Patent Publication Number: US-2023152263-A1

Title: Driving device provided with piezoelectric element deterioration detection circuit and deterioration detection method

Description:
TECHNICAL FIELD 
     The present invention relates to a driving device provided with a deterioration detection circuit of a piezoelectric element used as a driving source of a driving device such as an actuator and a method of detecting deterioration of the piezoelectric element, and more particularly, to a control device used for supplying a fluid such as a gas in a semiconductor manufacturing facility or a chemical plant or the like, i.e., a pressure type flow rate control device having a deterioration detection circuit of the piezoelectric element, and a method of detecting deterioration of the piezoelectric element of the pressure type flow rate control device. 
     BACKGROUND ART 
     A pressure type flow rate control device used for supplying a fluid such as a gas in a semiconductor manufacturing facility, a chemical plant, or the like is known (Patent Documents 1, 2, and the like). A control valve using a piezoelectric element can be used as the pressure type flow rate control device. For example. Patent Document 1 below discloses a pressure type flow rate control device having a piezoelectric element driven metal diaphragm type control valve shown in  FIG.  8    (hereinafter, simply referred to as a control valve). The control valve  100  includes a piezoelectric element  101 , a connector  102  connected to a power supply (not shown) for suppling voltage to the piezoelectric element  101 , and a diaphragm valve element  104  provided in a valve body  103 . The piezoelectric element  101  is housed in a cylindrical support tube  105  and deformed in the longitudinal direction by on/off controlling of the supply of a predetermined voltage via the connector  102 . The deformation of the piezoelectric element  101  causes the deformation of the diaphragm valve element  104 , and thereby the opening and closing of the valve is performed. 
     The pressure type flow rate control device includes a throttle portion  106 , such as an orifice, provided in a flow path  107  of a fluid G, under a critical expansion condition where an upstream pressure P 1  of the throttle portion  106  is held at approximately twice or more of a downstream pressure P 2  of the throttle portion  106 , the upstream pressure P 1  detected by a first pressure sensor  108  provided upstream of the throttle portion  106  is adjusted by the control valve  100  upstream of the throttle portion  106 . Thus, as a basic principle, the flow rate Qc downstream of the throttle portion  106  is calculated by Qc=KP 1 , where K is a constant depending on the type of fluid and the fluid temperature, and the flow rate Qc is controlled to become a predetermined set value. Further, even when the difference between the upstream pressure P 1  and the downstream pressure P 2  is small, and the critical expansion condition is not satisfied, it is possible to detect the downstream pressure P 2  by a second pressure sensor (not shown) provided downstream of the throttle portion  106 , and to determine the flow rate by calculation. That is, it is possible to obtain the flow rate Q based on the upstream pressure P 1  and downstream pressure P 2  measured by the first pressure sensor  108  and the second pressure sensor, from Q=K 2 *P 2   m  (P 1 −P 2 ) n (where K 2  is a constant depending on the type of the fluid and fluid temperature, m and n are index numbers derived from the actual flow rate). In addition, a reference numeral  109  in  FIG.  8    indicates a control circuit board (control unit). 
     The control valve as described above is continuously used for a long time in a semiconductor manufacturing facility, a chemical plant, or the like. Though the piezoelectric element used for the control valve fails by aged deterioration, it can not be predicted when the control valve fails due to the deterioration of the piezoelectric element. 
     The control valve is also used for the supply of gas or the like containing moisture, the life of the piezoelectric element is greatly different depending on whether or not the use environment containing moisture. Some control valves are filled with a member (moisture adsorbent) for absorbing moisture in the case containing the piezoelectric element. Therefore, even when the control valve is used for the same period of time and the same number of opening and closing times, the degree of the deterioration of the piezoelectric element differs depending on the environment, in which the control valve is used (i.e., the environment in which the piezoelectric element is used), and whether or not the piezoelectric element is provided with the moisture adsorbent. Therefore, it is difficult to predict the deterioration of the piezoelectric element and the replacement timing of the pressure type flow rate control device. 
