Patent Publication Number: US-11396178-B2

Title: Liquid discharge apparatus

Description:
The present application is based on, and claims priority from JP Application Serial Number 2019-198148, filed Oct. 31, 2019, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a liquid discharge apparatus. 
     2. Related Art 
     A technique relating to a liquid discharge apparatus is disclosed in JP-A-2017-43024. In the technique, when a nozzle discharge failure is detected, the print mode is switched from a normal mode to a discharge disabling mode. In the discharge disabling mode, driving of all piezoelectric devices for discharging ink from nozzles is disabled, and any ink micro-vibration operation is disabled to prevent thickened ink that is produced around the nozzles from entering pressure chambers. 
     The technique in JP-A-2017-43024 also describes that when the supply of a drive signal to the piezoelectric devices is interrupted, the piezoelectric devices hold the potentials at the time and maintain their deformed state. Accordingly, in the technique discussed in JP-A-2017-43024, when the print mode is switched from the normal mode to the discharge disabling mode, depending on the potential that is applied in the normal mode, the piezoelectric devices may be held in the deformed state. The piezoelectric devices maintained in the deformed state are subject to stress, and may result in deterioration. 
     SUMMARY 
     According to an aspect of the present disclosure, a liquid discharge apparatus is provided. The liquid discharge apparatus includes a first discharge section including a nozzle configured to discharge a liquid, a pressure chamber in communication with the nozzle, and a piezoelectric device configured to change a liquid pressure in the pressure chamber, a second discharge section including a nozzle configured to discharge the liquid, a pressure chamber in communication with the nozzle, and a piezoelectric device configured to change a liquid pressure in the pressure chamber, a drive waveform generator configured to generate a common drive waveform including, in one cycle, a first period having a discharge waveform to be supplied to the piezoelectric device to force the liquid out of the nozzle, the discharge waveform including a first element for changing a potential from a first potential to a second potential and a second element for changing a potential from the second potential to a third potential, and a second period having a first potential change waveform for decreasing deformation of the piezoelectric device as compared to deformation of the piezoelectric device when the first potential is supplied, the first potential change waveform including a first potential change element for changing a potential from the first potential to a fourth potential that is a potential between the first potential and the second potential, and a waveform selector configured to select the first period from the common drive waveform and supply the discharge waveform to the piezoelectric device in the first discharge section that discharges the liquid, and select the second period from the common drive waveform and supply the first potential change waveform to the piezoelectric device in the second discharge section that does not discharge the liquid. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating an electrical configuration of a liquid discharge apparatus according to a first embodiment. 
         FIG. 2  illustrates a schematic structure of a discharge section. 
         FIG. 3  illustrates a piezoelectric device that is in a bent state. 
         FIG. 4  illustrates a waveform selector. 
         FIG. 5  illustrates a common drive waveform and waveforms of drive voltages. 
         FIG. 6  illustrates a common drive waveform and waveforms of drive voltages according to a second embodiment. 
         FIG. 7  illustrates another example waveform in a common drive waveform. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     A. First Embodiment 
       FIG. 1  is a block diagram illustrating an electrical configuration of a liquid discharge apparatus  100  according to the first embodiment. The liquid discharge apparatus  100  is, for example, an ink jet printer. The liquid discharge apparatus  100  includes a power-supply circuit board  10 , a control circuit board  20 , a plurality of drive circuit boards  30 - 1  to  30 - n , and a plurality of discharge heads  40 - 1  to  40 - n , where n is an integer greater than or equal to two, and means plurality. 
     The drive circuit boards  30 - 1  to  30 - n  are referred to as a drive circuit board  30  when all of the drive circuit boards  30 - 1  to  30 - n  have the same configuration and it is not necessary to distinguish individual drive circuit boards. The discharge heads  40 - 1  to  40 - n  are referred to as a discharge head  40  when all of the discharge heads have the same structure and it is not necessary to distinguish individual discharge heads. In this embodiment, a drive circuit board  30 - i , where i=1 to n, and a discharge head  40 - i  are disposed to correspond to each other. 
     On the power-supply circuit board  10 , a high-voltage generation circuit  110  is provided. The power-supply circuit board  10  is electrically coupled to the control circuit board  20  via a first cable  65 . 
     Based on a power source voltage that is input from the outside of the liquid discharge apparatus  100 , the high-voltage generation circuit  110  generates a voltage HVH to be used in the liquid discharge apparatus  100 , for example, a 42-VDC voltage signal, and outputs the signal to the control circuit board  20 . 
     The power-supply circuit board  10  transmits a signal that is input from an external host computer of the liquid discharge apparatus  100  to the control circuit board  20 . 
     The control circuit board  20  includes a control circuit  210 , and is electrically coupled to the drive circuit board  30  via a board-to-board (BtoB) connector  83 . 
     The control circuit  210  includes a discharge-data generation circuit  211  and a drive-data generation circuit  212 . Based on various signals such as image data supplied from a host computer via the power-supply circuit board  10 , the control circuit  210  generates various control signals for controlling the drive circuit board  30  and the discharge head  40  and outputs the signals. 
