 23 is a rectangular plate made of, for example, a resin such as polyimide or a metal such as stainless steel. The diaphragm 41 is bonded to one surface of the pressure chamber substrate 4 opposite the nozzle plate 23. The diaphragm 41 has flexibility such that it deforms when an external force is applied. The diaphragm 41 is a rectangular plate made of, for example, a flexible polyimide film or metal.

[0022] The pressure chambers 42 are formed in the pressure chamber substrate 4. The pressure chambers 42 are arranged at the positions of the nozzles 24 and communicate with the nozzles 24, respectively. For example, the pressure chambers 42 are formed in the pressure chamber substrate 4 as rectangular openings penetrating in the second direction, for example, in the Z direction, and the openings on both sides are closed by the nozzle plate 23 and the vibration plate 41, respectively, to form spaces filled with ink. The pressure chambers 42 communicate with a guide flow path 43 having a narrowed portion, and further communicate with an ink supply manifold 45 via an ink supply port 44, which is an opening hole formed in the vibration plate 41. The guide flow path 43 is formed in a groove shape in the third direction, for example, in the Y direction, on one surface of the pressure chamber substrate 4 on the vibration plate 41 side, for each pressure chamber 42. The ink supply manifold 45 is formed in a frame 46 joined to one surface of the vibration plate 41. The ink supply manifold 45 extends in the X direction and communicates with the pressure chambers 42 of each channel via the ink supply ports 44 and guide channels 43 of each channel. The ink supply manifold 45, which serves as a common ink chamber, communicates with the ink channel 311 (see FIGS. 1 and 2).

[0023] The piezoelectric actuator 5 is disposed on one surface of the vibration plate 41 opposite to the pressure chamber 42. The piezoelectric actuators 5 of each channel are arranged at a position facing the pressure chamber 42 with the vibration plate 41 in between. The piezoelectric actuator 5 and the vibration plate 41 are bonded, for example, with an adhesive. Each piezoelectric actuator 5 is fixed by bonding one surface opposite to the vibration plate 41 in the Z direction to the support member 47. As shown in FIG. 3 in particular, the piezoelectric actuator 5 is a laminated piezoelectric actuator formed by alternately laminating a piezoelectric body 51 such as a piezo element, a first internal electrode 52, and a second internal electrode 53 in layers. The piezoelectric bodies 51 are arranged such that the polarization directions are opposite to each other in the Z direction, for example, and are deformed in the d33 mode. The first internal electrode 52 and the second internal electrode 53 are conductive films formed on the main surface of the piezoelectric body 51. The first internal electrode 52 is formed up to one end surface of the piezoelectric actuator 5 in the Y direction, and is connected to a first external electrode 54 formed on this end surface. The second internal electrodes 53 are formed up to the other end surface of the piezoelectric actuator 5 in the Y direction, and are connected to the second external electrodes 55 formed on this end surface. The dummy layer 58 is made of the same material as the piezoelectric body 51. The dummy layer 58 is not deformed because no internal electrodes are provided and no electric field is applied to it. The dummy layer 58 serves as a base for fixing the piezoelectric actuator 5 to the support member 47 (see FIG. 4), or serves as a polishing allowance for polishing to achieve precision during or after assembly. As shown in FIG. 4 in particular, a support 50 may be disposed between the piezoelectric actuators 5 of each channel via a groove 59. The support 50 may be constituted by a dummy actuator formed in the same manner as the driving piezoelectric actuator 5. The support 50 is disposed, for example, at a position corresponding to the partition wall 40 between adjacent pressure chambers 42. Instead of being formed by a dummy actuator, the support 50 may be formed by another member.

[0024] In the case of a piezoelectric actuator 5 in which a plurality of piezoelectric bodies 51 are laminated, as an example, a first internal electrode 52 and a second internal electrode 53 are formed on the main surface of each piezoelectric body 51 processed into a thin plate shape. The piezoelectric bodies 51 are then laminated together and fired to be integrated. After that, a first external electrode 54 and a second external electrode 55 are formed. The piezoelectric body 51 is then polarized by a polarization voltage, which will be described in detail later. The piezoelectric body 51 is formed of a lead-containing piezoelectric material such as lead zirconate titanate (PZT) or a lead-free piezoelectric material such as sodium potassium niobate. The first internal electrode 52 and the second internal electrode 53 are formed of a conductive material that can be fired, such as silver palladium. The first external electrode 54 and the second external electrode 55 are formed of Ni, Cr, Au, or the like, by a known method such as a plating method or a sputtering method.

[0025] The first external electrode 54 of each channel is connected to the individual wiring 56 of the flexible printed wiring board 21 (see FIG. 3). The flexible printed wiring board 21 has a base material 26, individual wiring 56, an adhesive layer 27, and an insulating layer 28. The flexible printed wiring board 21 is arranged so that the area where the solder plating layer 29 is formed faces the first external electrode 54, and the first external electrode 54 and the individual wiring 56 of each channel are electrically and mechanically connected by melting the solder. Instead of solder, they may be fixed by ACF (Anisotropic Conductive Film), ACP (Anisotropic Conductive Paste), NCF (Non-Conductive Film), NCP (Non-Conductive Paste), etc., and may be anisotropically conductively connected in the thickness direction. On the other hand, the second external electrode 55 of each channel is connected to a common wiring (not shown), and is connected to ground (GND) via the flexible printed wiring board 21, for example.

