# Patent

## Title
Printing apparatus

## Abstract

A printing apparatus that discharges liquid droplets onto a recording medium, includes a movable carriage; discharging units, which are installed on the carriage and include nozzles that discharge a liquid, pressure chambers that communicate with the nozzles, and piezoelectric elements provided for each of the pressure chambers; a first circuit substrate, which is installed outside of the carriage, and on which is installed a control signal supply unit that generates control signals; a second circuit substrate, which is installed on the carriage, and on which are installed a booster circuit that generates a plurality of voltages; in which a path length of the control wiring between the first circuit substrate and the second circuit substrate is longer than the path length of the wiring between the second circuit substrate and the piezoelectric elements.

## Background
This application claims priority to Japanese Patent Application No. 2013-059206 filed on Mar. 22, 2013. The entire disclosure of Japanese Patent Application No. 2013-059206 is hereby incorporated herein by reference.
BACKGROUND
1. Technical Field
The present invention relates to a printing technology in which liquid droplets are discharged onto a recording medium.
2. Related Art
In the related art, a serial-type printing apparatus is proposed which discharges ink droplets onto a recording medium from a plurality of nozzles of a print head while causing a carriage, on which the print head is mounted, to move reciprocally in an intra-surface direction of the recording medium such as paper (for example, refer to JP-A-2000-343690). A control unit that is installed on a housing of the printing apparatus and the print head on the carriage are electrically connected via a flexible flat cable (hereinafter referred to as an FFC).
An electronic circuit, which generates a control signal of a predetermined waveform, is installed in the control unit that is outside of the carriage. The control signal is supplied to the print head from the control unit via the FFC. A plurality of piezoelectric elements, which discharge the ink droplets from the nozzles by deforming according to the supply of a control signal, and a plurality of switches, which control the supply and cut-off of the control signals supplied from the control unit for each of the piezoelectric elements, are installed in the print head on the carriage.
However, in order to supply a control signal of an appropriate waveform to each of the piezoelectric elements even when the number of the piezoelectric elements that are supplied with the control signal in parallel is great (when the drive load is great), it is necessary to supply a control signal with an extremely large current to the print head from the control unit via the FFC. Therefore, the power loss on the FFC is great and the waveform of the control signal is not stable. As a result, there is a problem in that the print quality is reduced. In a Large Format Printer (LFP), in which the movement amount of the carriage is great, since the total length of the FFC can reach several meters, the power loss on the FFC becomes prominent and the reduction in the print quality becomes particularly serious.
SUMMARY
An advantage of some aspects of the invention is that a reduction in the print quality caused by power loss on the FFC is suppressed.
According to an aspect of the invention, there is provided a printing apparatus that discharges liquid droplets onto a recording medium, including a movable carriage; discharging units, which are installed on the carriage and include nozzles that discharge a liquid, pressure chambers that communicate with the nozzles, and piezoelectric elements provided for each of the pressure chambers; a first circuit substrate, which is installed outside of the carriage, and on which is installed a control signal supply unit that generates control signals; a second circuit substrate, which is installed on the carriage, and on which are installed a booster circuit that generates a plurality of voltages, and connection path selecting units that selectively supply the plurality of voltages generated by the booster circuit to the piezoelectric elements according to the control signals; and a flexible flat cable, on which is formed a control wiring, which transmits the control signals from the first circuit substrate to the second circuit substrate, in which a path length of the control wiring between the first circuit substrate and the second circuit substrate is longer than the path length of the wiring between the second circuit substrate and the piezoelectric elements.
In the configuration described above, the booster circuit and the connection path selecting units are installed on the second circuit substrate on the carriage; thus, in comparison to a configuration in which the booster circuit and the connection path selecting units are installed on the first circuit, it is possible to suppress a reduction in the print quality caused by the power loss on the FFC. Note that the phrase “selectively supply the plurality of voltages to the piezoelectric elements” means to select one of a plurality of voltages, and to supply the voltage to the piezoelectric element; specifically, this includes a configuration in which a plurality of wirings, to which different voltages are supplied from the booster circuit, are selectively electrically connected to the piezoelectric elements.
In a favorable aspect of the invention, the connection path selecting units may electrically connect the piezoelectric elements and the booster circuit using a first signal path or a second signal path according to the first signal path, to which a first voltage generated by the booster circuit is applied, the second signal path, to which the second voltage generated by the booster circuit that is higher than the first voltage is applied, voltages of the control signals, and the voltages held by the piezoelectric elements. In the aspect described above, the piezoelectric elements and the booster circuit are electrically connected by the first signal path or the second signal path according to the voltages of the control signals and the voltages held by the piezoelectric elements. Accordingly, it is possible to increase the energy efficiency in comparison to a configuration of the related art in which the charges of the piezoelectric elements are charged and discharged at once between the power supply voltages. There is also a merit in that it is possible to suppress EMI in comparison with D class amplification that switches a large current.
The printing apparatus according to a favorable aspect of the invention may further include detection units, which are installed on the second circuit substrate, and detect whether or not the voltages held by the piezoelectric elements are lower than the first voltage, or, whether or not the voltages held by the piezoelectric elements are equal to or higher than the first voltage and lower than the second voltage. In the aspect described above, it is detected whether or not the voltages maintained by the piezoelectric elements are lower than the first voltage, and whether or not the voltages are equal to or higher than the first voltage and lower than the second voltage. Note that, in the detection unit, a portion that detects whether or not the voltage held by the piezoelectric element is lower than the first voltage, and a portion that detects whether or not the voltage held by the piezoelectric element is equal to or higher than the first voltage and lower than the second voltage may be installed separately or integrally.
In a favorable aspect of the invention, in relation to the piezoelectric elements holding a voltage that is lower than the first voltage, the connection path selecting units may control charges to be charged to the piezoelectric elements via the first signal path according to the voltages of the control signals, and, in relation to the piezoelectric elements holding a voltage that is equal to or higher than the first voltage and lower than the second voltage, the connection path selecting units may control the charges to be discharged from the piezoelectric elements via the first signal path, or, may control the charges to be charged to the piezoelectric elements via the second signal path according to the voltages of the control signals. In the aspect described above, the path of the charge that is charged to or discharged from the piezoelectric element is controlled according to the voltage of the control signal.
The printing apparatus according to a favorable aspect of the invention may include a first transistor, a second transistor, and a third transistor, in which, in relation to the piezoelectric element holding a voltage that is lower than the first voltage, the first transistor may control a charge to be charged to the piezoelectric element via the first signal path according to a voltage that is obtained by shifting the voltage of the control signal to a low potential side by a predetermined value, and in which, in relation to the piezoelectric element holding a voltage that is equal to or higher than the first voltage and lower than the second voltage, the second transistor may control a charge to be discharged from the piezoelectric element via the first signal path according to a voltage that is obtained by shifting the voltage of the control signal to a high potential side by a predetermined value, and the third transistor may control a charge to be charged to the piezoelectric element via the second signal path according to a voltage that is obtained by shifting the voltage of the control signal to the low potential side by a predetermined value. Note that, in the aspect described above, the predetermined value described above may be set to zero if the first transistor, the second transistor, and the third transistor are ideal; however, if bipolar transistors are used, for example, the predetermined value is set to a voltage that is equivalent to the bias voltage. For example, if a Metal-Oxide Semiconductor Field-Effect Transistor (a MOSFET) is used, the predetermined value may be set to a voltage that is equivalent to the threshold voltage.
In a favorable aspect of the invention, if the voltage held by the piezoelectric element is not lower than the first voltage, the first transistor turns off, and if the voltage is not equal to or higher than the first voltage and lower than the second voltage, the second transistor and the third transistor turn off. In the aspect described above, if the voltage held by the piezoelectric element is not lower than the first voltage, the first transistor turns off; thus, the piezoelectric element is electrically isolated from the first signal path. If the voltage held by the piezoelectric element is not equal to or higher than the first voltage or lower than the second voltage, the second transistor and the third transistor turn off; thus the piezoelectric element is electrically isolated from the second signal path.
A configuration may also be adopted in which the charge to be charged to the piezoelectric element or the charge to be discharged from the piezoelectric element is controlled using a voltage, which is obtained by subtracting a voltage that corresponds to the piezoelectric element from the voltage of the control signal and multiplying the resulting voltage a predetermined number of times. In the aspect described above, it is possible to cause the voltage held by the piezoelectric element to follow a voltage that corresponds to the control signal in a highly precise and quick manner by using negative feedback control.
In a printing apparatus (a serial printer) in which a carriage may move in a main scanning direction, which intersects a sub-scanning direction in which a recording medium is transported, it is necessary to secure a sufficient length for the FFC; thus, there is in issue in that a reduction in the print quality caused by power loss on the FFC is likely to manifest. The invention, which can suppress the reduction in the print quality caused by power loss on the FFC, is especially effective in a printing apparatus of a configuration in which the carriage moves in the main scanning direction (in other words, a configuration in which it is necessary to secure a sufficient length for the FFC).



BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements.
FIG. 1 is a schematic view showing a portion of the structure of a printing apparatus according to one of the embodiments of the invention.
FIG. 2 is a block diagram showing an electrical configuration of the printing apparatus.
