Liquid ejection head

Abstract

To provide a liquid ejection head capable of stably ejecting liquid.SOLUTION: A liquid ejection head according to an embodiment, includes a plurality of piezoelectric actuators and a switch circuit. The switch circuit is inserted into a path of a current that flows commonly to the plurality of piezoelectric actuators. The switch circuit is a parallel circuit constituted of: a DMOS (Double-diffused MOSFET) transistor; and a diode whose forward direction is opposite to that of a body diode of the DMOS transistor.SELECTED DRAWING: Figure 6

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 ink droplets from an inkjet head to form images or the like on the surface of a recording medium. 3D printers eject modeling material droplets from a modeling material ejection head, harden them, and form three-dimensional objects. Dispensing devices eject sample droplets and supply them in predetermined amounts to multiple containers or the like.

[0003] A liquid ejection head has multiple channels for ejecting liquid. Each channel has a nozzle for ejecting liquid, a pressure chamber connected to the nozzle, and an actuator for changing the volume of the pressure chamber. The liquid ejection head selects a channel from among the multiple channels for ejecting liquid and drives the actuator by applying a drive waveform to it. When the actuator is driven, the volume of the pressure chamber filled with liquid changes, causing the liquid to be ejected from the nozzle.

[0004] After assembly, a liquid ejection head may be connected to a test circuit to check whether the actuator is operating normally. However, the voltage drop in the switch circuit that switches the current path to the test circuit can affect the drive waveform applied to the actuator, resulting in electrical crosstalk between channels. Electrical crosstalk can affect the liquid ejection characteristics. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-121689 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-55644 [Patent Document 3] JP 2016-55645 A [Patent Document 4] Japanese Patent Application Publication No. 2022-37469 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 that can eject liquid stably. [Means for solving the problem]

[0007] A liquid ejection head according to an embodiment of the present invention includes a plurality of piezoelectric actuators and a switch circuit. The switch circuit is inserted in a path of current flowing commonly through the plurality of piezoelectric actuators. The switch circuit is a parallel circuit of a DMOS transistor and a diode whose forward direction is opposite to that of the body diode of the DMOS transistor. In other words, the switch circuit is a parallel circuit of a DMOS transistor having a body diode and a diode whose forward direction is the direction in which the DMOS transistor cuts off when in an OFF state. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a diagram illustrating the overall configuration of an inkjet printer equipped with an inkjet head according to a first embodiment. [Figure 2] FIG. 2 is a perspective view of the inkjet head. [Figure 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. [Figure 5] FIG. 2 is a partially enlarged plan view of a head portion of the inkjet head. [Figure 6] FIG. 3 is a circuit diagram for inspecting the inkjet head. [Figure 7] 3A and 3B are explanatory diagrams of a drive waveform used in the above inspection and a current waveform to be detected. [Figure 8] 3A and 3B are explanatory diagrams of a drive waveform used in the above inspection and a residual vibration waveform to be detected. [Figure 9] FIG. 10 is a circuit diagram for carrying out an inspection of an inkjet head according to a second embodiment. [Figure 10] 3A and 3B are explanatory diagrams of a drive waveform used in the above inspection and a current waveform to be detected. [Figure 11] FIG. 10 is a circuit diagram for carrying out an inspection of an inkjet head according to a third embodiment. [Figure 12] This is a driving waveform applied to the piezoelectric actuator of the inkjet head. [Figure 13] 3A to 3C are diagrams illustrating the operation of the piezoelectric actuator. DETAILED DESCRIPTION OF THE INVENTION

[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) An inkjet printer 10 that prints an image on a recording medium will be described as an example of an image forming apparatus equipped with a liquid ejection head according to the first embodiment. Fig. 1 shows a schematic configuration of the inkjet printer 10. Inside a housing 11, the inkjet printer 10 has arranged therein: a cassette 12 that stores sheets S, which are an example of a recording medium; an upstream transport path 13 for the sheets S; a transport belt 14 that transports the sheets S removed from the cassette 12; a plurality of inkjet heads 100-103 that eject ink droplets toward the sheets S on the transport belt 14; a downstream transport path 15 for the sheets S; an ejection tray 16; and a control board 17. An operation unit 18 that serves as 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 by, for example, a computer 200, which is an externally connected device. The image data generated by the computer 200 is sent to the control board 17 of the inkjet printer 10 via a cable 201 and connectors 202 and 203.

