Patent Publication Number: US-11648775-B2

Title: Liquid coating apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a U.S. national stage of PCT Application No. PCT/JP2019/033696, filed on Aug. 28, 2019, with priority under 35 U.S.C. § 119(a) and 35 U.S.C. § 365(b) being claimed from Japanese Patent Application No. 2018-180760, filed on Sep. 26, 2018, the entire disclosures of which are hereby incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a liquid coating apparatus. 
     BACKGROUND 
     A liquid coating apparatus is known in which a liquid supplied from a liquid storage assembly is discharged to a material to be coated. Such a liquid coating apparatus changes the volume of a liquid chamber to discharge a liquid in the liquid chamber. As a conventional liquid coating apparatus, there is disclosed an example of the liquid coating apparatus in which the volume of a liquid chamber containing a liquid is changed using a flexible plate that is deformed by driving a piezoelectric element, thereby discharging the liquid through a nozzle. 
     In the case of a configuration in which a piezoelectric element is driven to deform a flexible body as in the configuration of a conventional liquid coating apparatus, it is conceivable to input a rectangular signal to the piezoelectric element to operate the piezoelectric element at a high speed in order to enhance responsiveness of liquid discharge. 
     Unfortunately, when a drive element including the piezoelectric element is operated at a high speed, the drive element may excessively expand and contract, and then an excessive load may be applied to the drive element. This may affect the life of the drive element. 
     SUMMARY 
     A liquid coating apparatus according to an example embodiment of the present disclosure includes a liquid chamber that stores a liquid, an inflow path that is connected to the liquid chamber to allow the liquid to be supplied into the liquid chamber, a diaphragm that defines a portion of a wall portion defining the liquid chamber and is deformed to change a volume of the liquid chamber, a driver that expands and contracts in at least one direction to deform the diaphragm in a thickness direction, a first support portion that is between the driver and the diaphragm in the one direction to support the driver on a diaphragm side, a second support portion that supports an end of the driver on an opposite side of the driver to the diaphragm in the one direction, a transmission that extends in the one direction between the driver and the diaphragm and passes through the first support portion to transmit expansion and contraction of the driver to the diaphragm, and a compressive force applicator that is between the driver and the first support portion and supported by the first support portion to apply a compressive force to the driver in the one direction. 
     The liquid coating apparatus according to one example embodiment of the present disclosure prevents an excessive load at a level affecting the life of a driver from being applied to the driver even when the driver is operated at a high speed. 
     The above and other elements, features, steps, characteristics and advantages of the present disclosure will become more apparent from the following detailed description of the example embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram illustrating a schematic configuration of a liquid coating apparatus according to an example embodiment of the present disclosure. 
         FIG.  2    is an enlarged view illustrating schematic structure of a discharge assembly according to an example embodiment of the present disclosure. 
         FIG.  3    is a flowchart illustrating an example of operation of a liquid coating apparatus according to an example embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, example embodiments of the present disclosure will be described in detail with reference to the drawings. The same or corresponding parts in the drawings are designated by the same reference numerals, and description thereof will not be duplicated. Each of the drawings shows dimensions of components that do not faithfully represent actual dimensions of the components and dimensional ratios of the respective components. 
       FIG.  1    is a diagram schematically illustrating a schematic configuration of a liquid coating apparatus  1  according to an example embodiment of the present disclosure.  FIG.  2    is a flowchart illustrating operation of the liquid coating apparatus  1 . 
     The liquid coating apparatus  1  is an ink-jet liquid coating apparatus that discharges a liquid in the form of droplets to the outside. Examples of the liquid include solder, thermosetting resin, ink, and a coating liquid for forming a functional thin film such as an alignment film, a resist, a color filter, and organic electroluminescence. 
     The liquid coating apparatus  1  includes a liquid storage assembly  10 , a pressure adjusting assembly  20 , a discharge assembly  30 , and a controller  60 . 
     The liquid storage assembly  10  is a container for storing a liquid inside. The liquid storage assembly  10  supplies the stored liquid to the discharge assembly  30 . That is, the liquid storage assembly  10  includes an outlet  10   a  for supplying the stored liquid to the discharge assembly  30 . Pressure in the liquid storage assembly  10  is adjusted by the pressure adjusting assembly  20 . The liquid storage assembly  10  includes a supply port (not illustrated) through which a liquid is supplied thereto. 
     The pressure adjusting assembly  20  adjusts the pressure in the liquid storage assembly  10  to any one of positive pressure higher than an atmospheric pressure, negative pressure lower than the atmospheric pressure, and the atmospheric pressure. When the pressure in the liquid storage assembly  10  is adjusted in this way, as described later, a liquid can be stably discharged from a discharge port  32   a  of the discharge assembly  30 , and the liquid can be prevented from leaking from the discharge port  32   a.    
     Specifically, the pressure adjusting assembly  20  includes a positive pressure generator  21 , a negative pressure generator  22 , a first switching valve  23 , a second switching valve  24 , an atmospheric opening assembly  25 , and a pressure sensor  26 . 
     The positive pressure generator  21  generates positive pressure higher than the atmospheric pressure. The positive pressure generator  21  includes a positive pressure pump  21   a  as a positive pressure generator. The positive pressure pump  21   a  generates positive pressure. 
     The negative pressure generator  22  generates negative pressure lower than the atmospheric pressure. The negative pressure generator  22  includes a negative pressure pump  22   a  as a negative pressure generator, and a negative pressure adjusting container  22   b.    