FIG. 3 is a view showing the main components of a discharging unit in a print head.
FIG. 4 is a diagram showing an example of the configuration of a driver in the print head.
FIGS. 5A and 5B are diagrams illustrating the operations of the driver.
FIGS. 6A to 6C are diagrams illustrating the operations of a level shifter in the driver.
FIG. 7 is a diagram for illustrating the flow of a current (a load) in the driver.
FIG. 8 is a diagram for illustrating the flow of a current (a load) in the driver.
FIG. 9 is a diagram for illustrating the flow of a current (a load) in the driver.
FIG. 10 is a diagram for illustrating the flow of a current (a load) in the driver.
FIGS. 11A and 11B are diagrams illustrating a loss during charging and discharging of the driver.
FIG. 12 is a diagram showing an example of the configuration of an auxiliary power supply unit.
FIGS. 13A and 13B are diagrams illustrating the operations of the auxiliary power supply unit.
FIGS. 14A and 14B are diagrams showing the voltage change of the auxiliary power supply unit.
FIG. 15 is a block diagram of a comparative example.
FIG. 16 is a schematic diagram for illustrating a problem of the comparative example.
FIG. 17 is a schematic diagram for illustrating the effect of the embodiment compared with the comparative example.
FIG. 18 is a diagram showing a configuration example of an application example of the driver.
FIG. 19 is a diagram showing a configuration example of an application example of the driver.



DESCRIPTION OF EXEMPLARY EMBODIMENTS
FIG. 1 is a schematic view showing a portion of a printing apparatus 100 of an ink jet type according to one of the embodiments of the invention. The printing apparatus 100 of this embodiment is a liquid discharging apparatus that discharges liquid droplets of an ink (hereinafter referred to as ink droplets) onto a recording medium 200 such as printing paper. The recording medium 200 is transported in a sub-scanning direction Y by a transporting mechanism (not shown). The printing apparatus 100 of this embodiment is a serial printer that includes a movable carriage 300. Specifically, the carriage 300 moves in a main scanning direction X, which intersects the sub-scanning direction Y in which the recording medium 200 is transported. In addition to a configuration in which a cartridge (not shown) that accommodates a liquid ink is installed on the carriage 300 (on-carriage), a configuration may also be adopted in which the ink is supplied to the carriage 300 from a cartridge installed outside of the carriage 300 (off-carriage) via a flow path.
FIG. 2 is a block diagram showing the electrical configuration of the printing apparatus 100. As shown in FIGS. 1 and 2, the printing apparatus 100 includes a control unit 10, a print head 20 and a Flexible Flat Cable (FFC) 70. The control unit 10 is installed outside of the carriage 300 (for example, on the housing of the printing apparatus 100), and the print head 20 is installed on the carriage 300 and moved in the main scanning direction X. The FFC 70 is a flexible wiring substrate on which a plurality of wirings 72 (722, 724, 726, and 728), which electrically connect the control unit 10 and the print head 20 together, is formed. In addition, the FFC 70 deforms together with the movement of the carriage 300 (the print head 20).
The control unit 10 is an element that executes a computation process and a control process for printing an image specified by image data supplied from a host computer and includes the control substrate 12 (the first circuit substrate) of FIG. 2. A print data generating unit 120, a control signal supply unit 140 and a main power supply unit 180 are installed on the control substrate 12. Furthermore, the main power supply unit 180 can also be installed outside of the control substrate 12.
The main power supply unit 180 generates a power supply voltage VH and a ground voltage G, and supplies the generated voltages to each of the elements on the control substrate 12 and to the print head 20. The ground voltage G is equivalent to a voltage reference value (voltage zero), and the power supply voltage VH is the voltage of the high potential side of the ground voltage G. The power supply voltage VH is supplied to the print head 20 via the wiring 726 of the FFC 70, and the ground voltage G is supplied to the print head 20 via the wiring (hereinafter referred to as the “ground wiring”) 728 of the FFC 70.
The print data generating unit 120 and the control signal supply unit 140 of FIG. 2 are, for example, realized by a computational processing apparatus (a CPU), which executes a program that is stored on a memory circuit such as RAM, or various logical circuits. Furthermore, an element that controls the transporting mechanism, which transports the recording medium 200 in the sub-scanning direction Y, or an element that controls the movement mechanism, which causes the carriage 300 to move in the main scanning direction X, can be installed on the control substrate 12. However, the illustration of such elements is omitted from FIG. 2 for convenience.
The print data generating unit 120 generates print data DP by executing various computational processes (for example, an image extraction process, a color conversion process, a color separation process, a half tone process and the like) in relation to the image data that is supplied from the host computer. The print data DP specifies the presence or absence of a discharge of ink droplets and the discharge amount of the ink droplets for each nozzle of the print head 20. The print data DP that is generated by the print data generating unit 120 is supplied to the print head 20 via the wiring 722 of the FFC 70.
The control signal supply unit 140 is an element that generates the control signal for causing the print head 20 to discharge ink droplets from each of the nozzles, and is configured to include a waveform generating unit 142 and a D/A converter 144. The waveform generating unit 142 generates a digital control signal dCOM that exhibits a predetermined waveform. The D/A converter 144 converts the control signal dCOM that is generated by the waveform generating unit 142 into an analogue control signal COM. The control signal COM that is generated by the control signal supply unit 140 is supplied to the print head 20 via the wiring (hereinafter referred to as the control wiring) 724 of the FFC 70.
The print head 20 is an element that discharges ink droplets from a plurality of nozzles under the control of the control unit 10, and includes a print head substrate 22 (the second circuit substrate) and a head module 24. A voltage amplifier 210, a head control unit 220, a selection unit 230, an element drive unit 240 and an auxiliary power supply unit 50 are installed on the print head substrate 22. Each of the elements on the print head substrate 22 is implemented on the print head substrate 22 in the form of being mounted on a single semiconductor integrated circuit (an IC chip), for example. However, it is also possible to mount the elements alternately on a plurality of separate semiconductor integrated circuits.
The head module 24 includes a plurality of piezoelectric elements (piezo elements) 40 that correspond to the distinct nozzles. Each of the piezoelectric elements 40 is a capacitive load disposed in a cavity (the ink chamber) into which the ink is supplied via the flow path. The piezoelectric elements 40 are deformed by charging and discharging and the volume of the cavity changes; therefore, the ink droplets are discharged from the nozzle that corresponds to the piezoelectric element 40.
FIG. 3 is a view showing the schematic configuration of a discharging unit 400 that corresponds to one of the nozzles in the print head 20. As shown in FIG. 3, the discharging unit 400 includes the piezoelectric element 40, a vibration plate 421, a cavity (a pressure chamber) 431, a reservoir 441 and a nozzle 451. Of these, the vibration plate 421 is deformed by the piezoelectric element 40 that is provided on the upper surface thereof in FIG. 3; therefore causing the internal volume of the cavity 431, which is filled with the ink, to expand or contract. The nozzle 451 is an opening portion that communicates with the cavity 431.
The piezoelectric element 40 shown in FIG. 3 is generally referred to as unimorphic (monomorphic), and the structure thereof is formed from a piezoelectric body 401 interposed between a pair of electrodes 411 and 412. In the piezoelectric body 401 of this structure, corresponding to a voltage that is applied between the electrodes 411 and 412, in FIG. 3, the central portion of the piezoelectric body 401 flexes in the upward or downward direction in relation to both terminal portions thereof together with the electrodes 411 and 412, and the vibration plate 421. Here, if the piezoelectric body 401 flexes in the upward direction, since the internal volume of the cavity 431 expands, the ink is drawn in from the reservoir 441. However, if the piezoelectric body 401 flexes in the downward direction, since the internal volume of the cavity 431 contracts, the ink is discharged from the nozzle 451. Furthermore, the piezoelectric element 40 is not limited to being unimorphic, and may be of any type, such as bimorphic or laminated, that is capable of deforming the piezoelectric element and discharging a liquid such as the ink.
The element drive unit 240 on the print head substrate 22 is an element that drives the plurality of piezoelectric elements 40, and is configured to include a plurality of drivers 30, as shown in FIG. 2. Each of the drivers 30 of the element drive unit 240 corresponds one-for-one with each of the piezoelectric elements 40 of the head module 24. In other words, the print head 20 includes a plurality of sets, each of which includes one of the piezoelectric elements 40 and one of the drivers 30. A first terminal of each of the piezoelectric elements 40 is connected to an output terminal of the driver 30 that corresponds to the piezoelectric element 40 via the wiring 52, and the second terminals of the piezoelectric elements 40 are connected in common to the ground wiring 728 (the ground voltage G) of the FFC 70.