[0012] A pickup roller 204 supplies 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 sheets S are fed via the upstream conveying path 13 onto the upper surface of the conveying belt 14. An arrow 104 in the figure indicates the conveying path of the sheets 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 drive roller 141 to rotate the conveyor belt 14. 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 disposed on the back side of the conveyor belt 14. The negative pressure container 206 is connected to a decompression fan 207. The fan 207 creates an airflow that creates a negative pressure inside the negative pressure container 206, causing the sheet S to be attracted and held 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 held by suction 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. As sheet S passes below inkjet heads 100-103, they print an image. Each of inkjet heads 100-103 has the same structure except for the color of ink they eject. 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 adjusters 321-324 via ink flow paths 311-314, respectively. Each ink tank 315-318 is located above the corresponding inkjet head 100-103. During standby, each ink supply pressure adjuster 321-324 adjusts the pressure inside each inkjet head 100-103 to a negative pressure, e.g., -1.2 kPa, relative to atmospheric pressure to prevent ink from leaking 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 adjusters 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 made up 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, we will explain the configuration of the inkjet heads 100 to 103. Below, we will explain the inkjet head 100 with reference to Figures 2 to 5, 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, but may be arranged in multiple rows. The detailed configuration of the head unit 2 will be described later.

[0020] The flexible printed wiring board 21 is a flexible printed wiring board made of a synthetic resin film such as polyimide. The flexible printed wiring board 21 is equipped with a driver chip, a driving integrated circuit (IC) 3 (hereinafter referred to as the driving IC). The printed circuit board 22 is a hard through-hole board made by laminating multiple layers of glass fiber-reinforced epoxy resin and copper wiring layers. The driving IC 3, which serves as the control unit for the inkjet head 100, 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 for the inkjet printer 10, and sends driving signals to each channel to eject ink at 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 is flexible enough to deform 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 are connected to the nozzles 24, respectively. For example, rectangular openings penetrating the pressure chambers 42 in the second direction, e.g., the Z direction, are formed in the pressure chamber substrate 4, and both openings are closed by the nozzle plate 23 and the diaphragm 41, respectively, to form spaces to be filled with ink. The pressure chambers 42 communicate with guide channels 43 having narrowed portions, and further communicate with an ink supply manifold 45 via ink supply ports 44, which are openings formed in the diaphragm 41. The guide channels 43 are formed in the shape of grooves in the third direction, e.g., the Y direction, on one surface of the pressure chamber substrate 4 facing the diaphragm 41, for each pressure chamber 42. The ink supply manifold 45 is formed in a frame 46 bonded to one surface of the diaphragm 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 of each channel and the guide channels 43. The ink supply manifold 45, which serves as a common ink chamber, communicates with the ink channels 311 (see FIGS. 1 and 2).

[0023] The piezoelectric actuator 5 is disposed on one side of the diaphragm 41 opposite the pressure chamber 42. The piezoelectric actuators 5 of each channel are arranged facing the pressure chamber 42 across the diaphragm 41. The piezoelectric actuators 5 and the diaphragm 41 are bonded together, for example, with an adhesive. Each piezoelectric actuator 5 is fixed by bonding its surface opposite the diaphragm 41 in the Z direction to a support member 47. As shown in FIG. 3 in particular, the piezoelectric actuator 5 is a multilayer piezoelectric actuator formed by alternately stacking piezoelectric bodies 51, such as piezo elements, first internal electrodes 52, and second internal electrodes 53. The piezoelectric bodies 51 are arranged with their polarization directions opposite to each other in the Z direction, for example, and are deformed in the d33 mode. The first internal electrodes 52 and second internal electrodes 53 are conductive films formed on the main surfaces of the piezoelectric bodies 51. The first internal electrodes 52 are each formed up to one end face of the piezoelectric actuator 5 in the Y direction and connected to a first external electrode 54 formed on this end face. 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.

[0024] The dummy layer 58 is made of the same material as the piezoelectric body 51. The dummy layer 58 does not have an internal electrode and is not subjected to an electric field, so it does not deform. The dummy layer 58 serves as a base for fixing the piezoelectric actuator 5 to the support member 47 (see FIG. 4), or 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 composed of 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 from a dummy actuator, the support 50 may be formed from a separate member.

[0025] In the case of a piezoelectric actuator 5 in which multiple piezoelectric bodies 51 are stacked, for example, a first internal electrode 52 and a second internal electrode 53 are formed on the main surfaces of each piezoelectric body 51 processed into a thin plate. The piezoelectric bodies 51 are then stacked together and fired to form a single body. A first external electrode 54 and a second external electrode 55 are then formed. The piezoelectric bodies 51 are then polarized using a polarization voltage. The piezoelectric bodies 51 are formed from 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 from a sinterable conductive material such as silver-palladium. The first external electrode 54 and the second external electrode 55 are formed from Ni, Cr, Au, or the like using a known method such as plating or sputtering.