     The negative pressure pump  22   a  generates negative pressure. Pressure inside the negative pressure adjusting container  22   b  becomes the negative pressure generated by the negative pressure pump  22   a.  The negative pressure adjusting container  22   b  is between the negative pressure pump  22   a  and a second switching valve  24 . When the negative pressure generator includes the negative pressure adjusting container  22   b,  the negative pressure generated by the negative pressure pump  22   a  is uniformed. 
     This enables not only reducing pulsation of the negative pressure generated by the negative pressure pump  22   a,  but also acquiring stable negative pressure in the negative pressure generator  22 . As described later, even when output of the negative pressure pump  22   a  changes in accordance with a detection result of pressure in the liquid storage assembly  10  acquired by the pressure sensor  26 , the negative pressure adjusting container  22   b  reduces pulsation of negative pressure generated by the negative pressure pump  22   a,  and uniform pressure can be acquired under the negative pressure having changed. Thus, when the negative pressure generator  22  is connected to the liquid storage assembly  10  as described later, pressure in the liquid storage assembly  10  can be quickly set to negative pressure. 
     The first switching valve  23  and the second switching valve  24  are each a three-way valve. That is, the first switching valve  23  and the second switching valve  24  each have three ports. The first switching valve  23  includes the three ports that are each connected to the corresponding one of the liquid storage assembly  10 , the positive pressure generator  21 , and the second switching valve  24 . The second switching valve  24  includes the three ports that are each connected to the corresponding one of the negative pressure generator  22 , the atmospheric opening assembly  25 , and the first switching valve  23 . 
     The first switching valve  23  and the second switching valve  24  each allow two ports of the corresponding three ports to be internally connected to each other. In the present example embodiment, the first switching valve  23  allows the port connected to the liquid storage assembly  10  to be connected to the port connected to the positive pressure generator  21  or the port connected to the second switching valve  24 . That is, the first switching valve  23  switches between a line connected to the positive pressure generator  21  and a line connected to the second switching valve  24  to connect the switched line to the liquid storage assembly  10 . The second switching valve  24  allows the port connected to the first switching valve  23  to be connected to the port connected to the negative pressure generator  22  or the port connected to the atmospheric opening assembly  25 . That is, the second switching valve  24  switches between a line connected to the negative pressure generator  22  and a line connected to the atmospheric opening assembly  25  to connect the switched line to the first switching valve  23 . 
     The first switching valve  23  and the second switching valve  24  each switch connection between the corresponding ports in response to an open-close signal output from the controller  60 . The open-close signal includes a first control signal, a second control signal, a third control signal, and a fourth control signal, which are described later. 
     The pressure sensor  26  detects pressure in the liquid storage assembly  10 . The pressure sensor  26  outputs the detected pressure in the liquid storage assembly  10  as a pressure signal to the controller  60 . Negative pressure to be detected by the pressure sensor  26  changes in accordance with a remaining amount of liquid in the liquid storage assembly  10 . That is, when the remaining amount of liquid in the liquid storage assembly  10  decreases, the negative pressure detected by the pressure sensor  26  increases more than when a large amount of liquid remains. The increase in negative pressure means, for example, a state in which the negative pressure has changed from −1 kPa to −1.1 kPa. 
     The controller  60  described later controls the drive of the negative pressure pump  22   a  in response to a pressure signal output from the pressure sensor  26 . When decrease in the remaining amount of liquid in the liquid storage assembly  10  is detected by the pressure sensor  26  as high negative pressure in the liquid storage assembly  10 , the controller  60  sets a negative pressure target value lower to bring negative pressure generated by the negative pressure pump  22   a  close to the atmospheric pressure. 
     The above configuration causes the pressure adjusting assembly  20  to switch the first switching valve  23  to connect the positive pressure generator  21  to the liquid storage assembly  10  when pressure in the liquid storage assembly  10  is made positive, i.e., when the pressure in the liquid storage assembly  10  is pressurized to positive pressure. This enables a liquid to be pushed out from the liquid storage assembly  10  to the discharge assembly  30 . Thus, the liquid can be stably supplied to the discharge assembly  30 . 
     When the pressure in the liquid storage assembly  10  is made negative, the pressure adjusting assembly  20  switches not only the second switching valve  24  to connect the negative pressure generator  22  to the first switching valve  23 , but also the first switching valve  23  to connect the second switching valve  24  to the liquid storage assembly  10 . This enables the liquid to be prevented from leaking from the discharge port  32   a  of the discharge assembly  30  by setting the pressure in the liquid storage assembly  10  to negative pressure. 
     When the pressure in the liquid storage assembly  10  is set to the atmospheric pressure, the pressure adjusting assembly switches the second switching valve  24  to connect the atmospheric opening assembly  25  to the first switching valve  23 . At this time, the first switching valve  23  is in a state in which the second switching valve  24  is connected to the liquid storage assembly  10 . This enables the pressure in the liquid storage assembly  10  to be set to the atmospheric pressure. 
     The discharge assembly  30  discharges the liquid supplied from the liquid storage assembly  10  to the outside in the form of droplets.  FIG.  2    is an enlarged view illustrating structure of the discharge assembly  30 . Hereinafter, the structure of the discharge assembly  30  will be described with reference to  FIG.  2   . 