[0026] Fig. 6 is a circuit diagram of the control system of the inkjet head 100. As shown in Fig. 6, the piezoelectric actuator 5 has a first external electrode 54 connected to an individual wiring 56. The individual wiring 56 from each piezoelectric actuator 5 is connected to an output terminal of a drive driver D (i.e., a drive circuit) of an output circuit of the drive IC 3. The connection point between the first external electrode 54 and the individual wiring 56 is one terminal of the piezoelectric actuator 5. Hereinafter, this one terminal will be referred to as an individual terminal.

[0027] The driving IC3 is connected to a first driving power supply 32 which is a power supply for a driving voltage V1, and a second driving power supply 33 which is a power supply for a driving voltage V2. The first driving power supply 32 and the second driving power supply 33 each have a positive pole connected to the driving IC3, and a negative pole connected to, for example, ground (GND). That is, in this example, the positive voltages are the driving voltages V1 and V2. The driving voltage V1 is, for example, 20V. The driving voltage V2 is, for example, 10V. The ground (GND) is, for example, 0V.

[0028] The driving IC 3 is further connected to a signal line for a control signal and a control power supply. The control signal is print data sent from the control board 17 (see FIG. 1) of the inkjet printer 10 when printing is performed by ejecting ink, and a signal for controlling the driving driver D when performing polarization, which will be described later. The control power supply is, for example, a power supply that operates the driving IC 3.

[0029] The driving IC 3 includes an output protection diode 31 that protects the driving driver D. As an example, the output protection diode 31 is a diode with a cathode connected to the output terminal of the driving driver D and an anode connected to the ground (GND) of the driving IC 3. The output protection diode 31 is provided for each driving driver D of each channel (#1ch to #nch) that ejects ink. In addition, when a parasitic diode is configured due to the structure of the transistor of the output circuit, the parasitic diode may be used instead of the output protection diode. The output protection diode 31 or the parasitic diode is an example of a diode connected between the output terminal of the driving IC 3 and a reference potential in a direction that does not conduct when the driving IC 3 outputs a driving voltage. The reference potential is, for example, ground (GND).

[0030] On the other hand, the second external electrodes 55 of each piezoelectric actuator 5 are commonly connected to a common wiring 57. The connection point between the second external electrodes 55 and the common wiring 57 is the other terminal of the piezoelectric actuator 5. Hereinafter, this other terminal will be referred to as a common terminal. The common wiring 57 is connected to the ground (GND) via the first Photo MOS switch 6. In other words, a common potential of, for example, 0V is applied to the common terminals of each piezoelectric actuator 5.

[0031] Furthermore, the common terminals of each piezoelectric actuator 5 are commonly connected to the polarization power supply 7 via the second Photo MOS switch 61 using a common wiring 57. The polarization power supply 7 has a negative pole connected to the first Photo MOS switch 6 and a positive pole connected to, for example, ground (GND). The polarization power supply 7 is configured with a boost circuit, which will be described in detail later. The boost circuit uses, for example, a positive voltage from the first driving power supply 32 as a high-voltage generating power supply to boost it and generate a negative voltage of the opposite polarity to the driving voltage. The generated negative voltage is commonly applied as a polarization voltage to the common terminals of each piezoelectric actuator 5 via the first Photo MOS switch 6. The polarization voltage is, for example, -50V to -70V. When applying a polarization voltage to the common terminals of each piezoelectric actuator 5, if the individual terminal side is fixed to, for example, the positive voltage of the driving voltage V1, the piezoelectric actuator 5 can be polarized with a higher voltage. In addition, it is preferable to set the voltage generated by the boost circuit higher in advance by the amount that drops when the piezoelectric actuator 5 is connected.

[0032] The first photo MOS switch 6 and the second photo MOS switch 61 switch between connecting the common terminal of the piezoelectric actuator 5 to a common potential or to a polarization power supply. That is, the common terminal of the piezoelectric actuator 5 is connected to the common potential when the first photo MOS switch 6 is turned ON, and is connected to the polarization power supply 7 when the first photo MOS switch 6 is turned OFF. The second photo MOS switch 61 is turned OFF when connecting to the common potential, and is turned ON when connecting to the polarization power supply 7. In this manner, by exclusively switching the first photo MOS switch 6 and the second photo MOS switch 61, it is possible to switch between connecting the common terminal of the piezoelectric actuator 5 to the common potential or to the polarization power supply 7. However, a configuration in which the polarization power supply 7 generates a polarization voltage only when the first photo MOS switch 6 is turned OFF may be used. In this case, the second photo MOS switch 61 does not necessarily have to be provided.

[0033] The first Photo MOS switch 6 is an example of a first switch element. The drive voltages V1 and V2 are examples of a drive power source for liquid ejection. As will be described in detail later, the first Photo MOS switch 6 is also an example of a switch element that switches between providing a common potential or blocking the common potential by connecting the other terminal of the piezoelectric actuator 5, which is a common terminal, to ground (GND) or a predetermined potential.

[0034] The first Photo MOS switch 6 and the second Photo MOS switch 61 are configured, for example, with a Photo MOS transistor. For example, the Photo MOS transistor used is "AQV255G3A" by Panasonic. Inside this Photo MOS transistor, two Nch DMOS transistors with pins 6 and 4 as drains and pin 5 as source are connected in series with a common source, and a body diode with the source side as the anode and the drain side as the cathode is connected in parallel to each DMOS transistor. The first Photo MOS switch 6 uses this connection as a one-way switch. Specifically, pins 6 and 4 of the Photo MOS transistor are shorted and the area between pin 5 and the pin is used as a switch. In this case, two Nch DMOS transistors with pins 6 and 4 as drains and pin 5 as source are used in parallel. In this connection, a body diode 62 with pin 5 as the anode and pins 6 and 4 as the cathode becomes a parallel diode in the switch. That is, it is the source side of the DMOS transistor that is connected to the common terminal of the piezoelectric actuator 5 , and the forward direction of the body diode 62 is the direction in which the piezoelectric actuator 5 is charged.

