Liquid dispensing head

Abstract

To provide a liquid dispensing head that can stably dispense liquid. [Solution] The liquid dispensing head comprises multiple piezoelectric actuators, multiple drive circuits, a DMOS transistor, a resistor, a switch, and a body diode for the DMOS transistor. The DMOS transistor connects the other terminal of the piezoelectric actuators, which are connected in common to the multiple piezoelectric actuators, to the source, and a predetermined potential to the drain. The resistor is placed between the gate and source of the DMOS transistor. The switch turns the gate of the DMOS transistor ON or OFF. The body diode of the DMOS transistor is configured to charge the piezoelectric actuators in the forward direction. The liquid dispensing head keeps the switch ON and turns on the DMOS transistor at all times, at least during the period when a drive waveform for dispensing liquid is applied to the piezoelectric actuators.

Description

Technical Field

[0001] Embodiments of the present invention relate to a liquid ejection head.

Background Art

[0002] A liquid ejection head that supplies a predetermined amount of liquid to a predetermined position is known. The liquid ejection head is mounted on, for example, an inkjet printer, a 3D printer, a dispensing device, or the like. An inkjet printer ejects ink droplets from an inkjet head to form an image or the like on the surface of a recording medium. A 3D printer ejects droplets of a modeling material from a modeling material ejection head and cures them to form a three-dimensional object. A dispensing device ejects droplets of a sample and supplies a predetermined amount to a plurality of containers or the like.

[0003] The liquid ejection head has a plurality of channels for ejecting liquid. Each channel includes a nozzle for ejecting liquid, a pressure chamber communicating with the nozzle, and a piezoelectric actuator for changing the volume of the pressure chamber. The liquid ejection head selects a channel for ejecting liquid from among the plurality of channels, and applies a drive signal to the piezoelectric actuator to drive it. When the piezoelectric actuator is driven, the volume of the pressure chamber filled with liquid changes, and liquid is ejected from the nozzle.

[0004] A piezoelectric actuator that is driven using the inverse piezoelectric effect of a piezoelectric body is polarized in the polarization direction that realizes the function of changing the volume of the pressure chamber. However, the piezoelectric body may undergo polarization degradation. When polarization degradation occurs, the liquid ejection performance may decrease. A piezoelectric actuator with polarization degradation can be depolarized by applying a high voltage for a predetermined time. However, the insertion loss depending on the switching circuit that connects the piezoelectric actuator to the polarization power supply 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

[0006] The problem that this invention aims to solve is to provide a liquid dispensing head that can stably dispense liquid. [Means for solving the problem]

[0007] The liquid dispensing head according to an embodiment of the present invention comprises a plurality of piezoelectric actuators, a plurality of drive circuits, a DMOS (Double-diffused MOSFET) transistor, a resistor, a switch, and a body diode of the DMOS transistor. The plurality of drive circuits each supply a drive waveform for dispensing liquid to one terminal of the plurality of piezoelectric actuators. The DMOS transistor has the other terminal of the piezoelectric actuator, which is connected in common to the plurality of piezoelectric actuators, connected as a source, and a predetermined potential connected to the drain. The resistor is placed between the gate and source of the DMOS transistor. The switch turns the gate of the DMOS transistor ON or OFF. The body diode of the DMOS transistor is configured to charge the piezoelectric actuator in the forward direction. The liquid dispensing head keeps the switch ON and the DMOS transistor ON at all times, at least during the period when the drive waveform for dispensing liquid is supplied to the piezoelectric actuator. [Brief explanation of the drawing]

[0008] [Figure 1] This is an overall configuration diagram of an inkjet printer equipped with an inkjet head according to the first embodiment. [Figure 2] The above is a perspective view of the inkjet head. [Figure 3]This is a partially enlarged cross-sectional view of the head portion of the inkjet head shown above. [Figure 4] This is a partially enlarged cross-sectional view of the head portion of the inkjet head shown above. [Figure 5] This is a partially enlarged plan view of the head portion of the inkjet head shown above. [Figure 6] This is a circuit diagram showing the control device connected to the inkjet head mentioned above. [Figure 7] This is a circuit diagram showing a polarization device connected to the inkjet head shown above. [Figure 8] This is a waveform diagram of the polarization voltage applied to the inkjet head shown above. [Figure 9] This is a circuit diagram showing a control device connected to the inkjet head of the second embodiment. [Figure 10] This is a circuit diagram showing a polarization device connected to the inkjet head shown above. [Figure 11] This is a circuit diagram showing a polarization device connected to the inkjet head of the third embodiment. [Figure 12] This is a circuit diagram showing a polarization device connected to the inkjet head of the fourth embodiment. [Figure 13] This is a circuit diagram showing a control device connected to the inkjet head of the fifth embodiment. [Figure 14] This is a circuit diagram showing a polarization device connected to the inkjet head shown above. [Figure 15] This is a circuit diagram showing a control device connected to the inkjet head of the sixth embodiment. [Figure 16] This is a circuit diagram showing a polarization device connected to the inkjet head shown above. [Figure 17] This is a circuit diagram showing a control device connected to the inkjet head of the seventh embodiment. [Figure 18] This is a circuit diagram showing a polarization device connected to the inkjet head shown above. [Figure 19] This is a circuit diagram showing a control device connected to the inkjet head of the eighth embodiment. [Figure 20]This is a circuit diagram in which a polarization device is connected to the inkjet head. [Figure 21] This shows the simulation results of the transition from the driving state to polarization.

