JP5471245B2 - Piezoelectric actuator drive device and liquid ejection device - Google Patents

Description

The present invention relates to a driving device that drives a piezoelectric actuator and a liquid ejection device .

  Conventionally, piezoelectric actuators that drive an object using piezoelectric deformation (also referred to as piezoelectric strain) of a piezoelectric layer have been widely used in various technical fields. Among them, Japanese Patent Application Laid-Open No. 2004-151867, which is the same application as the present application, discloses a piezoelectric actuator for an ink jet head.

  The piezoelectric actuator of Patent Document 1 is provided in a flow path unit having a plurality of pressure chambers that communicate with a plurality of nozzles, respectively, and applies pressure to ink in each pressure chamber to cause ink droplets to be ejected from the nozzles. It is to be injected. More specifically, the piezoelectric actuator of Patent Document 1 includes a piezoelectric layer disposed so as to cover a plurality of pressure chambers of a flow path unit, and two types of electrodes (a plurality of electrodes provided respectively on both surfaces of the piezoelectric layer). Individual electrode and common electrode). The plurality of individual electrodes are provided so as to face the plurality of pressure chambers, respectively, and the common electrode faces the plurality of individual electrodes in common with the piezoelectric layer interposed therebetween. When a voltage is applied between the individual electrode and the common electrode from the driving device (driver IC), piezoelectrics are applied to a plurality of piezoelectric layer portions (hereinafter referred to as active portions) sandwiched between the plurality of individual electrodes and the common electrode. Due to the deformation, pressure is applied to the ink in the pressure chamber.

  By the way, although not described in detail in Patent Document 1, the driving device (driver IC) for driving the conventional piezoelectric actuator connects two types of electrodes sandwiching each active portion to a power source of a constant voltage. Thus, a potential difference is generated between the electrodes. However, as a result of studies by the present inventors, it has been found that the following problems occur in the above-described conventional configuration.

  Each of the plurality of active portions of the piezoelectric layer stores a charge when a voltage is applied between the two types of electrodes (charge), and releases a stored charge when the voltage application between the two types of electrodes is canceled ( It acts as a capacitor having a certain capacitance. When the plurality of active portions are connected to a power source having a constant voltage, each of the plurality of active portions is instantaneously fully charged by a charging current supplied from the power source. At this time, the voltages of the plurality of active portions are all constant.

  By the way, the capacitance value often varies among a plurality of active portions. Factors that can be considered include variations in the electrode area, variations in the thickness of the active portion, variations in stress generated in the manufacturing process, variations in piezoelectric constants due to in-plane non-uniformity of the piezoelectric material, and the like. As described above, when the capacitance varies among the plurality of active portions, if the same voltage is applied to each of the plurality of active portions, the deformation amount of the active portion varies depending on the capacitance. For example, the amount of deformation is large in the active portion having a small thickness (large capacitance), and the amount of deformation is small in the active portion having a large thickness (small capacitance). In the case of the piezoelectric actuator for an ink jet head, when the amount of deformation varies between the plurality of active portions, the pressure applied to the ink varies among the plurality of pressure chambers, and as a result, the ink is ejected from the plurality of nozzles. The speed of the droplets will be different.

  Therefore, the inventors of the present application are studying a driving method of the piezoelectric actuator in which the deformation amount of the active portion is equal even if the capacitance varies among the plurality of active portions. In order to make the deformation amounts of the plurality of active portions equal, it is only necessary to decrease the applied voltage for active portions having a large capacitance and increase the applied voltage for active portions having a small capacitance. Here, it is well known that the relationship of Q = CV is established among the capacitance (C) of the capacitor, the applied voltage (V), and the stored charge amount (Q). Therefore, in order to obtain V (small) when C (large) and V (large) when C (small), the charge amount (Q) should be constant in a plurality of active portions. . Specifically, the charging current to the active part is kept constant using a constant current source, and the charging time is controlled to be equal so that the amount of charge stored in each of the active parts is the same.

  In this regard, Patent Document 2 includes a charge-side constant current circuit that maintains a constant charge current to the piezoelectric element, and a discharge-side constant current circuit that maintains a discharge current from the piezoelectric element constant. A drive circuit for a piezoelectric actuator for an inkjet head is disclosed.

JP 2006-256317 A JP 2001-150666 A

  As described above, when the charge amount is controlled to be constant in a plurality of active portions, it is assumed that a constant charging current is supplied from the constant current source to the active portion. Here, in order to continue to send a constant charging current from the constant current source, from the characteristics of the transistors constituting the constant current source, between the upstream side (power source side) and the downstream side (active part side) of the constant current source. It is necessary to ensure a potential difference of a certain level or more.

  However, when a charging current is supplied from the constant current source and charging of the active portion proceeds, and the voltage of the active portion increases as the amount of charge increases, the potential difference of the constant current source decreases. In particular, when the active part is charged up to its maximum charge amount (maximum charge amount that can be stored) (hereinafter also referred to as full charge), the potential difference of the constant current source at the end of charging is very small. As a result, the charging current becomes almost zero.

  Here, when the maximum charge amount Qmax stored in each active part when fully charged is the capacitance Cn of each active part and the maximum applied voltage Vmax (= power supply voltage connected to a constant current source), Qmax = Cn × Vmax, and when the capacitance Cn varies among the plurality of active portions, the maximum charge amount Qmax also differs. Therefore, for all active parts, when the stored charge amount is controlled to be equal to the predetermined charge amount, in the active part having a large capacitance, the predetermined charge amount is the maximum charge of the active part. Even if the amount is less than the amount, the predetermined charge amount may become the maximum charge amount in another active part having a small electrostatic capacity. At this time, in some active parts charged to the maximum charge amount, the charging current at the end of charging becomes smaller than in other active parts, resulting in a problem that the charging time becomes considerably long. Alternatively, even if a predetermined charging time set on the assumption that a constant charging current is maintained, there is a problem that the actual charge amount does not reach the desired charge amount. Patent Document 2 discloses a drive circuit that charges a piezoelectric element (active part) with a constant charging current, but does not describe any of the above-described problems during charging and the means for solving the problem.