     Part of the signals input to the control circuit  210  are input to the discharge data generation circuit  211 . Based on the input signal, the discharge data generation circuit  211  generates a plurality of types of control signals for controlling discharging of an ink from a discharge section  600 . 
     More specifically, the discharge data generation circuit  211  generates n print data signals SI 1  to SIn and n latch signals LAT 1  to LATn for controlling times for discharging the ink from the discharge section  600 , and outputs the signals to respective n drive circuit boards  30 - 1  to  30 - n . The discharge data generation circuit  211  also generates n selection control signals CH 1  to CHn, and outputs the signals to the respective n drive circuit boards  30 - 1  to  30 - n . The selection control signal CH is also referred to as a change signal. In addition, the discharge data generation circuit  211  commonly outputs a clock signal Sck to the n drive circuit boards  30 - 1  to  30 - n . To the drive circuit board  30 - i , the clock signal Sck, the print data signal SIi, the latch signal LATi, and the selection control signal CHi are input. In the following description, the print data signals SI 1  to SIn are collectively referred to as a print data signal SI, the latch signals LAT 1  to LATn are collectively referred to as a latch signal LAT, and the selection control signals CH 1  to CHn are collectively referred to as a selection control signal CH. 
     Part of the signals input to the control circuit  210  are input to the drive data generation circuit  212 . Based on the input signals, the drive data generation circuit  212  generates n drive data dA 1  to dAn that are original digital data for a common drive waveform COM for driving the discharge section  600 , and outputs the drive data dA 1  to dAn to n drive circuit boards  30 - 1  to  30 - n  respectively. To the drive circuit board  30 - i , drive data dAi is input. In the following description, the drive data dA 1  to dAn are collectively referred to as drive data dA. The drive data dA 1  to dAn may be digital data that are analog-to-digital converted data of waveforms of drive voltages, or digital data that indicate differences from last drive data. The drive data dA 1  to dAn may be digital data that define correspondence relationships between lengths of sections that have constant slopes and respective slopes in a drive waveform. 
     The control circuit board  20  has a wiring pattern that divides the voltage HVH that is generated in the high-voltage generation circuit  110 , and outputs the voltage HVH to each of n drive circuit boards  30 - 1  to  30 - n . The control circuit board  20  functions as a relay substrate that divides and transfers the voltage HVH. 
     The control circuit  210  on the control circuit board  20  may be provided on the power-supply circuit board  10 . More specifically, print data signals SI 1  to SIn, latch signals LAT 1  to LATn, selection control signals CH 1  to CHn, drive data dA 1  to dAn that are generated in the control circuit  210  may be generated in the power-supply circuit board  10  and input to the control circuit board  20  via the first cable  65 . 
     Various signals that are transferred from the power-supply circuit board  10  to the control circuit board  20  via the first cable  65  may be differential signals that are used in a low voltage differential signaling (LVDS) transmission mode, a low voltage positive emitter coupled logic (LVPECL) transmission mode, a current mode logic (CML) transmission mode, or the like that uses serial control signals. In such a case, the power-supply circuit board  10  is provided with a conversion circuit for converting various signals that are to be transferred to the control circuit board  20  into the differential signals, and the control circuit board  20  is provided with a restoration circuit for restoring the input differential signals. 
     The drive circuit board  30  is provided with a drive waveform generator  311  and a voltage generation circuit  320 . The drive circuit board  30  is electrically coupled to the discharge head  40  via a second cable  86  and a third cable  87 . 
     To the drive waveform generator  311 , the drive data dA and the voltage HVH are input. The drive waveform generator  311  includes a circuit that generates a common drive waveform COM for driving respective piezoelectric devices  60  in the discharge head  40  based on the input drive data dA and voltage HVH, and outputs the common drive waveform COM to the discharge head  40 . 
     For example, when drive data dA are digital data that are generated by performing analog-to-digital conversion to a common drive waveform COM, the drive waveform generator  311  performs digital-to-analog conversion to the drive data dA, and amplifies the converted data based on the voltage HVH to generate a common drive waveform COM. 
     Alternatively, for example, when drive data dA are digital data that define correspondence relationships between lengths of sections that have constant slopes and respective slopes in a waveform of a common drive waveform COM, the drive waveform generator  311  generates analog signals that satisfy the correspondence relationships between the lengths of the sections and the respective slopes that are defined by the drive data dA, and amplifies the generated signals based on the voltage HVH to generate a common drive waveform COM. 
     The voltage generation circuits  320  generate a plurality of voltage signals of a plurality of voltages based on the voltage HVH. More specifically, as a voltage signal, the voltage generation circuits  320  generate a voltage VBS that is supplied to the piezoelectric devices  60  in the discharge head  40 , and output the voltage VBS to the discharge head  40 . The voltage VBS is, for example, a voltage of 6 VDC. The voltage generation circuits  320  generate, as a voltage signal, a voltage VDD that is a power source voltage supplied to components in the discharge head  40 , and output the voltage VDD to the discharge head  40 . The voltage VDD is, for example, a voltage of 3.3 VDC. The voltage generation circuits  320  generate, as a voltage signal, a voltage GVDD for driving individual amplifiers in amplifier circuits in the drive waveform generators  311 , and output the voltage GVDD to the drive waveform generators  311 . The voltage GVDD is, for example, a voltage of 7.5 VDC. The voltage generation circuits  320  may generate voltage signals other than the above-described voltage signals. 