Embodiments for Carrying out the Invention

[0009] Hereinafter, the liquid ejection head according to the embodiment will be described in detail while referring to the accompanying drawings. In each figure, 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 includes, inside a housing 11, a cassette 12 that stores a sheet S, which is an example of a recording medium, an upstream conveyance path 13 of the sheet S, a conveyance belt 14 that conveys the sheet S taken out from the cassette 12, a plurality of inkjet heads 100 to 103 that eject ink droplets toward the sheet S on the conveyance belt 14, a downstream conveyance path 15 of the sheet S, a discharge tray 16, and a control board 17. An operation unit 18, which is a user interface, is disposed on the upper side of the housing 11.

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

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

[0013] The conveyor belt 14 is a mesh-like endless belt with numerous through holes formed on its surface. Three rollers, a drive roller 141 and driven rollers 142 and 143, rotatably support the conveyor belt 14. 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 direction of rotation of the conveyor belt 14. A negative pressure container 206 is placed on the back side of the conveyor belt 14. The negative pressure container 206 is connected to a pressure-reducing fan 207. The fan 207 creates negative pressure inside the negative pressure container 206 with the airflow it generates, causing the sheet S to adhere to and hold 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 examples of liquid ejection heads, are positioned opposite a sheet S held by suction on a transport belt 14, with a small gap of, for example, 1 mm between them. Each inkjet head 100-103 ejects droplets of ink toward the sheet S. The inkjet heads 100-103 print an image as the sheet S passes below them. Each inkjet head 100-103 has the same structure except that it ejects a different color of ink. The ink colors are, for example, cyan, magenta, yellow, and black.

[0015] Each of the inkjet heads 100-103 is connected to ink tanks 315-318 and ink supply pressure regulators 321-324 via ink channels 311-314. Each ink tank 315-318 is positioned above each of the inkjet heads 100-103. To prevent ink leakage from the nozzles 24 (see Figure 2) of the inkjet heads 100-103 during standby, each ink supply pressure regulator 321-324 adjusts the pressure inside each inkjet head 100-103 to a negative pressure relative to atmospheric pressure, for example, -1.2kPa. During image formation, the ink from each ink tank 315-318 is supplied to each of the inkjet heads 100-103 by the ink supply pressure regulators 321-324.

[0016] After image formation, the sheet S is sent from the conveyor belt 14 to the downstream conveyor path 15. The downstream conveyor path 15 consists of feed roller pairs 151, 152, 153, and 154, and sheet guide plates 155 and 156 that define the conveying path of the sheet S. The sheet S is sent via the downstream conveyor path 15 to the discharge tray 16 from the discharge port 157. In the figure, arrow 107 indicates the conveying path of the sheet S.

[0017] Next, we will explain the configuration of inkjet heads 100 to 103. The following explanation of inkjet head 100 is based on Figures 2 to 5, but inkjet heads 101 to 103 have the same structure as inkjet head 100.

[0018] As shown in Figure 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 circuit board 21, which is an example of a film wiring board. The flexible printed circuit 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 shown in Figure 1 via an ink flow path 311.

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

[0020] The flexible printed circuit board 21 is a flexible printed circuit board made of a synthetic resin film such as polyimide. The flexible printed circuit board 21 is equipped with a driver chip, which is an integrated circuit (IC) 3 (hereinafter referred to as the driver IC). The printed circuit board 22 is a rigid through-hole board made of multiple layers of glass fiber-reinforced epoxy resin and copper wiring layers. The driver IC 3, which acts as the control unit for the inkjet head 100, temporarily stores the print data sent from the control board 17, which is equipped with a CPU that acts as the control unit for the inkjet printer 10, via the printed circuit board 22, and provides drive signals to each channel to eject ink at predetermined timings.

[0021] Figures 3 to 5 are partial cross-sectional views of the head section 2. The nozzle plate 23 is bonded to one surface of the pressure chamber substrate 4. The nozzle plate 23 is a rectangular plate formed from, 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 to the nozzle plate 23. The diaphragm 41 is flexible and deforms when an external force is applied. The diaphragm 41 is a rectangular plate formed from, for example, a flexible polyimide film or metal.