     Due to the deterioration of the piezoelectric element, the control valve cannot control the flow rate as scheduled, and if it fails, the fluid may not be able to be supplied. If the control valve is replaced after a failure, the equipment will be shut down at an unscheduled timing. To prevent this, it is possible to replace the control valve at regular intervals. However, it will cause the replacement of a control valve even the piezoelectric element provided therein has not yet failed. 
     The Patent Document 3 described below discloses a piezoelectric actuator for detecting an abnormality of the piezoelectric element in advance. The piezoelectric actuator includes an abnormality detection circuit separate from the drive circuit for normal operation, and a switch for switching the drive circuit and the abnormality detection circuit. Before operating the piezoelectric actuator, the switch is switched to determine whether an abnormality has occurred in the piezoelectric element by the abnormality detection circuit. If there is no abnormality, the switch is switched to operate the piezoelectric actuator by the drive circuit. 
     PRIOR-ART DOCUMENT 
     Patent Literature 
     
         
         Patent Document 1: Japanese Patent No. 41 19109 
         Patent Document 2: Japanese Laid-Open Patent Application No. 2009-116904 
         Patent Document 3: Japanese Laid-Open Patent Application No. 2017-060356 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     As described above, it is preferable that the deterioration of the piezoelectric element can be detected in advance and the control valve can be replaced before its deterioration or failure. For this purpose, it is necessary to detect the deterioration of the piezoelectric element itself. 
     In Patent Document 3, even it is possible to detect the deterioration of the piezoelectric element, an abnormality detection circuit must be provided separately from the drive circuit for a normal operation, and there are problems that the configuration becomes complicated and the cost increases. Further, before performing the normal operation, it is necessary to determine the presence or absence of abnormality of the piezoelectric element by the abnormality detection circuit, there is a problem that it can not be applied to equipment continuously used for a long period in a semiconductor manufacturing facility or a chemical plant or the like. 
     An object of the present invention is to solves the above problems, and to provide a driving device having a piezoelectric element deterioration detection circuit that can detect deterioration of the piezoelectric element used in the driving device, without stopping the normal operation of the driving device, and a deterioration detection method. 
     Solution of Problem 
     To achieve the above object, the driving device according to a first embodiment of the present invention includes a piezoelectric element, a first resistor connected in series with the piezoelectric element, a voltage supply unit for supplying a DC voltage across the series connection circuit formed by the piezoelectric element and the first resistor, a measuring unit for measuring a voltage of the first resistor, and a control unit for controlling the voltage supply unit and the measuring unit, wherein the resistance value of the first resistor is smaller than an insulation resistance value of the piezoelectric element, the measuring unit measures the voltage of the first resistor, in a state where a predetermined voltage is supplied from the voltage supply unit, and the control unit calculates a resistance value of the piezoelectric element from the voltage value obtained by measurement of the measuring unit, and determines whether or not deterioration has occurred in the piezoelectric element based on the calculated resistance value. 
     The measuring unit measures the voltage of the first resistor for a plurality of times, in a state where the predetermined voltage is supplied from the voltage supply unit, and the control unit calculates the resistance value of the piezoelectric element from each of the plurality of voltage values obtained by the plurality of measurements, calculates a representative value of the plurality of calculated resistance values of the piezoelectric element, and determines whether or not deterioration has occurred in the piezoelectric element by comparing the representative value with a predetermined threshold. 
     The measuring unit can measure the voltage of the first resistor for a plurality of times, in a state where the predetermined voltage is supplied from the voltage supply unit, and the control unit can conduct repeatedly a process of calculating the resistance value of the piezoelectric element from each of a plurality of voltage values obtained by a plurality of measurements, and a process of calculating a representative value of the plurality of calculated resistance values of the piezoelectric element, calculate a slope of the plurality of calculated representative values, and determine whether or not deterioration has occurred in the piezoelectric element by comparing the slope with a predetermined threshold value. 
     The measuring unit can determine whether or not deterioration has occurred in the piezoelectric element in response to the fact that a predetermined period of time has elapsed after the voltage is first applied to the piezoelectric element. 