An object of the present invention is to provide a driving device for a piezoelectric actuator and a liquid ejection device capable of suppressing a decrease in charging current at the end of charging.

A drive device for a piezoelectric actuator according to a first aspect of the present invention includes a piezoelectric layer, a plurality of first electrodes and a plurality of second electrodes provided on the piezoelectric layer, and the plurality of first electrodes and the plurality of second electrodes. A drive device for driving a piezoelectric actuator, comprising a plurality of active portions composed of a piezoelectric layer portion sandwiched between and acting as a capacitor,
A charge-side constant current source connected to the plurality of active units, a discharge-side constant current source connected to the plurality of active units, and connection / cutoff of the charge-side constant current source and the plurality of active units. A plurality of charge switches that respectively switch, a plurality of discharge switches that respectively switch connection / disconnection of the discharge-side constant current source and the plurality of active parts, and a charge amount of the plurality of active parts that are charged and discharged are all equal. A charge / discharge control circuit for determining a switching timing of the charge switch and the discharge switch, and controlling a charge time and a discharge time of each active part,
The charge / discharge control circuit is configured so that the amount of charge charged in each active portion is less than the maximum amount of charge that can be stored in the active portion , and the potential difference of the charge-side constant current source is greater than or equal to a predetermined value. The charging time is controlled so that the charging of each active part is finished during a certain period .

According to the present invention, the charge / discharge control circuit determines the switching timing of the charge switch and the discharge switch so that the amount of charge charged in each active portion is less than the maximum amount of charge that can be stored in each active portion. The charging time for each active part is controlled. According to this, since the maximum charge amount is not charged in all the active parts, the voltage of the active part is low even at the end of charging, so that the potential difference of the charging side constant current source is ensured above a certain level. , A decrease in charging current can be suppressed.
According to a second aspect of the present invention, there is provided the piezoelectric actuator driving apparatus according to the first aspect, wherein the charge / discharge control circuit is charged in all active portions based on the capacitances of the plurality of active portions. The charging time is controlled so that is less than the maximum charge amount.

According to a third aspect of the present invention, there is provided the piezoelectric actuator driving apparatus according to the first or second aspect , wherein the discharge-side constant current source is composed of a depletion type FET.

If the next charge is performed without the charge charged in each active part being completely discharged, the amount of charge stored in each active part is controlled to a predetermined amount, and the voltage is kept constant among the plurality of active parts. Therefore, at the time of discharging, it is necessary to completely discharge until the charged amount of the active part becomes zero. However, at the end of the discharge where the amount of charge remaining in the active part is considerably small, the potential difference of the discharge-side constant current source becomes small, and the discharge current becomes very small. Then, there arises a problem that the discharge time is extended for a long time or the discharge cannot be completed completely. Therefore, in the present invention, by setting the FET constituting the discharge side constant current source to the depletion type, the discharge current can be ensured at a predetermined level or more even at the end of the discharge where the potential difference of the discharge side constant current source becomes low.
A liquid ejection apparatus according to a fourth aspect of the present invention includes a flow path unit in which a flow path including a plurality of nozzles is formed, a piezoelectric layer, a plurality of first electrodes and a plurality of second electrodes provided on the piezoelectric layer. , Comprising a plurality of active portions which are composed of piezoelectric layers sandwiched between the plurality of first electrodes and the plurality of second electrodes and act as capacitors, and each of the plurality of active portions discharges liquid from the plurality of nozzles. A piezoelectric actuator to be charged, a charge-side constant current source connected to the plurality of active portions, a discharge-side constant current source similarly connected to the plurality of active portions, the charge-side constant current source, and the plurality of active portions A plurality of charge switches that respectively switch connection / disconnection of the battery, a plurality of discharge switches that respectively switch connection / disconnection of the discharge-side constant current source and the plurality of active parts, and a charge amount that is charged / discharged by the plurality of active parts But A charge / discharge time determination unit for determining a charge time and a discharge time of each active part, and based on the charge time and the discharge time determined by the charge / discharge time determination unit, A charge / discharge control circuit that determines the switching timing of the discharge switch and controls charging and discharging of each active unit;
The charge / discharge time determining unit is configured so that the amount of charge charged in each active unit is less than the maximum amount of charge that can be stored in the active unit, and the potential difference of the charge-side constant current source is equal to or greater than a predetermined value. The charging time is determined so that charging of each active part is terminated during the period.
According to a fifth aspect of the present invention, the liquid ejection apparatus according to the fourth aspect further includes a storage unit that stores capacitance data of each of the plurality of active units, and the charge / discharge time determination unit is included in the storage unit. The charging time is determined based on the stored capacitance data of the plurality of active portions so that the charge amount to be charged is less than the maximum charge amount in all active portions. It is.

  According to the present invention, since the maximum charge amount is not charged in all active parts, the voltage of the active part is low even at the end of charging, so that the potential difference of the charging-side constant current source is ensured above a certain level, and the charging current Can be prevented from decreasing.