The voltage amplifier 210 of FIG. 2 amplifies the voltage of the control signal COM, which is supplied from the control signal supply unit 140 (the D/A converter 144) on the control substrate 12 via the control wiring 724 of the FFC 70. The selection unit 230 includes a plurality of switches 232. Each of the switches 232 corresponds one-for-one with each set that includes one of the drivers 30 of the element drive unit 240 and one of the piezoelectric elements 40 of the head module 24. After the control signal COM is amplified by the voltage amplifier 210, the first terminals of the switches 232 are supplied with the control signal COM in common, and the second terminal of each of the switches 232 is connected to an input terminal of the driver 30 that corresponds to the switch 232. Therefore, when one of the switches 232 is controlled to enter an ON state, the driver 30 of the subsequent stage of the switch 232 is supplied with the control signal COM. Conversely, when one of the switches 232 enters an OFF state, the supply of the control signal COM stops in relation to the driver 30 of the subsequent stage of the switch 232. The head control unit 220 of FIG. 2 controls each of the switches 232 of the selection unit 230 according to the print data DP that is supplied from the print data generating unit 120 on the control substrate 12 via the wiring 722 of the FFC 70. In other words, the selection unit 230 supplies the control signal COM, which is supplied from the control unit 10, to each of the drivers 30 that are selected according to the print data DP.
The auxiliary power supply unit 50 of FIG. 2 is a booster circuit, which generates a plurality of voltages from the voltage VH that is supplied from the main power supply unit 180 on the control substrate 12 via the wiring 726 of the FFC 70. Specifically, the auxiliary power supply unit 50 generates a 1/6 voltage, a 2/6 voltage, a 3/6 voltage, a 4/6 voltage, and a 5/6 voltage in relation to the voltage VH by using a charge pump circuit to perform voltage division and redistribution on the voltage VH. The auxiliary power supply unit 50 then supplies the generated voltages together with the voltage VH to the plurality of drivers 30 in common. The driver 30 is a circuit (a connection path selecting unit), which drives (charges or discharges) the piezoelectric element 40 according to the control signal supplied from the selection unit 230 by using the plurality of power supply voltages that are supplied from the auxiliary power supply unit 50. Furthermore, a configuration may also be adopted in which each of the switches 232 of the selection unit 230 selects one of the control signals COM of a plurality of systems that are supplied to the print head 20 in parallel from the control substrate 12, and supplies the selected control signal COM to the driver 30 of the subsequent stage.
The path length L1 of FIG. 2 is the path length of the control wiring 724 of 1 wire that transmits the control signal COM between the control substrate 12 and the print head substrate 22. Specifically, the path length L1 is equivalent to the entire length of the path spanning from the terminal portion of the control wiring 724 on the FFC 70, which is connected to the control substrate 12, to the terminal portion that is connected to the print head substrate 22. Note that, in this embodiment, a case is assumed in which the wirings 72 (722, 724, 726, and 728) of the FFC 70 have a common path length L1.
On the other hand, the path length L2 of FIG. 2 refers to the path length between the print head substrate 22 and the piezoelectric elements 40. Specifically, the path length L2 is equivalent to the entire length of the path spanning from the terminal portions that are connected to the print head substrate 22 of a wiring (a wiring pattern) 52, which connects the print head substrate 22 to the piezoelectric elements 40, to the terminal portions that are connected to the electrodes of the piezoelectric elements 40. The path length L1 is sufficiently long in comparison with the path length L2 (L1>L2). For example, when assuming that the printing apparatus 100, which can print onto the large format (for example, A2 size or bigger) recording medium 200, is used, the path length L1 is approximately 4 m; and conversely, the path length L2 is approximately 0.1 m. A path length L1A from the semiconductor integrated circuit on the control substrate 12 to the semiconductor integrated circuit on the print head substrate 22, and a path length L2A from the semiconductor integrated circuit on the print head substrate 22 to the piezoelectric elements 40 are taken into consideration with a focus on the semiconductor integrated circuit, on which is mounted the control signal supply unit 140 (the D/A converter 144) on the control substrate 12, and the semiconductor integrated circuit on which is mounted each of the elements (the voltage amplifier 210, the selection unit 230, the head control unit 220, the element drive unit 240, and the auxiliary power supply unit 50) on the print head substrate 22. Therefore, it is possible to adopt a configuration in which the path length L1A is sufficiently long in comparison with the path length L2A.
Driver 30
FIG. 4 is a diagram showing an example of the configuration of the driver 30 that drives one of the piezoelectric elements 40. As shown in FIG. 4, the driver 30 generates the voltage Vout using seven voltages including voltage zero; specifically, generates the voltage Vout using the following voltages in low-to-high order of voltage zero (ground voltage G), 1/6 VH, 2/6 VH, 3/6 VH, 4/6 VH, 5/6 VH, and VH. The voltage 1/6 VH is supplied to the driver 30 from the auxiliary power supply unit 50 via the power supply wiring 511. Similarly, the voltages 2/6 VH, 3/6 VH, 4/6 VH, and 5/6 VH are supplied to the respective drivers 30 from the auxiliary power supply unit 50 via power supply wirings 512, 513, 514, and 515. As shown in FIG. 4, the driver 30 includes an operational amplifier 32, unit circuits 34a to 34f, and comparators 38a to 38e. The driver 30 drives the piezoelectric element 40 according to the control signal Vin, from which the control signal COM is extracted by the selection unit 230.
The control signal Vin that is output from the selection unit 230 is supplied to the input terminal (+) of the operational amplifier 32, which is the input terminal of the driver 30. The output signal of the operational amplifier 32 is supplied to each of the unit circuits 34a to 34f, returns to the input terminal (−) of the operational amplifier 32 via the resistance Rf by negative feedback, and is further connected to the ground wiring 728 via the resistance Rin. Therefore, the operational amplifier 32 multiplies the control signal Vin by (1+Rf/Rin) using non-inverting amplification. It is possible to set the voltage amplification ratio of the operational amplifier 32 using the resistances Rf and Rin. However, for convenience, hereinafter Rf is set to zero and Rin is set to infinite. In other words, description is given with the assumption that the voltage amplification ratio of the operational amplifier 32 is “1”, and that the control signal Vin is supplied as-is to the unit circuits 34a to 34f. Note that the voltage amplification ratio may also be a value other than “1”.
Each of the unit circuits 34a to 34f is provided to correspond to two neighboring voltages, of the seven voltages described above, in low-to-high voltage order. Specifically, the unit circuit 34a corresponds to the voltage zero and the voltage 1/6 VH, the unit circuit 34b corresponds to the voltage 1/6 VH and the voltage 2/6 VH, the unit circuit 34c corresponds to the voltage 2/6 VH and the voltage 3/6 VH, the unit circuit 34d corresponds to the voltage 3/6 VH and the voltage 4/6 VH, the unit circuit 34e corresponds to the voltage 4/6 VH and the voltage 5/6 VH, the unit circuit 34f corresponds to the voltage 5/6 VH and the voltage VH.
The circuit configuration of each of the unit circuits 34a to 34f is the same as that of the others, and includes a level shifter that corresponds to one of the level shifters 36a to 36f, a bipolar NPN-type transistor 341 and a PNP-type transistor 342. Furthermore, when the unit circuits 34a to 34f are described generally without being specified, they will be described simply using the reference numeral “34”. Similarly, when the level shifters 36a to 36f are described generally without being specified, the will be described simply using the reference numeral “36”.
The level shifter 36 enters either an enable state or a disable state. Specifically, the level shifter 36 enters the enable state when the signal supplied to the negative control terminal, which is marked with a circular symbol, is an L level and the signal supplied to the positive control terminal, which is not marked with the circular symbol, is an H level. The level shifter 36 is in the disable state during other times.
As described below, of the seven voltages described above, the comparators 38a to 38e are associated one-for-one with the five voltages excluding the voltage zero and the voltage VH. Here, focusing on the unit circuit 34, the output signal of the comparator that is associated with, of the two voltages associated with the unit circuit 34, the voltage of the high potential side is supplied to the negative control terminal of the level shifter 36 in the unit circuit 34. Furthermore, the output signal of the comparator that is associated with, of the two voltages associated with the unit circuit 34, the voltage of the low potential side is supplied to the positive control terminal of the level shifter 36. However, the negative control terminal of the level shifter 36f in the unit circuit 34f is connected to the ground wiring 728 of the voltage zero (the ground voltage G), which is equivalent to the L level. Conversely, the positive control terminal of the level shifter 36a in the unit circuit 34a is connected to a power supply wiring 516 (the wiring 726) that supplies the voltage VH, which is equivalent to the H level.
In the enable state, the level shifter 36 causes the voltage of the input control signal Vin to shift in the negative direction by a predetermined value, and supplies the shifted voltage to the base terminal of the transistor 341, and causes the voltage of the control signal Vin to shift in the positive direction by a predetermined amount, and supplies the shifted voltage to the base terminal of the transistor 342. In the disable state, the level shifter 36 supplies a voltage that causes the transistor 341 to turn off, for example, the voltage VH to the base terminal of the transistor 341 regardless of the control signal Vin, and supplies a voltage that causes the transistor 342 to turn off, for example, the voltage zero to the base terminal of the transistor 342. Note that the predetermined value is set to the voltage (the bias voltage, approximately 0.6 volts) between the base and the emitter, when a current starts flowing to the emitter terminal. Therefore, the predetermined value is characterized according to the properties of the transistors 341 and 342, and is zero if the transistors 341 and 342 are ideal.