[0035] On the other hand, the second Photo MOS switch 61 uses the same Panasonic "AQV255G3A" as a bidirectional switch. Specifically, the switch is between pins 6 and 4 of the Photo MOS transistor. In this case, two Nch DMOS transistors are used in series. In this connection, one of the body diodes 63, which are reverse-connected, is always off.

[0036] When switching the common terminal of the piezoelectric actuator 5, the first Photo MOS switch 6 as a switch element must be turned on in both directions during normal operation, i.e., when printing is performed. Generally, to use a Photo MOS switch as a bidirectional switch, two Photo MOS transistors are connected in series, but when connected in series, the ON resistance becomes large. It is desirable that the ON resistance of the first Photo MOS switch 6 during printing is small. In this embodiment, the first Photo MOS switch 6 is connected as a unidirectional switch in which pins 6 and 4 are shorted and the MOS switch is connected between pin 5 and pin 6. In this case, two Photo MOS transistors are used in parallel, so the ON resistance is reduced to 1 / 4 of that in the case of a series connection. However, in this connection, the body diode 62, with pin 5 as the anode and pins 6 and 4 as the cathodes, enters the switch as a parallel diode. Since the common wiring 57 needs to be separated from the ground (GND) when polarization is performed, this body diode 62 needs to be turned off when polarization is performed. In this embodiment, during polarization, a polarization power supply 7 having a polarity opposite to that of the drive power supply for ejecting ink is connected to the common wiring 57, and a polarization voltage having a polarity opposite to that of the drive voltage is applied to the common terminal. As a result, during polarization, the body diode 62 is turned off in a direction that blocks the polarization voltage.

[0037] The first Photo MOS switch 6 is turned ON / OFF by turning ON / OFF the LED 64. The second Photo MOS switch 61 is turned ON / OFF by turning ON / OFF the LED 65. The LEDs 64, 65 may be turned ON / OFF via the driving IC 3 by providing a control port in the driving IC 3, for example, or may be turned ON / OFF by connecting a circuit used when assembling the inkjet head 100.

[0038] Next, the ink ejection operation and the polarization operation will be described with reference to FIG. First, in the normal operation of ejecting ink, the first Photo MOS switch 6 is turned ON and the second Photo MOS switch 61 is turned OFF. Each driver D of the driving IC 3 uses driving voltages V1, V2 and ground (GND) to apply driving waveforms to the individual terminals of each piezoelectric actuator 5. Which piezoelectric actuator 5 is to be driven is based on, for example, print data.

[0039] When driving the piezoelectric actuator 5, which has a ground (GND) potential applied to the common potential, a drive voltage V2, for example, is applied to the individual terminal to put it into a standby state. At this time, an electric field is applied in the direction of the polarization axis of the piezoelectric body 51, and the piezoelectric actuator 5 expands in the stacking direction (Z direction), and the volume of the pressure chamber 42 is reduced. This is performed prior to the timing of ink ejection. Then, at the timing of ink ejection, the potential of the individual terminal is first lowered to ground (GND), so that the expanded piezoelectric actuator 5 returns to its original state, that is, it contracts relatively, and the volume of the pressure chamber 42 expands relatively. Ink flows into the pressure chamber 42 through the guide flow path 43 by the amount of the expansion of the volume of the pressure chamber 42. Then, for example, after a time of 1 / 2 the pressure vibration period of the head part 2 has elapsed, when a drive voltage V2 is applied to the individual terminal, the piezoelectric actuator 5 expands, and the volume of the pressure chamber 42 is relatively reduced, and ink is ejected from the nozzle 24. Then, for example, after a time that is half the pressure vibration period of the head unit 2 has elapsed, a drive voltage V1 is applied to the individual terminal, and after a predetermined time, the drive voltage is returned to V2. The volume of the pressure chamber 42 is reduced and restored by the expansion and restoration of the piezoelectric actuator 5 at that time, and this operation damps the residual vibration. In this way, the volume of the pressure chamber 42 changes in accordance with the longitudinal vibration of the piezoelectric actuator 5 in the stacking direction, and ink can be ejected.

[0040] Here, a first Photo MOS switch 6 is provided between the common terminal of the piezoelectric actuator 5 and a common potential (e.g., GND) to switch the connection destination of the common terminal to the polarization power supply 7 during polarization. Since the current of the piezoelectric actuator 5 driven at the same time during printing is concentrated in this first Photo MOS switch 6, the ON resistance must be small to maintain print quality. Also, it is desirable that the ON resistance is small to suppress heat generation of the first Photo MOS switch 6. Therefore, the first Photo MOS switch 6 is used in a unidirectional switch mode to lower the ON resistance. On the other hand, since a negative polarization voltage is applied to the common terminal during polarization, the body diode 62 of the first Photo MOS switch 6 is oriented in a direction in which this negative voltage does not flow, that is, in a direction in which the common terminal side of the piezoelectric actuator 5 becomes the anode. The body diode 62 is turned ON at the timing when the piezoelectric actuator 5 is charged during printing, but the body diode 62 may be ON or OFF during printing. This is because the first Photo MOS switch 6 is turned ON when printing is executed. Since the voltage drop in the MOS transistor portion of the first Photo MOS switch 6 is smaller than the voltage drop when the body diode 62 is ON, the ON resistance of the entire switch is dominated by the ON resistance of the MOS transistor portion, and the body diode 62 has almost no effect on operation.

