JP5251896B2 - Piezoelectric actuator drive device and inkjet printer - Google Patents

Description

  The present invention relates to a driving device for driving a piezoelectric actuator and an ink jet printer.

  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

  By the way, since the piezoelectric layer is made of a ceramic which is a brittle material, cracks may occur in the piezoelectric layer during manufacturing. Here, if this crack exists in the active part of the piezoelectric layer sandwiched between two kinds of electrodes, leakage of electric charge (current) occurs in this active part. Therefore, in this active part, even if it is charged for the same time as the normal active part, a part of it leaks, so that the stored charge is reduced, and at the time of discharging, the stored charge is discharged in a short time. For example, the change in the amount of charge during charging / discharging (that is, the change in the applied voltage associated therewith) differs from the normal active part, and as a result, the behavior of the active part (piezoelectric deformation mode) differs. For the reasons described above, conventionally, a piezoelectric layer having a crack in the active portion is discarded without being used, and the yield is low.

  In addition, although the drive circuit which charges a piezoelectric element (active part) with a fixed charging current is disclosed in Patent Document 2, the above-described problem when a crack exists in the active part, and the solution thereof Is not described at all.

  An object of the present invention is to provide a driving device for a piezoelectric actuator capable of bringing the behavior of an active portion where charge leakage occurs to a normal active portion.

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 plurality of charging paths connected to each of the plurality of active parts; a plurality of discharge paths; a charge-side constant current source connected to each of the plurality of active parts via the plurality of charging paths; Discharge-side constant current sources connected to the plurality of active portions through a plurality of discharge paths, respectively, and provided to the plurality of charge paths, respectively. And a plurality of current interchange circuits that allow charge currents to be interchanged between the charge paths.

  When a crack exists in an active part and charge (current) leaks, current is supplied from the charging path of another active part to the charging path corresponding to the active part by the current accommodation circuit. can do. This makes it possible to compensate for the charge for leakage from another charging path, and to make the change in the amount of charge during charging / discharging (that is, the change in the applied voltage associated therewith) closer to the normal active part. Therefore, even when a crack exists in a part of the active portion of the piezoelectric layer, it can be used for the piezoelectric actuator, and the yield is improved.

  According to a piezoelectric actuator driving apparatus of a second invention, in the first invention, each of the plurality of current accommodation circuits is provided in an accommodation path that connects a corresponding charging path to another charging path, and the accommodation path. And a second switch provided between the accommodation path and the active part.

  When the first switch of the current accommodation circuit corresponding to the active part where leakage occurs during charging, the charging current is supplied from the other charging path to the active part, and thus supplied to the active part. Charging current increases, and the amount of charge after charging for a certain period of time approaches other normal active parts. In addition, when the first switch and the second switch corresponding to the active part where the leakage occurs are turned ON during discharging, the leakage current is compensated by the charging current from the other charging path, so the discharging time is normal. Approach the active part.

According to a third aspect of the present invention, there is provided a piezoelectric actuator driving apparatus comprising: the control circuit for performing ON / OFF control of the first switch and the second switch of the current accommodation circuit in the second invention;
The control circuit turns on the first switch provided in the accommodation path connected to the charging path of the active part when charging the predetermined active part, and charges the active part when discharging the predetermined active part. The first switch provided in the accommodation path connected to the path is turned on, the second switch between the active part and the accommodation path is turned on, and after a predetermined time has passed, the second switch is turned on again. It is characterized by being turned off.

  When the control circuit turns on the first switch of the current accommodation circuit corresponding to the active part in which leakage occurs during charging, a charging current is also supplied to the active part from other charging paths. In addition, when the first switch and the second switch corresponding to the active part where the leak occurs are turned on during discharging, the leaked charge is compensated by the charging current from the other charging path. In addition, when a certain amount of charge has been compensated for the active part where leakage occurs from another charging path after a certain time has elapsed from the start of discharge, the second switch is turned off to stop the charge compensation, and normal The discharge is completed in approximately the same time as the active part.

  According to a fourth aspect of the present invention, in the piezoelectric actuator driving apparatus according to the third aspect, the control circuit receives leak current information indicating a degree of leak current for each of the plurality of active portions, and the control circuit Is the first switch that is turned on among the first switches provided in the accommodation path connected to the charging path of the active part based on the leakage current information about the active part when charging the predetermined active part. The number of switches is determined, and the amount of charging current accommodated from other charging paths is controlled.