[0022] The pressure chambers 42 are formed in the pressure chamber substrate 4. Multiple pressure chambers 42 are arranged at the position of each nozzle 24 and communicate with each nozzle 24. As an example, the pressure chambers 42 have rectangular openings formed in the pressure chamber substrate 4 that penetrate in a second direction, for example in the Z direction, and the openings on both sides are closed by the nozzle plate 23 and the diaphragm 41, respectively, to form a space for filling with ink. The pressure chambers 42 communicate with guide channels 43 having a constricted section, and further communicate with the ink supply manifold 45 via an ink supply port 44, which is an opening formed in the diaphragm 41. The guide channels 43 are formed in a groove shape in a third direction, for example in the Y direction, on one surface of the pressure chamber substrate 4 on the diaphragm 41 side for each pressure chamber 42. The ink supply manifold 45 is formed in a frame 46 joined to one surface of the diaphragm 41. The ink supply manifold 45 extends in the X direction and communicates with the pressure chamber 42 of each channel via the ink supply port 44 and guide flow path 43 of each channel. The ink supply manifold 45, acting as a common ink chamber, communicates with the ink flow path 311 (see Figures 1 and 2).

[0023] The piezoelectric actuator 5 is positioned on one side of the diaphragm 41 opposite to the pressure chamber 42. The piezoelectric actuators 5 for each channel are arranged in positions facing the pressure chamber 42, with the diaphragm 41 in between. The piezoelectric actuators 5 and the diaphragm 41 are joined together, for example, with an adhesive. Each piezoelectric actuator 5 is fixed by joining one side opposite to the diaphragm 41 in the Z direction to a support member 47. In particular, as shown in Figure 3, the piezoelectric actuator 5 is a laminated piezoelectric actuator formed by alternately stacking piezoelectric elements 51, such as a piezo element, a first internal electrode 52, and a second internal electrode 53 in layers. Each piezoelectric element 51 is positioned with its polarization direction opposite to that of the others in the Z direction, for example, and is deformed in d33 mode. The first internal electrode 52 and the second internal electrode 53 are conductive films formed on the main surface of the piezoelectric element 51, respectively. The first internal electrode 52 is formed to one end face of the piezoelectric actuator 5 in the Y direction and is connected to a first external electrode 54 formed on this end face. The second internal electrodes 53 are formed to the other end face of the piezoelectric actuator 5 in the Y direction and are connected to the second external electrodes 55 formed on this end face.

[0024] The dummy layer 58 is made of the same material as the piezoelectric element 51. The dummy layer 58 does not have internal electrodes and does not deform because 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 Figure 4), or as a polishing surface for polishing during or after assembly to achieve accuracy. In particular, as shown in Figure 4, support columns 50 may be placed between the piezoelectric actuators 5 of each channel via grooves 59. The support columns 50 may be made of dummy actuators formed in the same way as the piezoelectric actuators 5 used for driving. The support columns 50 are placed, for example, at a position corresponding to the partition wall 40 between adjacent pressure chambers 42. The support columns 50 may be made of a different material instead of being made of dummy actuators.

[0025] In the case of a piezoelectric actuator 5 in which multiple piezoelectric elements 51 are stacked, as an example, a first internal electrode 52 and a second internal electrode 53 are deposited on the main surface of each piezoelectric element 51 that has been processed into a thin plate shape. Then the piezoelectric elements 51 are stacked and fired to form a single unit. After that, a first external electrode 54 and a second external electrode 55 are deposited. After that, the piezoelectric elements 51 are polarized with a polarization voltage. The piezoelectric elements 51 are formed from lead-containing piezoelectric materials such as lead zirconate titanate (PZT) or lead-free piezoelectric materials such as sodium potassium niobate. The first internal electrode 52 and the second internal electrode 53 are deposited from a sinterable conductive material such as silver palladium. The first external electrode 54 and the second external electrode 55 are deposited from Ni, Cr, Au, etc., by known methods such as plating or sputtering.

[0026] The first external electrode 54 of each channel is connected to the individual wiring 56 of the flexible printed circuit board 21 (see Figure 3). The flexible printed circuit board 21 has a base material 26, individual wiring 56, an adhesive layer 27, and an insulating layer 28. The flexible printed circuit 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 of each channel and the individual wiring 56 are electrically and mechanically connected by melting solder. Instead of solder, the connections may be fixed with ACF (Anisotropic Conductive Film), ACP (Anisotropic Conductive Paste), NCF (Non-Conductive Film), NCP (Non-Conductive Paste), etc., and anisotropic conductive connection may be made 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, for example, to ground (GND) via the flexible printed circuit board 21.