1 is a plan view schematically showing an ink jet printer according to an embodiment. It is a top view of an inkjet head. FIG. 3 is a partially enlarged view of FIG. 2. FIG. 4 is a sectional view taken along line IV-IV in FIG. 3. It is a circuit diagram of a driver IC. It is a figure which shows the characteristic of the voltage (Vds) -current (Ids) of general FET. It is a figure which shows the characteristic of the voltage (Vgs) -current (Ids) of a depletion type FET. FIG. 8 is a diagram comparing the relationship between a drain-source voltage Vds and a drain-source current Ids of a depletion type FET having the characteristics of FIG. It is a block diagram which shows the control system of an inkjet printer.

  Next, an embodiment of the present invention will be described. This embodiment is an example in which the present invention is applied to an inkjet printer including an inkjet head that ejects ink droplets onto a recording sheet.

  First, a schematic configuration of the inkjet printer 1 of the present embodiment will be described. FIG. 1 is a schematic plan view of the ink jet printer of the present embodiment. As shown in FIG. 1, a printer 1 includes a carriage 2 configured to be reciprocally movable along a predetermined scanning direction (left-right direction in FIG. 1), an inkjet head 3 mounted on the carriage 2, and a recording. A transport mechanism 4 that transports the paper P in a transport direction orthogonal to the scanning direction is provided.

  The carriage 2 is configured to be able to reciprocate along two guide shafts 17 extending in parallel with the scanning direction (left-right direction in FIG. 1). An endless belt 18 is connected to the carriage 2. When the endless belt 18 is driven to travel by the carriage drive motor 19, the carriage 2 moves in the scanning direction as the endless belt 18 travels. It has become. The printer 1 is provided with a linear encoder 10 having a large number of light transmitting portions (slits) arranged at intervals in the scanning direction. On the other hand, the carriage 2 is provided with a transmissive photosensor 11 having a light emitting element and a light receiving element. The printer 1 can recognize the current position in the scanning direction of the carriage 2 from the count value (number of detections) of the light transmitting portion of the linear encoder 10 detected by the photosensor 11 while the carriage 2 is moving. .

  An ink jet head 3 is mounted on the carriage 2. The ink-jet head 3 includes a large number of nozzles 30 (see FIGS. 2 to 4) on the lower surface (the surface on the opposite side of FIG. 1). The inkjet head 3 is configured to eject ink supplied from an ink cartridge (not shown) from a large number of nozzles 30 onto a recording paper P that is conveyed downward (conveying direction) in FIG. ing.

  The transport mechanism 4 includes a paper feed roller 12 disposed on the upstream side in the transport direction with respect to the ink jet head 3 and a paper discharge roller 13 disposed on the downstream side in the transport direction with respect to the ink jet head 3. The paper feed roller 12 and the paper discharge roller 13 are rotationally driven by a paper feed motor 14 and a paper discharge motor 15, respectively. The transport mechanism 4 transports the recording paper P from above in FIG. 1 to the ink jet head 3 by the paper feed roller 12, and records the images, characters, etc. recorded by the ink jet head 3 by the paper discharge roller 13. The paper P is discharged downward in FIG.

  Next, the inkjet head 3 will be described. 2 is a plan view of the inkjet head, FIG. 3 is a partially enlarged view of FIG. 2, and FIG. 4 is a sectional view taken along line IV-IV of FIG. As shown in FIGS. 2 to 4, the inkjet head 3 includes a flow path unit 6 in which an ink flow path including a nozzle 30 and a pressure chamber 24 is formed, and a piezoelectric actuator 7 that applies pressure to the ink in the pressure chamber 24. And.

  First, the flow path unit 6 will be described. As shown in FIG. 4, the flow path unit 6 includes a cavity plate 20, a base plate 21, a manifold plate 22, and a nozzle plate 23, and these four plates 20 to 23 are joined in a laminated state. Among these, the cavity plate 20, the base plate 21, and the manifold plate 22 are substantially rectangular plates in plan view made of a metal material such as stainless steel. Therefore, ink flow paths such as a manifold 27 and a pressure chamber 24 described later can be easily formed on these three plates 20 to 22 by etching. Further, the nozzle plate 23 is formed of, for example, a polymer synthetic resin material such as polyimide, and is bonded to the lower surface of the manifold plate 22 with an adhesive. Or this nozzle plate 23 may be formed with metal materials, such as stainless steel, similarly to the other three plates 20-22.

  As shown in FIGS. 2 to 4, among the four plates 20 to 23, the uppermost cavity plate 20 has holes in which a plurality of pressure chambers 24 arranged along a plane penetrate the plate 20. It is formed by. The plurality of pressure chambers 24 are arranged in two rows in a staggered manner in the transport direction (the vertical direction in FIG. 2). Further, as shown in FIG. 4, the plurality of pressure chambers 24 are respectively covered with a diaphragm 40 and a base plate 21 described later from above and below. Furthermore, each pressure chamber 24 is formed in a substantially elliptical shape that is long in the scanning direction (left-right direction in FIG. 2) in plan view.

  As shown in FIGS. 3 and 4, communication holes 25 and 26 are respectively formed at positions where the base plate 21 overlaps both longitudinal ends of the pressure chamber 24 in plan view. The manifold plate 22 is formed with two manifolds 27 extending in the transport direction so as to overlap with the communication hole 25 side portions of the pressure chambers 24 arranged in two rows in a plan view. These two manifolds 27 communicate with an ink supply port 28 formed in a vibration plate 40 described later, and ink is supplied to the manifold 27 from an ink tank (not shown) via the ink supply port 28. Further, a plurality of communication holes 29 that are continuous with the plurality of communication holes 26 are formed at positions where the manifold plate 22 overlaps the ends of the plurality of pressure chambers 24 opposite to the manifolds 27 in plan view.