[0026] The first external electrodes 54 of each channel are connected to the individual wiring 56 of the flexible printed wiring board 21 (see FIG. 3). The flexible printed wiring board 21 includes a substrate 26, individual wiring 56, an adhesive layer 27, and an insulating layer 28. The flexible printed wiring board 21 is disposed so that the area where the solder plating layer 29 is formed faces the first external electrodes 54, and the first external electrodes 54 and the individual wiring 56 of each channel are electrically and mechanically connected by melting the solder. Instead of solder, anisotropic conductive film (ACF), anisotropic conductive paste (ACP), non-conductive film (NCF), non-conductive paste (NCP), or the like may be used for fixing and anisotropic conductive connection in the thickness direction. Meanwhile, the second external electrodes 55 of each channel are connected to a common wiring (not shown) and, for example, connected to ground (GND) via the flexible printed wiring board 21.

[0027] 6 is a diagram showing the overall configuration of a circuit for inspecting the inkjet head 100. The inspection is performed, for example, after the inkjet head 100 is assembled or when the inkjet head 100 has been in use for a predetermined period of time, by detachably connecting an inspection jig 6 to the inkjet head 100. The inspection jig 6 includes, for example, a control device 61, a differential amplifier 62, an A / D converter 63, and a personal computer (PC) 64.

[0028] The inspection items include checking whether the piezoelectric actuator 5 functions normally, and observing the natural vibration period λ of the inkjet head 100, which is determined by the ink characteristics and the internal structure of the head. If the inkjet head 100 passes the inspection, the inspection jig 6 is removed and the inkjet head 100 is mounted in the inkjet printer 10 (see Figure 1).

[0029] As shown in Figure 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 the 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. This one terminal is referred to as an individual terminal.

[0030] The driving IC 3 is connected to a first driving power supply 31, which is the power source for driving voltage V1, and a second driving power supply 32, which is the power source for driving voltage V2. The positive poles of the first driving power supply 31 and the second driving power supply 32 are connected to the driving IC 3, and the negative poles are 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, 20 V. The driving voltage V2 is, for example, 10 V. The ground (GND) is, for example, 0 V.

[0031] The driver IC 3 is further connected to a control power supply 33, which supplies a control voltage Vdd, a signal line 34 for a control signal, and a test mode line 35. The control power supply 33 is the power source that drives the driver IC 3. The control voltage Vdd is, for example, 3.3 V. The control signal corresponds to print data sent from the control board 17 (see Figure 1) of the inkjet printer 10, and controls which channel of the piezoelectric actuator 5 is driven. The test mode operates a switch circuit 7, described below, when a test is performed.

[0032] One terminal of bypass capacitors C1 and C2 is connected to each of the wirings connecting the first drive power supply 31 to the drive IC 3. The other terminal of bypass capacitors C1 and C2 is connected to GND. Similarly, one terminal of bypass capacitors C3 and C4 is connected to each of the wirings connecting the second drive power supply 32 to the drive IC 3. The other terminal of bypass capacitors C3 and C4 is connected to ground. One terminal of bypass capacitors C5 and C6 is also connected to each of the wirings connecting the control power supply 33 to the drive IC 3. The other terminal of bypass capacitors C5 and C6 is connected to ground. The bypass capacitors C1, C3, and C5 function as power supply bypass capacitors. The capacitance of each of bypass capacitors C1, C3, and C5 is set to, for example, 10 times or more the total capacitance of the piezoelectric actuators 5 from channel 1 to channel nch. This allows the voltages V1, V2, and Vdd to be stabilized even when the piezoelectric actuators 5 from channel 1 to channel nch are driven simultaneously. In this example, the total capacitance of the piezoelectric actuators 5 from 1ch to nch is 0.3μF (=1000pF×300ch), so the capacitance of each of the bypass capacitors C1, C3, and C5 is set to, for example, 10μF. The capacitance of each of the bypass capacitors C2, C4, and C6 is set to, for example, 0.1μF.

[0033] 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. This other terminal is called the common terminal. The common wiring 57 is connected to ground (GND). In other words, a common potential of, for example, 0 V is applied to the common terminal of each piezoelectric actuator 5.

[0034] The switch circuit 7 is a parallel circuit of a DMOS transistor Q1 and a diode D1. The switch circuit 7 is provided on the common wiring 57 of the inkjet head 100. That is, the switch circuit 7 is inserted into a path of current flowing commonly to multiple piezoelectric actuators 5. In this embodiment, the path of current flowing commonly to multiple piezoelectric actuators 5 into which the switch circuit 7 is inserted is a circuit connecting the common electrode 55 of the piezoelectric actuators 5 to a power supply terminal. The DMOS transistor Q1 is, for example, an N-channel DMOS FET (Field Effect Transistor). When an N-channel DMOS transistor Q1 is used, the drain is connected to the common terminal of the piezoelectric actuator 5 and the source is connected to the negative side of the power supply terminal. The gate is connected to the test mode wiring 35 from the control device 61 and to the wiring 34 of the control voltage Vdd via a pull-up resistor R2 (for example, 10 kΩ).