The collector terminal of the transistor 341 is connected to the power supply wiring that supplies the high potential side voltage of the two corresponding voltages. The collector terminal of the transistor 342 is connected to the power supply wiring that supplies the low potential side voltage. For example, in the unit circuit 34a that corresponds to the voltage zero and the voltage 1/6 VH, the collector terminal of the transistor 341 is connected to the power supply wiring 511 that supplies the voltage 1/6 VH, and the collector terminal of the transistor 342 is connected to the ground wiring 728 of the voltage zero (the ground voltage G). For example, in the unit circuit 34b that corresponds to the voltage 1/6 VH and the voltage 2/6 VH, the collector terminal of the transistor 341 is connected to the power supply wiring 512 that supplies the voltage 2/6 VH, and the collector terminal of the transistor 342 is connected to the power supply wiring 511 that supplies the voltage 1/6 VH. Furthermore, in the unit circuit 34f that corresponds to the voltage 5/6 VH and the voltage VH, the collector terminal of the transistor 341 is connected to the power supply wiring 516 that supplies the voltage VH, and the collector terminal of the transistor 342 is connected to the power supply wiring 515 that supplies the voltage 5/6 VH.
Meanwhile, the emitter terminals of the transistors 341 and 342 in the unit circuits 34a to 34f are connected in common to the first terminal of the piezoelectric element 40. In other words, the common connection point of the emitter terminals of the transistors 341 and 342 is connected to the first terminal of the piezoelectric element 40 as the output terminal of the driver 30. Therefore, the voltage of the first terminal of the piezoelectric element 40, that is, the voltage held by the piezoelectric element 40 is represented as the voltage Vout to include the meaning of the output voltage of the driver 30.
Of the seven voltages described above, the comparators 38a to 38e correspond to the five voltages excluding the voltage zero and the voltage VH of 1/6 VH, 2/6 VH, 3/6 VH, 4/6 VH, and 5/6 VH, the levels of voltages supplied to the two input terminals are compared with one another and a signal indicating the comparison results is output. Here, of the two input terminals in the comparators 38a to 38e, the first terminal is connected to the power supply wiring that supplies the voltage that corresponds to itself, and the second terminal is connected to each emitter terminal of the transistors 341 and 342 and is connected in common to the first terminal of the piezoelectric element 40. For example, in regard to the comparator 38a that corresponds to the voltage 1/6 VH, of the two input terminals thereof, the first terminal is connected to the power supply wiring 511, which supplies the voltage 1/6 VH corresponding to itself. In addition, for example, in regard to the comparator 38b that corresponds to the voltage 2/6 VH, of the two input terminals, the first terminal is connected to the power supply wiring 512, which supplies the voltage 2/6 VH corresponding to itself.
In relation to the input terminal thereof, each of the comparators 38a to 38e outputs a signal of the H level if the voltage Vout of the second terminal is equal to or higher than the voltage of the first terminal, or of the L level if the voltage Vout is lower than the voltage of the first terminal. Specifically, for example, the comparator 38a that corresponds to the voltage 1/6 VH sets the output signal to the H level if the voltage Vout is equal to or higher than the voltage 1/6 VH, and to the L level if the voltage Vout is lower than the voltage 1/6 VH. For example, the comparator 38b that corresponds to the voltage 2/6 VH sets the output signal to the H level if the voltage Vout is equal to or higher than the voltage 2/6 VH, and to the L level if the voltage Vout is lower than the voltage 2/6 VH.
Focusing on one of the five voltages, the output signal of the comparator that corresponds to the voltage being focused on is supplied to the negative input terminal of the level shifter 36 of the unit circuit, in which the voltage is set to the high potential side voltage, and the positive input terminal of the level shifter 36 of the unit circuit, in which the voltage is set to the low potential side voltage. For example, the output signal of the comparator 38a that corresponds to the voltage 1/6 VH is supplied to the negative input terminal of the level shifter 36a of the unit circuit 34a, in which the voltage 1/6 VH is associated with the high potential side voltage, and the positive input terminal of the level shifter 36b of the unit circuit 34b, in which the voltage 1/6 VH is associated with the low potential side voltage. In addition, for example, the output signal of the comparator 38b that corresponds to the voltage 2/6 VH is supplied to the negative input terminal of the level shifter 36b of the unit circuit 34b, in which the voltage 2/6 VH is associated with the high potential side voltage, and the positive input terminal of the level shifter 36c of the unit circuit 34c, in which the voltage 2/6 VH is associated with the low potential side voltage.
Next, description will be given of the operations of the driver 30. First, description will be given of the states that the comparators 38a to 38e and the level shifter 36 enter in relation to the voltage Vout held by the piezoelectric element 40.
All of the output signals of the comparators 38a to 38e are the L level in a state (a first state) in which the voltage Vout is equal to or higher than the voltage zero and lower than the voltage 1/6 VH. Therefore, in the first state, only the level shifter 36a enters the enable state, and the other level shifters 36b to 36f enter the disable state.
The output signal of the comparator 38a is the H level and the output signals of the other comparators 38b to 38e are the L level in a state (a second state) in which the voltage Vout is equal to or higher than the voltage 1/6 VH and lower than the voltage 2/6 VH. Therefore, in the second state, only the level shifter 36b enters the enable state, and the other level shifters 36a and 36c to 36f enter the disable state.
The output signals of the comparators 38a and 38b are the H level and the output signals of the other comparators 38c to 38e are the L level in a state (a third state) in which the voltage Vout is equal to or higher than the voltage 2/6 VH and lower than the voltage 3/6 VH. Therefore, in the third state, only the level shifter 36c enters the enable state, and the other level shifters 36a, 36b and 36d to 36f enter the disable state.
The output signals of the comparators 38a to 38c are the H level and the output signals of the other comparators 38d and 38e are the L level in a state (a fourth state) in which the voltage Vout is equal to or higher than the voltage 3/6 VH and lower than the voltage 4/6 VH. Therefore, in the fourth state, only the level shifter 36d enters the enable state, and the other level shifters 36a to 36c, 36e and 36f enter the disable state.
The output signals of the comparators 38a to 38d are the H level and the output signals of the other comparator 38e is the L level in a state (a fifth state) in which the voltage Vout is equal to or higher than the voltage 4/6 VH and lower than the voltage 5/6 VH. Therefore, in the fifth state, only the level shifter 36e enters the enable state, and the other level shifters 36a to 36d and 36f enter the disable state.
All of the output signals of the comparators 38a to 38e are the H level in a state (a sixth state) in which the voltage Vout is equal to or higher than the voltage 5/6 VH and lower than the voltage VH. Therefore, in the sixth state, only the level shifter 36f enters the enable state, and the other level shifters 36a to 36e enter the disable state.
In this manner, in the first state, only the level shifter 36a enters the enable state, and similarly hereinafter, in the second state, the third state, the fourth state, the fifth state, and the sixth state, only the level shifter 36b, the level shifter 36c, the level shifter 36d, the level shifter 36e, and the level shifter 36f enter the enable state, respectively.
Note that, while the states from the first state to the sixth state are defined using the voltage Vout, the states can also be referred to as the state of the charge held (accumulated) in the piezoelectric element 40.
In the first state, when the level shifter 36a is in the enable state, the level shifter 36a supplies a voltage signal, which is obtained by level shifting the control signal Vin in the negative direction by a predetermined value, to the base terminal of the transistor 341 in the unit circuit 34a, and supplies a voltage signal, which is obtained by level shifting the control signal Vin in the positive direction by a predetermined value, to the base terminal of the transistor 342 in the unit circuit 34a. 
Here, when the voltage of the control signal Vin is higher than the voltage Vout (the connection point voltage between the emitter terminals), a current that corresponds to the difference therebetween (the voltage between the base and the emitter, strictly speaking, the voltage obtained by subtracting the predetermined value from the voltage between the base and the emitter) flows from the collector terminal of the transistor 341 to the emitter terminal. Therefore, the voltage Vout slowly rises and approaches the voltage of the control signal Vin. When the voltage Vout eventually matches the voltage of the control signal Vin, the current flowing in the transistor 341 becomes zero at this point in time.
On the other hand, when the voltage of the control signal Vin is lower than the voltage Vout, a current that corresponds to the difference therebetween flows from the emitter terminal of the transistor 342 to the collector terminal. Therefore, the voltage Vout slowly falls and approaches the voltage of the control signal Vin. When the voltage Vout eventually matches the voltage of the control signal Vin, the current flowing in the transistor 342 becomes zero at this point in time.
Accordingly, in the first state, the transistors 341 and 342 of the unit circuit 34a execute control such that the voltage Vout is caused to match the control signal Vin.
Also in the first state, in the unit circuits 34b to 34f other than the unit circuit 34a, since the level shifter 36 enters the disable state, the voltage VH is supplied to the base terminal of the transistor 341 and the voltage zero is supplied to the base terminal of the transistor 342. Therefore, in the first state, in the unit circuits 34b to 34f, since the transistors 341 and 342 turn off, the transistors 341 and 342 do not influence the control of the voltage Vout.
Note that, here, description is given of the first state; however, the same operations are also performed in the second state to the sixth state. Specifically, according to the voltage Vout held by the piezoelectric element 40, the transistors 341 and 342 of the unit circuit, which becomes active together with one of the unit circuits 34a to 34f becoming active, perform control to cause the voltage Vout to match the control signal Vin. Therefore, when viewed as a whole, the driver 30 operates such that the voltage Vout follows the control signal Vin.