     The discharge assembly  30  includes a liquid supply assembly  31 , a diaphragm  35 , and a drive  40 . 
     The liquid supply assembly  31  includes a base  32  provided inside with a liquid chamber  33  and an inflow path  34 , and a heater  36 . The liquid storage assembly  10  is located on the base  32 . The inflow path  34  of the base  32  is connected to an outlet  10   a  of the liquid storage assembly  10 . The inflow path  34  is connected to the liquid chamber  33 . That is, the inflow path  34  is connected to the liquid chamber  33  and allows the liquid to be supplied from the liquid storage assembly  10  into the liquid chamber  33 . The liquid chamber  33  stores the liquid. 
     The base  32  includes the discharge port  32   a  connected to the liquid chamber  33 . The discharge port  32   a  is an opening for discharging the liquid supplied into the liquid chamber  33  to the outside. In the present example embodiment, the discharge port  32   a  opens downward, so that the liquid supplied into the inflow path  34  and the liquid chamber  33  has a liquid level protruding downward caused by a meniscus in the discharge port  32   a.    
     The heater  36  is located near the inflow path  34  in the base  32 . The heater  36  heats the liquid in the inflow path  34 . Although not particularly illustrated, the heater  36  includes, for example, a plate-shaped heater and a heat transfer block. The heater  36  may include another component such as a rod-shaped heater or a Peltier element as long as it can heat the liquid in the inflow path. 
     Heating the fluid in the inflow path  34  with the heater  36  enables temperature of the liquid to be maintained at a constant temperature higher than room temperature. This enables preventing physical characteristics of the liquid from changing with temperature. 
     Although not particularly illustrated, the liquid coating apparatus  1  may include a temperature sensor for controlling heating of the heater  36 , being located near the heater  36  or near the discharge port  32   a.  The heater  36  may be located on the base  32  as long as the fluid in the inflow path  34  can be heated. 
     The diaphragm  35  constitutes a part of a wall portion defining the liquid chamber  33 . The diaphragm  35  is located on an opposite side to the discharge port  32   a  across the liquid chamber  33 . The diaphragm  35  is supported by the base  32  in a deformable manner in its thickness direction. The diaphragm  35  constitutes the part of the wall portion defining the liquid chamber  33 , and is deformed to change the volume of the liquid chamber  33 . When the diaphragm  35  is deformed in the thickness direction to change the volume of the liquid chamber  33 , the liquid in the liquid chamber  33  is discharged to the outside through the discharge port  32   a.    
     The drive  40  deforms the diaphragm  35  in the thickness direction. Specifically, the drive  40  includes a piezoelectric element  41 , a first base  42 , a second base  43 , a plunger  44 , a coil spring  45 , and a casing  46 . 
     The piezoelectric element  41  extends in one direction by receiving predetermined voltage. That is, the piezoelectric element  41  is stretchable in the one direction. The piezoelectric element  41  deforms the diaphragm  35  in the thickness direction by expanding and contracting in the one direction. That is, the piezoelectric element  41  is a driving element that generates a driving force that deforms the diaphragm  35  in the thickness direction. The driving force for deforming the diaphragm  35  in the thickness direction may be generated by another driving element such as a magnetostrictive element. 
     The piezoelectric element  41  of the present example embodiment has a rectangular parallelepiped shape that is long in the one direction. Although not particularly illustrated, the piezoelectric element  41  of the present example embodiment is formed by electrically connecting multiple piezoelectric bodies  41   a  made of piezoelectric ceramics such as lead zirconate titanate (PZT), being laminated in the one direction. That is, the piezoelectric element  41  includes the multiple piezoelectric bodies  41   a  laminated in the one direction. This enables increasing the amount of expansion and contraction of the piezoelectric element  41  in the one direction as compared with the piezoelectric element  41  including one piezoelectric body. The shape of a piezoelectric element is not limited to a rectangular parallelepiped shape, and another shape such as a columnar shape may be used. 
     The multiple piezoelectric bodies  41   a  are electrically connected by side electrodes (not illustrated) located opposite to each other in a direction intersecting the one direction. Thus, the piezoelectric element  41  extends in the one direction when the side electrodes receive predetermined voltage. The predetermined voltage applied to the piezoelectric element  41  is a drive signal received from the controller  60  described later. 
     The structure of the piezoelectric element  41  is similar to that of a conventional piezoelectric element, so that detailed description thereof will be eliminated. The piezoelectric element  41  may have only one piezoelectric body. 
     The plunger  44  is a rod-shaped member. The plunger  44  has one end in its axial direction, being in contact with the diaphragm  35 . The plunger  44  has the other end in the axial direction, being in contact with the first base  42  described later, the first base  42  covering an end of the piezoelectric element  41  in the one direction. That is, the one direction of the piezoelectric element  41  aligns with the axial direction of the plunger  44 . The plunger  44  is between the piezoelectric element  41  and the diaphragm  35 . This allows expansion and contraction of the piezoelectric element  41  to be transmitted to the diaphragm  35  via the plunger  44 . The plunger  44  is a rod-shaped transmission. 
     The other end of the plunger  44  is in a hemispherical shape. That is, the plunger  44  is in a rod shape, and has a leading end close to the piezoelectric element  41 , being in a hemispherical shape. This enables the expansion and contraction of the piezoelectric element  41  to be reliably transmitted by the diaphragm  35  via the plunger  44 . 