  Further, a plurality of nozzles 30 are formed at positions where the nozzle plate 23 overlaps the plurality of communication holes 29 in plan view. As shown in FIG. 2, the plurality of nozzles 30 are arranged so as to overlap with the end portions on the opposite side of the manifold 27 of the plurality of pressure chambers 24 arranged in two rows along the transport direction, respectively. A nozzle row is configured.

  As shown in FIG. 4, the manifold 27 communicates with the pressure chamber 24 through the communication hole 25, and the pressure chamber 24 communicates with the nozzle 30 through the communication holes 26 and 29. As described above, a plurality of individual ink flow paths 31 from the manifold 27 to the nozzles 30 through the pressure chambers 24 are formed in the flow path unit 6.

  In FIG. 2, only one type of flow path structure (manifold 27, pressure chamber 24, nozzle 30, etc.) connected to one ink supply port 28 is shown for simplicity of explanation, but the inkjet head 3 However, it has a configuration in which a plurality of flow path structures shown in FIG. 2 are arranged in the scanning direction and can eject a plurality of colors (for example, four colors of black, yellow, cyan, and magenta). An inkjet head may be used.

  Next, the piezoelectric actuator 7 will be described. As shown in FIGS. 2 to 4, the piezoelectric actuator 7 includes a vibration plate 40 disposed on the upper surface of the flow path unit 6 (cavity plate 20) so as to cover the plurality of pressure chambers 24, and an upper surface of the vibration plate 40. In addition, a piezoelectric layer 41 disposed to face the plurality of pressure chambers 24 and a plurality of individual electrodes 42 disposed on the upper surface of the piezoelectric layer 41 are provided.

  The diaphragm 40 is a substantially rectangular metal plate in plan view, and is made of, for example, an iron-based alloy such as stainless steel, a copper-based alloy, a nickel-based alloy, or a titanium-based alloy. The vibration plate 40 is joined to the cavity plate 20 in a state of being disposed on the upper surface of the cavity plate 20 so as to cover the plurality of pressure chambers 24. In addition, the upper surface of the conductive diaphragm 40 is disposed on the lower surface side of the piezoelectric layer 41, thereby generating an electric field in the thickness direction in the piezoelectric layer 41 with the plurality of individual electrodes 42 on the upper surface. Also serves as an electrode. The diaphragm 40 as the common electrode is connected to the ground wiring of the driver IC 47 that drives the piezoelectric actuator 7 and is always held at the ground potential.

  The piezoelectric layer 41 is made of a piezoelectric material mainly composed of lead zirconate titanate (PZT), which is a solid solution of lead titanate and lead zirconate and is a ferroelectric substance. As shown in FIG. 2, the piezoelectric layer 41 is continuously formed across the plurality of pressure chambers 24 on the upper surface of the vibration plate 40. The piezoelectric layer 41 is polarized in the thickness direction at least in a region facing the pressure chamber 24.

  A plurality of individual electrodes 42 are respectively disposed in regions on the upper surface of the piezoelectric layer 41 facing the plurality of pressure chambers 24. Each individual electrode 42 has a substantially oval planar shape that is slightly smaller than the pressure chamber 24, and faces the central portion of the pressure chamber 24. In addition, a plurality of contact portions 45 connected to a flexible wiring board (not shown) on which the driver IC 47 is mounted are drawn from the end portions of the plurality of individual electrodes 42 along the longitudinal direction of the individual electrodes 42. . Note that the plurality of individual electrodes 42 described above correspond to the plurality of first electrodes in the present application, and a plurality of diaphragms 40 as common electrodes that sandwich the piezoelectric layer 41 respectively facing the plurality of individual electrodes 42. This part corresponds to a plurality of second electrodes in the present application.

  A plurality of piezoelectric layer portions (active portions 41a) sandwiched between the plurality of individual electrodes 42 and the diaphragm 40 as a common electrode are previously polarized in the thickness direction. When a potential difference (voltage) is generated between the individual electrode 42 and the diaphragm 40, piezoelectric deformation (piezoelectric distortion) occurs in the active portion 41a, and the pressure chamber facing the active portion 41a is generated by this deformation. Pressure is applied to the ink in 24.

  The piezoelectric actuator 7 is connected to a flexible wiring board (FPC) (not shown) on which a driver IC 47 (driving device) for driving the piezoelectric actuator 7 is mounted. The driver IC 47 and a plurality of individual ICs are connected to the piezoelectric actuator 7 via wiring on the FPC. The electrode 42 and the diaphragm 40 as a common electrode are electrically connected.

  As will be described in detail later, the driver IC 47 that drives the piezoelectric actuator 7 supplies a constant charging current to the plurality of active portions 41a of the piezoelectric actuator 7 to equalize the amount of charge between the plurality of active portions 41a. Then, a voltage corresponding to the capacitance is generated in each active part 41a (between the individual electrode 42 and the common electrode (diaphragm 40)). Moreover, the voltage of each active part 41a is set to 0 by discharging the electric charge stored in the active part 41a with a constant discharge current. By this charging / discharging, voltage application to the active part 41a and release thereof are repeated to drive the active part 41a. A specific configuration of the driver IC 47 will be described later.

  Next, the operation of the piezoelectric actuator 7 during ink ejection will be described. In each active portion 41a sandwiched between the individual electrode 42 and the diaphragm 40 as a common electrode, the electric potential of the individual electrode 42 is the same ground potential as that of the diaphragm 40 when no charge is stored. . At this time, an electric field does not act on the active portion 41a, and piezoelectric distortion does not occur.