[0035] The DMOS transistor Q1 has a body diode. A body diode is a diode that DMOS FETs normally have. A body diode is also called a parasitic diode. When the source and drain of the DMOS transistor Q1 are connected as shown in Figure 6, the cathode of the body diode faces the common terminal side of the piezoelectric actuator 5, and the anode faces the power supply terminal side. Diode D1, which forms a parallel circuit with the DMOS transistor Q1, is connected so that its forward direction is opposite to that of the body diode. In other words, the forward direction of diode D1 is the direction in which it cuts off when the N-channel DMOS transistor Q1 is in the OFF state.

[0036] For example, the body diode and diode D1 each have a dead band of approximately 0.6 V. Because the forward direction of the body diode and diode D1 is reversed, the switch circuit 7 itself has a dead band of approximately ±0.6 V. In Figure 6, the switch circuit 7 is configured using a parallel circuit of the DMOS transistor Q1 and diode D1, which allows the use of a DMOS transistor Q1 with a low withstand voltage, thereby reducing the ON resistance of the DMOS transistor Q1. This means that the voltage drop across the switch circuit 7 is prevented from affecting the drive waveform applied to the piezoelectric actuator 5. This prevents electrical crosstalk from occurring between channels, stabilizing the ink ejection characteristics.

[0037] An inspection resistor R1 (for example, 1 Ω) is connected in parallel with the switch circuit 7. A differential amplifier 62 and an A / D converter 63 of the inspection jig 6 detect the voltages across the resistor R1 during inspection, convert the detected voltages into digital data, and send it to the PC 64. The PC 64 uses the detected voltage values ​​to perform calculations related to each inspection item.

[0038] Next, the operation of the test circuit in Figure 6 will be described. When testing begins, the control device 61 sets the potential of the test mode wiring 35 to low (e.g., ground potential). This causes the gate and source of the DMOS transistor Q1 to have the same potential, turning the DMOS transistor Q1 OFF. Note that in the normal state after installation in the inkjet printer 10, the test mode wiring 35 is not connected to the inkjet head 100, and a control voltage Vdd is applied to the gate of the DMOS transistor via a pull-up resistor R2. Therefore, the DMOS transistor Q1 remains ON. As mentioned above, the ON resistance of the DMOS transistor Q1 is small, and the voltage drop when the charging / discharging current of the piezoelectric actuator 5 flows through the switch circuit 7 is small.

[0039] Next, the control device 61 sends a control signal to the driver IC 3 to drive the piezoelectric actuators 5 one channel at a time. The driver IC 3 operates the driver D in sequence in response to the control signal, and sequentially sends a drive waveform to the piezoelectric actuator 5 of each channel. FIG. 7(a) shows an example of a drive waveform sent to the piezoelectric actuator 5. When the drive waveform of FIG. 7(a) is sent, the piezoelectric actuator 5 is charged and discharged at the rising and falling edges of the waveform. The charge / discharge current flowing through the common wiring 57 at this time is measured using the inspection resistor R1, the differential amplifier 62 of the inspection jig 6, the A / D converter 63, and the PC 64, and is used to determine each inspection item, such as whether the piezoelectric actuator 5 is functioning normally.

[0040] Specifically, when the piezoelectric actuator 5 under test is driven, the charge / discharge current of the piezoelectric actuator 5 flowing through the common wiring 57 generates a voltage drop across resistor R1 (e.g., 1 Ω). The voltage drop across resistor R1 is proportional to the charge / discharge current unless diode D1 and the body diode of DMOS transistor Q1 are turned on. Because the forward voltages of the body diode and diode D1 are each approximately 0.6 V, the diodes will not turn on unless the charge / discharge current is less than approximately ±0.6 A. Therefore, the piezoelectric actuator 5 is driven one channel at a time to ensure that the absolute value of the charge / discharge current does not exceed approximately 0.6 V. This allows the differential amplifier 62 and A / D converter 63 to accurately acquire a waveform proportional to the charge / discharge current and send it to the PC 64.

[0041] Figure 7(b) shows an example of a waveform proportional to the acquired charge and discharge current. By integrating the discharge, charge, discharge, and charge waveforms using the PC64's calculation function, the charge transferred during each charge and discharge can be calculated. Dividing this by the change in drive voltage yields the capacitance of the piezoelectric actuator 5. For the drive waveform shown in Figure 7(a), the voltage change corresponding to each charge and discharge is 10 V. Once the capacitance is determined, it can be determined whether the piezoelectric actuator 5 is functioning properly based on whether it is within the tolerance range of the design value. Note that capacitor C7 (e.g., 0.1 μF) connected in parallel to the switch circuit 7 removes high-speed ringing due to the inductance of the common wiring 57 from the charge and discharge current waveform, reducing the peak of the current waveform and lengthening the charging time, making it easier to integrate the current waveform. Since the capacitive piezoelectric actuator 5 may resonate, capacitor C7 is provided to remove ringing from the current waveform, allowing for accurate integration of the charge during charging and discharging.