     The piezoelectric element  41  has an end close to the diaphragm  35  in the one direction, the end being covered with the first base  42 . The first base  42  is in contact with the plunger  44 . The piezoelectric element  41  has an end on an opposite side to the diaphragm  35  in the one direction, the end being covered with the second base  43 . The second base  43  is supported by a fixed casing bottom-wall portion  47   a  of a fixed casing  47  described later. 
     The first base  42  and the second base  43  include bottom portions  42   a  and  43   a,  and vertical wall portions  42   b  and  43   b  located on their outer peripheral sides, respectively. The bottom portions  42   a  and  43   a  each have a size covering corresponding one of end surfaces of the piezoelectric element  41  in the one direction. The vertical wall portions  42   b  and  43   b  are each located covering a part of a side surface of the piezoelectric element  41 . 
     The first base  42  and the second base  43  are each made of a wear-resistant material. At least one of the first base  42  and the second base  43  may be made of a sintered material in order to improve wear resistance. The first base  42  and the second base  43  may be different in hardness from each other. 
     The piezoelectric element  41  is housed in the casing  46 . The casing  46  includes the fixed casing  47  and a pressurized casing  48 . The pressurized casing  48  is housed in the fixed casing  47 . The piezoelectric element  41  is housed in the pressurized casing  48 . The fixed casing  47  and the pressurized casing  48  are fixed with bolts or the like (not illustrated). 
     The fixed casing  47  has a box shape opening toward the diaphragm  35 . Specifically, the fixed casing  47  includes a fixed casing bottom-wall portion  47   a  and a fixed casing side-wall portion  47   b.    
     The fixed casing bottom-wall portion  47   a  is located on the opposite side to the diaphragm  35  across the piezoelectric element  41 . The fixed casing bottom-wall portion  47   a  includes a hemispherical protrusion  47   c  that supports one of the ends of the piezoelectric element  41  in the one direction. That is, the liquid coating apparatus  1  includes the hemispherical protrusion  47   c  protruding from the fixed casing bottom-wall portion  47   a  toward the piezoelectric element  41  in the one direction and supporting the end of the piezoelectric element  41  on the opposite side to the diaphragm  35 . This enables the end of the piezoelectric element  41  on the opposite side to the diaphragm  35  to be supported by the protrusion  47   c  of the fixed casing bottom-wall portion  47   a  without partial contact. Thus, the end of the piezoelectric element  41  on the opposite side to the diaphragm  35  can be more reliably supported by the fixed casing bottom-wall portion  47   a . The fixed casing bottom-wall portion  47   a  is a second support portion that supports the end of the piezoelectric element  41  on the side opposite to the diaphragm  35  in the one direction. 
     The second base  43  is between the piezoelectric element  41  and the protrusion  47   c.  That is, the liquid coating apparatus  1  includes the second base  43  between the piezoelectric element  41  and the protrusion  47   c.  This enables the end of the piezoelectric element  41  on the opposite side to the diaphragm  35  to be reliably supported by the protrusion  47   c  with the second base  43  interposed therebetween while the end of the piezoelectric element  41  on the opposite side to the diaphragm  35  is held by the second base  43 . 
     The pressurized casing  48  has a box shape opening toward the opposite side to the diaphragm  35  across the piezoelectric element  41 . Thus, in a state where the pressurized casing  48  is housed in the fixed casing  47 , a part of the fixed casing bottom-wall portion  47   a  is exposed in the casing  46 . The protrusion  47   c  described above is located in the exposed part of the fixed casing bottom-wall portion  47   a.    
     The pressurized casing  48  includes a pressurized casing bottom-wall portion  48   a  and a pressurized casing side-wall portion  48   b.    
     The pressurized casing bottom-wall portion  48   a  is located close to the diaphragm  35 . The pressurized casing bottom-wall portion  48   a  includes a through-hole allowing the plunger  44  to pass therethrough. Thus, the plunger  44  extends in the one direction between the piezoelectric element  41  and the diaphragm  35 , and passes through the pressurized casing bottom-wall portion  48   a,  thereby transmitting expansion and contraction of the piezoelectric element  41  to the diaphragm  35 . 
     The pressurized casing bottom-wall portion  48   a  is supported on an upper surface of the base  32 . This does not allow force generated by the coil spring  45  described later and sandwiched between the pressurized casing bottom-wall portion  48   a  and the first base  42  to act on the diaphragm  35  supported by the base  32 , or allows the force even to act on the diaphragm  35  slightly. 
     The coil spring  45  described later is held between the pressurized casing bottom-wall portion  48   a  and the first base  42 . The pressurized casing bottom-wall portion  48   a  is a first support portion that is between the piezoelectric element  41  and the diaphragm  35  in the one direction and supports the piezoelectric element  41  from a side close to the diaphragm  35 . 
     The pressurized casing side-wall portion  48   b  has an outer surface in contact with an inner surface of the fixed casing side-wall portion  47   b,  and the pressurized casing side-wall portion  48   b  has an inner surface in contact with the vertical wall portions  42   b  and  43   b  of the first base  42  and second base  43 , respectively. This enables the first base  42  and the second base  43  to be held by the pressurized casing side-wall portion  48   b . Thus, even when predetermined voltage is applied to the piezoelectric element  41 , deformation of the piezoelectric element  41  in a direction orthogonal to the one direction is reduced. 