  From this state, when a constant charging current is supplied from a driver IC 47 to a certain active portion 41a for a predetermined time, a predetermined amount of charge is stored in the active portion 41a and the potential of the individual electrode 42 is equal to the ground potential. It becomes higher with respect to the diaphragm 40. Therefore, a predetermined voltage determined by the capacitance of the active portion 41a and the amount of stored charge is applied between the individual electrode 42 and the diaphragm 40 sandwiching the active portion 41a, and the active portion An electric field in the thickness direction acts on 41a. Since the direction of the electric field is parallel to the polarization direction of the piezoelectric layer 41, the active portion 41a contracts in a plane direction perpendicular to the thickness direction. Here, since the lower vibration plate 40 of the piezoelectric layer 41 is fixed to the cavity plate 20, as the piezoelectric layer 41 positioned on the upper surface of the vibration plate 40 contracts in the surface direction, the vibration plate 40. The portion covering the pressure chamber 24 is deformed so as to protrude toward the pressure chamber 24 (unimorph deformation). At this time, since the volume in the pressure chamber 24 decreases, the ink pressure in the pressure chamber 24 rises, and ink is ejected from the nozzle 30 communicating with the pressure chamber 24.

  Further, when the driver IC 47 discharges the charge with a constant discharge current from the state where a predetermined amount of charge is stored in the active part 41a, the potential of the individual electrode 42 becomes the ground potential again, and the electric field is generated in the active part 41a. Since it no longer acts, the deformed state of the active part 41a is eliminated, and the diaphragm 40 returns to its original state (a state parallel to the cavity plate 10).

  Next, a specific configuration of the driver IC 47 will be described. FIG. 5 is a circuit diagram of the driver IC 47. As shown in FIG. 5, the driver IC 47 includes two constant current sources (a charge-side constant current source 50 and a discharge-side constant current source 51) connected to a power supply voltage (VDD), a charge-side constant current source 50, and a plurality of them. Charge switch SW1 for switching connection / disconnection with the active portion 41a, a discharge switch SW2 for switching connection / disconnection between the discharge-side constant current source 51 and the plurality of active portions 41a, a plurality of charge switches SW1, and a plurality of charge switches SW1 And a charge / discharge control circuit 52 for controlling switching of the discharge switch SW2.

  The charge-side constant current source 50 and the discharge-side constant current source 51 are respectively a MOSFET type transistor Ta, a resistor R connected to the source of the transistor Ta, and a plurality of MOSFET type transistors Tb respectively corresponding to the plurality of active portions 41a. And have. The drains of the transistors Ta of the constant current sources 50 and 51 are connected to a power supply (VDD), and the sources are connected to the ground via a resistor R.

  The drains of the plurality of transistors Tb of the charging-side constant current source 50 are connected to a power supply (VDD). The sources of the plurality of transistors Tb of the discharge side constant current source 51 are connected to the ground. Further, the sources of the plurality of transistors Tb of the charge-side constant current source 50 and the drains of the plurality of transistors Tb of the discharge-side constant current source 51 are respectively connected to charge and discharge the plurality of active portions 41a described above. A plurality of charge / discharge paths 53 are configured. Moreover, from each charging / discharging path | route 53, the connection path | route 54 connected with the separate electrode 42 of the active part 41a has branched.

  In each of the constant current sources 50 and 51, the gate terminals of the transistors Ta and Tb are connected to each other, and a power supply voltage (VDD) is applied to these gate terminals. The transistors Ta and Tb are current mirror circuits. Is configured. As a result, a constant current determined by the resistor R flows between the drain and source of the transistor Ta, while the same applies to the transistor Ta between the drain and source of the plurality of transistors Tb that form a current mirror circuit with the transistor Ta. A constant current will flow. As a result, the charging current supplied to the active part 41a by the charging-side constant current source 50 is kept constant when charging the active part 41a, and the discharge current from the active part 41a is discharged when discharging from the active part 41a. It is kept constant by the side constant current source 51.

  MOSFET type transistors T1 and T2 are formed between the branch point P1 from the charge / discharge path 53 of the connection path 54 and the charge-side constant current source 50 and between the branch point P1 and the discharge-side constant current source 51. A charge switch SW1 and a discharge switch SW2 are provided. When the charging switch SW1 is ON, the charging side constant current source 50 and the active part 41a are connected to charge the active part 41a. Further, when the discharge switch SW2 is ON, the discharge side constant current source 51 and the active part 41a are connected to discharge the active part 41a.

  The charge / discharge control circuit 52 applies a gate voltage to each gate terminal of the transistor T1 constituting the charge switch SW1 and the transistor T2 constituting the discharge switch SW2 provided in each charge / discharge path 53, thereby SW1 and discharge switch SW2 are switched on / off. More specifically, the charge / discharge control circuit 52 turns on the charge switch SW1 and turns off the discharge switch SW2 when applying voltage to the active part 41a to cause piezoelectric deformation in the active part 41a. The charging-side constant current source 50 and the active part 41a are connected to supply a constant charging current to the active part 41a through a path indicated by an arrow A in FIG. Further, when the voltage of the active part 41a is set to 0 and the deformation of the active part 41a is restored, the charge switch SW1 is turned off and the discharge switch SW2 is turned on, and the active part is switched along the path indicated by arrow B in FIG. The electric charge stored in 41a is discharged with a constant discharge current.

  Here, the pressure applied to the ink in each pressure chamber 24 (that is, the energy applied to the liquid droplets ejected from the nozzle 30) is applied to each active portion 41a by a voltage (vibrating plate as an individual electrode 42 and a common electrode). (Potential difference with respect to 40) is determined by the amount of piezoelectric deformation when applied. However, when there are variations in capacitance among the plurality of active portions 41a, the amount of piezoelectric deformation varies depending on the size of the capacitance even when the same voltage is applied. For example, the amount of deformation is large in the active portion 41a having a small thickness (large capacitance), and the amount of deformation is small in the active portion 41a having a large thickness (small capacitance).