  According to the present invention, when charging the active part where current leakage occurs, the number of first switches to be turned on is determined according to the degree of the leakage current, thereby charging supplied from another charging path. The amount of current can be controlled in stages.

  According to a fifth aspect of the present invention, there is provided the piezoelectric actuator drive device according to the first aspect, wherein the charging path is connected to another charging path and the charging current is interchanged between the two charging paths. And a control circuit for controlling.

  When there is a crack in an active part and a charge (current) leak occurs, the control circuit controls the current accommodation circuit to connect the charging path of the active part where the leak occurs and another charging path. Then, the current is allowed to pass from the other charging path to the charging path of the active part.

An inkjet printer according to a sixth aspect of the invention is an inkjet printer that performs printing by ejecting ink droplets from a plurality of nozzles onto a printing medium,
A capacitor comprising a piezoelectric layer and a plurality of first electrodes and a plurality of second electrodes provided on the piezoelectric layer, the piezoelectric layer being sandwiched between the plurality of first electrodes and the plurality of second electrodes A plurality of active portions that act as a piezoelectric actuator that ejects ink droplets from the plurality of nozzles by the plurality of active portions, and any one of the third to fifth inventions that drives the piezoelectric actuator. The degree of leakage current for each of the driving device and each of the plurality of active portions is at least three levels of a normal level, a first abnormal level, and a second abnormal level having a leakage current higher than the first abnormal level. A storage unit for storing leakage current information indicated by
The control circuit of the driving device, when a low image quality print command is input, only charges the active current whose leakage current information is at the second abnormal level by the current interchange circuit. When flexible supply is performed and a high-quality print command that requires higher image quality than the low-quality print command is input, not only the active portion whose leakage current information is at the second abnormal level but also the first abnormality The active portion which is a level is also controlled so that the charging current is interchanged and supplied from another charging path by the current accommodating circuit.

  In order to conduct current accommodation to the active portion where leakage occurs, data for conducting current accommodation (data for identifying the active portion where leakage occurs and the degree of leakage current) are separated from printing data. Data), and it takes time for data transfer and processing, so the printing time becomes longer. Therefore, in the present invention, among low-quality printing that places more importance on printing speed than image quality, among the abnormal active portions where leakage occurs, those that have been determined to be the first abnormal level with a relatively small leakage current Does not allow current exchange from other charging paths.

  According to the present invention, it becomes possible to supplement the active portion where charge (current) leakage occurs from the charging path of the other active portion, and the change in the amount of charge during charging / discharging (that is, The change in the applied voltage (according thereto) can be brought close to a normal active part. Therefore, even when a crack exists in a part of the active portion of the piezoelectric layer, it can be used for the piezoelectric actuator, and the yield is improved.

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. It is a circuit diagram of driver IC (at the time of charge). It is a circuit diagram of driver IC (at the time of discharge). 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 cover. A transport mechanism 4 that transports the recording paper P, which is a print medium, 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 46) 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 strain) occurs in the active portion 46, and the pressure chamber facing the active portion 46 due to 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 46 of the piezoelectric actuator 7 to equalize the amount of charge stored between the plurality of active portions 46. As a result, a voltage corresponding to the capacitance is generated in each active portion 46 (between the individual electrode 42 and the common electrode (diaphragm 40)). In addition, by discharging the charge stored in the active part 46 with a constant discharge current, the voltage of each active part 46 is set to zero. By this charging / discharging, voltage application to the active part 46 and release thereof are repeated to drive the active part 46. A specific circuit 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 46 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 46, and piezoelectric distortion does not occur.

  From this state, when a certain charging current is supplied from a driver IC 47 to a certain active portion 46 for a predetermined time, a predetermined amount of charge is stored in the active portion 46 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 voltage (V = Q / C) determined by the electrostatic capacity (C) of the active portion 46 and the stored charge amount (Q) between the individual electrode 42 and the diaphragm 40 sandwiching the active portion 46. ) Is applied, and an electric field in the thickness direction acts on the active portion 46. Since the direction of the electric field is parallel to the polarization direction of the piezoelectric layer 41, the active portion 46 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.