[0027] Next, the circuit for driving the inkjet head 100 will be described. As shown in Figure 6, the entire circuit for driving the inkjet head 100 consists of the inkjet head 100 and the control device 6 of the inkjet printer 10 that is equipped with the inkjet head 100. The inkjet head 100 and the control device 6 are detachably connected to each other via an external connection terminal 61. The control device 6 is formed on a control board 17 equipped with a CPU, which is the control unit of the inkjet printer 10 (see Figure 1). As an example, a computer 200 shown in Figure 1 is connected to the control circuit 62 of the control device 6 to receive the image data to be printed. The image data may be received from a source other than the computer 200.

[0028] The piezoelectric actuator 5 connects its first external electrode 54 to an individual wiring 56. Each individual wiring 56 from the piezoelectric actuator 5 is connected to the output terminal of the drive driver D (i.e., the drive circuit) of the drive IC 3's output circuit. The connection point between the first external electrode 54 and the individual wiring 56 is one of the terminals of the piezoelectric actuator 5. This one terminal is referred to as the individual terminal. The drive driver D for each channel (1ch~nch) provides a drive waveform that ejects ink to the individual terminal of the piezoelectric actuator 5 for the corresponding channel.

[0029] The driver IC 3 is connected to a first driver power supply 31, which supplies the drive voltage V1, and a second driver power supply 32, which supplies the drive voltage V2. The first driver power supply 31 and the second driver power supply 32 each have their positive terminals connected to the driver IC 3 and their negative terminals connected to, for example, ground (GND). In other words, in this example, the positive voltages are the drive voltages V1 and V2. The drive voltage V1 is, for example, 20V. The drive voltage V2 is, for example, 10V. Ground (GND) is, for example, 0V.

[0030] The driver IC 3 is further connected to a control power supply 33, which is the power source for the control voltage Vdd, and a signal line 34 for the control signal from the control circuit 62. The control power supply 33 is the power source that drives the driver IC 3. The control voltage Vdd is, for example, 3.3V. The control signal is the print data sent from the control circuit 62 when printing is performed and ink is ejected.

[0031] The drive IC 3 includes a first output protection diode 35 and a second output protection diode 36 to protect the drive driver D. The first output protection diode 35 is a diode whose cathode is connected to the output terminal of the drive driver D and whose anode is connected to the ground (GND) of the drive IC 3. The first output protection diode 35 is provided for each drive driver D of each channel that ejects ink. For example, if a parasitic diode is formed by the structure of the transistor in the output circuit, the parasitic diode may be used instead of the first output protection diode 35.

[0032] The second output protection diode 36 is a diode in which the anode is connected to the output terminal of the drive driver D and the cathode is connected to the voltage V1 of the drive IC 3. The second output protection diode 36 is provided for each drive driver D of each channel that ejects ink. For example, if a parasitic diode is formed by the structure of the transistor in the output circuit, the parasitic diode may be used instead of the second output protection diode 36. The first output protection diode 35, the second output protection diode 36, or a substitute parasitic diode is an example of a diode connected between the output terminal of the drive IC 3 and a reference potential in a direction that does not conduct when the drive IC 3 outputs a drive voltage. An example of the reference potential for the first output protection diode 35 is ground (GND). An example of the reference potential for the second output protection diode 36 is voltage V1.

[0033] On the other hand, the second external electrode 55 of each piezoelectric actuator 5 is commonly connected to a common wiring 57. The connection point between the second external electrode 55 and the common wiring 57 is the other terminal of the piezoelectric actuator 5. This other terminal is called the common terminal. The common wiring 57 is connected to ground (GND) via a DMOS transistor U1. That is, 0V is applied to the common terminal of each piezoelectric actuator 5 as an example of a common potential.

[0034] The DMOS transistor U1 is inserted into the current path that flows in common to multiple piezoelectric actuators 5. That is, the DMOS transistor U1 is provided in the common wiring 57 within the inkjet head 100. The DMOS transistor U1 is, for example, an N-channel DMOS FET (Field Effect Transistor). When using an N-channel DMOS transistor U1, the source is connected to the common terminal that connects the multiple piezoelectric actuators 5 in common, and the drain is connected to ground (GND). The ground potential is an example of a predetermined potential to which the drain is connected. The gate of the DMOS transistor U1 is connected to voltage V1 via resistor R1 and switch SW. Resistor R2 is provided between the gate and source of the DMOS transistor U1.