Accordingly, as shown in FIG. 5A, when the control signal Vin rises from the voltage zero to the voltage VH, for example, the voltage Vout also changes from the voltage zero to the voltage VH, following the control signal Vin. As shown in FIG. 5B, when the control signal Vin drops from the voltage VH to the voltage zero, the voltage Vout also changes from the voltage VH to the voltage zero, following the control signal Vin.
FIGS. 6A to 6C are diagrams illustrating the operations of the level shifter.
When the control signal Vin rises from the voltage zero to the voltage VH, the voltage Vout also rises, following the control signal Vin. In the process of rising, when in the first state, in which the voltage Vout is equal to or higher than the voltage zero and lower than the voltage 1/6 VH, the level shifter 36a is in the enable state. Therefore, as shown in FIG. 6A, the voltage (represented as “P-type”) that is supplied to the base terminal of the transistor 341 by the level shifter 36a becomes a voltage obtained by shifting the control signal Vin in the negative direction by a predetermined amount, and the voltage (represented as “N-type”) that is supplied to the base terminal of the transistor 342 becomes a voltage obtained by shifting the control signal Vin in the positive direction by a predetermined amount. On the other hand, when in a state other than the first state, since the level shifter 36a enters the disable state, the voltage that is supplied to the base terminal of the transistor 341 becomes VH, and the voltage that is supplied to the base terminal of the transistor 342 becomes zero.
Note that, FIG. 6B shows a voltage waveform that is output by the level shifter 36b, and FIG. 6C shows a voltage waveform that is output by the level shifter 36f. Considering that, when in the second state, in which the voltage Vout is equal to or higher than the voltage zero and lower than the voltage 2/6 VH, the level shifter 36b enters the enable state, and when in the sixth state, in which the voltage Vout is equal to or higher than the voltage 5/6 VH and less than the voltage VH, the level shifter 36f enters the enable state, it is evident that description thereof is not particularly necessary.
Description of the operations of the level shifters 36c to 36e in the rising process of the voltage of the control signal Vin (or the voltage Vout), and description of the operations of the level shifters 36a to 36f in the falling process of the voltage of the control signal Vin (or the voltage Vout) will also be omitted.
Next, description will be given of the flow of current (charge) in the unit circuits 34a to 34f, using the unit circuits 34a and 34b as examples. Note that the description will be divided into when charging and when discharging take place.
FIG. 7 is a diagram illustrating the operation when the piezoelectric element 40 is charged, when in the first state (the state in which the voltage Vout is equal to or higher than the voltage zero and lower than the voltage 1/6 VH).
In the first state, the level shifter 36a enters the enable state, and the other level shifters 36b to 36f enter the disable state; therefore, only the unit circuit 34a may be focused on.
In the first state, when the voltage of the control signal Vin is higher than the voltage Vout, a current that corresponds to the voltage between the base and the emitter of the transistor 341 of the unit circuit 34a flows. Accordingly, the transistor 341 of the unit circuit 34a functions as the first transistor. Note that the transistor 342 of the unit circuit 34a is off at this time.
At this time, the current flows along a path of the power supply wiring 511→the transistor 341 (of the unit circuit 34a)→the piezoelectric element 40, as shown by the arrows in FIG. 7, and the piezoelectric element 40 is charged with a charge. The voltage Vout rises according to the charging.
When the voltage Vout matches the voltage of the control signal Vin, the transistor 341 of the unit circuit 34a turns off; thus, the charging to the piezoelectric element 40 stops.
On the other hand, when the control signal Vin rises to be equal to or higher than the voltage 1/6 VH, since the voltage Vout follows the control signal Vin, the voltage Vout becomes equal to or higher than the voltage 1/6 VH, and transitions from the first state to the second state (the state in which the voltage Vout is equal to or higher than the voltage 1/6 VH and lower than the voltage 2/6 VH).
FIG. 8 is a diagram illustrating the operation in the second state, when the piezoelectric element 40 is charged.
In the second state, the level shifter 36b enters the enable state, and the other level shifters 36a and 36c to 36f enter the disable state; therefore, only the unit circuit 34b may be focused on.
In the second state, when the control signal Vin is higher than the voltage Vout, a current that corresponds to the voltage between the base and the emitter of the transistor 341 of the unit circuit 34b flows. Accordingly, the transistor 341 of the unit circuit 34b functions as the third transistor. Note that the transistor 342 of the unit circuit 34b is off at this time.
At this time, the current flows along a path of the power supply wiring 512→the transistor 341 (of the unit circuit 34b)→the piezoelectric element 40, as shown by the arrows in FIG. 8, and the piezoelectric element 40 is charged with a charge. In other words, in the second state, when the piezoelectric element 40 is charged, the first terminal of the piezoelectric element 40 is electrically connected to the auxiliary power supply unit 50 via the power supply wiring 512.
In this manner, when transitioning from the first state to the second state during the rising of the voltage Vout, the supply source of the current switches from the power supply wiring 511 to the power supply wiring 512.
When the voltage Vout matches the voltage of the control signal Vin, the transistor 341 of the unit circuit 34b turns off; thus, the charging to the piezoelectric element 40 stops.
On the other hand, when the control signal Vin rises to be equal to or higher than the voltage 2/6 VH, since the voltage Vout follows the control signal Vin, the voltage Vout becomes equal to or higher than the voltage 2/6 VH, and, as a result, transitions from the second state to the third state (the state in which the voltage Vout is equal to or higher than the voltage 2/6 VH and lower than the voltage 3/6 VH).
Note that, while the charging operations from the third state to the sixth state are not particularly shown in the drawings, the supply source of the current switches in stages between the power supply wirings 513, 514, 515, and 516.
FIG. 9 is a diagram illustrating the operation when the piezoelectric element 40 is discharged in the second state.
In the second state, the level shifter 36b enters the enable state. In this state, when the control signal Vin is lower than the voltage Vout, a current that corresponds to the voltage between the base and the emitter of the transistor 342 of the unit circuit 34b flows. Accordingly, the transistor 341 of the unit circuit 34b functions as the second transistor. Note that the transistor 341 of the unit circuit 34b is off at this time.
At this time, the current flows along a path of the piezoelectric element 40→the transistor 342 (of the unit circuit 34b)→the power supply wiring 511, as shown by the arrows in FIG. 9, and a charge is discharged from the piezoelectric element 40. In other words, when the piezoelectric element 40 is charged with a charge in the first state, and, when a charge is discharged from the piezoelectric element 40 in the second state, the first terminal of the piezoelectric element 40 is electrically connected to the auxiliary power supply unit 50 via the power supply wiring 511. In addition, the power supply wiring 511 supplies a current (a charge) during the charging of the first state, and recovers the current (the charge) during the discharging of the second state. Note that the recovered charge is redistributed by the auxiliary power supply unit 50, which is described later, and reused.
When the voltage Vout matches the voltage of the control signal Vin, the transistor 342 of the unit circuit 34b turns off; thus, the discharging from the piezoelectric element 40 stops.
On the other hand, when the control signal Vin falls to lower than the voltage 1/6 VH, since the voltage Vout follows the control signal Vin, the voltage Vout becomes lower than the voltage 1/6 VH, and transitions from the second state to the first state.
FIG. 10 is a diagram illustrating the operation when the piezoelectric element 40 is discharged in the first state.
In the first state, the level shifter 36a enters the enable state. In this state, when the control signal Vin is lower than the voltage Vout, a current that corresponds to the voltage between the base and the emitter of the transistor 342 of the unit circuit 34a flows.
Note that the transistor 341 of the unit circuit 34a is off at this time.
At this time, the current flows along a path of the piezoelectric element 40→the transistor 342 (of the unit circuit 34a)→the ground wiring 728, as shown by the arrow in FIG. 10, and a charge is discharged from the piezoelectric element 40.
Note that, here, description is given divided into when charging and when discharging take place using the unit circuits 34a and 34b as examples; however, the unit circuits 34c to 34f operate in approximately the same manner, except that the transistors 341 and 342 that control the current are different.
In other words, the power supply wiring 512 supplies a current (a charge) during the charging of the second state and recovers the current (the charge) during the discharging of the third state, the power supply wiring 513 supplies a current (a charge) during the charging of the third state and recovers the current (the charge) during the discharging of the fourth state, the power supply wiring 514 supplies a current (a charge) during the charging of the fourth state and recovers the current (the charge) during the discharging of the fifth state, the power supply wiring 515 supplies a current (a charge) during the charging of the fifth state and recovers the current (the charge) during the discharging of the sixth state, the power supply wiring 516 supplies a current (a charge) during the charging of the sixth state, and the recovered charge is redistributed by the auxiliary power supply unit 50 and reused.
As can be understood from the above description, each of the drivers 30 of the element drive unit 240 functions as an element that executes an operation in which the auxiliary power supply unit 50 is caused to supply a charge that corresponds to the control signal COM to the piezoelectric element 40, and an operation in which the piezoelectric element 40 is caused to discharge a charge that corresponds to the control signal COM to the auxiliary power supply unit 50. Note that, in the discharging paths and the charging paths in each of the states, the path is common from the first terminal of the piezoelectric element 40 to the connection point of the emitter terminals in the transistors 341 and 342.
In general, when the capacity of a capacitive load such as the piezoelectric element 40 is represented as C, and the voltage amplitude as E, the energy P that is accumulated in the capacitive load is represented by P=(C·E2)/2.