     The above driving device may have a mechanism for opening and closing a valve by the piezoelectric element, and the measuring unit may determine whether or not deterioration has occurred in the piezoelectric element in response to the fact that the number of times of opening and closing the valve has exceeded a predetermined number of times. 
     Preferably, the above driving device further includes a second resistor, wherein the second resistor is connected in series with the piezoelectric element and the first resistor, the second resistor is connected between a positive terminal of the voltage supply unit and the first resistor, and a resistance value of the second resistor is smaller than the insulation resistance value of the piezoelectric element and larger than the resistance value of the first resistor. 
     A deterioration detection method according to a second aspect of the present invention is a method of detecting deterioration of a piezoelectric element in the driving device including the piezoelectric element, a first resistor connected in series with the piezoelectric element, a voltage supply unit for supplying a DC voltage, and a measuring unit for measuring the voltage including a step of supplying, by the voltage supply unit, a predetermined DC voltage across the series connection circuit formed by the piezoelectric element and the first resistor, a measurement step of measuring a voltage of the first resistor by the measuring unit, in a state of supplying the predetermined DC voltage, a calculation step of calculating a resistance value of the piezoelectric element from the voltage value obtained by the measurement step, and a step of determining whether or not deterioration has occurred in the piezoelectric element, based on the resistance value calculated by the calculation step. 
     Advantageous Effect of Invention 
     According to the present invention, whether or not the piezoelectric element is deteriorated may be determined without affecting the normal operation of the driving device. That is, since the detection circuit is provided in the middle of a wiring for normally controlling the piezoelectric element, so as not to affect the normal control, it is possible to determine the presence or absence of deterioration of the piezoelectric element during normal control without providing an extra equipment or circuit. Therefore, whether or not the driving device itself should be replaced may be determined during normal control of the driving device. 
     In addition, whether or not degradation of the piezoelectric element has occurred and whether or not the driving device should be replaced may be determined regardless of the applying environment of the driving device (piezoelectric element) and whether or not the driving device is provided with a moisture adsorbent. 
     Further, the presence or absence of deterioration of the piezoelectric element may also be determined when stopping the driving device, or when the gas supply is completed. 
     When determining the presence or absence of deterioration of the piezoelectric element by comparing a resistance value of the piezoelectric element calculated from one measurement with a predetermined threshold value, erroneous determination of deterioration may occur even when the resistance value exceeds the threshold value by accident. On the other hand, erroneous determination may be prevented by comparing the calculated representative value (e.g., average value) of the plurality of resistance values, or the tendency (e.g., slope) of the representative value with a predetermined reference (threshold value). 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a block diagram showing a schematic configuration of a driving device provided with a piezoelectric element deterioration detection circuit according to an embodiment of the present invention. 
         FIG.  2    is a circuit diagram showing an equivalent circuit of a circuit for supplying a voltage to the piezoelectric element of  FIG.  1   . 
         FIG.  3    is a block diagram showing a control portion related to the deterioration detection of the piezoelectric element. 
         FIG.  4    is a flowchart showing a deterioration detection method of the piezoelectric element. 
         FIG.  5    is a graph showing a measurement result by a data logger. 
         FIG.  6    is a block diagram showing a schematic configuration of a driving device provided with a deterioration detection circuit different from that of  FIG.  1   . 
         FIG.  7    is a graph showing the resistance value of the piezoelectric element calculated from the measurement result. 
         FIG.  8    is a longitudinal sectional view showing a configuration of a conventional piezoelectric element driven metal diaphragm type control valve. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of a driving device provided with a piezoelectric element deterioration detection circuit and a deterioration detection method according to the present invention will be described with reference to the drawings below. The same or similar components are denoted by the same reference numerals throughout the drawings and the embodiments. 
       FIG.  1    shows a driving device provided with a piezoelectric element deterioration detecting circuit according to an embodiment of the present invention. The driving device  1  is, for example, a pressure type flow rate control device. The driving device  1  includes a piezoelectric element  2 , a control unit  3 , a power supply unit  4 , a first connector  5 , a second connector  6 , a first positive terminal  7 , a first negative terminal  8 , a second positive terminal  9 , a second negative terminal  10 , a first resistor  11 , a second resistor  12 , a first terminal  13  and a second terminal  14 . 