[0035] Therefore, when the switch SW is turned ON, the intermediate voltage obtained by dividing the voltage V1 by the resistance ratio of resistors R1 and R2 is applied to the gate of the DMOS transistor U1, and the DMOS transistor U1 turns ON. By adjusting the voltage division ratio of resistors R1 and R2 and providing a sufficient gate voltage to the DMOS transistor U1, the ON resistance of the DMOS transistor U1 can be kept low. The resistance value of resistor R1 is, for example, 48kΩ, and the resistance value of resistor R2 is, for example, 22kΩ. On the other hand, when the switch SW is turned OFF, the source and gate of the DMOS transistor U1 become at the same potential due to the resistor R2 placed between the gate and source, and the DMOS transistor U1 turns OFF.

[0036] The DMOS transistor U1 is equipped with a body diode. The body diode is a diode typically found in DMOS FETs. The body diode is also called a parasitic diode. When the source and drain of the N-channel DMOS transistor U1 are connected as shown in Figure 6, the anode of the body diode faces the common terminal side of the piezoelectric actuator 5, and the cathode faces the power supply terminal side. In other words, the body diode's forward direction is the direction that charges the piezoelectric actuator 5. Therefore, the current used to charge the piezoelectric actuator 5 flows in the direction that turns the body diode ON.

[0037] If the charging current path were limited to the body diode only, and the DMOS transistor U1 is a silicon semiconductor, there would be a voltage drop of approximately 0.6V across the body diode, resulting in a large insertion loss. However, in the circuit shown in Figure 6, a sufficient gate voltage can be applied, and the transistor itself operates with low resistance in both charging and discharging directions. As a result, the voltage drop between the drain and source of the DMOS transistor U1 is significantly lower than approximately 0.6V. Therefore, the insertion loss of the DMOS transistor U1 is small. In other words, the voltage drop across the DMOS transistor U1 is suppressed from affecting the drive waveform supplied to the piezoelectric actuator 5 during ink ejection. This suppresses electrical crosstalk between channels, stabilizing the ink ejection characteristics. Therefore, in this embodiment, the switch SW is always turned ON to turn on the DMOS transistor U1 during the period when the drive waveform is supplied to at least one of the piezoelectric actuators 5 of any one channel.

[0038] As mentioned above, ink ejection is performed with the DMOS transistor U1 turned ON. Each drive driver D of the drive IC3 uses drive voltages V1, V2 and ground (GND) to apply a drive waveform to the individual terminals of the piezoelectric actuator 5. Note that Figure 6 shows an example of a drive waveform for ejecting ink by pull-and-shoot, but it is not limited to this. When using this drive waveform, first a voltage V2 is applied to the individual terminals to put them into a standby state. When a voltage V2 is applied to the individual terminals, an electric field is applied in the direction of the polarization axis of the piezoelectric element 51, and due to the inverse piezoelectric effect of the piezoelectric element, the piezoelectric actuator 5 expands in the stacking direction (Z direction), and the volume of the pressure chamber 42 decreases. This is done prior to the ink ejection timing. Then, at the ink ejection timing, the potential of the individual terminals is first lowered to ground (GND), so that the expanded piezoelectric actuator 5 returns to its original position, that is, it contracts relatively, and the volume of the pressure chamber 42 expands relatively. As the volume of the pressure chamber 42 expands, ink flows into the pressure chamber 42 via the guide channel 43.

[0039] For example, after a time elapsed that is half the pressure vibration period of the head unit 2, when a voltage V2 is applied to the individual terminals, the piezoelectric actuator 5 extends in the stacking direction (Z direction), and the volume of the pressure chamber 42 relatively contracts, causing ink to be ejected from the nozzle 24. Then, for example, after a time elapsed that is half the pressure vibration period of the head unit 2, a voltage V1 is applied to the individual terminals, and after a predetermined time, the voltage is returned to V2. The extension and return of the piezoelectric actuator 5 at this time causes the volume of the pressure chamber 42 to contract and return, and this operation dampens residual vibrations. 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] Next, the circuit for polarizing the piezoelectric actuator 5 will be explained with reference to Figure 7. Polarization of the piezoelectric actuator 5 includes not only the repolarization of the piezoelectric element 51 after polarization degradation, but also the initial polarization of the piezoelectric element 51. The entire circuit for polarizing the inkjet head 100 consists of the inkjet head 100 and the polarization device 63. The inkjet head 100 and the polarization device 63 are detachably connected to each other via the external connection terminal 61 after removing the control device 6. For this reason, the external connection terminal 61 of the inkjet head 100 is provided with a connection terminal that allows the connection point between the other terminal of the piezoelectric actuator 5 and the source of the DMOS transistor U1 to be connected externally (VCOM Open in Figure 6). The terminals for the drive power supply V1, V2, control power supply Vdd, and control signal are connected to ground. The polarization device 63 is connected using this connection terminal. The polarization device 63 does not need to be installed inside the inkjet printer 10 and may be prepared separately when polarization is required.