     The drive circuit board  30  transfers a print data signal SI, a latch signal LAT, a selection control signal CH, and a clock signal Sck that are input from the discharge data generation circuit  211  to the discharge head  40 . 
     The drive circuit board  30  and the discharge head  40  are electrically coupled to each other with the second cable  86  and the third cable  87 . The second cable  86  transfers the common drive waveform COM, the voltage VDD, and the voltage VBS from the drive circuit board  30  to the discharge head  40 , and the third cable  87  transfers the print data signal SI, the latch signal LAT, the selection control signal CH, and the clock signal Sck. The second cable  86  and the third cable  87  may be integrated into one cable. 
     The discharge head  40  includes a plurality of discharge modules  500 . Each discharge module  500  includes a waveform selector  510  and a plurality of discharge sections  600 . 
     The waveform selector  510  includes a selection controller  520  and a plurality of selection circuits  530 . The waveform selector  510  is, for example, an integrated circuit (IC) and operates on the voltage VDD. 
     To the selection controller  520 , the print data signal SI, the latch signal LAT, the selection control signal CH, and the clock signal Sck that are input. 
     The selection controller  520  generates, to each of the selection circuits  530 , a selection signal for controlling an output of a waveform in the common drive waveform COM based on the print data signal SI, and outputs the selection signal at a time determined by the latch signal LAT and the selection control signal CH. 
     To each selection circuit  530 , the common drive waveform COM that is generated in the drive waveform generator  311  is input. The selection circuit  530  generates, based on the selection signal that is output from the selection controller  520 , a drive voltage Vout from the common drive waveform COM and outputs the drive voltage Vout to the corresponding discharge section  600 . 
     The discharge sections  600  includes a first discharge section  601  and a second discharge section  602 . Each of the discharge sections  600  includes the piezoelectric device  60 , and the discharge sections  600  correspond to the respective selection circuits  530 . To one end of the piezoelectric device  60 , the drive voltage Vout that is output from the selection circuit  530  is applied, and to the other end, the voltage VBS is applied. The piezoelectric device  60  deforms due to a potential difference between the drive voltage Vout and the voltage VBS, and the deformation forces an ink out of a nozzle  651  in the discharge section  600 . 
       FIG. 2  illustrates a schematic structure of a discharge section  600  in the discharge module  500 . The discharge module  500  includes the discharge section  600  and a reservoir  641 . 
     The reservoir  641  is provided for each color of ink, and the ink is supplied from a supply port  661  into the reservoir  641 . The supply port  661  is coupled to an ink cartridge or an ink tank. 
     The discharge section  600  includes the nozzle  651  that discharges an ink as a liquid, a cavity  631  that functions as a pressure chamber and is in communication with the nozzle  651 , the piezoelectric device  60  that functions as a pressure generating element that changes the ink pressure in the cavity  631 , and a vibration plate  621 . The piezoelectric device  60  that is disposed on an upper surface of the vibration plate  621  makes the vibration plate  621  bend and vibrate, and thereby the vibration plate  621  functions as a diaphragm that increases or decreases the internal volume of the cavity  631  that is filled with the ink. The nozzle  651  is an opening that is provided in a nozzle plate  632  and in communication with the cavity  631 . The cavity  631  is filled with an ink and its internal volume changes as the piezoelectric device  60  deforms. The nozzle  651  is in communication with the cavity  631  and discharges the ink in the cavity  631  as an ink droplet as the internal volume of the cavity  631  changes. 
     The piezoelectric device  60  according to the embodiment includes a piezoelectric element  61 , a first electrode  62  that is disposed on one side of the piezoelectric element  61 , and a second electrode  63  that is disposed on the other side of the piezoelectric element  61 . In other words, the piezoelectric device  60  includes the piezoelectric element  61  that is disposed between a pair of electrodes. To the first electrode  62 , a drive voltage Vout is applied, and to the second electrode  63 , a fifth potential that is higher than a second potential and lower than or equal to the voltage VBS is applied. In this embodiment, the fifth potential is the voltage VBS. A first potential to a fourth potential will be described below. In the description below, as long as not specifically mentioned, applying a voltage, potential, or waveform to the piezoelectric device  60  means applying a voltage, potential, or waveform to the first electrode  62  of the discharge section  600 . 