[0041] During polarization, the first Photo MOS switch 6 is turned OFF and the second Photo MOS switch 61 is turned ON, and the common terminal of the piezoelectric actuator 5 is connected to the polarization power supply 7. The polarization power supply 7 applies a negative voltage, for example -50V, to the common terminal of the piezoelectric actuator 5. At this time, the body diode 62 of the first Photo MOS switch 6 is turned OFF and no current flows because a negative voltage is applied to the anode. In other words, it turns OFF in the direction that blocks the polarization voltage. On the other hand, the second Photo MOS switch 61 is used in bidirectional switch mode. Therefore, the ON resistance of the second Photo MOS switch 61 is four times that of the first Photo MOS switch 6, but since current flows only at the start of polarization during polarization and stops thereafter, no particular problem occurs even if the ON resistance is large.

[0042] It is desirable to polarize the piezoelectric actuator 5 when there is concern about polarization degradation, for example, when the inkjet head 100 has been used for a predetermined period of time after assembly of the inkjet head 100. Note that the polarization may be re-polarization of the piezoelectric body 51 or may be the initial polarization.

[0043] The common terminal of the piezoelectric actuator 5 does not have to be ground (GND). As shown in a modified example in FIG. 7, the common terminal of the piezoelectric actuator 5 may be connected to the drive power supply 34 of the drive voltage V3 instead of ground (GND). In this case, the connection destination of the first Photo MOS switch 6 becomes the positive voltage of the voltage V3 instead of 0V. Even with this connection, the current flowing to the LED 65 is stopped and a current is passed through the LED 64 during the ejection operation, so that the first Photo MOS switch 6 can be turned on in both directions, and when the current flowing to the LED 64 is stopped and a current is passed through the LED 65 during polarization, a negative voltage is applied from the polarization power supply 7 to the common wiring 57, so that the first Photo MOS switch 6 and its body diode 62 are turned off. In the case of this connection, a voltage in the opposite direction to the polarization direction is applied to the piezoelectric actuator 5 at the timing when 0V is applied to the individual terminals. That is, since the voltage applied to the piezoelectric actuator 5 crosses zero, the hysteresis of the piezoelectric body 51 has the advantage that the amount of deformation of the piezoelectric actuator 5 can be made large even with the same voltage amplitude. Voltage V3 is set to a potential between the driving power supply voltage and 0V, for example 3V. With this configuration, there is a timing for applying a voltage in the opposite direction to the polarization direction, which increases the risk of polarization deterioration, but if the value of voltage V3 is small, the effect is small, and even if polarization deterioration does occur, with this configuration it is possible to restore the original polarization state by performing a re-polarization operation.

[0044] Next, the boost circuit constituting the polarized power supply 7 will be described with reference to FIG. 8. FIG. 8 is an equivalent circuit including the boost circuit in the polarized power supply 7, which has the configuration shown in FIG. 6. As an example, the boost circuit is composed of a flyback circuit 71 using an inductor L1 and a diode D9. In the equivalent circuit, the switch S1 is the first Photo MOS switch 6, and the diode D1 is the body diode 62. The switch S3 is the second Photo MOS switch 61. The power supply V1 is branched from the first drive power supply 32 as a high-voltage generating power supply and supplied to the boost circuit. The drive terminal is an individual terminal of the piezoelectric actuator 5. VCOM is a common terminal of the piezoelectric actuator 5. However, for simplicity, the equivalent circuit is described assuming that, for example, 300 piezoelectric actuators 5 are in parallel. If the capacitance of each piezoelectric actuator 5 is, for example, 1000 pF, the total capacitance of the capacitor C2 is 0.3 uF.

[0045] The flyback circuit 71 uses the voltage V1 (=20V) from the first driving power supply 32 as a high voltage generating power supply, and generates a negative voltage in the inductor L1 by switching on the switch S2. The generated negative voltage is stored in the capacitor C8. The stored negative voltage is applied to the capacitor C2 via the resistor R5 (100 kΩ) by turning on the switch S3. In other words, the negative voltage generated by the flyback circuit 71 is applied to the piezoelectric actuator 5 as a polarization voltage. Such a boost circuit is provided, for example, inside the inkjet head 100 (i.e., inside the head unit).

[0046] The capacitance of capacitor C8 is set to be slightly larger than 10 times the total capacitance of the piezoelectric actuator 5. When switch S3 is turned on, the negative charge stored in capacitor C8 is supplied to the capacitive piezoelectric actuator 5, so the polarization voltage becomes smaller than the value initially stored in capacitor C8. If the reduction rate is large, the requirement for withstand voltage on the circuit when trying to obtain a specified polarization voltage increases. Therefore, it is preferable to keep the reduction rate as small as possible. To reduce the reduction rate, it is sufficient to increase the ratio of the value of capacitor C8 to the value of capacitor C2, which corresponds to the total capacitance of the piezoelectric actuator 5.

[0047] With the circuit configuration described above, a polarization voltage (e.g., -50V) is generated from the ink ejection voltage V1 (e.g., 20V) and is applied to the common terminal of the piezoelectric actuator 5. During polarization, if the driving IC 3 is controlled to fix the voltage of the individual terminal to the driving voltage V1 (e.g., 20V), the piezoelectric actuator 5 can be polarized with a voltage of 70V. The polarization time is, for example, 5 seconds. 70V is the voltage at which the magnitude of the electric field applied to the piezoelectric body 51 of the piezoelectric actuator 5 exceeds the coercive electric field.