[0041] The polarization device 63 consists of a polarization power supply 64, a current limiter 65, and a resistor R3 connected in series. The polarization power supply 64 is a variable power supply capable of outputting the polarization voltage of the piezoelectric actuator 5, for example, a variable power supply from 0V to 70V. The polarization power supply 64 has its positive terminal connected to the drive IC 3 and its negative terminal connected to the connection point between the other terminal of the piezoelectric actuator 5 and the source of the DMOS transistor U1. The current limiter 65 and resistor R3 protect the drive IC 3 by preventing large currents from flowing during polarization. The current limit value of the current limiter 65 is, for example, 0.3mA. The resistance value of resistor R3 is, for example, 100kΩ.

[0042] When the polarization device 63 is connected to the inkjet head 100, the switch SW is turned OFF to turn OFF the DMOS transistor U1. Then, a high voltage for polarization is supplied to the piezoelectric actuator 5 from the polarization power supply 64. That is, by using a variable power supply for the polarization power supply 64, the voltage is increased to 62V at approximately 1V / sec, as shown in Figure 8. Although Figure 8 shows the voltage being increased in a step-like manner, it is desirable to increase the voltage more smoothly. The current during polarization is supplied via the first output protection diode 35 of the drive IC 3 and charges the piezoelectric actuator 5. If this current becomes too large, the drive IC 3 may be damaged, so the voltage rise is made gradual as shown in Figure 8, and the current is suppressed by a 0.3mA current limit and a 100kΩ series resistor.

[0043] If the total capacitance of the piezoelectric actuators 5 from channel 1 to channel 2 is 0.3 μF (= 1000 pF × 300 channels), the charging current will peak at around 25 V, depending on the thickness and number of piezoelectric elements 51, and then decrease. At 62 V, the charging current will stop flowing, but by continuing to apply a voltage of 62 V for a period of, for example, 100 sec to 1000 sec, the polarization of the piezoelectric elements 51 will be stabilized. After this period of constant voltage application has elapsed, as shown in Figure 8, the voltage will be lowered to 0 V at approximately 1 V / sec to discharge the piezoelectric actuators 5 and end the polarization. The discharge current flows in the opposite direction to that during charging through the second output protection diode 36 of the drive IC 3. When polarization is performed in the manufacturing process of an inkjet printer 10 or an inkjet head 100, a polarization voltage supply circuit may be provided in the manufacturing equipment. Alternatively, the piezoelectric actuators 5 may be driven before polarization to raise their temperature. By raising the temperature, reliable polarization can be effectively achieved quickly and with a lower voltage.

[0044] (Second Embodiment) Next, the inkjet head 100 of the second embodiment will be described. In contrast to the inkjet head 100 of the first embodiment, where the common is on the low side, the inkjet head 100 of the second embodiment has the common on the high side, as shown in Figure 9.

[0045] Specifically, a voltage V1 is applied to the common terminal of the piezoelectric actuator 5 as a common potential. The DMOS transistor U1 is, for example, a P-channel DMOS FET. The source of the P-channel DMOS transistor U1 is connected to the common terminal where multiple piezoelectric actuators 5 are connected in common, and the drain is connected to the wiring of voltage V1. Voltage V1 is an example of a predetermined potential to which the drain is connected. The body diode has its cathode facing the common terminal side of the piezoelectric actuator 5 and its anode facing the power supply terminal side. That is, the body diode is forward in the direction that charges the piezoelectric actuator 5. The gate of the DMOS transistor U1 is connected to ground (GND) via resistor R1 and switch SW. In this way, when the common is high-side, the polarity of the DMOS transistor U1 is reversed, as shown in Figure 9, and the drive waveform applied to the piezoelectric actuator 5 is also reversed. Other configurations may be the same as in the first embodiment where the common is low-side.

[0046] The circuit for polarizing the piezoelectric actuator 5 can be configured by reversing the polarity of the polarization power supply 64, as shown in Figure 10. Other configurations may be the same as in the first embodiment where the common is the low side. During polarization, a polarization voltage with the opposite polarity to that in the first embodiment is supplied from the polarization power supply 64. In this case as well, the voltage rise is made gradual, as shown in Figure 8. Current flows through the second protection diode 36 during charging and through the first protection diode 35 during discharge. This is the opposite flow compared to when the common is the low side.

[0047] (Third embodiment) Next, the inkjet head 100 of the third embodiment will be described. The inkjet head 100 of the third embodiment has the same configuration as the inkjet head 100 of the first embodiment, except that the external connection terminal 61 for connecting the control device 6 and the external connection terminal for connecting the polarization device 63 are separate, as shown in Figure 11. The polarization device 63 is connected to the polarization input terminal 66. The wiring for voltage V1 is connected to ground within the polarization device 63. In other words, the inkjet head 100 of the third embodiment is a modified example in which the common is low-side.