     The power supply unit  4  is included in the control unit  3 . The first positive terminal  7  and the second positive terminal  9  are electrically connected by electrical wiring. Hereinafter, unless otherwise specified, the term “connection” means electrical connection. The first negative terminal  8  is connected to the second negative terminal  10  via the first resistor  11  and the second resistor  12  connected in series. The piezoelectric element  2  may be composed of one piezoelectric body, or may be composed of a plurality of piezoelectric bodies constructed by laminating. 
     The power supply unit  4  outputs a DC voltage (hereinafter simply referred to as a voltage) of a predetermined magnitude. Thus, the predetermined voltage is applied between the second positive terminal  9  and the second negative terminal  10  via the first positive terminal  7  and the first negative terminal  8 , so that the piezoelectric element  2  deforms. Therefore, it may cause deformation or displacement of a displacement portion (such as a diaphragm valve element) to control the operation of the driving unit (opening and closing of the valve, etc.), and, for example, to control the supply of a fluid such as a gas. 
     Both ends of the first resistor  11  are respectively connected to the first terminal  13  and the second terminal  14 . Thus, the first resistor  11 , the second resistor  12 , the first terminal  13  and the second terminal  14  constitute a deterioration detection circuit  15 . The first resistor  11  and the second resistor  12  are voltage dividing resistors for dropping the voltage of the second terminal  14  to a voltage value that can be input to a measuring device, which will be described later. 
       FIG.  2    shows an equivalent circuit of a circuit including the power supply unit  4 , the piezoelectric element  2 , the first resistor  11  and the second resistor  12  of  FIG.  1   . Each resistance value of the first resistor  11 , the second resistor  12  and the piezoelectric element  2  is respectively denoted by R 1 , R 2  and R 3 . The first resistor  11 , the second resistor  12  and the piezoelectric element  2  are connected in series, and a predetermined voltage is supplied from the power supply unit  4  across the formed series circuit. 
     An insulation resistance of the piezoelectric element  2  that is not deteriorated is very large, the resistance value R 3  is, for example, R 3 &gt;1×109 (Ω)=1-103 (MΩ). As the first resistor  11  and the second resistor  12 , resistance values that are sufficiently smaller than the resistance value R 3  of the piezoelectric element  2  under a non-deteriorated state are used (R 1 &lt;&lt;R 3 , R 2 &lt;R 3 ). Here, R 1 =10 kΩ, R 2 =39 kΩ. 
     When the voltage between the first terminal  13  and the second terminal  14  is represented by V 1 , the current I flowing through the circuit of  FIG.  2    becomes I=V 1 /R 1 . Therefore, if R 1 &lt;&lt;R 3 , R 2  R 3 , when the output voltage value of the power supply unit  4  is V 0 , the resistance value R 3  of the piezoelectric element  2  can be obtained by R 3 =V 0 /I=V 0 /(V 1 /R 1 ). That is, in a state of supplying a predetermined voltage (V 0 ) from the power supply unit  4 , by measuring the voltage (V 1 ) between the first terminal  13  and the second terminal  14 , the resistance value of the piezoelectric element  2  may be calculated from the measured voltage. 
     With respect to the piezoelectric element  2 , when voltage is repetitively applied for a long time, the piezoelectric element  2  will be deteriorated, and the insulation resistance value will be lowered. Therefore, in a state of normally operating the driving device  1  and supplying a predetermined voltage from the power supply unit  4 , the presence or absence of deterioration of the piezoelectric element  2  may be determined by measuring the voltage between the first terminal  13  and the second terminal  14 , calculating the resistance value R 3  of the piezoelectric element  2 , and comparing the calculated value (R 3 ) with a predetermined threshold value Rth. For example, it can be determined that the piezoelectric element  2  is not deteriorated when R 3 ≥Rth, and deterioration has occurred in the piezoelectric element  2  when R 3 &lt;Rth. 