[0048] Next, the results of a simulation using the equivalent circuit of FIG. 8 will be described with reference to FIG. 9 to FIG. 14. B1 in the equivalent circuit of FIG. 8 is the output voltage from the driving driver D of the driving IC 3. The output voltage V(drive) is controlled, for example, as shown in FIG. 9. FIG. 10 is an enlarged view of the first 50 μs of FIG. 9. FIG. 10 shows the output voltage V(drive) of a driving waveform for ejecting ink. The driving waveform of FIG. 10 is a waveform in which, for example, a voltage V2 (10 V) is applied to the piezoelectric actuator 5 in standby, and 0 V is applied for 2 μs, which corresponds to 1 / 2 the pressure vibration period of the head part 2, to relatively expand the volume of the pressure chamber 42, a voltage V2 (10 V) is applied to return the volume to eject ink, and after waiting for 2 μs, a voltage V1 (20 V) is applied for 2 μs to contract the pressure chamber 42, and then the voltage is returned to V2 (10 V) to attenuate the residual vibration. This driving waveform may be output during any period as long as repolarization is not being performed. Of course, the drive waveform for driving the piezoelectric actuator 5 is not limited to the drive waveform of FIG.

[0049] During polarization, the switches S1, S2, and S3 are controlled as shown in FIG. The switch S2 is switched on and off at a period of 5 ms from 1 second onward to generate a negative voltage in the 10 mH inductor L1. FIG. 12 is an enlarged view of the part where the switch S2 is switching. When the desired voltage is generated, the switching is stopped. In this example, the switching is stopped after 350 times, but the switching may be stopped when the voltage of the capacitor C8 is monitored and a predetermined voltage is reached. After that, as shown in FIG. 11, the switch S1 is turned off and the switch S3 is turned on, and the negative charge stored in the capacitor C8 is applied to the common terminal VCOM of the piezoelectric actuator 5 via the resistor R5 (100 kΩ), and the polarization is started at -50 V. The polarization time is, for example, 5 seconds. The switch S1 may be turned off before the timing shown in FIG. 11. For example, the switch S1 may be turned off immediately after the driving operation for executing printing is completed.

[0050] V(n005) in FIG. 13 is the upper terminal voltage of inductor L1. V(n002) is the upper terminal voltage of capacitor C8 in the same figure. When the polarization process of piezoelectric actuator 5 is completed, switch S3 is turned OFF, switch S1 is turned ON, and then the potential of the individual terminal is returned to a steady-state voltage V2 (e.g. 10V) such as a standby state. At this time, the potential V(drive) of each individual terminal and the potential V(vcom) of the common terminal are as shown in FIG. 14. From the results in FIG. 14, it can be seen that the potential difference between V(drive) and V(vcom) is maintained at 70V for 5 seconds, allowing the polarization process to be performed.

[0051] Second embodiment Next, an inkjet head 100 according to a second embodiment will be described. As shown in Fig. 15, the inkjet head 100 according to the second embodiment has the same configuration as that of the first embodiment, except that the boost circuit is configured as a Cockcroft-Walton circuit 72. Therefore, a detailed description of the configuration similar to that of the first embodiment will be omitted.

[0052] The waveforms of each part in the circuit in which the boost circuit is configured with the Cockcroft-Walton circuit 72 are as shown in the simulation results in FIG. 16. The driving waveform of the first 50 μs is the same as that in the first embodiment, so an enlarged view is omitted. V(n003) in FIG. 16 is the waveform of the oscillator B2 input to the boost part configured with the capacitors C3, C1, C4, C5, C6, and C7 and the diodes D2, D3, D4, D5, D6, and D7 of the Cockcroft-Walton circuit 72. The oscillator B2 switches 350 times with a period of 5 ms, as in the case of the flyback circuit 71, and is configured to be able to flow in and out with an amplitude of 20 V by using, for example, a transistor. The voltage V1 can be used as the power source.

[0053] V(n002) in FIG. 16 is the voltage waveform of the right terminal of the capacitor C7, which is the output of the Cockcroft-Walton circuit 72. For the reasons already mentioned, this voltage is reduced when the switch S3 is turned on. Since the capacitors C3, C1, C4, C5, C6, and C7 are connected in series, the reduction rate is greater than that of the flyback circuit 71. For this reason, the capacitors C3, C1, C4, C5, C6, and C7 have a larger capacitance than that of the flyback circuit 71. Since the voltage applied to each capacitor is distributed, the withstand voltage of the capacitor is smaller than that of the flyback circuit 71, which has the advantage that it is easy to use a large-capacity capacitor. As shown in FIG. 16, even if the boost circuit is configured with the Cockcroft-Walton circuit 72, it can be seen that a polarization voltage of 70V is applied for 5 seconds by looking at the difference between V(drive) and V(vcom).

[0054] Third embodiment Next, an inkjet head 100 according to a third embodiment will be described. In the inkjet head 100 of the third embodiment, a high voltage generation input waveform corresponding to the waveform of the oscillator B2 input to the booster section of the Cockcroft-Walton circuit 72 in the second embodiment is input from the common terminal of the piezoelectric actuator 5 via a common wiring 57. A third photo MOS switch 66 is provided between the common terminal and the high voltage generation input terminal. The third photo MOS switch 66 is used in a bidirectional switch mode, similar to the second photo MOS switch 61.

[0055] In this case, no voltage is supplied from the first driving power supply 32 to the polarization power supply 7, but instead the second Photo MOS switch 61 and the third Photo MOS switch 66 are switched ON / OFF to selectively connect the common terminal of the piezoelectric actuator 5 to the input and output of the polarization power supply 7. That is, a switching waveform with a 5 ms period that is generated by the Cockcroft-Walton circuit 72 to generate a negative voltage is generated using the output circuit of the driving IC 3 and input to the booster section 73 of the Cockcroft-Walton circuit via the piezoelectric actuator 5. Note that detailed explanations of other configurations that are the same as those in the first embodiment will be omitted.