[0048] (Fourth Embodiment) Next, the inkjet head 100 of the fourth embodiment will be described. The inkjet head 100 of the fourth embodiment has the same configuration as the inkjet head 100 of the second embodiment, except that the external connection terminal 61 for connecting the control device 6 and the external connection terminal for connecting the polarization device 63 are separate, as shown in Figure 12. The polarization device 63 is connected to the polarization input terminal 66. The wiring for voltage V1 is connected to ground within the polarization device 63. In other words, the inkjet head 100 of the fourth embodiment is a modified example in which the common is high-side.

[0049] (Fifth embodiment) Next, the inkjet head 100 of the fifth embodiment will be described. The inkjet head 100 of the fifth embodiment has the same configuration as the inkjet head 100 of the third embodiment, except that a bipolar transistor U2 is used as the switch SW that turns the DMOS transistor U1 ON / OFF, as shown in Figures 13 and 14. In other words, it is a modified version of the switch SW with the common side being the low side.

[0050] The bipolar transistor U2 is, for example, a PNP transistor. When using a PNP transistor, the emitter is connected to voltage V1, and the collector is connected to the gate of the DMOS transistor U1 via resistor R1. The base of the PNP transistor is connected to ground (GND) via resistor R4. Furthermore, a resistor R5 is provided between the emitter and base of the PNP transistor. With this configuration, as shown in Figure 13, simply connecting the inkjet head 100 and the control device 6 will cause an intermediate voltage, obtained by dividing voltage V1 by the resistance ratio of resistors R4 and R5, to be applied to the base, turning on the PNP transistor. As a result, the DMOS transistor U1 can be kept ON automatically when the drive connection is made, and always while the control device 6 is connected. Consequently, the circuit becomes simple, inexpensive, and compact. The ink ejection operation, etc., is the same as in the third embodiment.

[0051] On the other hand, when polarization is performed, the control device 6 is disconnected from the external connection terminal 61, as shown in Figure 14. When the control device 6 is disconnected, the voltage V1 applied to the PNP transistor is removed, the PNP transistor turns OFF, and the DMOS transistor U1 automatically turns OFF. The polarization device 63 is connected to the polarization input terminal 66. The operation during polarization is the same as in the third embodiment.

[0052] (Sixth Embodiment) Next, the inkjet head 100 of the sixth embodiment will be described. The inkjet head 100 of the sixth embodiment has the same configuration as the inkjet head 100 of the fourth embodiment, except that a bipolar transistor U2 is used as the switch SW for turning the DMOS transistor U1 ON / OFF, as shown in Figures 15 and 16. In other words, it is a modified version of the switch SW with the common side being high-side.

[0053] The bipolar transistor U2 is, for example, an NPN transistor. When using an NPN transistor, the emitter is connected to ground (GND), and the collector is connected to the gate of the DMOS transistor U1 via resistor R1. The base of the NPN transistor is connected to voltage V1 via resistor R4. Furthermore, a resistor R5 is provided between the emitter and base of the NPN transistor. With this configuration, as shown in Figure 15, simply connecting the inkjet head 100 and the control device 6 causes an intermediate voltage, obtained by dividing voltage V1 by the resistance ratio of resistors R4 and R5, to be applied to the base, turning the NPN transistor ON. As a result, the DMOS transistor U1 can be kept ON automatically when the drive connection is made, and always while the control device 6 is connected. Consequently, the circuit becomes simple, inexpensive, and compact. The ink ejection operation, etc., is the same as in the fourth embodiment.

[0054] On the other hand, when polarization is performed, the control device 6 is disconnected from the external connection terminal 61, as shown in Figure 16. When the control device 6 is disconnected, the voltage V1 applied to the NPN transistor is removed, and the NPN transistor turns OFF, so the DMOS transistor U1 automatically turns OFF. The polarization device 63 is connected to the polarization input terminal 66. The operation during polarization is the same as in the fourth embodiment.

[0055] (Seventh Embodiment) Next, the inkjet head 100 of the seventh embodiment will be described. The inkjet head 100 of the seventh embodiment has the same configuration as the inkjet head 100 of the first embodiment, except that the physical switch SW for turning it ON / OFF is omitted, as shown in Figures 17 and 18. In other words, it is a modified version of the switch SW with the common side being the low side.

[0056] As shown in Figure 17, the switch SW that was provided inside the inkjet head 100 in the first embodiment is omitted, and a connection terminal that allows one end of resistor R1 to be connected to the outside is provided at the external connection terminal 61. The control device 6 is wired so that a power supply of voltage V1 can be connected to this connection terminal. With this configuration, the DMOS transistor U1 can be kept ON automatically simply by connecting the inkjet head 100 and the control device 6, and while the control device 6 is connected, it can be kept ON at all times. The ink ejection operation, etc., is the same as in the first embodiment.