     A potential difference between a voltage applied by the first electrode  62  and a voltage applied by the second electrode  63  makes the piezoelectric element  61  deform in the vertical direction in a central portion with respect to both end portions in  FIG. 2  together with the first electrode  62 , the second electrode  63 , and the vibration plate  621 . More specifically, the piezoelectric device  60  according to the embodiment deforms upward when the voltage of the drive voltage Vout becomes low and deforms downward when the voltage of the drive voltage Vout becomes high. In this structure, the piezoelectric device  60  that deforms upward increases the internal volume of the cavity  631 , and thus the ink is drawn from the reservoir  641 . On the other hand, the piezoelectric device  60  that deforms downward decreases the internal volume of the cavity  631 , and depending on the extent of the contraction, the ink is discharged from the nozzle  651 . 
       FIG. 3  illustrates the piezoelectric device  60  that is in a bent state. In this embodiment, in a state in which discharging of an ink is not performed during a print period, to the first electrode  62  of the piezoelectric device  60 , an intermediate potential Vm that is higher than the voltage VBS is applied. The application produces a potential difference between the first electrode  62  and the second electrode  63 , and thus the discharge section  600  slightly bends toward the nozzle  651  side as illustrated in  FIG. 3 . This application is performed prior to the ink discharge to increase the volume of the cavity  631  to sufficiently supply the ink from the reservoir  641  to the cavity  631 . The intermediate potential Vm can be determined by experiments, simulations, or the like depending on the structure of the discharge section  600  and an amount of ink to be discharged. 
     The structure of the piezoelectric device  60  is not limited to the structure illustrated in  FIG. 2  and  FIG. 3 , and may be any structure that deforms the piezoelectric device  60  to discharge an ink. The piezoelectric device  60  is not limited to the device that bends and vibrates, and may be a device that longitudinally vibrates. 
     In the discharge module  500 , the piezoelectric devices  60  correspond to the respective cavities  631  and nozzles  651 , and also correspond to the respective selection circuits  530 . In the discharge module  500 , accordingly, a set of the piezoelectric device  60 , the cavity  631 , the nozzle  651 , and the selection circuit  530  is provided for each nozzle  651 . 
       FIG. 4  illustrates a structure of the waveform selector  510 . The waveform selector  510  includes the selection controller  520  and the plurality of selection circuits  530 . 
     To the selection controller  520 , the clock signal Sck, the print data signal SI, the latch signal LAT, and the selection control signal CH are supplied. In the selection controller  520 , a set of a shift register  222 , a latch circuit  224 , and a decoder  226  is provided for each piezoelectric device  60 . In one waveform selector  510 , accordingly, the number of sets of the shift registers  222 , the latch circuits  224 , and the decoders  226  is the same as the total number m of the nozzles  651 . 
     The print data signal SI is in synchronization with the clock signal Sck, and contains data that causes respective m discharge sections  600  to discharge or not to discharge an ink. 
     The shift register  222  temporarily holds the print data signal SI. More specifically, the shift registers  222  of the same number of the stages of the piezoelectric devices  60  are cascaded and the serially supplied print data signal SI is sequentially transferred in accordance with the clock signal Sck to the latter stage. In  FIG. 3 , in order to distinguish the shift registers  222 , the shift registers  222  are expressed as SR 1 , SR 2 , . . . SRm from the upstream side from which the print data signal SI is supplied. 
     The respective m latch circuits  224  latch the print data signal SI that is stored in the respective m shift registers  222  on the rising edge of the latch signal LAT. 
     In accordance with the print data signal SI that is latched in the individual m latch circuits  224 , the m decoders  226  switch outputs of the selection signals to the selection circuits  530  to a H level or a L level for each period that is defined by the latch signal LAT and the selection control signal CH. 
     The discharge sections  530  are provided for the respective piezoelectric devices  60 . In one waveform selector  510 , accordingly, the number the selection circuits  530  is the same as the total number m of the nozzles  651 . The selection circuit  530  selects, based on a selection signal, whether to output the common drive waveform COM as the drive voltage Vout. More specifically, when a selection signal is at the H level, the selection circuit  530  brings the drive waveform generator  311  and the piezoelectric device  60  into conduction to output a corresponding part of the common drive waveform COM as the drive voltage Vout. On the other hand, when the selection signal is at the L level, the selection circuit  530  brings the drive waveform generator  311  and the piezoelectric device  60  out of conduction. When the drive waveform generator  311  and the discharge section  600  are disconnected, the last voltage is maintained by the capacitiveness of the piezoelectric device  60 , and the last voltage is the drive voltage Vout. 
       FIG. 5  illustrates the common drive waveform COM and waveforms of drive voltages Vout. The common drive waveform COM according to the embodiment includes, in one cycle, a plurality of periods that are divided by the latch signal LAT and the selection control signal CH. The periods include a first period T 1 , a second period T 2 , a third period T 3 , and a fourth period T 4 . These periods are included in the cycle of the common drive waveform COM, and their time-based relationship is not particularly limited. 