[0042] The test circuit in Figure 6 can also be used to observe pressure vibrations in the inkjet head 100, for example, by filling it with test ink. However, because the impact of residual vibrations on the drive current is smaller than the charge / discharge current described above, it is difficult to detect by driving only one channel. Therefore, measurements are taken by simultaneously driving the piezoelectric actuators 5 of multiple channels. Figure 8(a) shows an example of a drive waveform applied to simultaneously driven piezoelectric actuators 5. When the drive waveform shown in Figure 8(a) is applied, the piezoelectric actuators 5 are charged at the rising edge of the waveform. At this time, as shown in Figure 8(b), the charging current i_COM flowing through the common wiring 57 is measured using the test resistor R1, the differential amplifier 62 of the test jig 6, the A / D converter 63, and the PC 64.

[0043] However, when piezoelectric actuators 5 of multiple channels are driven simultaneously, the charging current i_COM flowing through the common wiring 57 is a multiple of the number of simultaneously driven channels. When observing small pressure vibrations, a large charging / discharging current would interfere with the inspection, but the diode D1 and the body diode of the DMOS transistor, which are oriented in opposite directions, act as clamp diodes, and their dead zone makes it possible to clamp a voltage waveform equivalent to a current exceeding approximately ±0.6 A, as shown in Figure 8(c).

[0044] Then, by using the calculation function of the PC64 to extract the vibration period from the detected residual vibration waveform, the natural vibration period λ of the inkjet head 100 can be determined. The natural vibration period λ of the inkjet head 100 is also useful for adjusting the pulse width of the drive waveform applied to the piezoelectric actuator 5 when ejecting ink. In addition, the state (for example, rigidity) of the pressure chamber 42 can be detected from the amplitude and damping rate of the residual vibration waveform. Furthermore, by observing the change in residual vibration when the drive waveform is changed, the state of the pressure chamber 42 when that drive waveform is applied can be known.

[0045] (Second embodiment) Next, an inkjet head 100 according to a second embodiment will be described. The inkjet head 100 of the second embodiment has the same configuration as the first embodiment, except that the configuration of the inspection circuit is changed, as shown in Fig. 9. Therefore, a detailed description of the configuration that is the same as the first embodiment will be omitted.

[0046] As shown in FIG. 9 , the switch circuit 7 of this embodiment is disposed midway along the wiring for the drive voltage V2. While the switch circuit 7 was disposed on the ground (GND) side in the first embodiment, the switch circuit 7 is disposed on the power supply side in this embodiment. That is, in this embodiment, the path of the current common to the multiple piezoelectric actuators into which the switch circuit 7 is inserted is a circuit connecting a terminal that applies a drive voltage to a drive circuit that drives the multiple piezoelectric actuators to a power supply terminal for that drive voltage. When the switch circuit 7 is disposed on the power supply side, the power supply is positive in this example, so the DMOS transistor Q2 is, for example, a P-channel DMOS FET (Field Effect Transistor). The drain of the P-channel DMOS transistor Q2 is connected to the individual terminal of the piezoelectric actuator 5, and the source is connected to the positive terminal of the drive power supply 32. A voltage-dividing resistor R2 (e.g., 10 kΩ), a resistor R3 (e.g., 22 kΩ), and a transistor Q3 are connected in series to the wiring connecting the drive voltage V2 to ground (GND), and the gate of the DMOS transistor Q2 is connected to the junction between the resistors R2 and R3. The transistor Q3 is a switch that turns the DMOS transistor Q2 ON / OFF. The base of the transistor Q3 is connected to the test mode line 35 from the control device 61 and to the line of the control voltage Vdd via a pull-up resistor R4 (e.g., 10 kΩ).

[0047] The P-channel DMOS transistor Q2 has a body diode. As mentioned above, a body diode is a diode that DMOS FETs normally have. When the source and drain of the DMOS transistor Q2 are connected as shown in Figure 9, the cathode of the body diode faces the power supply terminal and the anode faces the individual terminal of the piezoelectric actuator 5. Diode D1 is connected so that its direction is opposite to that of the body diode. In other words, the forward direction of diode D1 is the direction in which the P-channel DMOS transistor Q2 cuts off when it is in OFF operation. As mentioned above, diode D1 is a component of switch circuit 7, and forms a parallel circuit with DMOS transistor Q2.