  In addition, when the driver IC 47 discharges the charge with a constant discharge current from a state where a predetermined amount of charge is stored in the active part 46, the potential of the individual electrode 42 becomes the ground potential again, and an electric field is applied to the active part 46. Since it no longer acts, the deformed state of the active part 46 is eliminated, and the diaphragm 40 returns to the original state (a state parallel to the cavity plate 10).

  Next, a specific configuration of the driver IC 47 will be described. FIGS. 5 and 6 are circuit diagrams of the driver IC 47. FIG. 5 shows charging at the active portion and FIG. 6 shows discharging at the active portion. A plurality of active portions 46 of the piezoelectric layer 41 sandwiched between the individual electrode 42 and the diaphragm 40 as a common electrode each act as a capacitor (capacitance component) that accumulates electric charge by applying a voltage between both electrodes. Therefore, in FIG. 5, the active portion 46 is indicated by a capacitor symbol. As shown in FIGS. 5 and 6, 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), and a charge side constant current source. 50, a plurality of charging paths 55 that connect the plurality of active portions 46, a plurality of discharge paths 56 that connect the discharge-side constant current source 51 and the plurality of active portions 46, and a charge switch provided in the charging path 55 SWa, a discharge switch SWb provided in the discharge path 56, and a charge / discharge control circuit 52 that controls switching of the plurality of charge switches SWa and the plurality of discharge switches SWb.

  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 46. 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. Furthermore, the sources of the plurality of transistors Tb of the charging-side constant current source 50 and the drains of the plurality of transistors Tb of the discharging-side constant current source 51 are connected to each other, and a plurality of charging units for charging the plurality of active units 46 described above. A charging path 55 and a plurality of discharging paths 56 for discharging from the plurality of active portions 46 are configured. Further, a connection path 54 connected to the individual electrode 42 of the active portion 46 branches off from a connection point P1 between the charge path 55 and the discharge path 56.

  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 (arrow A in FIG. 5) supplied from the charging-side constant current source 50 to the active unit 46 via the charging path 55 is kept constant during charging of the active unit 46. At the time of discharging, the discharge current (arrow B in FIG. 6) flowing out from the active portion 46 through the discharge path 56 is kept constant by the discharge-side constant current source 51.

  The charging path 55 (between the connection point P1 and the charging-side constant current source 50) and the discharging path 56 (between the connection point P1 and the discharging-side constant current source 51) include a charge switch SWa composed of a MOSFET type transistor. And a discharge switch SWb. When the charging switch SWa is ON, the charging side constant current source 50 and the active part 46 are connected to charge the active part 46. Further, when the discharge switch SWb is ON, the discharge side constant current source 51 and the active part 46 are connected to discharge the active part 46.

  The charge / discharge control circuit 52 applies a gate voltage to each of the gate terminals of the transistor constituting the charge switch SWa and the transistor constituting the discharge switch SWb provided in the charge path 55 and the discharge path 56, SWa and discharge switch SWb are switched ON / OFF. More specifically, the charge / discharge control circuit 52 turns on the charge switch SWa and turns off the discharge switch SWb when applying a voltage to the active unit 46 to cause piezoelectric deformation in the active unit 46. The charging-side constant current source 50 and the active part 46 are connected to supply a constant charging current to the active part 46 through a path indicated by an arrow A in FIG. Further, when the voltage of the active part 46 is set to 0 and the deformation of the active part 46 is restored, the charge switch SWa is turned OFF and the discharge switch SWb is turned ON, and the active part is shown in the path indicated by arrow B in FIG. The electric charge stored in 46 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 46 with a voltage (the vibration plate as the individual electrode 42 and the 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 46, 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 46 having a small thickness (large capacitance), and the amount of deformation is small in the active portion 46 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 SWa and the discharge switch SWb so that the charge amounts of the plurality of active units 46 are equalized. Thereby, when Q is constant in the relationship of Q = CV, the applied voltage to the active part 46 having a large electrostatic capacity is small, and conversely, the applied voltage to the active part 46 having a small electrostatic capacity is large. Therefore, variations in the amount of piezoelectric deformation among the plurality of active portions 46 can be suppressed.

  The above contents will be described more specifically. The charging current and discharging current of each active unit 46 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 and the discharging current have the same value, but the charging current and the discharging current have different values. In addition, the configuration of the charging-side constant current source 50 and the discharging-side constant current source 51 (for example, the electrical resistance value of the resistor R) may be different.