     The first period T 1  includes a discharge waveform WE that is supplied to the piezoelectric device  60  to force the ink out of the nozzle  651 . The discharge waveform WE includes a first element EA for changing a potential from a first potential V 1  to a second potential V 2 , and a second element EB for changing a potential from the second potential V 2  to a third potential V 3 . The discharge waveform WE according to the embodiment includes a third element EC for changing a potential from the third potential V 3  to the first potential. The first potential V 1  according to the embodiment is the intermediate potential Vm. The second potential V 2  according to the embodiment is lower than the first potential V 1  and lower than the voltage VBS. The third potential V 3  is higher than the first potential V 1  and the second potential V 2 . 
       FIG. 5  illustrates a drive voltage Vout 1  that is a drive voltage Vout selected from the common drive waveform COM in the first period T 1  and output. When the discharge waveform WE is supplied in accordance with the drive voltage Vout 1  to the piezoelectric device  60 , in accordance with the first element EA, the piezoelectric device  60  deforms such that the volume in the cavity  631  increases from a normal volume that corresponds to the intermediate potential Vm to an expansion volume that corresponds to the second potential V 2  and thereby the pressure of the ink in the cavity  631  fluctuates at a natural frequency. Then, in accordance with the second element EB, the piezoelectric device  60  deforms such that the volume in the cavity  631  rapidly decreases to a contraction volume that corresponds to the third potential V 3 . The amount of an ink droplet and the flying speed of the ink droplet discharged from the nozzle  651  depend on the contraction timing with respect to the fluctuation occurring in the ink pressure in the cavity  631 . The pressure of the ink in the cavity  631  that decreases due to the ink droplet discharge fluctuates at a natural frequency. Then, in accordance with the third element EC, the piezoelectric device  60  deforms such that the volume in the cavity  631  expands to the volume that corresponds to the intermediate potential Vm. 
     The second period T 2  includes a first potential change waveform WC 1 . The first potential change waveform WC 1  includes a first potential change element E 1  for changing a potential from the first potential V 1  to the fourth potential V 4 . The fourth potential V 4  according to the embodiment is lower than the first potential V 1  and corresponds to the voltage VBS.  FIG. 5  illustrates a drive voltage Vout 2  that is a drive voltage Vout selected from the common drive waveform COM in the second period T 2  and output. When the first potential change waveform WC 1  is applied in accordance with the drive voltage Vout 2  to the piezoelectric device  60 , the potential that is applied to the piezoelectric device  60  changes from the intermediate potential Vm to the voltage VBS. As a result, the potential difference between the first electrode  62  and the second electrode  63  becomes zero, and the shape of the piezoelectric device  60  is changed from the bent state illustrated in  FIG. 3  into a flat state illustrated in  FIG. 2 . In this specification, “flat” is not limited to a completely horizontal state, and may be a state flatter than a state in which the intermediate potential Vm is applied to the first electrode  62 . 
     In this embodiment, an amount of change in potential per unit time of the first potential change element E 1  in the first potential change waveform WC 1  is smaller than an amount of change in potential per unit time of the first element EA for changing a potential from the first potential V 1  to the second potential V 2  in the discharge waveform WE. Accordingly, when the first potential change waveform WC 1  is applied to the piezoelectric device  60 , the piezoelectric device  60  relatively gently changes its shape from the bent state to the flat state. 
     The third period T 3  includes a potential maintaining waveform WK for maintaining the voltage VBS.  FIG. 5  illustrates a drive voltage Vout 3  that is a drive voltage Vout selected from the common drive waveform COM in the third period T 3  and output. When the drive voltage Vout 3  is applied to the piezoelectric device  60 , the piezoelectric device  60  maintains a flat state as illustrated in  FIG. 2 . 
     The fourth period T 4  includes a second potential change waveform WC 2 . The second potential change waveform WC 2  includes a second potential change element E 2  for changing a potential from the fourth potential V 4  to the first potential V 1 .  FIG. 5  illustrates a drive voltage Vout 4  that is a drive voltage Vout selected from the common drive waveform COM in the fourth period T 4  and output. When the second potential change waveform WC 2  is applied to the piezoelectric device  60  in accordance with the drive voltage Vout 4 , the potential that is applied to the piezoelectric device  60  changes from the voltage VBS to the intermediate potential Vm. As a result, the shape of the piezoelectric device  60  is changed from the flat state illustrated in  FIG. 2  into a bent state illustrated in  FIG. 3 . 
     In this embodiment, an amount of change in potential per unit time of the second potential change element E 2  in the second potential change waveform WC 2  is smaller than an amount of change in potential per unit time of the first element EA for changing a potential from the first potential V 1  to the second potential V 2  in the discharge waveform WE. Accordingly, when the second potential change waveform WC 2  is applied to the piezoelectric device  60 , the piezoelectric device  60  relatively gently changes its shape from the flat state to the bent state. 
     The selection circuits  530  illustrated in  FIG. 4  supply a waveform in a period that is selected from the first period T 1  to the fourth period T 4  in the common drive waveform COM to the piezoelectric devices  60  in a corresponding discharge section  600  among the discharge sections  600  that include the first discharge section  601  and the second discharge section  602 . 