[0048] For example, the body diode and diode D1 each have a dead band of approximately 0.6 V. Because the forward directions of the body diode and diode D1 are reversed, the switch circuit 7 itself has a dead band of approximately ±0.6 V. As described above, configuring the switch circuit 7 with a parallel circuit of the DMOS transistor Q2 and diode D1 allows the use of a DMOS transistor Q2 with a low breakdown voltage. As a result, the ON resistance of the DMOS transistor Q2 is reduced.

[0049] A test resistor R1 (e.g., 1 Ω) is connected in parallel with the switch circuit 7. In this embodiment, the test resistor R1 is located in the control device 61 of the testing jig 6. A differential amplifier 62 and an A / D converter 63 detect the voltage across the resistor R1 during testing, convert the detected voltage into digital data, and send it to the PC 64. The PC 64 uses the detected voltage values ​​to perform calculations related to each test item. The body diode of the DMOS transistor Q2 and the diode D1 are connected in parallel to the resistor R1 with their forward directions reversed, so that they function as a clamp circuit with a dead band of approximately ±0.6 V (see FIG. 8). If there is a risk that the cable connecting the inkjet head 100 and the testing jig 6 will be long, diodes D2 and D3 may also be connected in parallel with the resistor R1 on the testing jig 6 side (see FIG. 11).

[0050] The bypass capacitor C3 is set to, for example, 10 times or more the total capacitance of the piezoelectric actuators 5 from channel 1 to channel 2 (e.g., 0.3 μF = 1000 pF × 300 ch) to stabilize the drive voltage V2. However, since this bypass capacitor C3 gets in the way during testing, it is placed on the source side of the DMOS transistor Q2. After installation in the inkjet printer 10, when ink is ejected, the DMOS transistor Q2 is turned on to activate the bypass capacitor C3 and stabilize the drive voltage V2. Similar to capacitor C7 in the first embodiment, bypass capacitor C4 (e.g., 0.1 μF) removes high-frequency vibrations such as ringing from the detected waveform.

[0051] Next, the operation of the test circuit in Figure 9 will be described. When testing begins, the control device 61 sets the potential of the test mode wiring 35 to low (e.g., ground potential). This causes the base and emitter of transistor Q3 to have the same potential, turning transistor Q3 OFF. When transistor Q3 turns OFF, DMOS transistor Q2 also turns OFF. Note that in the normal state after installation in the inkjet printer 10, the test mode wiring 35 is not connected to the inkjet head 100, and a control voltage Vdd is applied to the base of transistor Q3 via pull-up resistor R4. Therefore, transistor Q3 remains ON. When transistor Q3 remains ON, a voltage determined by the resistance ratio of voltage-dividing resistors R2 and R3 is applied to the gate of DMOS transistor Q2, causing DMOS transistor Q2 to also remain ON. As mentioned above, the ON resistance of DMOS transistor Q2 is low, resulting in a small voltage drop when charging / discharging current flows through piezoelectric actuator 5.

[0052] Next, the control device 61 sends a control signal to the driver IC 3 to drive the piezoelectric actuators 5 one channel at a time. The driver IC 3 sequentially operates the driver D in response to the control signal and sequentially sends drive waveforms to the piezoelectric actuators 5 of each channel. Figure 10(a) shows an example of a drive waveform sent to the piezoelectric actuator 5. When the drive waveform of Figure 10(a) is sent, the piezoelectric actuator 5 charges and discharges at the rising and falling edges of the waveform. At this time, the charge / discharge current flowing through the common wiring 57 is as shown in Figure 10(b). However, because the inspection resistor R1 is connected to the wiring for the drive voltage V2, charge / discharge waveforms not involving the drive voltage V2 cannot be detected. In other words, the waveform acquired using the differential amplifier 62, A / D converter 63, and PC 64 of the inspection jig 6 is as shown in Figure 10(c). As such, this embodiment can detect charge / discharge waveforms involving the drive voltage V2. Furthermore, this embodiment also allows for the observation of residual vibration waveforms. The method for detecting residual vibration is the same as in the first embodiment.

[0053] In this embodiment, the common wiring 57 is not essential, and therefore the present invention is applicable not only to the inkjet head 100 shown in FIGS. 2 to 5, but also to the inspection of a share-mode shared-wall type inkjet head.

[0054] (Third embodiment) Next, an inkjet head 100 according to a third embodiment will be described. The inkjet head 100 according to the third embodiment has the same configuration as the first and second embodiments, except that the configuration of the inspection circuit is changed, as shown in Fig. 11. Therefore, detailed description of the configuration that is the same as the first and second embodiments will be omitted.