     The above structure allows the piezoelectric element  41  to be sandwiched between the plunger  44  and the protrusion  47   c  of the fixed casing bottom-wall portion  47   a  in the one direction. This enables expansion and contraction of the piezoelectric element  41  to be transmitted to the diaphragm  35  with the plunger  44  when the piezoelectric element  41  expands and contracts in the one direction. Thus, the diaphragm  35  can be deformed in its thickness direction by the expansion and contraction of the piezoelectric element  41 .  FIG.  2    illustrates movement of the plunger  44  due to the expansion and contraction of the piezoelectric element  41  in the one direction with a solid arrow. 
     The coil spring  45  is a spring member that spirally extends along the axis in the one direction. The coil spring  45  is sandwiched in the one direction between the first base  42  and the pressurized casing bottom-wall portion  48   a.  The plunger  44  in a rod-like shape passes through inside the coil spring  45  in the axial direction. That is, the first base  42  is between the piezoelectric element  41  and the plunger  44  together with the coil spring  45 . The coil spring  45  extends along the axis of the plunger  44  between the piezoelectric element  41  and the pressurized casing bottom-wall portion  48   a.    
     This allows the coil spring  45  to apply force to compress the piezoelectric element  41  in the one direction via the first base  42 .  FIG.  2    illustrates compressive force of the coil spring  45  with a white arrow. The coil spring  45  is a compressive force applying assembly that is between the piezoelectric element  41  and the pressurized casing bottom-wall portion  48   a  and supported by the pressurized casing bottom-wall portion  48   a  to apply a compressive force to the piezoelectric element  41  in the one direction. The compressive force generated by the coil spring  45  preferably allows the first base  42  to be located in contact with the plunger  44  in a state where no voltage is applied to the piezoelectric element  41 . For example, the compressive force is preferably 30 to 50% of force generated in the piezoelectric element  41  when rated voltage is applied to the piezoelectric element  41 . 
     When the first base  42  is between the piezoelectric element  41  and the plunger  44  together with the coil spring  45 , the expansion and contraction of the piezoelectric element  41  can be stably transmitted to the plunger  44  via the first base  42 . At the same time, the compressive force of the coil spring  45  can be stably transmitted to the piezoelectric element  41  via the first base  42 . 
     Here, when the liquid has a high viscosity, the piezoelectric element  41  is required to operate at high speed. Thus, it is conceivable to improve responsiveness of the piezoelectric element  41  by inputting a drive signal with a rectangular wave to the piezoelectric element  41 . In this case, when the piezoelectric element  41  expands and contracts at high speed, the piezoelectric element  41  may expand and contract excessively, causing internal damage such as peeling. In particular, when the piezoelectric element  41  has multiple piezoelectric bodies  41   a  laminated in an expansion-contraction direction, high-speed operation of the piezoelectric element  41  tends to cause damage such as peeling inside the piezoelectric element  41 . The excessive expansion and contraction of the piezoelectric element  41  means that the amount of expansion and contraction of the piezoelectric element  41  is larger than the maximum amount of expansion and contraction when the rated voltage is applied to the piezoelectric element  41 . 
     In contrast, when the piezoelectric element  41  is compressed in the one direction by the coil spring  45  as in the present example embodiment, damage such as peeling due to expansion and contraction of the piezoelectric element  41  can be prevented from occurring inside the piezoelectric element  41  even when the piezoelectric element  41  receives a drive signal with a rectangular wave. That is, the coil spring  45  can suppress excessive expansion and contraction of the piezoelectric element  41 , and can prevent occurrence of internal damage of the piezoelectric element  41  due to its expansion and contraction. This enables improving durability of the piezoelectric element  41 . 
     When the coil spring  45  is between the piezoelectric element  41  and the pressurized casing bottom-wall portion  48   a  as described above, the pressurized casing bottom-wall portion  48   a  can receive elastic restoring force of the coil spring  45 . Thus, the diaphragm  35  can be prevented from being deformed by the elastic restoring force of the coil spring  45 . This enables preventing a liquid from leaking from the discharge port  32   a  and liquid discharge performance from being deteriorated. 
     When the plunger  44  passes through inside the coil spring  45  spirally extending along the axis in the axial direction, the plunger  44  and the coil spring  45  can be compactly disposed. This enables the liquid coating apparatus  1  to be miniaturized. 
     Next, a configuration of the controller  60  will be described below. 
     The controller  60  controls drive of the liquid coating apparatus  1 . That is, the controller  60  controls drive of each of the pressure adjusting assembly  20  and the drive  40 . 
     The controller  60  includes a pressure adjustment controller  61  and a drive controller  62 . 
     The pressure adjustment controller  61  outputs a control signal to the first switching valve  23  and the second switching valve  24  of the pressure adjusting assembly  20 . The pressure adjustment controller  61  also outputs a positive pressure pump drive signal to the positive pressure pump  21   a.  The pressure adjustment controller  61  further outputs a negative pressure pump drive signal to the negative pressure pump  22   a.  The pressure adjustment controller  61  outputs the control signal to the first switching valve  23  and the second switching valve  24  to control pressure in the liquid storage assembly  10 . 