     The method of detecting the deterioration of the piezoelectric element  2  will be described in more details. Of the configuration of the driving device  1 , a configuration related to the detection of deterioration of the piezoelectric element  2  is shown in  FIG.  3   . Referring to  FIG.  3   , the control unit  3  controls the entire driving device  1  to normally operate. The control unit  3  is configured with a CPU (Central Processing Unit)  20 , an ROM (Read Only Memory)  21 , an RAM (Random Access Memory)  22 , an I/O unit  23 , a bus  24  and a power supply unit  4 . 
     An external measuring unit  25  and an information presentation unit  26  are connected to the I/O unit  23 . Here, with respect to the deterioration detection circuit  15 , only the first terminal  13  and second terminal  14  are shown, other components are not shown. 
     The CPU  20  realizes the function of the driving device  1  by executing a program recorded in the ROM  21 . The ROM  21  is, for example, an electrically writable non-volatile memory that stores the predetermined program and the data required to execute the program. The required data are, for example, the determination threshold Rth, the voltage value V 0  supplied from the power supply unit  4  to the piezoelectric element  2 , the resistance value R 1  of the first resistor  11  and the like. The RAM  22  is a volatile memory being used as a work area when CPU  20  executes the program and for temporarily storing the value of the calculation result. 
     The I/O unit  23  is an interface for exchanging data with the outside of the control unit  3  (the measuring unit  25  and the information presentation unit  26 ). The CPU  20  outputs a signal for causing the measuring unit  25  to start measuring (control code or the like, hereinafter, also referred to as a measurement start signal), to the measuring unit  25  via the I/O unit  23 . The I/O unit  23  acquires data (measurement data) outputted from the measuring unit  25 , and stores the data (measurement data) in the RAM  22 . Further, the I/O unit  23  outputs predetermined data outputted from the CPU  20  to the information presentation unit  26 . 
     Although not shown, the I/O unit  23  may include an interface for exchanging information with an external device such as a computer. This allows program and data to be written to the ROM  21  through the interface with the external device. In the case where an interface with the external device is not provided, if the ROM  21  is configured to be removable, programs and parameters can be updated by replacing the ROM  21  with a new one. It is also possible to update the data of the removed ROM  21  using an external device. 
     The buses  24  are parallel electric wirings for exchanging data among the CPU  20 , the ROM  21 , the RAM  22 , the I/O unit  23 , and the power supply unit  4 . Further, although not shown in  FIGS.  1  and  3   , the driving device  1  also includes components necessary for its operation, such as a clock signal generator for operating the respective units in synchronization. 
     The measurement terminal of the measuring unit  25  is connected to the first terminal  13  and second terminal  14  of the deterioration detection circuit  15 . The measuring unit  25  measures the voltage between the first terminal  13  and the second terminal  14  upon receiving the measurement start signal from the CPU  20 . The measuring unit  25  is, for example, a known tester or data logger capable of measuring the voltage. 
     The information presentation unit  26  can present information and is, for example, a display device (liquid crystal panel. LED panel, etc.) capable of displaying information such as text, or a lighting device (LED lamp, etc.). 
     Being configured in this way, the driving device  1  can drive the driving unit by supplying a voltage from the power supply unit  4  to the piezoelectric element  2 , at a predetermined timing set by the CPU  20 . Further, while supplying a voltage from the power supply unit  4  to the piezoelectric element  2 , the measuring unit  25  is controlled by the CPU  20 , the voltage between the first terminal  13  and the second terminal  14  (voltage across the first resistor  11 ) is measured, and as described above, it is possible to determine the presence or absence of deterioration of the piezoelectric element  2  by calculating the resistance value R 3  of the piezoelectric element  2 . 
     Referring to the flowchart of  FIG.  4   , a method for detecting deterioration of the piezoelectric element  2  in the driving device  1  will be described. Each step of the flow chart of  FIG.  4    is realized by turning on the power of the driving device  1  and causing the CPU  20  to execute predetermined programs read out from ROM  21 . Here, it is assumed that the CPU  20  reads a control program (hereinafter, also referred to as a normal drive program) for normally operating the driving device  1  and a control program (hereinafter, also referred to as a detection program) for detecting deterioration of the piezoelectric element  2 , and executes these control programs in parallel.  FIG.  4    shows the detection program, and does not include the normal drive program. 