[0055] As shown in FIG. 11, the switch circuit 7 of this embodiment is disposed on the ground (GND) side of the bypass capacitor C3. That is, in this embodiment, the path of current flowing commonly to the multiple piezoelectric actuators 5 into which the switch circuit 7 is inserted is a circuit that supplies charge / discharge current from the bypass capacitor C3 to a drive circuit that drives the multiple piezoelectric actuators. When the switch circuit 7 is disposed on the ground (GND) side of the bypass capacitor C3, the DMOS transistor Q1 is, for example, an N-channel DMOS FET (Field Effect Transistor). The drain of the N-channel DMOS transistor Q1 is connected to a terminal of the bypass capacitor C3, and the source is connected to ground (GND). The gate is connected to the test mode line 35 from the control device 61 and to the line of the control voltage Vdd via a pull-up resistor R2 (e.g., 10 kΩ).

[0056] As mentioned above, the DMOS transistor Q1 has a body diode. When the source and drain of the DMOS transistor Q1 are connected as shown in Figure 11, the cathode of the body diode faces the bypass capacitor C3 and the anode faces ground (GND). The diode D1, which forms a parallel circuit with the DMOS transistor Q1, is connected so that its forward direction is opposite to that of the body diode. As mentioned above, by configuring the switch circuit 7 with the parallel circuit of the DMOS transistor Q1 and diode D1, the voltage drop in the circuit that supplies charge and discharge current from the bypass capacitor C3 is small.

[0057] A test resistor R1 (e.g., 1 Ω) is placed in the control device 61 of the test jig 6. A differential amplifier 62 and an A / D converter 63 detect the voltage across the resistor R1 during testing, convert the detected voltage into digital data, and send it to the PC 64. The PC 64 uses the detected voltage values ​​to perform calculations related to each test item. Diodes D2 and D3 are connected in parallel with the resistor R1. The diodes D2 and D3 are placed so that their forward directions are opposite to each other. For example, the diodes D2 and D3 each have a dead band of approximately 0.6 V. The diodes D2 and D3 are clamp diodes placed to measure residual vibration (see Figure 8).

[0058] The current waveform and detected voltage in the inspection mode in this embodiment are the same as those in the second embodiment (see FIG. 10). That is, since the inspection resistor R1 is provided in the wiring for the drive voltage V2, it is not possible to detect charge / discharge waveforms that do not involve the drive voltage V2, but it is possible to detect charge / discharge waveforms that involve the drive voltage V2 (FIG. 10(c)). Furthermore, it is possible to observe residual vibration waveforms. The advantage of this embodiment is that there are fewer components on the inkjet head 100 side than in the other embodiments, and the number of wires connecting the inkjet head 100 and the control device 61 is reduced.

[0059] After installation in the inkjet printer 10, the DMOS transistor Q1 is turned ON when ink is to be ejected. When the DMOS transistor Q1 is turned ON, the bypass capacitor C3 acts as a power supply bypass capacitor, stabilizing the drive voltage. On the other hand, in the test mode, the DMOS transistor Q1 is turned OFF. When the DMOS transistor Q1 is turned OFF, the ground side of the bypass capacitor C3 is connected to ground (Gnd) via a parallel circuit consisting of the diode D1 and the body diode of the DMOS transistor Q1, providing a dead band of the diode forward voltage, i.e., approximately ±0.6 V. Measurements of capacitance and residual vibration are performed within this dead band. This is the same as in the first and second embodiments.

[0060] As in the second embodiment, the common wiring 57 is not essential in this embodiment, and therefore it can be used to inspect not only the inkjet head 100 of FIGS. 2 to 5 but also inkjet heads of a shared mode shared wall type.

[0061] As previously mentioned with respect to the first through third embodiments, the ON resistance of the DMOS transistors Q1 and Q2 in the switch circuit 7 can be made sufficiently small. An ON resistance of 200 mΩ or less is desirable. Otherwise, the drive voltage waveform will vary depending on the number of simultaneously driven channels, resulting in electrical crosstalk, which can lead to variations in print density and reduced print quality. In all of the first through third embodiments, diode D1 is provided in parallel with the DMOS transistors Q1 and Q2, controlling their conduction and cutoff. This ensures that the drain-source voltage only rises to the forward voltage of diode D1. This allows the use of DMOS transistors Q1 and Q2 with low breakdown voltages. Because low-voltage DMOS transistors Q1 and Q2 have low ON resistance even when small, employing the switch circuit 7 illustrated in the first through third embodiments reduces the ON resistance of the switch circuit 7, enabling the provision of a compact inkjet head 100 with excellent print quality.