[0056] In this embodiment, the polarization power supply 7 is a Cockcroft-Walton booster 73 shown in FIG. 18. The input of the Cockcroft-Walton circuit booster 73 is on the left side of the capacitor C1, and the output is on the right side of the capacitor C7. The switch S4 is a switch that connects the common terminal of the piezoelectric actuator 5 to the input of the Cockcroft-Walton circuit booster 73. The switch S3 is a switch that connects the output of the Cockcroft-Walton circuit booster 73 to the common terminal of the piezoelectric actuator 5. That is, the switch S4 is the third Photo MOS switch 66. The switch S3 is the second Photo MOS switch 61. The combination of the third Photo MOS switch 66 and the second Photo MOS switch 61 is an example of a second switch element.

[0057] 19 to 22 show the results of a simulation of the equivalent circuit of FIG. As shown in FIG. 19, one second after the output waveform V(drive) of the driving IC 3, a 20V waveform with a 5 ms period for boosting is applied 350 times. The output waveform V(drive) is output from the driving driver D of all channels. During this time, there is a risk that ink may be ejected from the nozzle 24, but for example, the nozzle surface may be capped. Alternatively, the waveform may be fine-tuned so that ink is not ejected. If there is a risk that air may be sucked from the nozzle 24 due to the vibration of the ink meniscus formed in the nozzle 24, a weak positive pressure may be applied to the ink to prevent the meniscus from being drawn inward. This prevents air from being mixed in from the nozzle 24. During this time, the piezoelectric actuator 5 generates heat due to charging and discharging, and the polarization function improves as the temperature rises. The period of the waveform may be increased to increase the temperature. The piezoelectric actuator 5 may be driven for the purpose of increasing the temperature, and then a voltage may be applied to perform polarization processing.

[0058] The switches S1, S3, and S4 are controlled as shown in FIG. During normal operation, switch S1 is turned ON, and switches S3 and S4 are turned OFF. During generation of the polarization voltage, switches S1 and S3 are turned OFF, and switch S4 is turned ON to provide the voltage of the common terminal of the piezoelectric actuator 5 to the input of the booster 73 of the Cockcroft-Walton circuit. When the boost is complete, switch S4 is turned OFF and switch S3 is turned ON to provide the output voltage of the booster 73 of the Cockcroft-Walton circuit to the common terminal of the piezoelectric actuator 5, and polarization processing is started. After that, switch S3 is turned OFF and switch S1 is turned ON to end the polarization processing. The reason why switch S4 is turned ON at the last timing is that the simulation becomes unstable if the input of the booster 73 of the Cockcroft-Walton circuit is made high impedance. Switch S4 may remain OFF, or may be turned ON once and then OFF as in this example. While switch S1 is ON, the ON / OFF of switch S4 does not significantly affect the operation of the circuit.

[0059] In this circuit, the input of the boost unit 73 of the Cockcroft-Walton circuit, i.e., the voltage at the left terminal V(n003) of the capacitor C1, and the output of the boost unit 73 of the Cockcroft-Walton circuit, i.e., the voltage at the right terminal V(n002) of the capacitor C7, transition as shown in Figure 21. The voltages at the individual terminal (drive terminal) and common terminal (VCOM terminal) of the piezoelectric actuator 5 transition as shown in Figure 22. As shown in the simulation results in Figure 22, it can be seen that a polarization voltage of 70V is applied between the voltage V(drive) of the individual terminal and the voltage V(vcom) of the common terminal of the piezoelectric actuator 5 for 5 seconds in this circuit as well.

[0060] The polarization of the piezoelectric actuator 5 requires a voltage higher than the drive voltage for ink ejection. If a polarization power source is prepared outside the inkjet head 100, a high-voltage power line must be provided to the head unit 2, but the connectors and cables of the inkjet head 100 have many poles and a fine pitch, and are not suitable for wiring a high-voltage power source. A semiconductor switch or the like must be used to switch between the case where a drive waveform for ejecting ink is applied and the case where a polarization voltage is applied, but the drive voltage for ejecting ink must be precisely controlled, so a high ON resistance of the switch element is not acceptable. On the other hand, the switch element that switches to the high-voltage power source must withstand a high voltage. It is difficult to achieve both such low ON resistance and high withstand voltage, and the switch element and its control circuit tend to be expensive. Any of the above-mentioned embodiments has the advantage that the piezoelectric actuator 5 can be polarized without a high-voltage power source for polarization being separately prepared outside the inkjet head 100.

[0061] (Fourth embodiment) Next, an inkjet head 100 according to a fourth embodiment will be described. The inkjet head 100 of the fourth embodiment has the circuit of the head module 8 of FIG. 23. The polarization power supply 7 is not shown in FIG. 1, and is separated from the inkjet head 100 so that it can be connected to the inkjet head 100 only when necessary. FIG. 23 shows, as a preferred example, a configuration in which the polarization power supply 7 and the second Photo MOS switch 61 are separated from the head module 8 so that they can be detachably connected to the head module 8 via an external connection terminal 81. The first Photo MOS switch 6 is disposed in the head module 8.

[0062] The polarization power supply 7 and the second Photo MOS switch 61 separated from the head module 8 may be modularized as a deposition polarization device 82, as shown in FIG. 23. When the first Photo MOS switch 6 is turned OFF and the polarization power supply 7 is connected during deposition polarization, the second Photo MOS switch 61 may be removed from the deposition polarization device 82. The polarization power supply 7 may be disposed, for example, outside the head module 8, inside the inkjet printer 10. It may not be disposed inside the inkjet printer 10, but may be prepared when necessary, or may be prepared during the manufacturing process of the inkjet head 100 and not connected after manufacturing. The polarization power supply 7 may be configured with the boost circuit described in the first to third embodiments, but is connected to the high voltage generating power supply outside the head module 8.