     In step  40  of the detection program, the CPU  20  performs the initialization. For example, the CPU  20  reads out a predetermined threshold value (Rth) from the ROM  21  to the RAM  22 , secures an area to be used as a counter in the RAM  22 , reads out a value indicating the number of times the driving unit of the driving device  1  was driven (hereinafter, referred to as the number of driving times, for example, in the case of a control valve, the number of times of opening and closing the valve) stored in the ROM  21 , and sets the value as an initialization value of the counter. The counter is incremented by “1” each time the driving unit of the driving device  1  is driven by the normal drive program executed by the CPU  20 . When the power supply of the driving device  1  is turned off, the present value of the counter in the RAM  22  is stored in advance in the ROM  21  as the number of driving times. 
     In step  41 , the CPU  20  determines whether or not to measure the voltage of the first resistor  11  by the measuring unit  25 . If it determines to perform the voltage measurement, the control proceeds to step  42 , otherwise, the control proceeds to step  47 . The voltage measurement is performed by determining whether the value of the counter exceeds a predetermined value. For example, if the value exceeds an integer multiple of a predetermined number of times (500,000 times, 1,000,000 times, etc.), it determines to execute the voltage measurement. Otherwise, it determines not to execute the voltage measurement. 
     In step  42 , the CPU  20  performs the voltage measurement by the voltage measuring unit  25 . Specifically, the CPU  20  outputs the measurement start signal to the measuring unit  25  via the I/O unit  23 , while supplying the predetermined voltage (V 0 ) from the power supply unit  4  to the piezoelectric element  2 . The measuring unit  25 , upon receiving the measurement start signal, performs repeated voltage measurement at predetermined time intervals. The measuring unit  25  outputs the measurement data V 1  to the I/O unit  23 , and the I/O unit  23  stores the received data in the RAM  22 . 
     When stopping the operation of the driving unit, prior to stopping the voltage supply from the power supply unit  4  to the piezoelectric element  2 , the CPU  20  outputs a control signal for instructing the measurement stop (hereinafter, referred to as measurement stop signal) to the measuring unit  25  via the I/O unit  23 . Once receiving the measurement stop signal, the measuring unit  25  stops the voltage measurement. The method, in which the measuring unit  25  outputs the measurement data to the I/O unit  23 , is arbitrary. For example, the measuring unit  25  may output the measurement data to the I/O unit  23  every time the measurement data is obtained, or may temporarily store the measurement data in a storage unit (such as a buffer) in the measuring unit  25 , and collectively output the measurement data to the I/O unit  23  when the measurement stop signal is received, or when the amount of the temporarily stored measurement data reaches a predetermined amount. 
     As described above, since the normal drive program is executed simultaneously with the main detection program, the supply of the voltage from the power supply unit  4  and the stoppage thereof can be transmitted from the normal drive program to the main detection program by interruption or the like. 
     In step  43 , the CPU  20  reads out the measurement data (V 1 ) from the RAM  22 , and calculates the resistance value of the piezoelectric element  2  from each of the measurement data V 1   i  (i is a data number, and i=1 to n). Specifically, representing the calculated value of the resistor by Ri, it is calculated by Ri=V 0 /(V 1   i /R 1 ). V 0  and R 1  are values read out from the ROM  21 , and are respectively the voltage supplied to the piezoelectric element  2  and the resistance value of the first resistor  11 . The calculated resistances values Ri (i=1 to n) are stored in the RAM  22 . 
     In step  44 , the CPU  20  reads out the resistance values Ri (i=1 to n) calculated in step  43  from the RAM  22 , and calculates an evaluation value A for determining the presence or absence of deterioration of the piezoelectric device  2 . The evaluation value A is a value representing the set of calculated resistance values Ri (i=1 to n), for example, an average value of Ri (i=1 to n), a median value (median), and the like. 