  Therefore, the charge / discharge control circuit 52 controls the switching timing (that is, the charge time and the discharge time) of the charge switch SW1 and the discharge switch SW2 so that the charge amounts of the plurality of active portions 41a are equalized. Thereby, when Q is constant in the relationship of Q = CV, the applied voltage to the active part 41a having a large electrostatic capacity is small, and conversely, the applied voltage to the active part 41a having a small electrostatic capacity is large. Therefore, variation in the amount of piezoelectric deformation among the plurality of active portions 41a can be suppressed.

  The above contents will be described more specifically. The charging current I1 and the discharging current I2 of each active part 41a are kept constant by the charging side constant current source 50 and the discharging side constant current source 51, respectively. In the present embodiment, since the charging-side constant current source 50 and the discharging-side constant current source 51 have the same configuration, the charging current I1 and the discharging current I2 have the same value, but the charging current I1 and the discharging current I2 are different. The charge-side constant current source 50 and the discharge-side constant current source 51 may have different configurations (for example, an electric resistance value of the resistor R) so that the values are equal.

  Thus, since the charging current I1 and the discharging current I2 are constant, the charging time T1 and discharging time T2 of each active part 41a for charging and discharging a predetermined charge amount Q are T1 = Q / I1. , T2 = Q / I2. Therefore, in the charge / discharge control circuit 52, first, the charge switch SW1 is ON (the connection state between the charge-side constant current source 50 and the active part 41a), and the discharge switch SW2 is OFF (the discharge-side constant current source 51 and the active part). The switching timing of the charge switch SW1 and the discharge switch SW2 is controlled so that the time when it is in the state of being cut off from 41a becomes T1. Next, the charge switch SW1 is OFF (the charge-side constant current source 50 and the active part 41a are disconnected), and the discharge switch SW2 is ON (the connection state between the discharge-side constant current source 51 and the active part 41a). The switching timing of the charge switch SW1 and the discharge switch SW2 is controlled so that the time becomes T2.

  As a result, when each active part 41a is charged, a constant charging current I1 flows to each active part 41a for the charging time T1, and a charge Q is stored in each active part 41a. Each active part 41a has its capacitance. A voltage corresponding to is applied. Further, when each active part 41a is charged, a constant discharge current I2 flows from each active part 41a for a discharge time T2, and the charge Q is discharged from each active part 41a, so that the voltage becomes zero.

  By the way, in order to control the charge amount Q charged / discharged to / from each active part 41a to a predetermined value by controlling the charging / discharging time, it is assumed that the charging current and the discharging current are always kept constant by the constant current source. Actually, however, a situation may occur in which the charging current and the discharging current are not kept constant during charging and discharging. FIG. 6 is a diagram showing current characteristics of a general transistor constituting the constant current source. In FIG. 6, the horizontal axis indicates the drain-source voltage (Vds), and the vertical axis indicates the drain-source current (Ids). As shown in FIG. 6, when the drain-source voltage Vds is large (Vds = Va), the current Ids flowing between the drain and the source is maintained at a constant current Ia. However, when Vds becomes small, for example, Vb or less, Ids becomes smaller than Ia. In the constant current sources 50 and 51 described above, a phenomenon occurs in which Vds of the transistor Tb becomes small at the end of charging and at the end of discharging.

  At the end of charging when charging of the active part 41a has progressed, the voltage of the active part 41a is increased. That is, the potential (potential at the branch point P1) on the downstream side (active portion 41a side) of the charging-side constant current source 50 is increased. In particular, when the active portion 41a is fully charged up to its maximum charge amount (maximum charge amount that can be stored), the potential difference of the constant current source (Vds of the transistor Tb) at the end of charging becomes very small. , The charging current is almost zero.

  As described above, the maximum charge amount Qmax stored in each active portion 41a when fully charged is the capacitance Cn of each active portion 41a and the maximum applied voltage Vmax (= connected to a constant current source). Qmax = Cn × Vmax, and the maximum charge amount Qmax is different if the capacitance varies among the plurality of active portions 41a. That is, the larger the capacitance of the active part 41a, the larger the maximum charge amount Qmax. Therefore, when all of the charge charges to the plurality of active portions 41a are set equal to the predetermined charge amount Q, in the active portion 41a having a large electrostatic capacity, the predetermined charge amount Q is less than that of the active portion 41a. Even if the charge amount is less than the maximum charge amount, the predetermined charge amount Q may be the maximum charge amount in the active part 41a having a small electrostatic capacity, which is different from the above. In some of the active portions 41a charged up to the maximum charge amount, the potential at the branch point P1 becomes as close as possible to the power supply voltage VDD at the end of charging, so that Vds is set to Vds in the charging-side constant current source 50 in FIG. It becomes impossible to ensure more than Vb, and a charging current will become small. If the charging current is reduced in this way, the charging time will be considerably delayed, and even if a predetermined charging time set assuming that a constant charging current is maintained, the actual charging charge will elapse. The problem arises that the amount does not reach the desired amount of charge.

  Further, at the end of the discharge in which the discharge from the active part 41a has advanced, the voltage of the active part 41a is small (almost 0). That is, since the potential on the upstream side (active portion 41a side) (potential at the branch point P1) is lower than the discharge-side constant current source 51, the discharge-side constant current source 51 also secures Vds to Vb or more. The discharge current becomes small. In this case, there arises a problem that the discharge time is extended for a long time or the discharge cannot be completed completely. In addition, if the next charging starts before the battery is completely discharged, when the predetermined charge amount is completely charged, not only the predetermined charge amount but also the electric charge that could not be discharged. As a result, the charge is actually stored in the active part 41a, the total amount of charges actually stored in the active part 41a exceeds the set value, and the voltage of the active part 41a becomes larger than the predetermined voltage.