  As described above, since the charging current and the discharging current are constant, the charging time T1 and discharging time T2 of each active part 46 for charging and discharging the predetermined charge amount Q are the charging current I1 and the discharging current. Is I2, T1 = Q / I1 and T2 = Q / I2. Therefore, in the charge / discharge control circuit 52, first, the charge switch SWa is ON (the connection state between the charge-side constant current source 50 and the active part 46), and the discharge switch SWb is OFF (the discharge-side constant current source 51 and the active part). The switching timing of the charging switch SWa and the discharging switch SWb is controlled so that the time when the switching state is 46) is T1. Next, the charge switch SWa is OFF (the charge-side constant current source 50 and the active part 46 are disconnected), and the discharge switch SWb is ON (the connection state between the discharge-side constant current source 51 and the active part 46). The switching timing of the charge switch SWa and the discharge switch SWb is controlled so that the time becomes T2.

  By the way, there is no problem if all of the plurality of active portions 46 are normal active portions 46 with no cracks. However, if any active portion 46 has cracks, this active portion 46 has no problem. In this case, charge (current) leaks. Therefore, at the time of charging, even if a constant charging current is supplied for the predetermined time T1 described above, the amount of charge stored in the abnormal active portion 46 is larger than that of the normal active portion 46 by the amount of charge leakage. Less. In addition, at the time of discharge, since the electric charge is reduced due to leakage in addition to the discharge with a constant discharge current, the discharge ends in a time shorter than the normal discharge time T2. Therefore, in the active portion 46 where the crack exists, the change in charge amount (that is, the change in applied voltage) during charge / discharge differs from the normal active portion 46, and the behavior (piezoelectric deformation) also differs.

  Therefore, the driver IC 47 of the present embodiment is configured to be able to compensate for the leaked charge by passing current from the charging path 55 of the other active part 46 to the abnormal active part 46 in which charge leakage occurs. Has been. That is, as shown in FIGS. 5 and 6, the driver IC 47 is provided for each of the plurality of charging paths 55, and the corresponding charging path 55 is connected to another charging path 55 between the charging paths 55. A plurality of current accommodation circuits 57 are provided to exchange charging currents.

  The circuit configuration of the current accommodation circuit 57 will be specifically described. Each of the plurality of current accommodation circuits 57 includes an accommodation path 58 that connects the corresponding charging path 55 to another charging path 55, a first switch SW1 provided in the accommodation path 58, the accommodation path 58, and the active portion 46. And a second switch SW2 provided therebetween. The first switch SW1 and the second switch SW2 are both constituted by MOSFET type transistors. As a result, the charging path 55 of one active unit 46 is connected to the charging path 55 of the adjacent active unit 46 by the interchange path 58 of the current accommodation circuit 57 corresponding to the active unit 46. Part of the charging current flowing through the charging path 55 of the active unit 46 can be accommodated.

  In addition to the ON / OFF control of the charge switch SWa and the discharge switch SWb described above, the charge / discharge control circuit 52 charges / discharges the active part 46 when there is an abnormal active part 46 that causes charge leakage. Occasionally, the ON / OFF of the first switch SW1 and the second switch SW2 of the current accommodation circuit 57 is controlled to compensate for the charge from the other charging path 55 to the active portion 46.

  In the following description, it is assumed that among the three active portions 46a, 46b, and 46c shown in FIGS. 5 and 6, the active portion 46b located at the center is an abnormal active portion 46 in which current leakage occurs. Note that information (leakage current information) indicating whether each of the plurality of active units 46 is normal or abnormal is sent from the control device 8 of the printer 1 to be described later to the charge / discharge control circuit 52, and based on this information, The charge / discharge control circuit 52 determines to which active part 46 the current interchange should be performed.

  At the time of charging, as shown in FIG. 5, the charging switch SWa is ON (the discharging switch SWb is OFF) in each of the charging paths 55 of the three active portions 46a, 46b, 46c, and charging is performed as indicated by an arrow A. Charging currents are respectively supplied to the active portions 46a, 46b, and 46c through the path 55. Here, the abnormal active part 46b is connected to the charging path 55 of the right active part 46c by the accommodation path 58 of the current accommodation circuit 57 corresponding to the active part 46b. Furthermore, the active part 46b is also connected to the charging path 55 of the active part 46a by the accommodation path 58 of the current accommodation circuit 57 corresponding to the left side active part 46a.