[0063] In the head module 8, the connection point between the first Photo MOS switch 6 and the common terminal, which is the other terminal of the piezoelectric actuator 5, is drawn out to the external connection terminal 81 so that the polarization power supply 7 can be connected between VCOM and GND. As described above, the first Photo MOS switch 6 is, for example, a Photo MOS transistor. A transistor with a DMOS structure can be used for the Photo MOS transistor. As shown in FIG. 23, the connection point between the first Photo MOS switch 6 and the common terminal of the piezoelectric actuator 5 is the source terminal side (pin 5 side) of the transistor with a DMOS structure. On the other hand, the drain terminal side (pins 6 and 4 side) of the transistor with a DMOS structure is connected to GND in the example of FIG. 23, but in order to increase the efficiency with respect to the drive voltage, a potential higher than GND, such as the voltage V3 described above, may be provided and connected to that potential. This potential higher than GND is preferably a positive potential of several volts (for example, 3 V) that does not cause depolarization. That is, the first Photo MOS switch 6 is an example of a switch element that switches between connecting the other terminal of the piezoelectric actuator 5, which is a common terminal, to ground (GND) or a predetermined potential to provide a common potential or blocking it.

[0064] As described above, the first Photo MOS switch 6 in the head module 8 has a built-in body diode 62 that serves as a parallel diode in the switch. The forward direction of this body diode 62 is the direction in which the piezoelectric actuator 5 is charged.

[0065] During ink ejection operation, which is the normal operation of the inkjet head 100, the polarization power supply 7 is not connected to the external connection terminal 81, and the first photo MOS switch 6 is turned ON. Since the first photo MOS switch 6 is inside the head module 8, the wiring path from the common electrode 57 of the piezoelectric actuator 5 to the GND is short, and the impedance between the common electrode 57 of the piezoelectric actuator 5 and the GND when the first photo MOS switch 6 is turned ON can be kept small. In addition, since the body diode 62 is allowed to be built in, a transistor with a DMOS structure having low ON resistance can be used for the photo MOS transistor, which is an example of the first photo MOS switch 6. In other words, the ON resistance of the first photo MOS switch 6 can be reduced. With this configuration, during normal operation in which the first photo MOS switch 6 is turned ON, it is possible to minimize the increase in crosstalk and the deterioration of print quality caused by the insertion loss of the first photo MOS switch 6, which is an example of a switching element.

[0066] On the other hand, when performing polarization, the polarization power supply 7 is connected between VCOM and GND via the external connection terminal 81. At this time, the first Photo MOS switch 6 is turned OFF, and the second Photo MOS switch 61 is turned ON. As described above in detail, since the polarization power supply 7 applies a negative voltage to VCOM, the body diode 62 is also turned OFF, and the first Photo MOS switch 6 does not affect the polarization.

[0067] As shown in FIG. 23, the head module 8 may connect the drive voltage V1, the drive voltage V2, GND, which is an example of a common potential, a signal line for a control signal, and a control power supply via an external connection terminal 83. The connection via the external connection terminal 81 or the external connection terminal 83 may be applied to the first to third embodiments. The external connection terminal 81 may be assigned to another terminal of the same connector as the external connection terminal 83, and the connection may be switched at the other end of the connector (not shown). Alternatively, the connector itself may be a separate entity, and the connector between VCOM and GND may be connected only during polarization. Also, the external connection terminal 81 may be a test pad on a printed circuit board instead of a connector, and the polarization power supply 7 may be brought into contact with the test pad during polarization.

[0068] The VCOM-GND of the external connection terminal 81 of the head module 8 can be used for purposes other than polarization. FIG. 24 shows an example of using the VCOM-GND for testing the piezoelectric actuator 5. That is, if a current waveform measuring device 9 is connected to the VCOM-GND, and the first photo MOS switch 6 is turned off, and a driving power supply and a control signal are applied to the driving IC 3 to sequentially drive the piezoelectric actuators 5, it is possible to divert the device to a purpose of measuring the current waveform for each channel, for example, by using the method disclosed by the applicant in Japanese Patent No. 3637246. The current waveform measuring device 9 includes, for example, an oscilloscope 91 for measuring the current waveform, a current detection resistor 92 of, for example, 1 Ω, and a diode 93 connected in parallel to the detection circuit. A signal is applied to each external connection terminal 83 to sequentially charge and discharge the piezoelectric actuators 5 one by one, and the quality of each piezoelectric actuator 5 can be determined by observing the current waveform during charging and discharging with the oscilloscope. If the voltage generated across the current detection resistor 92 by the current to be detected is smaller than the forward voltages of the diode 62 and the diode 93 connected in parallel to the detection circuit, the waveform of the current flowing through the piezoelectric actuator 5 can be detected without any problems.

[0069] As described above, according to any of the above-mentioned embodiments, it is possible to suppress the insertion loss due to the first Photo MOS switch 6, which is an example of a switch element, from affecting the ink ejection, which is the normal operation of the inkjet head 100. As a result, it is possible to stably eject ink.

[0070] As for polarization, as explained in the third embodiment, by driving the piezoelectric actuator 5 and raising the temperature of the piezoelectric actuator 5 before polarization, it is possible to effectively perform reliable polarization in a short time with a smaller voltage. This configuration is not only effective in the third embodiment. It may be combined with the first, second, and fourth embodiments to drive the piezoelectric actuator 5 and raise the temperature of the piezoelectric actuator 5 before polarization according to each embodiment.