     In the description below, the first discharge section  601  is a discharge section  600  that discharges an ink, and the second discharge section  602  is a discharge section  600  that does not discharge an ink. In this specification, “discharge section  600  that discharges an ink” means a discharge section  600  that is used during a print period among the discharge sections  600  in the discharge head  40 . On the other hand, “discharge section  600  that does not discharge an ink” means a discharge section  600  that is not used during a print period among the discharge sections  600  in the discharge head  40 . In addition, the discharge sections  600  that do not discharge an ink may include discharge sections  600  that are determined to be faulty in a discharge failure inspection performed with the piezoelectric devices  60 , or discharge sections  600  that are located outside a recording medium. 
     When a printing process is started in the liquid discharge apparatus  100 , to the piezoelectric device  60  in the first discharge section  601  that discharges an ink, the waveform selector  510  selects the first period T 1  from the common drive waveform COM and supplies the discharge waveform WE. By the processing, to the first discharge section  601 , the drive voltage Vout 1  illustrated in  FIG. 5  is supplied, the ink is discharged from the nozzle  651 , and a dot is formed on the recording medium. 
     Immediately after the printing process is started in the liquid discharge apparatus  100 , to the piezoelectric device  60  in the second discharge section  602  that does not discharge an ink, the waveform selector  510  selects the second period T 2  from the common drive waveform COM and supplies the first potential change waveform WC 1 . By the processing, to the second discharge section  602 , the drive voltage Vout 2  illustrated in  FIG. 5  is supplied. When the drive voltage Vout 2  is supplied to the second discharge section  602 , the voltage applied to the piezoelectric device  60  is changed from the intermediate potential Vm to the voltage VBS, and the piezoelectric device  60  becomes a flat state as illustrated in  FIG. 2 . 
     Furthermore, in this embodiment, in a period after the first potential change waveform WC 1  is supplied to the piezoelectric device  60  of the second discharge section  602  that does not discharge the ink, the waveform selector  510  controls the selection circuit  530  such that the potential maintaining waveform WK in the third period T 3  in the common drive waveform COM is supplied to the piezoelectric device  60  of the second discharge section  602 . By the processing, to the second discharge section  602 , the drive voltage Vout 3  illustrated in  FIG. 5  is supplied, and thus the voltage VBS is applied to the piezoelectric device  60  of the second discharge section  602 , and the piezoelectric device  60  is maintained in a flat state. 
     In this embodiment, in completing the printing, in a period after the potential maintaining waveform WK is supplied to the piezoelectric device  60  of the second discharge section  602  that does not discharge the ink, the waveform selector  510  supplies the second potential change waveform WC 2  in the fourth period T 4  in the common drive waveform COM to the piezoelectric device  60  of the second discharge section  602 . By the processing, to the second discharge section  602 , the drive voltage Vout 4  illustrated in  FIG. 5  is supplied, and thus the intermediate potential Vm is applied to the piezoelectric device  60  of the second discharge section  602 . As a result, when the printing is completed, the piezoelectric device  60  returns from the flat state to the bent state illustrated in  FIG. 3 . 
     The print data signal SI that is sent from the control circuit  210  specifies a period in the common drive waveform COM to be selected by the waveform selector  510 . More specifically, the print data signal SI includes data that causes, in synchronization with the clock signal Sck, to perform at least one of the following processes: 1. forming a dot; 2. outputting the first potential change waveform WC 1 ; 3. outputting the second potential change waveform WC 2 ; 4. outputting the potential maintaining waveform WK; and 5. not selecting any of the first to fourth periods. The decoder  226  selects a period that corresponds to the data from the common drive waveform COM, and outputs a selection signal for selecting the period to the selection circuit  530 . The waveform selector  510  thus controls the selection circuits  530  to output desired waveforms to the first discharge section  601  and the second discharge section  602  respectively. 
     The liquid discharge apparatus  100  according to the embodiment supplies, to the piezoelectric device  60  of the second discharge section  602  that does not discharge the ink, the first potential change waveform WC 1  for decreasing the deformation of the piezoelectric device  60  as compared to that when the intermediate potential Vm is applied. Consequently, the piezoelectric device  60  is not always subject to stress, and deterioration of the piezoelectric device  60  can be suppressed. In particular, in this embodiment, the fifth potential that is applied to the second electrode  63  of the piezoelectric device  60  is the voltage VBS, and the fourth potential that is applied to the first electrode  62  after the supply of the first potential change waveform WC 1  is also the voltage VBS. Accordingly, the deformation of the piezoelectric element  61  in the second discharge section  602  can be minimized, and possible deterioration in the piezoelectric device  60  can be largely reduced. Such an effect is significant when the nozzles  651  are arranged in the nozzle plate  632  at a high density, and a thin piezoelectric device  60  that has a thickness of 10 μm or less that tends to crack is employed. 
     Furthermore, in this embodiment, after the first potential change waveform WC 1  is supplied to the piezoelectric device  60  of the second discharge section  602  that does not discharge the ink, the potential maintaining waveform WK in the third period T 3  in the common drive waveform COM is supplied. Accordingly, during the print period, the piezoelectric device  60  of the second discharge section  602  that does not discharge the ink is less subject to deformation. 