  Therefore, in the present embodiment, it is possible to suppress a decrease in charging current and discharging current during charging and discharging.

  First, a configuration for suppressing a decrease in charging current will be described. As described above, for each of the plurality of active portions 41a, when the charge amount is set so that the voltages are equal, if there is an active portion 41a in which the set charge amount is the maximum charge amount, A decrease in charging current in the active part 41a becomes a problem. Therefore, the charge / discharge control circuit 52 turns on / off the charge switch SW1 and the discharge switch SW2 so that the charge amount Q charged in each active portion 41a is less than the maximum charge amount that the active portion 41a can store. Switch off and control the charging time. As described above, if all the active portions 41a are not charged up to the maximum charge amount Qmax, the voltage of the active portion 41a is in a low state even at the end of charging compared to the case of full charge. Therefore, the potential difference of the charging-side constant current source 50 is ensured to be equal to or higher than Vb in FIG. 6, and the charging current is maintained at a constant value (Ia).

  Next, a configuration for suppressing a decrease in discharge current will be described. The decrease in the discharge current occurs when the amount of charge stored in the active portion 41a is considerably reduced at the end of the discharge, and is different from the decrease in the charge current described above, regardless of the charge amount. It occurs in all active parts 41a. In other words, even if the charge amount is limited, the decrease in the discharge current cannot be suppressed.

  Therefore, a depletion type FET is employed as the transistor Tb constituting the discharge side constant current source 51. FIG. 7 is a diagram showing the relationship between Vgs (gate-source voltage) and Ids (drain-source current) of a depletion type FET. Note that a depletion type FET means that a current Ids is generated between a drain and a source even when the gate-source voltage Vgs is 0 as shown in FIG. It is an FET having a flowing characteristic.

  Further, FIG. 8 shows a relationship between the drain-source voltage Vds and the drain-source current Ids of the depletion type FET having the characteristics shown in FIG. Under the condition that the same Vgs (gate-source voltage) is applied, as shown in FIG. 8, the depletion type FET has more current than a general FET (normal: broken line in the figure). (Ids) can flow. Therefore, even in a range where Ids is smaller than the constant current Ia and Vds is smaller than Vb, more current flows than in a general FET, and a decrease in discharge current at the end of discharge is suppressed.

  In the above description, the depletion type FET is used only for the discharge-side constant current source 51. However, it is also possible to suppress the decrease in the charge current at the end of the charge by using the depletion type FET for the charge-side constant current source 50 as well. It is. However, from the original inventive idea that the charge charge amount Q is controlled to be constant so that the piezoelectric deformation amount is constant even if the capacitances of the plurality of active portions 41a vary, all of the plurality of active portions 41a are controlled. It is unlikely that a situation occurs in which the charge amount Qn is set to the maximum charge amount Qmax. Therefore, in the present embodiment, the charging current amount Q is set to be less than the maximum charge amount Qmax in all the active portions 41a, thereby suppressing a decrease in charging current. In addition, the production of the depletion type FET requires a special process for injecting impurities into the gate, which increases the cost. Also from this point, it can be said that it is not practical to make all the transistors Tb of the charging-side constant current source 50 depletion type FETs.

  Next, the control system of the printer 1 will be described with reference to the block diagram of FIG. The control device 8 includes a microcomputer including a central processing unit (CPU) 60, a read only memory (ROM) 61, a random access memory (RAM) 62, and a bus 63 that connects them. The bus 63 also has a driver IC 47 for the inkjet head 3, a carriage drive motor 19 for driving the carriage 2, a paper feed motor 14 for the transport mechanism 4, a paper discharge motor 15, and the like, and an ASIC (Application Specific Integrated Circuit) 64. Is connected. The ASIC 64 is connected to an external device PC (personal computer) 69 via an input / output interface (I / F) 68 so that data communication is possible.

  The ASIC 54 also has a head control circuit 71 that controls the driver IC 47 and the carriage drive motor 19 of the inkjet head 3 based on the print data input from the PC 69, and the paper feed of the transport mechanism 4 based on the print data. A conveyance control circuit 72 and the like for controlling the motor 14 and the paper discharge motor 15 are incorporated.

  Further, the head control circuit 71 includes a storage unit 73, and various data used for controlling the driver IC 47 and the like are stored in the storage unit 73. Among them, the storage unit 73 stores data related to the capacitance Cn of the plurality of active units 41 a of the piezoelectric actuator 7. Each capacitance Cn of the plurality of active portions 41 a is measured in a pre-shipment inspection or the like of the inkjet head 3, and the measured value is stored in the storage unit 73.

  Further, the head control circuit 71 includes a charge / discharge time determination unit 74. The charging / discharging time determination unit 74 has a plurality of charge discharge times that can realize a predetermined droplet ejection speed for all the nozzles 30 based on the capacitance Cn data of the plurality of active units 41 a stored in the storage unit 73. The charge amount Q common to the active portions 41a is set, and the charge time T1 and the discharge time T2 are determined from the charge amount Q. In addition, the charge / discharge time determination unit 74 sends a signal related to the charge time T1 and the discharge time T2 of each active unit 41a to the driver IC 47. Based on these signals, the charge / discharge control circuit 52 of the driver IC 47 described above is configured such that the charging time and discharging time of each active unit 41a become the times T1 and T2 determined by the charge / discharge time determining unit 74. It controls ON / OFF switching of the charge switch SW1 and the discharge switch SW2.

  Next, modified embodiments in which various modifications are made to the embodiment will be described. However, the same reference numerals are given to those having the same configuration as in the above embodiment, and the description thereof is omitted as appropriate.