     For example, when positive pressure is applied to the liquid storage assembly  10 , the pressure adjustment controller  61  outputs a first control signal for connecting the positive pressure generator  21  to the liquid storage assembly  10  to the first switching valve  23 . When negative pressure is applied to the liquid storage assembly  10 , the pressure adjustment controller  61  outputs a second control signal for connecting the second switching valve  24  to the liquid storage assembly  10  to the first switching valve  23 , and outputs a third control signal for connecting the negative pressure generator  22  to the first switching valve  23  to the second switching valve  24 . When pressure inside the liquid storage assembly  10  is set to the atmospheric pressure, the pressure adjustment controller  61  outputs the second control signal for connecting the second switching valve  24  to the liquid storage assembly  10  to the first switching valve  23 , and outputs a fourth control signal for connecting the atmospheric opening assembly  25  to the first switching valve  23  to the second switching valve  24 . 
     The pressure adjustment controller  61  controls drive of the negative pressure pump  22   a  in response to a pressure signal output from the pressure sensor  26 . That is, when driving the negative pressure pump  22   a  does not allow pressure detected by the pressure sensor  26  to reach the negative pressure target value, the pressure adjustment controller  61  sets the negative pressure target value lower and causes the negative pressure pump  22   a  to be driven in accordance with a new negative pressure target value. In this way, when decrease in the remaining amount of liquid in the liquid storage assembly  10  is detected by the pressure sensor  26  as high negative pressure in the liquid storage assembly  10 , the pressure adjustment controller  61  sets the negative pressure target value lower to bring negative pressure generated by the negative pressure pump  22   a  close to the atmospheric pressure. 
     The pressure adjustment controller  61  also controls drive of the positive pressure pump  21   a.  The drive of the positive pressure pump  21   a  is similar to that of a conventional configuration, so that detailed description thereof will be eliminated. 
     The drive controller  62  controls drive of the piezoelectric element  41 . That is, the drive controller  62  outputs a drive signal to the piezoelectric element  41 . This drive signal includes a discharge signal. 
     The discharge signal allows the piezoelectric element  41  to expand and contract to vibrate the diaphragm  35  as described later, thereby discharging the liquid in the liquid chamber  33  to the outside through the discharge port  32   a.    
     The controller  60  controls timing of allowing the drive controller  62  to output the discharge signal to the piezoelectric element  41  and timing of outputting the control signals to the pressure adjusting assembly  20 . 
       FIG.  3    is a flowchart illustrating an example of operation of discharging a liquid with the discharge assembly  30  and adjusting pressure in the liquid storage assembly  10  with the pressure adjusting assembly  20 . Control of the timing of allowing the drive controller  62  to output the discharge signal to the piezoelectric element  41  and the timing of outputting the control signals to the pressure adjusting assembly  20 , the control being performed by the controller  60 , will be described. 
     As illustrated in  FIG.  3   , the controller  60  first determines whether an external signal instructing discharge is received (step S 1 ). This external signal is received by the controller  60  from a controller or the like higher than the controller  60 . 
     When the controller  60  receives an external signal (YES in step S 1 ), in step S 2 , the pressure adjustment controller  61  of the controller  60  generates the first control signal for connecting the positive pressure generator  21  to the liquid storage assembly in the first switching valve  23  of the pressure adjusting assembly  20  and outputs it to the first switching valve  23 . The first switching valve  23  is driven in response to the first control signal. This causes the inside of the liquid storage assembly  10  to be pressurized to positive pressure. In contrast, when the controller  60  receives no external signal (NO in step S 1 ), the determination in step S 1  is repeated until the controller  60  receives an external signal. 
     After step S 2 , the drive controller  62  of the controller  60  outputs a discharge signal to the piezoelectric element  44  to discharge the liquid to the discharge assembly  30  through the discharge port  32   a  (step S 3 ). 
     After the drive controller  62  outputs the discharge signal to the piezoelectric element  44 , the pressure adjustment controller  61  may output the first control signal to the first switching valve  23 . That is, discharge of the discharge assembly  30  may be performed before pressurization of positive pressure in the liquid storage assembly  10 . 
     After that, the pressure adjustment controller  61  generates the second control signal for connecting the second switching valve  24  to the liquid storage assembly  10  in the first switching valve  23  of the pressure adjusting assembly  20 , and outputs it to the first switching valve  23 . The pressure adjustment controller  61  also generates the third control signal for connecting the atmospheric opening assembly  25  to the first switching valve  23  in the second switching valve  24 , and outputs it to the second switching valve  24  (step S 4 ). The first switching valve  23  is driven in response to the second control signal. The second switching valve  24  is driven in response to the third control signal. This causes the pressure in the liquid storage assembly  10  to be the atmospheric pressure. 
     Subsequently, the pressure adjustment controller  61  generates the fourth control signal for connecting the negative pressure generator  22  to the first switching valve  23  in the second switching valve  24 , and outputs it to the second switching valve  24  (step S 5 ). The second switching valve  24  is driven in response to the fourth control signal. This causes the pressure in the liquid storage assembly  10  to be negative pressure. Thus, the liquid can be prevented from leaking through the discharge port  32   a  of the discharge assembly  30 . Then, this flow is ended (END). The controller  60  repeatedly performs the above-mentioned flow as necessary. 
     When the pressure in the liquid storage assembly  10  is controlled as described above, the liquid can be stably discharged through the discharge port  32   a  at appropriate timing without leakage of the liquid through the discharge port  32   a  of the discharge assembly  30 . 