[0062] As described above, according to any of the above-mentioned embodiments, the switch circuit 7 inserted in the path of the current flowing in common to the plurality of piezoelectric actuators 5 is configured as a parallel circuit of DMOS transistors Q1 / Q2 having body diodes and a diode D1 whose forward direction is the direction in which the DMOS transistors Q1 / Q2 are cut off when they are in an OFF state, thereby making it possible to provide an inkjet head 100 that can stably eject ink.

[0063] The piezoelectric actuator 5 is not limited to a laminated type in which multiple piezoelectric bodies 51 are stacked. 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 system, and may be applied to a continuous system. As explained in the second and third embodiments, the actuator can also be applied to a share-mode shared-wall type inkjet head.

[0064] Next, the ink ejection operation of the inkjet head 100 will be described with reference to Figures 12 and 13. The ink ejection operation is the same in the first to third embodiments. Each driver D of the driver IC 3 applies a drive waveform to the individual electrode 54 of the piezoelectric actuator 5 using drive voltages V1, V2 and ground (GND). The voltage V1 is, for example, 20V. The voltage V2 is, for example, 10V. The ground (GND) is, for example, 0V. Which channel's piezoelectric actuator 5 is driven is based on, for example, print data. Figure 12 shows an example of a drive waveform applied to the piezoelectric actuator 5.

[0065] As shown in FIG. 12, when the piezoelectric actuator 5 is driven with a ground potential applied to the common electrode 55, a voltage V2 is applied to the individual electrode 54 to place the piezoelectric actuator 5 in a standby state. When the voltage V2 is applied, an electric field is applied in the direction of the polarization axis of the piezoelectric body 51, and as shown in FIG. 13(a), the piezoelectric actuator 5 expands in the stacking direction (Z direction), reducing the volume of the pressure chamber 42. This is performed prior to the timing of ink ejection. Thereafter, by first lowering the potential of the individual electrode 54 to ground (GND) at the timing of ink ejection (time t1 in FIG. 12), the expanded piezoelectric actuator 5 returns to its original state, i.e., contracts relatively, and the volume of the pressure chamber 42 expands relatively, as shown in FIG. 13(b). Ink flows into the pressure chamber 42 via the guide channel 43 by the amount of expansion of the volume of the pressure chamber 42.

[0066] Then, for example, after half the pressure vibration period of the head unit 2 has elapsed, if a voltage V2 is applied to the individual electrode 54 at time t2 in FIG. 12, the piezoelectric actuator 5 expands in the stacking direction (Z direction) as shown in FIG. 13(c), causing the volume of the pressure chamber 42 to relatively shrink, resulting in the ejection of an ink droplet R from the nozzle 24. Then, for example, after half the pressure vibration period of the head unit 2 has elapsed, if a voltage V1 is applied to the individual electrode 54 at time t3 in FIG. 12, then returned to voltage V2 a predetermined time later at time t4. The volume of the pressure chamber 42 shrinks and then returns to its original state as the piezoelectric actuator 5 expands (FIG. 13(d)) and returns (FIG. 13(a)) at this 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, allowing ink to be ejected.

[0067] 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.

[0068] The embodiments of the present invention are presented as examples and are not intended to limit the scope of the invention. These novel embodiments may be embodied in various other forms, and various omissions, substitutions, and modifications may be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, and are also included in the scope of the inventions and their equivalents as defined in the claims. [Explanation of symbols]

[0069] 10. Inkjet printer 100~103 Inkjet head 5 Piezoelectric Actuators 7 Switch Circuit Q1 DMOS transistor (N-channel) Q2 DMOS transistor (P-channel) D1 Diode C1~C6 bypass capacitors

Claims

1. a plurality of piezoelectric actuators; a switch circuit inserted in a path of a current that flows in common to the plurality of piezoelectric actuators; The liquid ejection head is characterized in that the switch circuit is a parallel circuit of a DMOS (Double-diffused MOSFET) transistor and a diode whose forward direction is opposite to that of the body diode of the DMOS transistor.

2. 2. The liquid ejection head according to claim 1, wherein the path of the current flowing in common to the plurality of piezoelectric actuators is a circuit connecting a common electrode of the piezoelectric actuators to a power supply terminal.

3. A liquid ejection head as described in claim 1, characterized in that the path of current flowing commonly to the multiple piezoelectric actuators is a circuit connecting a terminal that applies a drive voltage to a drive circuit that drives the multiple piezoelectric actuators and a power supply terminal for that drive voltage.

4. The liquid ejection head according to claim 1, characterized in that the path of current flowing commonly to the plurality of piezoelectric actuators is a circuit that connects a power supply terminal that provides a drive voltage to a drive circuit that drives the plurality of piezoelectric actuators to another terminal of the power supply via a capacitor.

5. 5. The liquid ejection head according to claim 4, wherein the capacitance of the capacitor is 10 times or more the total capacitance of the piezoelectric actuators that are driven simultaneously.

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