  Therefore, the charging / discharging control circuit 52 turns on the first switches SW1 of the two interchange paths 58 connected to the active part 46b, respectively, so that the charging path 55 of the active part 46a is indicated by an arrow α in the figure. A part of the charging current is supplied (conversed) from the charging path 55 of the active part 46c to the charging path 55 of the active part 46b. As a result, the charging current supplied to the active part 46b becomes larger than the original current value, the charge leaked by the active part 46b is compensated, and the amount of charge stored in the active part 46b is normal. It approaches 46c.

  Further, at the time of discharging, as shown in FIG. 6, the discharge switch SWb is ON (the charge switch SWa is OFF) in each of the discharge paths 56 of the three active portions 46a, 46b, and 46c, as indicated by an arrow B. In addition, a discharge current flows out from the three active portions 46a, 46b, and 46c through the respective discharge paths 56. At this time, the charge / discharge control circuit 52 turns on the first switch SW1 of the accommodation path 58 connected to the abnormal active part 46b, and also turns on the second switch SW2 between the accommodation path 58 and the active part 46b. To. As a result, as indicated by an arrow β, the charging current is supplied from the charging path 55 of the normal active part 46 c to the active part 46 b through the accommodation path 58. Note that the reason for turning on not only the first switch SW1 but also the second switch SW2 unlike the charging in FIG. 5 is that the charging switch SWa is OFF at the time of discharging, so simply turning on the first switch SW1. This is because the charging current cannot be supplied to the active part 46b.

  If the supply of the charging current from the other charging path 55 is continued, the discharge of the active part 46b will not end. Therefore, after a certain time has elapsed from the start of discharging, the other charging path 55 Therefore, when a certain amount of charge is compensated for the active part 46b where the leak occurs, the second switch SW2 is turned off to stop the charge compensation. In this way, since the charge that is leaked from the other charging path 55 to the abnormal active part 46b is compensated in the early stage of discharge, the discharge time of the active part 46b becomes longer, and the normal active parts 46a, 46c. Get closer to.

  In this way, by compensating the charge from the charging path 55 of the other active portions 46a and 46c with respect to the active portion 46b in which charge (current) leakage occurs, a change in the amount of charge during charge / discharge (that is, The change in applied voltage) can be brought close to normal active portions 46a and 46c. Therefore, even if there is an abnormal active portion 46 in which leakage occurs in the piezoelectric layer 41, the piezoelectric layer 41 can be used for the piezoelectric actuator 7 and the yield is improved.

  In addition, when a crack exists in a certain active portion 46 and current leaks, the degree of current leaking varies depending on the state of the crack (size, length, etc.). Therefore, it is preferable that the charge / discharge control circuit 52 controls the amount of charging current that can be accommodated in the abnormal active portion 46 in accordance with the degree of leakage current of the active portion 46. Specifically, information (leakage current information) indicating the degree of leakage current of the active unit 46 is input from the control device 8 of the printer 1 described later, and the charge / discharge control circuit 52 is based on the information. To control the amount of charging current supplied (accommodated).

  In the present embodiment, one active part 46 b is connected to charging paths 55 of two adjacent active parts 46 a and 46 b by two interchange paths 58. Further, the charging path 55 of the active unit 46 c on the right side is not shown, but is connected to another charging path 55 and an accommodation path 58. That is, a plurality of charging paths 55 respectively corresponding to the plurality of active portions 46 are connected in series via an accommodation path 58 therebetween. Accordingly, when a charging current is supplied to a certain abnormal active part 46 from another charging path 55, a plurality of first switches SW1 provided in a plurality of interchange paths 58 connected in series to the active part 46, respectively. If the number of the first switches SW1 to be turned on is adjusted, the amount of charging current supplied to the abnormal active part 46 can be adjusted stepwise. For example, if the two first switches SW1 are turned on, the charging current is interchanged from the two charging paths 55 to the one active portion 46. Almost twice as much as

  Next, the control system of the printer 1 will be described with reference to the block diagram of FIG. The control device 8 of the printer 1 includes a microcomputer including a central processing unit (CPU) 60, a ROM (Read Only Memory) 61, a RAM (Random Access Memory) 62, and a bus 63 for connecting them. Have. 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 64 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 plurality of active units 46 of the piezoelectric actuator 7.