     The drive controller  62  may repolarize the piezoelectric element  41 . The piezoelectric element  41  includes multiple piezoelectric bodies  41   a  that are made of a polarized sintered material and are electrically connected. Thus, the piezoelectric element  41  has characteristics in which when the piezoelectric element  41  is left for a long time without being used or when the piezoelectric element  41  is at a high temperature, for example, an electric field is generated inside the piezoelectric element  41  and the amount of displacement of the piezoelectric element when voltage is applied gradually decreases. When displacement characteristics of the piezoelectric element  41  deteriorate as described above, the piezoelectric element  41  needs to be repolarized to recover the displacement characteristics of the piezoelectric element  41 . 
     When the piezoelectric element  41  is repolarized, the drive controller  62  outputs a drive signal for applying rated voltage to the piezoelectric element  41  for a certain period of time, and then turns off the drive signal for a predetermined period of time. In this case, the drive controller  62  generates, as the drive signal, a drive signal capable of preventing a steep rise and fall of the rated voltage applied to the piezoelectric element  41 . The rated voltage is predetermined voltage. The voltage applied to the piezoelectric element  41  by the drive controller  62  when the piezoelectric element  41  is repolarized may be voltage other than the rated voltage of the piezoelectric element  41  as long as the voltage enables repolarization of the piezoelectric element  41 . 
     As described above, the liquid coating apparatus  1  may include the controller  60  that performs drive control of the piezoelectric element  41  and performs a repolarization process of applying the rated voltage to the piezoelectric element  41  for a certain period of time and then setting voltage to be applied to zero. 
     This enables the displacement characteristics of the piezoelectric element  41  to be recovered without using a dedicated circuit when the controller  60  repolarizes the piezoelectric element  41 . 
     The piezoelectric element  41  may be repolarized at any timing other than timing at which a liquid is discharged, such as when the liquid coating apparatus  1  is started or when the liquid coating apparatus  1  receives an external signal instructing liquid discharge. 
     The liquid coating apparatus  1  according to the present example embodiment includes the liquid chamber  33  that stores a liquid, the inflow path  34  that is connected to the liquid chamber  33  and allows the liquid to be supplied from the liquid storage assembly  10  into the liquid chamber  33 , the diaphragm  35  that constitutes a part of a wall portion defining the liquid chamber  33  and is deformed in a thickness direction to change a volume of the liquid chamber  33 , the piezoelectric element  41  that expands and contracts in at least one direction to deform the diaphragm  35  in the thickness direction, the pressurized casing bottom-wall portion  48   a  that is between the piezoelectric element  41  and the diaphragm  35  in the one direction to support the piezoelectric element  41  from a diaphragm  35  side, the fixed casing bottom-wall portion  47   a  that supports an end of the piezoelectric element  41  on the opposite side to the diaphragm  35  in the one direction, the plunger  44  that extends in the one direction between the piezoelectric element  41  and the diaphragm  35  and passes through the pressurized casing bottom-wall portion  48   a  to transmit expansion and contraction of the piezoelectric element  41  to the diaphragm  35 , and the coil spring  45  that is between the piezoelectric element  41  and the pressurized casing bottom-wall portion  48   a  and is supported by the pressurized casing bottom-wall portion  48   a  to apply a compressive force to the piezoelectric element  41  in the one direction. 
     This enables the piezoelectric element  41  to be compressed in one direction in which the piezoelectric element  41  expands and contracts by the coil spring  45 . Thus, even when the piezoelectric element  41  is operated with a high response, the piezoelectric element  41  is prevented from excessively expanding and contracting, and thus an excessive load at a level affecting the life of the piezoelectric element  41  can be prevented from being applied to the inside of the piezoelectric element  41 . Additionally, the coil spring  45  is supported by the pressurized casing bottom-wall portion  48   a,  so that a force generated by the coil spring  45  is not transmitted to the diaphragm  35 . This enables the diaphragm  35  to be prevented from being deformed by the force generated by the coil spring  45 . 
     In particular, the piezoelectric element  41  includes the multiple piezoelectric bodies  41   a  laminated in the one direction. This enables increasing a length of expansion and contraction of the piezoelectric element  41  in the one direction as compared with the piezoelectric element  41  including one piezoelectric body  41   a . Unfortunately, the multiple piezoelectric bodies  41   a  laminated in the one direction as described above cause an excessive load to be likely to be applied to the inside of the piezoelectric element  41  when the piezoelectric element  41  is operated with a high response to cause the piezoelectric element  41  to be excessively expanded and contracted. In contrast, when the coil spring  45  compresses the piezoelectric element  41  in the one direction as described above, an excessive load at a level affecting the life of the piezoelectric element  41  can be prevented from being applied to the inside of the piezoelectric element  41 . That is, the above-described structure is particularly effective in a structure in which the piezoelectric element  41  includes the multiple piezoelectric bodies  41   a  laminated in the one direction. 
     In the present example embodiment, the plunger  44  has a rod shape extending along the axis. The coil spring  45  extends along the axis of the plunger  44  between the piezoelectric element  41  and the pressurized casing bottom-wall portion  48   a  to apply a compressive force to the piezoelectric element  41  in the one direction. 
     This enables a compressive force of the coil spring  45  to be applied to the piezoelectric element  41  in a direction in which the piezoelectric element  41  expands and contracts to apply a force to the plunger  44 . Thus, even when the piezoelectric element  41  is operated with a high response, the piezoelectric element  41  is prevented from excessively expanding and contracting, and thus an excessive load at a level affecting the life of the piezoelectric element  41  can be prevented from being applied to the inside of the piezoelectric element  41 . 