     Furthermore, in this embodiment, after the potential maintaining waveform WK is supplied to the piezoelectric device  60  of the second discharge section  602  that does not discharge the ink, the second potential change waveform WC 2  in the fourth period T 4  in the common drive waveform COM is supplied. Accordingly, after the printing is completed, the deformation of the piezoelectric device  60  of the second discharge section  602  that does not discharge the ink can be returned to the original state. Consequently, for example, maintenance processing such as flushing processing or cleaning processing that is performed after completion of printing can be started in a state in which the piezoelectric devices  60  in the first discharge section  601  and the second discharge section  602  are in the same deformation state. 
     In this embodiment, an amount of change in potential per unit time of the first potential change element E 1  in the first potential change waveform WC 1  is smaller than an amount of change in potential per unit time of the first element EA in the discharge waveform WE. Accordingly, when the first potential change waveform WC 1  is applied, the pressure fluctuation in the nozzle  651  and the cavity  631  due to the deformation of the piezoelectric device  60  can be suppressed as compared to the pressure fluctuation during the discharging. As a result, spreading of thickened ink into the cavity  631  can be reduced and the discharging of the thickened ink by flushing processing or cleaning processing after the completion of the printing can be readily performed. 
     B. Second Embodiment 
       FIG. 6  illustrates a common drive waveform COM and waveforms of drive voltages Vout according to the second embodiment. A structure of the liquid discharge apparatus  100  according to the second embodiment is similar to that in the first embodiment. The common drive waveform COM according to the embodiment includes a fifth period T 5  instead of the fourth period T 4  in the first embodiment. The fifth period T 5  includes an inverted trapezoidal mocrovibration waveform WS that falls from the intermediate potential Vm and after a short time, rises to the intermediate potential Vm. The potential after the fall of the mocrovibration waveform WS is lower than the intermediate potential Vm and higher than the voltage VBS. 
     When the waveform selector  510  receives data according to the print data signal SI that causes the first discharge section  601  that discharges an ink not to form a dot, the waveform selector  510  selects the fifth period T 5  from the common drive waveform COM with the decoder  226  and outputs a selection signal for selecting the fifth period T 5  to the selection circuit  530 . By the processing, to the first discharge section  601  that discharges the ink, the drive voltage Vout 5  illustrated in  FIG. 6  is supplied, and thus the mocrovibration waveform WS is applied to the piezoelectric device  60 . Accordingly, thickening of the ink around the nozzle  651  of the first discharge section  601  can be suppressed. 
     It should be noted that the common drive waveform COM according to the embodiment includes the fifth period T 5  instead of the fourth period T 4  in the first embodiment; however, the common drive waveform COM may be a waveform that includes both of the fourth period T 4  and the fifth period T 5 . 
     C. Other Embodiments 
     C-1. The common drive waveform COM according to the embodiments may include the discharge waveform WE illustrated in  FIG. 5 , the mocrovibration waveform WS illustrated in  FIG. 6 , or other waveforms. For example, as illustrated in  FIG. 7 , the common drive waveform COM may include a waveform that falls, rises once, falls again, and rises again. According to the waveform, an ink droplet smaller than that according to the discharge waveform WE illustrated in  FIG. 5  can be discharged. Furthermore, the waveform of the discharge waveform WE may be any waveform that forces the ink out of the nozzle  651 , and the waveform is not limited to the waveforms illustrated in  FIG. 5  to  FIG. 7 . For example, a waveform that has a simple shape such as a trapezoid or a rectangle may be used. The magnitude relation in potential of the waveform elements in the respective waveforms may be reversed depending on the structure of the discharge section  600  or the structure of the piezoelectric device  60 . 
     C-2. In the above-described embodiments, the common drive waveform COM includes the potential maintaining waveform WK. The common drive waveform COM, however, may not include the potential maintaining waveform WK. In such a case, after the first potential change waveform WC 1  is supplied to the piezoelectric device  60 , the waveform selector  510  may disconnect the discharge data generation circuit  211  and the discharge section  600  to substantially maintain a state in which the voltage VBS is applied to the first electrode  62 . 
     C-3. In the above-described embodiments, the common drive waveform COM may not include the second potential change waveform WC 2 . In such a case, until the liquid discharge apparatus  100  is restarted, a state in which the voltage VBS is applied to the second discharge section  602  may be maintained. 
     C-4. In the above-described embodiments, the voltage that is applied to the second electrode  63  of the piezoelectric device  60  may be a voltage other than the voltage VBS. For example, a potential that is lower than the voltage VBS and higher than 0 V may be applied. That is, when the first potential change waveform WC 1  is applied to the piezoelectric device  60  in the second discharge section  602 , a potential difference between the first electrode  62  and the second electrode  63  may be any potential difference at which deterioration of the piezoelectric device  60  can be suppressed, and may be a potential difference other than zero. 