  As data relating to such an active portion 46, there is leakage current information indicating the presence / absence of abnormality (leakage) and the degree of leakage current for each of the plurality of active portions 46. The leakage current information is information that divides the plurality of active portions 46 into at least three levels of a normal level, a first abnormal level, and a second abnormal level according to the magnitude of leak current generated in the plurality of active portions 46. . In the active portion 46 at the normal level, the leakage current is less than the predetermined first threshold value which is a very small value, and the leakage current hardly occurs. Further, in the active part 46 at the first abnormal level, the leakage current is not less than the first threshold and less than the second threshold higher than the first threshold, and some leakage current is generated. Further, in the active portion 46 at the second abnormal level, the leak current is not less than the second threshold value, and a considerably large leak current is generated. The leakage current information is determined according to the measurement result in the pre-shipment inspection of the inkjet head 3 and stored in the storage unit 73.

  Further, the head control circuit 71 includes a charge / discharge time determination unit 74. The charge / discharge time determination unit 74 sets a charge amount Q common to the plurality of active units 46, and determines a charge time T1 and a discharge time T2 from the charge amount Q. Then, the head control circuit 71 uses the charging time T1 and the discharging time T2 determined by the charging / discharging time determination unit 74 and the leakage current information for each of the plurality of active units 46 stored in the storage unit 73. Transmit to the driver IC 47.

  Based on the input signal, the charge / discharge control circuit 52 of the driver IC 47 described above makes the charging time and discharging time of each active unit 46 become the times T1 and T2 determined by the charge / discharge time determining unit 74. The ON / OFF switching of the charge switch SWa and the discharge switch SWb is controlled. Further, based on the leakage current information of the plurality of active portions 46, the current accommodation circuit 57 is controlled during charging / discharging of the active portion 46 at the abnormal level to compensate the charging current from the other charging path 55. Further, the number of first switches SW1 to be turned on is determined according to the level of leakage current of the active part 46 (whether the abnormal level of the active part 46 is the first abnormal level or the second abnormal level). On the other hand, the amount of charging current supplied from the other charging path 55 is adjusted.

  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] In order to carry out current accommodation to the active part 46 in which leakage occurs, data for conducting current accommodation (leakage current information in the above embodiment) is required in addition to the print data. Since it takes time to transfer and process data from the printer 1 to the printer 1, the printing time becomes longer. Conventionally, there are known printers that use a low-quality print mode that is used when printing text and a high-quality print mode that is used when printing an image. Since high image quality is not required, even if a leak current is generated in some active portions 46 and the performance (the ejection characteristics of the nozzles 30) is different from that of other active portions 46, printing is not significantly affected. Therefore, such a printer may be configured to limit current interchange to the abnormal active unit 46 when the low image quality printing mode is selected.

  For example, as in the above-described embodiment, when three levels of leakage current information are set for each of the plurality of active units 46, a low-quality printing command that places more importance on printing speed than image quality is input from the PC 69. In this case, the charge / discharge control circuit 52 of the driver IC 47 controls the current accommodation circuit 57 only for the active part 46 whose leakage current information is at the second abnormal level to allow the interchange supply of the charging current from the other charging path 55. If it is determined that the leakage current is relatively small and the first abnormality level is determined, current exchange from the other charging path 55 is not performed. On the other hand, when a high-quality print command that requires higher image quality than the low-quality print is input from the PC 69, the charge / discharge control circuit 52 of the driver IC 47 causes the active part whose leakage current information is the second abnormal level. In addition to the active portion 46 that is at the first abnormal level, the current accommodation circuit 57 is controlled so that the charging current is supplied from the other charging path 55.

  Alternatively, when a low image quality print command is input, charging current is not interchanged for all the abnormal active portions 46, and only when a high image quality print command is input, the charging current to the abnormal active portion 46 is charged. It may be configured so as to accommodate the following.

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 these two types of electrodes 42 and 40 in the thickness direction acts as the active portion 46. However, the piezoelectric actuator to which the present invention can be applied is not limited to the above configuration.

  For example, the diaphragm 40 does not necessarily have to serve as a common electrode, and the common electrode may be provided on the upper surface of the diaphragm 40 separately from the diaphragm 40 that covers the plurality of pressure chambers 24. In addition, the second electrode does not have to be a common electrode formed over a wide area of the piezoelectric layer 41, that is, a so-called solid electrode, and the plurality of second electrodes are arranged at positions facing each of the plurality of first electrodes. It may be provided separately.