The piezoelectric element 40 works by deforming according to the energy P; however, the amount of work spent causing the ink to be discharged accounts for 1% or less of the energy P. Accordingly, the piezoelectric element 40 can be perceived as a simple capacitance. When the capacity C is charged by a fixed power supply, an energy equivalent to (C·E2)/2 is consumed by the charging circuit. When discharging, an equal amount of energy is also consumed by the discharge circuit.
Merits of Driver 30
In this embodiment, when the piezoelectric element 40 is charged from the voltage zero to the voltage VH, the piezoelectric element 40 is charged through the six stages of from the voltage zero to the voltage 1/6 VH, from the voltage 1/6 VH to the voltage 2/6 VH, from the voltage 2/6 VH to the voltage 3/6 VH, from the voltage 3/6 VH to the voltage 4/6 VH, from the voltage 4/6 VH to the voltage 5/6 VH, and from the voltage 5/6 VH to the voltage VH. Therefore, in this embodiment, the loss during the charging is merely an amount that is equivalent to the area of the shaded region in FIG. 11A. Specifically, in this embodiment, the loss during the charging in the piezoelectric element 40 is merely 6/36 (=16.7%) in comparison with linear amplification, in which the charging is performed at once from the voltage zero to the voltage VH.
On the other hand, in this embodiment, since the discharging is also performed in stages, the loss during the discharging, as shown by a portion equivalent to the area of the shaded region in FIG. 11B, is also 6/36 (=16.7%) in comparison to a linear system, in which discharging is performed at once from the voltage VH to the voltage zero.
However, in this embodiment, since the total charge calculated as the loss during the discharging is recovered and redistributed by the auxiliary power supply unit 50 (described below) and reused, except for a case in which discharging is performed from the voltage 1/6 VH to the voltage zero, it is possible to obtain further power consumption reduction.
Auxiliary Power Supply Unit 50
FIG. 12 is a diagram showing an example of the configuration of the auxiliary power supply unit 50.
As shown in FIG. 12, the auxiliary power supply unit 50 is configured to include switches Sw1d, Sw1u, Sw2d, Sw2u, Sw3d, Sw3u, Sw4d, Sw4u, Sw5d, and Sw5u, and capacitive elements C12, C23, C34, C45, C56, C1, C2, C3, C4, C5, and C6.
Of these, the switches are all single pole double throw switches, and the common terminal connects to one of the terminals a or b according to the control signal A or B. The control signal A or B can be described in a simplified manner as, for example, a pulse signal with a duty ratio of approximately 50% in which the frequency thereof is set to, for example, approximately 20 times the frequency of the control signal COM. The control signal A or B may be generated by an internal oscillator (not shown) in the auxiliary power supply unit 50, and may also be supplied from the control unit 10 via the FFC 70.
Meanwhile, the capacitive elements C12, C23, C34, C45, and C56 are for moving charges, and the capacitive elements C1, C2, C3, C4, and C5 are used as backups. Note that the capacitive element C6 is for supplying the power supply voltage VH.
The switches described above are actually configured by combining transistors in the semiconductor integrated circuit, and the capacitive elements are implemented externally in relation to the semiconductor integrated circuit. Furthermore, a configuration in which a plurality of the drivers 30 described above are formed on the semiconductor integrated circuit is desirable.
In the auxiliary power supply unit 50, the power supply wiring 516 that supplies the voltage VH is connected between the first terminal of the capacitive element C6 and a terminal a of the switch Sw5u. The common terminal of the switch Sw5u is connected to the first terminal of the capacitive element C56, and the second terminal of the capacitive element C56 is connected to the common terminal of the switch Sw5d. The terminal a of the switch Sw5d is connected between the first terminal of the capacitive element C5 and the terminal a of the switch Sw4u. The common terminal of the switch Sw4u is connected to the first terminal of the capacitive element C45, and the second terminal of the capacitive element C45 is connected to the common terminal of the switch Sw4d. The terminal a of the switch Sw4d is connected between the first terminal of the capacitive element C4 and the terminal a of the switch Sw3u. The common terminal of the switch Sw3u is connected to the first terminal of the capacitive element C34, and the second terminal of the capacitive element C34 is connected to the common terminal of the switch Sw3d. The terminal a of the switch Sw3d is connected between the first terminal of the capacitive element C3 and the terminal a of the switch Sw2u. The common terminal of the switch Sw2u is connected to the first terminal of the capacitive element C23, and the second terminal of the capacitive element C23 is connected to the common terminal of the switch Sw2d. The terminal a of the switch Sw2d is connected between the first terminal of the capacitive element C2 and the terminal a of the switch Sw1u. The common terminal of the switch Sw1u is connected to the first terminal of the capacitive element C12, and the second terminal of the capacitive element C12 is connected to the common terminal of the switch Sw1d. The terminal a of the switch Sw1d is connected to the first terminal of the capacitive element C1.
The first terminal of the capacitive element C5 is connected to the power supply wiring 515. Similarly, the first terminals of the capacitive elements C4, C3, C2, and C1 are respectively connected to the power supply wirings 514, 513, 512, and 511.
Furthermore, each of the terminals b of the switches Sw5u, Sw4u, Sw3u, Sw2u, and Sw1u are connected, together with the terminal a of the switch Sw1d, to the first terminal of the capacitive element C1. Each of the second terminals of the capacitive elements C6, C5, C4, C3, C2, and C1 and each of the terminals b of the switches Sw5d, Sw4d, Sw3d, Sw2d, and Sw1d are connected in common to the ground wiring 728.
FIGS. 13A and 13B are diagrams showing the connection state of the switches in the auxiliary power supply unit 50.
According to the control signal A or B, each of the switches assumes one of two states of a state (state A) in which the common terminal is connected to the terminal a, and a state (state B) in which the common terminal is connected to the terminal b. FIG. 13A shows the connections of the state A in the auxiliary power supply unit 50 and FIG. 13B shows the connections of the state B. Both FIGS. 13A and 13B show simplified equivalent circuits.
In the state A, the capacitive elements C56, C45, C34, C23, C12, and C1 are connected in series between the wiring 726 (the voltage VH) and the ground wiring 728 (the ground voltage G). In the state B, the first terminals of the capacitive elements C56, C45, C34, C23, C12, and C1 are connected to one another; thus, the capacitive elements thereof are connected in parallel, and the voltages held therein are equalized.
Accordingly, when the states A and B are repeated alternately, the voltages 1/6 VH that are equalized during the state B are multiplied from 1 to 5 times by the series connections of the state A, and the voltages held at this time are supplied to the driver 30 via the power supply wirings 511 to 515 in addition to being held in the capacitive elements C1 to C5.
Here, when the piezoelectric element 40 is charged by the driver 30, of the capacitive elements C1 to C5, some emerge in which the held voltage decreases. The capacitive elements in which the held voltage decreased are supplied with a charge from the power supply due to the series connection of the state A, and since the capacitive elements are equalized by the redistribution due to the parallel connection of the state B, a balance is maintained at the voltage 1/6 VH, 2/6 VH, 3/6 VH, 4/6 VH, and 5/6 VH from the perspective of the entire auxiliary power supply unit 50.
On the other hand, when the piezoelectric element 40 is discharged by the driver 30, of the capacitive elements C1 to C5, some emerge in which the held voltage rises; however, the charge is swept out due to the series connection of the state A, and since the capacitive elements are equalized by the redistribution due to the parallel connection of the state B, a balance is maintained at the voltage 1/6 VH, 2/6 VH, 3/6 VH, 4/6 VH, and 5/6 VH from the perspective of the entire auxiliary power supply unit 50. Note that, when the current that is swept out cannot be absorbed by the capacitive elements C56, C45, C34, C23, C12, and C1, and thus an excess charge remains, the excess charge is absorbed by the capacitive element C6, that is, returned to the power supply system. Therefore, if there is another load, other than the piezoelectric element 40, the charge is used to drive the load. If there is not another load, since the charge is absorbed by the other capacitive elements including the capacitive element C6, the power supply voltage VH rises, that is, rippling occurs; however, such rippling can be practically avoided by increasing the capacity of the coupling capacitors including the capacitive element C6. As can be understood from the above description, the auxiliary power supply unit 50 (the capacitive elements C1, C2, C3, C4, and C5) functions as an element (a charge supply source) that supplies a charge to each of the drivers (each of the piezoelectric elements 40).
When the voltages 1/6 VH, 2/6 VH, 3/6 VH, 4/6 VH, and 5/6 VH, which are generated by the auxiliary power supply unit 50, are supplied to the drivers 30, in addition to being able to obtain a reduction in power consumption, the following merits are also obtained. In other words, even when the voltage VH that is supplied from the main power supply unit 180 is changed, the voltages 1/6 VH, 2/6 VH, 3/6 VH, 4/6 VH, and 5/6 VH are changed to correspond to the changed voltage VH.
The amplitude of the power supply voltage VH has a characteristic in that the amplitude is to be set according to the individual performance of the piezoelectric elements 40. Therefore, the (high efficiency) piezoelectric element 40, which has high performance, may be driven using a relatively low amplitude such as that indicated as rank A in FIG. 14A. In contrast, the (low efficiency) piezoelectric element 40, which has low performance, has to be driven using a relatively high amplitude such as that indicated as rank B.