     C-5. In the above-described embodiments, an amount of change in potential per unit time of the first potential change element E 1  in the first potential change waveform WC 1  is smaller than an amount of change in potential per unit time of the first element EA in the discharge waveform WE. However, an amount of change in potential per unit time of the first potential change element E 1  may be larger than an amount of change in potential per unit time of the first element EA. Alternatively, the amounts of change in potential per unit time may be the same amount. 
     C-6. In the above-described embodiments, an amount of change in potential per unit time of the second potential change element E 2  in the second potential change waveform WC 2  is smaller than an amount of change in potential per unit time of the first element EA in the discharge waveform WE. However, an amount of change in potential per unit time of the second potential change element E 2  may be larger than an amount of change in potential per unit time of the first element EA. Alternatively, the amounts of change in potential per unit time may be the same amount. 
     C-7. The liquid discharge apparatus  100  according to the above-described embodiments is an apparatus that discharges an ink. The liquid discharge apparatus  100 , however, may discharge not only ink but may discharge liquids other than ink. 
     D. Other Aspects 
     The present disclosure is not limited to the above-described embodiments, and various modifications may be made without departing from the scope of the present disclosure. For example, technical features in the embodiments corresponding to the technical features in aspects described below may be replaced or combined to solve some or all of the above-described problems or to achieve some or all of the above-described effects. Unless the technical features are described as essential in this specification, the technical features may be omitted as appropriate. 
     1. According to an aspect of the present disclosure, a liquid discharge apparatus is provided. The liquid discharge apparatus includes a first discharge section including a nozzle configured to discharge a liquid, a pressure chamber in communication with the nozzle, and a piezoelectric device configured to change a liquid pressure in the pressure chamber, a second discharge section including a nozzle configured to discharge the liquid, a pressure chamber in communication with the nozzle, and a piezoelectric device configured to change a liquid pressure in the pressure chamber, a drive waveform generator configured to generate a common drive waveform including, in one cycle, a first period having a discharge waveform to be supplied to the piezoelectric device to force the liquid out of the nozzle, the discharge waveform including a first element for changing a potential from a first potential to a second potential and a second element for changing a potential from the second potential to a third potential, and a second period having a first potential change waveform for decreasing deformation of the piezoelectric device as compared to deformation of the piezoelectric device when the first potential is supplied, the first potential change waveform including a first potential change element for changing a potential from the first potential to a fourth potential that is a potential between the first potential and the second potential, and a waveform selector configured to select the first period from the common drive waveform and supply the discharge waveform to the piezoelectric device in the first discharge section that discharges the liquid, and select the second period from the common drive waveform and supply the first potential change waveform to the piezoelectric device in the second discharge section that does not discharge the liquid. According to the aspect, to the piezoelectric device of the second discharge section that does not discharge the liquid, the first potential change waveform for decreasing the deformation of the piezoelectric device is supplied. As a result, the piezoelectric device is not always subject to stress, and deterioration of the piezoelectric device can be suppressed. 
     2. In this aspect, the common drive waveform may further include, in the cycle, a third period having a potential maintaining waveform for maintaining the fourth potential, and the waveform selector may be configured to supply, after the first potential change waveform is supplied to the piezoelectric device in the second discharge section that does not discharge the liquid, the potential maintaining waveform in the third period from the common drive waveform to the piezoelectric device in the second discharge section that does not discharge the liquid. According to the aspect, a state in which the piezoelectric device of the second discharge section that does not discharge the liquid is less deformed can be maintained. 
     3. In this aspect, the common drive waveform may further include, in the cycle, a fourth period having a second potential change waveform including a second potential change element for changing a potential from the fourth potential to the first potential, and the waveform selector may be configured to supply, after the potential maintaining waveform is supplied to the piezoelectric device in the second discharge section that does not discharge the liquid, the second potential change waveform in the fourth period from the common drive waveform to the piezoelectric device in the second discharge section that does not discharge the liquid. According to the aspect, the deformation of the piezoelectric device of the second discharge section that does not discharge the liquid can be returned to the original state. 
     4. In this aspect, the piezoelectric device may include a piezoelectric element, a first electrode that is disposed on one side of the piezoelectric element, and a second electrode that is disposed on the other side of the piezoelectric element. To the first electrode, the waveform in a period selected from the common drive waveform may be applied, and to the second electrode, a fifth potential that is lower than or equal to the fourth potential may be applied. According to the aspect, the piezoelectric element can be driven based on a differential pressure between the waveform that is applied to the first electrode and the fifth potential. 
     5. In this aspect, the fourth potential may be equal to the fifth potential. According to the aspect, the deformation of the piezoelectric element of the second discharge section that does not discharge the liquid is can be suppressed. 
     6. In this aspect, an amount of change in potential per unit time of the first potential change element in the first potential change waveform may be smaller than an amount of change in potential per unit time of the first element in the discharge waveform. According to the aspect, a rapid deformation of the piezoelectric device upon the application of the first potential change waveform can be suppressed. 
     The present disclosure is not limited to the above-described embodiments as the liquid discharge apparatus, and for example, may be various methods for controlling the liquid discharge apparatus, or methods for driving the piezoelectric device in the liquid discharge apparatus.