[0057] On the other hand, when polarization is performed, the control device 6 is disconnected from the external connection terminal 61, as shown in Figure 18. When the control device 6 is disconnected, the DMOS transistor U1 automatically turns OFF. The polarization device 63 is connected to the external connection terminal 61. The operation during polarization is the same as in the fourth embodiment.

[0058] (Eighth embodiment) Next, the inkjet head 100 of the eighth embodiment will be described. The inkjet head 100 of the eighth embodiment has the same configuration as the inkjet head 100 of the second embodiment, except that the physical switch SW for turning it ON / OFF is omitted, as shown in Figures 19 and 20. In other words, it is a modified version of the switch SW with the common side being high-side.

[0059] As shown in Figure 19, the switch SW that was provided inside the inkjet head 100 in the second embodiment is omitted, and a connection terminal that allows one end of resistor R1 to be connected to the outside is provided at the external connection terminal 61. The control device 6 is wired so that ground (GND) can be connected to this connection terminal. With this configuration, the DMOS transistor U1 can be automatically turned ON simply by connecting the inkjet head 100 and the control device 6, and can be kept ON at all times while the control device 6 is connected. The ink ejection operation, etc., is the same as in the second embodiment.

[0060] On the other hand, when polarization is performed, the control device 6 is disconnected from the external connection terminal 61, as shown in Figure 20. When the control device 6 is disconnected, the DMOS transistor U1 automatically turns OFF. The polarization device 63 is connected to the external connection terminal 61. The operation during polarization is the same as in the second embodiment.

[0061] The inkjet head 100 of the first to eighth embodiments described above can transition from a state in which it is driven by applying drive voltages V1 and V2 to the piezoelectric actuator 5 to a polarized state by setting V1=V2=0 and applying a polarization voltage from the polarization device 63. Figure 21 shows an example of the simulation results.

[0062] As described above, according to any of the embodiments described, the insertion loss caused by the switching circuit connecting the piezoelectric actuator 5 to the polarization device 63 can be suppressed from affecting the ink ejection, which is the normal operation of the inkjet head 100. As a result, stable ink ejection is possible.

[0063] Furthermore, the piezoelectric actuator 5 is not limited to a laminated type in which multiple piezoelectric elements 51 are stacked. It may also be a piezoelectric actuator with a single layer of piezoelectric elements 51. In addition, the operation of the actuator when a driving voltage is applied is not limited to longitudinal vibration. Moreover, it may be applied not only to the drop-on-demand piezoelectric method but also to the continuous method.

[0064] In the above-described embodiment, the inkjet head 100 of the inkjet printer 10 was described as an example of a liquid ejection device, but the liquid ejection device may also be the material ejection head of a 3D printer or the sample ejection head of a dispensing device.

[0065] 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 carried out 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 variations are included in the scope and spirit of the invention, as well as in the claims and their equivalents. [Explanation of Symbols]

[0066] 10 Inkjet Printers 100-103 Inkjet head 2 Head section 24 nozzles 3. Drive IC 5. Piezoelectric actuator 56 Individual wiring 57 Common Wiring 63 Polarized Power Supply U1 DMOS transistor U2 Bipolar Transistor SW Switch

Claims

1. Multiple piezoelectric actuators, Multiple drive circuits that provide a drive waveform for discharging liquid to one terminal of each of the multiple piezoelectric actuators, A DMOS (Double-diffused MOSFET) transistor is formed by connecting multiple piezoelectric actuators in common, with the other terminal of the piezoelectric actuator connected to the source and a predetermined potential connected to the drain, A resistor placed between the gate and source of the aforementioned DMOS transistor, A switch for turning the gate of the DMOS transistor ON or OFF, The DMOS transistor's body diode is configured to charge the piezoelectric actuator in the forward direction, A liquid dispensing head characterized in that the switch is always turned ON to turn on the DMOS transistor, at least during the period during which the drive waveform is supplied to the piezoelectric actuator.

2. The liquid dispensing head according to claim 1, further characterized by comprising a connection terminal that allows the connection point between the other terminal of the piezoelectric actuator and the source of the DMOS transistor to be connected to an external source.

3. The other terminal of the piezoelectric actuator is the low side, The liquid dispensing head according to claim 1 or 2, characterized in that the polarity of the DMOS transistor is Nch and the switch is a PNP transistor.

4. The liquid dispensing head according to claim 3, characterized in that the predetermined potential connected to the drain of the DMOS transistor is the ground potential.

5. The other terminal of the piezoelectric actuator is the high side, The liquid dispensing head according to claim 1 or 2, characterized in that the polarity of the DMOS transistor is Pch and the switch is an NPN transistor.

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