When the voltage VH is fixed in a high state to accommodate rank B in order to drive the piezoelectric elements 40 of both ranks A and B, the loss increases. In particular, there is a great amount of waste when driving the rank A piezoelectric elements, for which a low amplitude is sufficient.
Therefore, as shown in FIG. 14B, when the voltage VH is set appropriately to accommodate the performance (the efficiency) of the piezoelectric elements 40, in particular, it is possible to suppress wasteful loss even when driving the rank A piezoelectric elements.
Note that, in the auxiliary power supply unit 50, when the common terminal of each of the switches is switched from connecting to one of the terminals a and b to connecting to the other, if there are variations in the properties of the plurality (10 in FIG. 12) of switches, it is possible that a state in which the switches do not all switch at once will occur, causing short circuiting between the two terminals of the capacitive elements. For example, during the switching, when the terminal a of each of the switches Sw1u, Sw1d, and Sw2d are connected to the common terminal, if a state occurs in which the terminal b of the switch Sw2u is connected to the common terminal, the two terminals of each of the capacitive elements C12 and C23 in the series connection are short circuited with one another.
Therefore, it is preferable to adopt a configuration in which the occurrence of short circuiting is suppressed by, during the switching of the switches, temporarily entering a neutral state in which neither the terminal a or b is connected. The above description is the configuration of the auxiliary power supply unit 50.
In this embodiment, the element drive unit 240 and the auxiliary power supply unit 50 of the configuration described above are implemented on the print head substrate 22; however, it is also conceivable to adopt the configuration shown in FIG. 15 (hereinafter referred to as the “comparative example”) as the configuration that drives the plurality of piezoelectric elements 40. In the comparative example of FIG. 15, in addition to the print data generating unit 120 and the control signal supply unit 140, a voltage amplifier 192 and a current amplifier 194 are installed on the control substrate 12. The voltage amplifier 192 amplifies the voltage of the control signal COM, which is generated by the control signal supply unit 140, and the current amplifier 194 amplifies the current of the control signal COM after the amplification performed by the voltage amplifier 192. After being amplified by the current amplifier 194, the control signal COMa passes through the control wiring 724 of the FFC 70 and is supplied to the print head substrate 22.
In addition, in the comparative example, a plurality of high breakdown voltage switches 234 and a head control unit 220 are installed on the print head substrate 22. Each of the high breakdown voltage switches 234 corresponds one-for-one with each of the piezoelectric elements 40, and switches between the supply and cut-off of the control signal COM in relation to the corresponding piezoelectric element 40. The head control unit 220 controls each of the high breakdown voltage switches 234 according to the print data DP that is generated by the print data generating unit 120.
In the comparative example of FIG. 15, since the control signal COMa is supplied to each of the piezoelectric elements 40 via the high breakdown voltage switch 234, focusing on the current path from the control wiring 724 to the ground wiring 728 that passes through each of the piezoelectric elements 40, the drive load fluctuates according to the total number of piezoelectric elements 40 to which the control signal COMa is supplied. In order to supply the control signal COMa of an appropriate waveform to each of the piezoelectric elements 40 even when the drive load is great (when the control signal COMa is supplied to a large number of the piezoelectric elements 40 in parallel), it is necessary to sufficiently amplify the current amount of the control signal COMa, which is supplied from the control substrate 12 to the print head substrate 22, using the current amplifier 194 on the control substrate 12. Accordingly, it becomes a problem to secure the control wiring 724 capable of transmitting an extremely large current in the FFC 70.
The current path of the comparative example is modeled as shown in FIG. 16. The symbol Z1 of FIG. 16 refers to the impedance of, within the FFC 70, the control wiring 724 that transmits the control signal COMa, and the symbol F4 refers to the impedance of, within the FFC 70, the ground wiring 728 that transmits the ground voltage G. The symbol Z2 of FIG. 16 refers to the on-state resistance (Z2=120Ω) of one of the high breakdown voltage switches 234, and the symbol Z3 refers to the impedance of the wiring 52 from one of the high breakdown voltage switches 234 to the piezoelectric element 40. The symbol ZL is the impedance of one of the piezoelectric elements 40.
A case is assumed in which 1600 of the piezoelectric elements 40 with an electrostatic capacitance of 300 pF are installed on the head module 24, and a voltage of 33 V is applied to one of the piezoelectric elements 40 to supply a current of 5 mA. In a situation in which the control signal COM is supplied in parallel to all 1600 of the piezoelectric elements 40, it is necessary to allow a current of 8 A (5 mA×1600) to flow through the control wiring 724 and the ground wiring 728 within the FFC 70. Even in the situation described above, in order to suppress the fall in the voltage applied to the piezoelectric elements 40 to within 5% (1.65 V or lower) of the expected voltage (33 V), it is necessary to suppress the total value of the resistance components of the impedance Z1 of the control wiring 724 and the impedance Z4 of the ground wiring 728 to 0.21Ω.
Now, a case will be considered in which the FFC 70 of a general-use type, which is formed from a wiring of a plurality of parallel wires (3Ω/wire), each of which has a width of 700 μm and a thickness of 35 μm, the total length of which spans 4 m, is adopted for the connection between the control substrate 12 and the print head substrate 22. Since a sufficient current may not be transmitted by a wiring of only one wire, a collection (a bundle) of a wiring of a plurality of wires is used for the control wiring 724 and the ground wiring 728. In order to achieve the previously described conditions (Z1+Z4=0.21Ω) under the configuration described above, it is necessary to use a wiring of 21 wires of the FFC 70 for the control wiring 724 and to use a wiring of 42 wires for the ground wiring 728 (a total of 63 wires). For example, in a configuration in which the control signals COM of two systems are supplied from the control substrate 12 to the print head substrate 22 and selectively supplied to the piezoelectric elements 40, since it is necessary to use the control wiring 724 of 42 wires (21 wires×2 systems) for the transmission of the control signal COM, it is necessary to use the FFC 70 with a wiring of 84 wires.
On the other hand, in this embodiment, the transfer of charges between the auxiliary power supply unit 50 and the piezoelectric elements 40 is executed on the print head substrate 22. FIG. 17 is a schematic diagram modeling the current path of this embodiment. The element drive unit 240 and the auxiliary power supply unit 50 of the print head substrate 22 generate a current (a charge) that is higher than the current supplied from the FFC 70 and use the generated current for driving the piezoelectric elements 40. Specifically, a case is considered in which a current, which is five times the current supplied from the FFC 70, is generated by the print head substrate 22 (a current ratio of 1:5).
As described earlier in the comparative example, in a situation in which a current of 5 mA is supplied to 1600 of the piezoelectric elements 40 (a situation in which a total current of 8 A is necessary), it is necessary to allow a current of 1.6 A (8 A/5) to flow through the control wiring 724 and the ground wiring 728. In a situation in which a current of 1.6 A flows through the control wiring 724 and the ground wiring 728, in order to suppress a fall in the voltage between the control wiring 724 and the ground wiring 728 to approximately 2 V, it is sufficient to use a wiring of approximately 4 to 5 wires for each of the control wiring 724 and the ground wiring 728 in the FFC 70. In other words, in contrast to the comparative example in which a wiring of 84 wires is necessary, in this embodiment, a total number of wires necessary in the wiring for the transmission of the control signal COM and the ground voltage G is from 8 wires to 10 wires. Accordingly, there is a merit in that the number of connection points between each of the control substrate 12 and the print head substrate 22 and the FFC 70 is reduced.
As can be understood from the example described above, according to this embodiment, each of the piezoelectric elements 40 is driven by the transfer of charges between the auxiliary power supply unit 50 on the print head substrate 22 and each of the piezoelectric elements 40. Therefore, in principle, load fluctuation does not occur in the control wiring 724 or the ground wiring 728; thus, the current amount of the control signal COM to be transmitted by the FFC 70 and the fluctuation amount of the current are decreased. Accordingly, the power loss on the FFC 70 is greatly reduced in comparison to the comparative example; thus, it is possible to supply a control signal of an expected waveform to each of the piezoelectric elements 40 in a stable and highly precise manner regardless of the total number of the piezoelectric elements 40, which are the driving target. In other words, according to this embodiment, there is a merit in that it is possible to suppress a reduction in the print quality caused by power loss on the FFC 70. Note that, in the comparative example, since the drive load differs according to the total number (the number of nozzles) of the piezoelectric elements 40 of the print head 20, for example, it is necessary to carry out the evaluation and testing of the drive state of each of the piezoelectric elements 40, and the waveform correction and the like of the control signal separately for each type of the print head 20, in which the total number of the piezoelectric elements 40 differs. On the other hand, since load fluctuation does not occur in this embodiment, there is also a merit in that it is not necessary to carry out the evaluation and testing of the drive states of the piezoelectric elements 40, and the waveform correction and the like of the control signal separately for each type (for each total number of the piezoelectric elements 40) of the print head 20 (consequently, the manufacturing cost of the printing apparatus 100 is also reduced). Electromagnetic Interference (EMI) is also effectively suppressed by reducing the current fluctuation on the FFC 70. Accordingly, the structure for counteracting EMI, which is a problem in the comparative example, (for example, a ferrite core) can be rendered unnecessary or simplified.