     In the present example embodiment, the plunger  44  is in a rod shape, and has a leading end in a hemispherical shape on a piezoelectric element  41  side. The liquid coating apparatus  1  includes the protrusion  47   c  in a hemispherical shape protruding from the fixed casing bottom-wall portion  47   a  toward the piezoelectric element  41  in the one direction and supporting the end of the piezoelectric element  41  on the opposite side to the diaphragm  35 . 
     This enables a compression direction by the coil spring to be set to the one direction in which the piezoelectric element  41  expands and contracts, when the piezoelectric element  41  is compressed in the one direction by the coil spring  45 . The piezoelectric element  41  is likely to be damaged by a compressive force in a direction other than the one direction. Thus, when the compression direction by the coil spring  45  is set to the one direction as described above, the piezoelectric element  41  can be prevented from being damaged by the compressive force of the coil spring  45 . The compression direction by the coil spring  45  does not need to completely align with the one direction, and may be a direction in which the compressive force generated by the coil spring  45  includes a force of a component in the one direction. 
     Although the example embodiment of the present disclosure is described above, the above-described example embodiment is merely an example for implementing the present disclosure. Thus, the above-described example embodiment can be appropriately modified and implemented within a range without departing from the gist thereof and being limited to the above-described example embodiment. 
     In the example embodiment, the coil spring  45  compresses the piezoelectric element  41  in one direction. However, when the piezoelectric element can be compressed in one direction, the piezoelectric element may be compressed by a configuration other than a coil spring. That is, although in the above example embodiment, the coil spring  45 , which is a spiral spring member, is described as an example of a compressive force applying assembly, besides this, the spiral spring member may be, for example, a so-called coiled wave spring in which a wire rod or a flat plate, having a predetermined length and a wavy shape, is spirally wound. The compressive force applying assembly may have a structure other than the spiral shape as long as the piezoelectric element can be compressed in one direction. The compressive force applying assembly is preferably disposed preventing interference with the plunger regardless of structure. 
     In the above example embodiment, the plunger  44  passes through the coil spring  45  extending spirally along the axis. However, the placement of the coil spring is not particularly limited as long as the coil spring extends parallel to one direction that is a direction of expansion and contraction of the piezoelectric element with respect to the plunger. 
     In the above example embodiment, both ends of the piezoelectric element  41  are each covered with the corresponding one of the first base  42  and the second base  43  in one direction in which the piezoelectric element  41  expands and contracts. However, in the one direction, only one of both the ends of the piezoelectric element may be covered with a base. In the one direction, each end of the piezoelectric element may not be covered with a base. 
     In the above example embodiment, the piezoelectric element  41  is supported by the protrusion  47   c  in a hemispherical shape of the fixed casing bottom-wall portion  47   a  and the leading end in a hemispherical shape of the plunger  44  on the piezoelectric element  41  side. However, the liquid coating apparatus may not have at least one of the protrusion in a hemispherical shape and the leading end in a hemispherical shape of the plunger as long as the direction of expansion and contraction of the piezoelectric element is parallel to the compression direction of the coil spring. The shape of each of the protrusion and the leading end of the plunger is not limited to the hemispherical shape, and may be any shape as long as the shape can support the piezoelectric element. 
     In the above example embodiment, the casing  46  housing the piezoelectric element  41  includes the pressurized casing  48  housed in the fixed casing  47 . However, the casing may not include a pressurized casing. In this case, the piezoelectric element is housed in the fixed casing. The coil spring has an end on a diaphragm side that is supported by the upper surface of the base. That is, an upper wall portion of the base functions as the first support portion. 
     In the above example embodiment, the discharge assembly  30  includes the heater  36  that heats a liquid in the inflow path  34 . However, the discharge assembly may not include the heater. 
     In the above example embodiment, the pressure adjusting assembly  20  includes the first switching valve  23  that is connected to the liquid storage assembly  10  by switching between a line connected to the positive pressure generator  21  and a line connected to the second switching valve  24 , and the second switching valve  24  that is connected to the first switching valve  23  by switching between a line connected to the negative pressure generator  22  and a line connected to the atmospheric opening assembly  25 . 
     However, the pressure adjusting assembly may include a switching valve that connects each of the positive pressure generator, the negative pressure generator, and the atmospheric opening assembly, to the liquid storage assembly. The pressure adjusting assembly may have any configuration as long as the positive pressure generator, the negative pressure generator, and the atmospheric opening assembly can be each connected to the liquid storage assembly. 
     In the above example embodiment, the liquid storage assembly  10  can be connected to the atmospheric opening assembly by the pressure adjusting assembly  20 . However, the pressure adjusting assembly may have a configuration in which the atmospheric opening assembly cannot be connected to the liquid storage assembly. 
     In the above example embodiment, the liquid storage assembly  10  can be connected to the positive pressure generator  21  by the pressure adjusting assembly  20 . However, the liquid coating apparatus may not include a positive pressure generator. That is, the liquid coating apparatus may control pressure in the liquid storage assembly using negative pressure and the atmospheric pressure. 
     The present disclosure is available for a liquid coating apparatus that discharges a liquid from a discharge assembly. 
     Features of the above-described preferred example embodiments and the modifications thereof may be combined appropriately as long as no conflict arises. 
     While example embodiments of the present disclosure have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present disclosure. The scope of the present disclosure, therefore, is to be determined solely by the following claims.