[0071] In the above embodiment, the drive voltages V1 and V2 are positive voltages, but they are negative voltages when the polarization direction is reversed from that of this embodiment. When the drive voltages V1 and V2 are negative voltages, for example, in the circuit of Fig. 6, the connections of the positive and negative poles of the first drive power supply 32 and the second drive power supply 33, the positive and negative poles of the polarization power supply 7, and the first Photo MOS switch 6 are reversed, and the connections of the cathode and anode of the parallel diodes are reversed. Furthermore, the connections of the cathode and anode of each output protection diode 31 are reversed.

[0072] The piezoelectric actuator 5 is not limited to a laminated type in which a plurality of piezoelectric bodies 51 are laminated. The piezoelectric body 51 may be a single-layer piezoelectric actuator. The operation of the actuator when a drive voltage is applied is not limited to longitudinal vibration. Furthermore, the actuator is not limited to a drop-on-demand piezoelectric type, and may be applied to a continuous type.

[0073] In the above embodiment, the inkjet head 100 of the inkjet printer 10 has been described as an example of a liquid ejection device, but the liquid ejection device may also be a modeling material ejection head of a 3D printer or a sample ejection head of a dispensing device.

[0074] The above embodiment can be expressed as follows. (1) A plurality of piezoelectric actuators each including a piezoelectric body, one terminal for applying a drive waveform, and the other terminal; a drive circuit that applies the drive waveform to each of the piezoelectric actuators; a switch element that connects the other terminals of the piezoelectric actuators in common and connects the connection point to ground (GND) or a predetermined potential to switch between providing and blocking the common potential; and a parallel diode of the switch element, the forward direction being the direction in which the piezoelectric actuator is charged. (2) the switch element is a transistor having a DMOS structure, The connection point between the switch element and the other terminal is on the source terminal side of the transistor having the DMOS structure. (3) the switch element is a transistor having a DMOS structure, The parallel diode is the body diode of the transistor of the DMOS structure. (4) the drive circuit and the switch element are mounted on a head module, The head module includes an external connection terminal that draws out a connection point between the switch element and the other terminal to the outside of the head module. (5) A connection point between the switch element and the other terminal is connected to a polarization power source that applies a polarization potential to the piezoelectric actuator. (6) a piezoelectric actuator including a piezoelectric body, one terminal for applying a drive voltage from a liquid ejection driving power supply, and another terminal for applying a common potential; a first switch element that switches between connecting the other terminal of the piezoelectric actuator to ground (GND) or a predetermined potential to provide a common potential, or connecting the other terminal to a polarized power source having a polarity opposite to that of the liquid ejection driving power source; and a parallel diode of the switch element that operates to turn off in a direction to block the polarization voltage from the polarization power supply. (7) The polarized power supply is configured as a boost circuit within the head unit, The boost circuit uses the voltage from the liquid ejection driving power supply as a high voltage generating power supply to generate a voltage of opposite polarity to the driving voltage, and supplies this voltage to the other terminal of the piezoelectric actuator as a polarization voltage. (8) The boost circuit is a flyback circuit using an inductor and a diode. (9) The boost circuit is a Cockcroft-Walton circuit using multiple diodes and capacitors. (10) The input of the boost circuit is provided from the other terminal of the piezoelectric actuator via a second switch element, and the boost circuit generates a voltage of opposite polarity to the drive voltage, switches the second switch element, and returns the voltage to the other terminal of the piezoelectric actuator. (11) Prior to applying the polarization voltage to the other terminal of the piezoelectric actuator, the piezoelectric actuator is driven a number of times to increase the temperature of the piezoelectric actuator.

[0075] The embodiments of the present invention are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and spirit of the invention, and are included in the scope of the invention and its equivalents described in the claims. [Explanation of symbols]

[0076] 10. Inkjet Printer 100~103 Inkjet head 2 Head section 23 Nozzle plate 3. Driver IC 31 Output protection diode 5 Piezoelectric Actuator 56 Individual wiring 57 Common Wiring 6 First Photo MOS Switch 61 Second Photo MOS Switch 62 Body Diode 66 3rd Photo MOS Switch 7 Polarized Power Supply 71 Flyback Circuit 72 Cockcroft-Walton Circuit 73 Boost section of Cockcroft-Walton circuit

Claims

1. a plurality of piezoelectric actuators each including a piezoelectric body, one terminal for applying a drive waveform, and the other terminal; a drive circuit that applies the drive waveform to each of the piezoelectric actuators; a switch element that connects the other terminals of the piezoelectric actuators in common and connects the connection point to ground (GND) or a predetermined potential to switch between providing and blocking the common potential; a parallel diode of the switch element, the forward direction being the direction in which the piezoelectric actuator is charged.

2. the switch element is a transistor having a DMOS structure, 2. The liquid ejection head according to claim 1, wherein the connection point between the switch element and the other terminal is on the source terminal side of the transistor having the DMOS structure.

3. the switch element is a transistor having a DMOS structure, 3. The liquid ejection head according to claim 1, wherein the parallel diode is a body diode of the transistor having the DMOS structure.

4. The drive circuit and the switch element are mounted on a head module, 2. The liquid ejection head according to claim 1, wherein the head module includes an external connection terminal that leads out a connection point between the switch element and the other terminal to the outside of the head module.

5. 5. The liquid ejection head according to claim 4, wherein a connection point between the switch element and the other terminal is connected to a polarization power source that applies a polarization potential to the piezoelectric actuator.

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