  Alternatively, two types of electrodes may be arranged on one surface of the piezoelectric layer 41, and a piezoelectric layer portion sandwiched in the surface direction by these two types of electrodes may act as the active portion 46.

3] The circuit configuration of the driver IC is not limited to that of the above embodiment, and can be changed as appropriate without departing from the spirit of the present invention. For example, the connection method between the charging paths by the accommodation path, the installation position of the switch, or the like may be appropriately changed so that the current accommodation circuit can mutually exchange the current between the three or more charging paths.

  In the embodiment, at least one of the charge-side constant current source, the discharge-side constant current source, the charge switch, the discharge switch, the first switch and the second switch of the current accommodation circuit, which is configured by a MOSFET type transistor. Alternatively, all 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 (second electrode)
41 Piezoelectric layer 42 Individual electrode (first electrode)
46 Active part 47 Driver IC
50 charge side constant current source 51 discharge side constant current source 52 charge / discharge control circuit 55 charge path 56 discharge path 57 current accommodation circuit 58 accommodation path 73 storage unit SW1 first switch SW2 second switch

Claims (6)

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 plurality of charging paths connected to each of the plurality of active portions; and a plurality of discharging paths;
A charging-side constant current source connected to each of the plurality of active units via the plurality of charging paths;
A discharge-side constant current source connected to each of the plurality of active portions via the plurality of discharge paths;
A plurality of current accommodation circuits provided for each of the plurality of charging paths, and connecting a corresponding charging path to another charging path to allow charging current to be interchanged between the two charging paths;
A drive device for a piezoelectric actuator, comprising: Each of the plurality of current accommodation circuits is:
It has an accommodation path connecting a corresponding charging path to another charging path, a first switch provided in the accommodation path, and a second switch provided between the accommodation path and the active part. The drive device for a piezoelectric actuator according to claim 1. A control circuit for performing ON / OFF control of the first switch and the second switch of the current accommodation circuit;
The control circuit includes:
When charging a predetermined active part, turn on the first switch provided in the accommodation path connected to the charging path of the active part,
When discharging the predetermined active part, the first switch provided in the accommodation path connected to the charging path of the active part is turned ON, and the second switch between the active part and the accommodation path is turned on. 3. The piezoelectric actuator driving device according to claim 2, wherein the second switch is turned off again after a predetermined time has elapsed. Leak current information indicating the degree of leak current for each of the plurality of active portions is input to the control circuit,
The control circuit turns ON among the first switches provided in the accommodation path connected to the charging path of the active part based on leakage current information about the active part when charging the predetermined active part. 4. The piezoelectric actuator driving apparatus according to claim 3, wherein the number of the first switches to be determined is determined, and the amount of charging current accommodated from another charging path is controlled.   2. The control circuit according to claim 1, further comprising: a control circuit that controls the current accommodation circuit so that the charging path is connected to another charging path and the charging current is interchanged between the two charging paths. The drive device of the piezoelectric actuator as described. An inkjet printer that performs printing by ejecting ink droplets from a plurality of nozzles onto a printing medium,
A capacitor comprising a piezoelectric layer and a plurality of first electrodes and a plurality of second electrodes provided on the piezoelectric layer, the piezoelectric layer being sandwiched between the plurality of first electrodes and the plurality of second electrodes A plurality of active portions that act as piezoelectric actuators that eject ink droplets from the plurality of nozzles by the plurality of active portions,
The driving device according to any one of claims 3 to 5, which drives the piezoelectric actuator;
Leakage current indicating the degree of leakage current for each of the plurality of active portions in at least three stages: a normal level, a first abnormal level, and a second abnormal level having a leakage current higher than the first abnormal level A storage unit for storing information;
The control circuit of the driving device is:
When a low image quality print command is input, only for the active portion where the leakage current information is at the second abnormal level, the current accommodation circuit performs charging supply of charging current from another charging path,
When a high image quality print command that requires higher image quality than the low image quality print command is input, not only the active portion having the leakage current information is the second abnormal level but also the active portion having the first abnormal level. The inkjet printer is also characterized in that the current accommodation circuit controls the supply of charging current from other charging paths.

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