Since transistors, electrolyte capacitors and the like are necessary on a large scale for the voltage amplifier 192 and the current amplifier 194, which are necessary in the comparative example, a problem may arise in that the circuit scale and the number of parts increases. In this embodiment, there is a merit in that, since the voltage amplifier 192 and the current amplifier 194 are unnecessary on the control substrate 12, the circuit scale and the number of parts on the control substrate 12 are reduced. Note that, in a configuration in which a large scale circuit such as the one shown in the comparative example is implemented on the control substrate 12, it is difficult to realize the control substrate 12 with a single circuit substrate; thus, it may become necessary to realize the control substrate 12 using a plurality of circuit substrates, and to electrically connect the circuit substrates to one another. In this embodiment, since the circuit scale on the control substrate 12 is minimized, it is possible to sufficiently realize the control substrate 12 using a single circuit substrate. In addition, in the comparative example, since the amount of heat output by the circuits on the control substrate 12 is great, a structure for heat radiation (such as a fan or fins) is necessary, and there is a problem in that the structure becomes complicated. Since it is not necessary to generate a large current on the control substrate 12, in this embodiment, there is also a merit in that the amount of heat output by the control substrate 12 is reduced in comparison to the comparative example. Note that, in this embodiment, while the amount of heat output on the control substrate 12 is reduced, the amount of heat output on the print head substrate 22 is increased in comparison with the comparative example. However, it is possible to effectively use the heat generated on the print head substrate 22 to heat the ink within the print head 20, for example, in order to reduce the viscosity thereof. In other words, there is a merit to outputting heat on the print head substrate 22 in comparison to outputting heat on the control substrate 12.
Note that the configuration of the circuit, which is installed on the print head substrate 22 and charges or discharges the piezoelectric elements 40 according to a control signal COM, is arbitrary. For example, it is possible to install various amplifiers (such as AB class and D class) that amplify the control signal COM, which is supplied from the control substrate 12, and a selection unit that selectively supplies the control signal COM, after the control signal COM undergoes amplification, to each of the piezoelectric elements on the print head substrate 22 instead of the elements (the voltage amplifier 210, the selection unit 230, the head control unit 220, the element drive unit 240, and the auxiliary power supply unit 50) on the print head substrate 22, as exemplified in the embodiment described above.
Application and Modification Examples
The invention is not limited to the embodiments described above; for example, various applications and modifications as described below are possible. Furthermore, the forms of the applications and modifications described below can be arbitrarily selected, or a plurality thereof can be appropriately combined.
Negative Feedback Control
FIG. 18 is a diagram showing an example of the configuration of the driver 30 according to an (a first) application example of the embodiment. As shown in FIG. 18, in this application example, a configuration is adopted in which the voltage Vout of the first terminal of the piezoelectric element 40 returns to the input terminal (−) of the operational amplifier 32 by negative feedback. In this configuration, when the voltage of the control signal Vin and the voltage Vout are different, the transistors 341 and 342 are controlled in the direction that removes the difference. Therefore, even when the response properties of the level shifters 36a to 36f and the transistors 341 and 342 are poor, it is possible to cause the voltage Vout to follow the control signal Vin in a relatively fast and precise manner.
Note that it is preferable to adopt a configuration in which it is possible to appropriately set the negative return amount to accommodate the properties of the level shifters 36a to 36f and the transistors 341 and 342. For example, in the example of FIG. 18, the operational amplifier 32 is configured to output a voltage that is obtained by subtracting the voltage Vout from the control signal Vin; however, the operational amplifier 32 may also be configured to multiply the obtained voltage by an appropriate factor and supply the result to the level shifters 36a to 36f. 
FIG. 19 is a diagram showing an example of the configuration of the driver 30 according to another (a second) application example of the embodiment. In the driver 30 described in FIG. 4, the transistors 341 and 342 of the unit circuits 34a to 34f are bipolar transistors; however, in the (second) application example shown in FIG. 19, the transistors 341 and 342 are respectively P and N channel Metal-Oxide Semiconductor Field-Effect Transistors (MOSFETs) 351 and 352.
When using the MOSFETs 351 and 352, a diode for preventing a reverse current may be provided between each of the drain terminals and the first terminals of the piezoelectric elements 40. When the MOSFETs 351 and 352 are used, a configuration is adopted in which, if the level shifters 36a to 36f are in the enable state, the voltage of the control signal Vin is shifted in the negative direction by an amount that is equivalent to a threshold voltage as a predetermined value, and the shifted voltage is supplied to the gate terminal of the P channel MOSFET 351; whereas the voltage of the control signal Vin is shifted in the positive direction by an amount that is equivalent to the threshold voltage, and the shifted voltage is supplied to the gate terminal of the N channel MOSFET 352.
As shown in FIG. 18, when the MOSFETs 351 and 352 are used, a configuration may be applied in which the voltage Vout is returned by negative feedback.
Driving Target
In the embodiment, the piezoelectric element 40, which discharges an ink, is described as an example of the driving target of the driver 30. The invention is not limited to the driving target being the piezoelectric element 40, and is applicable to all loads that have a capacitive component.
Number of Unit Circuit Stages
In the embodiment, a configuration is adopted in which six stages of the unit circuits 34a to 34f are provided in low-to-high voltage order to correspond to two neighboring voltages, of the seven voltages; however, in the invention, the number of unit circuit stages is not limited thereto, and may also be two or more stages. Furthermore, the voltages need not necessarily be at equal intervals from one another.
Comparator
In the embodiment, a configuration is adopted in which, for example, if the determination result of the comparator 38a is false (the output signal is the L level), the first state is detected, and if the determination result of the comparator 38a is true (the output signal is the H level) and the determination result of the comparator 38b is false, the second state is detected. In other words, the configuration that detects the first state and the second state is not separated for each state, and a portion of the configuration overlaps; thus, the configuration detects from the first state to the sixth state using all the comparators 38a to 38e. The invention is not limited thereto, and a configuration may also be adopted in which each state is detected separately.
Disable State Level Shifter
In the embodiment, a configuration is adopted in which each of the level shifters 36a to 36f that are in the disable state supply the voltage zero to the base (the gate) terminal of the transistor 341 (351), and supply the voltage VH to the base (the gate) terminal of the transistor 342 (352); however, as long as the transistors 341 and 342 can be switched off, the invention is not limited to this configuration. For example, in the disable state, each of the level shifters 36a to 36f may supply an off signal, which is obtained by causing the voltage of the input control signal Vin to shift in the positive direction by a predetermined value, to the base (the gate) terminal of the transistor 341 (351), and may supply an off signal, which is obtained by causing the voltage of the control signal Vin to shift in the negative direction by a predetermined value, to the base (the gate) terminal of the transistor 342 (352).
According to this configuration, since a low breakdown voltage is sufficient for the transistors 341 (351) and 342 (352), it is possible to reduce the transistor size when forming the semiconductor substrate.

## Claims

1. A printing apparatus that discharges liquid droplets onto a recording medium, comprising:
a movable carriage;
discharging units, which are installed on the carriage and include nozzles that discharge a liquid, pressure chambers that communicate with the nozzles, and piezoelectric elements provided for each of the pressure chambers;
a first circuit substrate, which is installed outside of the carriage, and on which is installed a control signal supply unit that generates control signals;
a second circuit substrate, which is installed on the carriage, and on which are installed a booster circuit that generates a plurality of voltages, and connection path selecting units that selectively supply the plurality of voltages generated by the booster circuit to the piezoelectric elements according to the control signals; and
a flexible flat cable, on which is formed a control wiring, which transmits the control signals from the first circuit substrate to the second circuit substrate,
wherein a path length of the control wiring between the first circuit substrate and the second circuit substrate is longer than the path length of the wiring between the second circuit substrate and the piezoelectric elements.

2. The printing apparatus according to claim 1,
wherein the connection path selecting units electrically connect the piezoelectric elements and the booster circuit using a first signal path or a second signal path according to the first signal path, to which a first voltage generated by the booster circuit is applied, the second signal path, to which the second voltage generated by the booster circuit that is higher than the first voltage is applied, voltages of the control signals, and the voltages held by the piezoelectric elements.

3. The printing apparatus according to claim 2, further comprising:
detection units, which are installed on the second circuit substrate, and detect whether or not the voltages held by the piezoelectric elements are lower than the first voltage, or, whether or not the voltages held by the piezoelectric elements are equal to or higher than the first voltage and lower than the second voltage.

4. The printing apparatus according to claim 2,
wherein, in relation to the piezoelectric elements holding a voltage that is lower than the first voltage, the connection path selecting units control charges to be charged to the piezoelectric elements via the first signal path according to the voltages of the control signals, and
wherein, in relation to the piezoelectric elements holding a voltage that is equal to or higher than the first voltage and lower than the second voltage, the connection path selecting units control the charges to be discharged from the piezoelectric elements via the first signal path, or, control the charges to be charged to the piezoelectric elements via the second signal path according to the voltages of the control signals.

5. The printing apparatus according to claim 1,
wherein the carriage moves in a main scanning direction that intersects a sub-scanning direction in which the recording medium is transported.