1] Although the depletion type FET is employed as the transistor Tb of the discharge-side constant current source 51 in the above embodiment, this configuration is not always necessary. That is, although a problem such as an increase in discharge time due to a decrease in the discharge current occurs, the transistor Tb of the discharge-side constant current source 51 is not a depletion type like the charge-side constant current source 50 from the viewpoint of cost reduction or the like. A general FET having the characteristics shown in FIG. 6 may be employed.

2] In the piezoelectric actuator 7 of the above-described embodiment, a plurality of individual electrodes 42 (first electrodes) on one surface of the piezoelectric layer 41 and a diaphragm 40 (second electrode) as a common electrode on the other surface of the piezoelectric layer 41. The piezoelectric layer portion sandwiched between the two types of electrodes 42 and 40 in the thickness direction acts as the active portion 41a. However, the piezoelectric actuator to which the present invention can be applied is not limited to the above configuration.

  For example, the diaphragm does not necessarily have to serve as the common electrode, and the common electrode may be provided on the upper surface of the diaphragm separately from the diaphragm that covers the plurality of pressure chambers. In addition, the second electrode does not need to be a so-called solid electrode formed on the entire surface of the piezoelectric layer, and the plurality of second electrodes are provided separately from each other at positions facing the plurality of first electrodes. May be.

  Alternatively, both of the two types of electrodes may be disposed on one surface of the piezoelectric layer, and the piezoelectric layer portion sandwiched in the surface direction by these two types of electrodes may act as the active portion.

3] At least one or all of the charge-side constant current source, the discharge-side constant current source, the charge switch, and the discharge switch may be composed of bipolar transistors.

  The embodiment described above is an example in which the present invention is applied to a piezoelectric actuator driving apparatus for an ink jet head, but the application target of the present invention is not limited to a piezoelectric actuator used in a liquid handling apparatus. For example, the present invention can also be applied to a drive device for a piezoelectric actuator that vibrates a solid drive object by piezoelectric distortion generated in the active portion.

7 Piezoelectric Actuator 40 Diaphragm 41 Piezoelectric Layer 41a Active Part 42 Individual Electrode 50 Charge-side Constant Current Source 51 Discharge-side Constant Current Source 52 Charge / Discharge Control Circuit 47 Driver IC
SW1 Charge switch SW2 Discharge switch Tb Transistor

Claims (5)

A capacitor comprising a piezoelectric layer and a plurality of first electrodes and a plurality of second electrodes provided on the piezoelectric layer, and comprising a piezoelectric layer portion sandwiched between the plurality of first electrodes and the plurality of second electrodes. A drive device for driving a piezoelectric actuator, comprising a plurality of active parts that act,
A charging-side constant current source connected to the plurality of active units;
Similarly, a discharge-side constant current source connected to the plurality of active portions,
A plurality of charge switches for switching connection / disconnection of the charging-side constant current source and the plurality of active units, respectively;
A plurality of discharge switches that respectively switch connection / disconnection of the discharge-side constant current source and the plurality of active units;
A charge / discharge control circuit that determines a switching timing of the charge switch and the discharge switch so that all of the charge amounts of the plurality of active portions are equalized, and controls a charging time and a discharging time of each active portion; With
The charge / discharge control circuit includes:
While the amount of charge charged in each active part is less than the maximum amount of charge that can be stored in the active part , and while the potential difference of the charging-side constant current source is equal to or greater than a predetermined value, The drive device for a piezoelectric actuator , wherein the charging time is controlled so as to end the charging . The charge / discharge control circuit controls the charging time based on the capacitances of the plurality of active portions so that the charge amount to be charged is less than the maximum charge amount in all active portions. The piezoelectric actuator driving device according to claim 1, wherein the driving device is a piezoelectric actuator. The discharge-side constant current source, the depletion-type piezoelectric actuator drive device according to claim 1 or 2, characterized in that it is constituted by a FET. A flow path unit in which a flow path including a plurality of nozzles is formed;
A capacitor comprising a piezoelectric layer and a plurality of first electrodes and a plurality of second electrodes provided on the piezoelectric layer, and comprising a piezoelectric layer portion sandwiched between the plurality of first electrodes and the plurality of second electrodes. A plurality of active portions that act, and a piezoelectric actuator that discharges liquid from the plurality of nozzles by the plurality of active portions, and
A charging-side constant current source connected to the plurality of active units;
Similarly, a discharge-side constant current source connected to the plurality of active portions,
A plurality of charge switches for switching connection / disconnection of the charging-side constant current source and the plurality of active units, respectively;
A plurality of discharge switches that respectively switch connection / disconnection of the discharge-side constant current source and the plurality of active units;
A charge / discharge time determination unit that determines a charge time and a discharge time of each active part so that the charge amounts of the plurality of active parts are all equalized;
A charge / discharge control circuit that determines switching timing of the charge switch and the discharge switch based on the charge time and the discharge time determined by the charge / discharge time determination unit, and controls charging and discharging of each active unit; With
The charge / discharge time determination unit
While the amount of charge charged in each active part is less than the maximum amount of charge that can be stored in the active part, and while the potential difference of the charging-side constant current source is equal to or greater than a predetermined value, The liquid discharging apparatus according to claim 1, wherein the charging time is determined so as to end the charging. A storage unit for storing capacitance data of each of the plurality of active units;
The charge / discharge time determination unit
Based on the capacitance data of the plurality of active parts stored in the storage part, determining the charging time so that the charge amount to be charged is less than the maximum charge amount in all active parts. The liquid ejecting apparatus according to claim 4, wherein the liquid ejecting apparatus is a liquid ejecting apparatus.

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