       FIG.  5    shows an example of the measured result of the resistance value of the first resistor  11 . As will be described later, for each of the three types of the piezoelectric elements  2  (shown by ID 1  to ID 3 ), the voltage was measured by the data logger at intervals of 2 seconds. The vertical and horizontal axes represent voltage (in mV) and time (in seconds) respectively.  FIG.  5    shows data in 100 seconds. As can be seen from  FIG.  5   , since the resistance value of the first resistor  11  fluctuates with a certain width, it is preferable to calculate a representative value (e.g., an average value) of a plurality of measured values during a predetermined time and compare the value with the threshold value, rather than comparing one-time measured value with the threshold value. 
     In step  45 , the CPU  20  reads out the threshold value Rth from the RAM  22 , and determines whether or not the evaluation value A calculated in step  44  is smaller than the threshold value Rth. If A&lt;Rth, the control proceeds to step  46 ; otherwise (A≥Rth), the control proceeds to step  47 . 
     In step  46 , the CPU  20  reads out the predetermined information from the ROM  21  and outputs the read-out information to the information presentation unit  26  through the I/O unit  23 . The predetermined information is information indicating the possibility of deterioration of the piezoelectric element  2 . If the information presentation unit  26  is a liquid crystal display device, it is text data (“the piezoelectric element of the driving device is deteriorated,” “replace the driving device,” or the like), and if the information presentation unit  26  is a lighting device, it is a signal for instructing the lighting. By the lighting of the lighting device, it can be indicated that the piezoelectric element of the driving device is deteriorated, and replacement of the driving device is required. 
     In step  47 , the CPU  20  determines whether or not an end instruction has been issued, and if the end instruction has been received, ends the detecting program, otherwise, the control returns to step  41 . The end instruction is issued by, for example, the power switch OFF of the driving device  1 . 
     Thus, during a normal operation of the driving device  1 , and in a state where a voltage is applied to the piezoelectric element  2 , by measuring the voltage across the first resistor  11  and calculating the resistance value of the piezoelectric element  2 , it is possible to determine the presence or absence of deterioration of the piezoelectric element  2 . If the threshold Rth is appropriately set, by repeating the voltage measurement and the determination of deterioration in accordance with an increase in the number of driving times of the driving unit of the driving device  1 , it is possible to detect the occurrence of the deterioration of the piezoelectric element  2  and to recommend the replacement of the piezoelectric element  2  or the driving device  1  having the piezoelectric element  2 , before the driving device  1  becomes unable to operate normally. 
     As shown in  FIG.  5   , since the measured voltage value fluctuates (vibrates), the resistance value of the calculated piezoelectric element also fluctuates. Therefore, when the resistance value calculated from one measurement is compared with the predetermined threshold value to determine a deterioration, erroneous determination of deterioration may occur even if the resistance value accidentally exceeds the threshold value. On the other hand, as described above, by comparing the representative value (e.g., average value) of a plurality of calculated resistance values of the piezoelectric element with a predetermined reference (threshold value), erroneous determination may be suppressed. 
     In the above description of step  41 , the case where whether or not to perform the voltage measurement is determined by the number of driving times has been described, but the present invention is not limited thereto. Whether or not to perform the voltage measurement may be determined by time. In this case, when the elapsed time from the driving device  1  first started operating (supplying voltage to the piezoelectric element  2 ) exceeds a specified time, it is determined to perform the measurement, and steps  42  to  45  may be executed. 
     In the above description, the case of using the representative value (e.g., average value) of a plurality of measured values as the evaluation value has been described, but the present invention is not limited thereto. A tendency of change in the representative value may be compared with a predetermined reference (threshold value). For example, the rate of change (a slope) of the representative values may be calculated as the evaluation value. In this case, the representative value calculated every time the step  44  is executed as described above may be stored in the RAM  22  (when the power is turned off, the representative values are stored in the ROM  21  in advance). After the voltage is measured and the representative value is calculated, the previously calculated representative values are read out from the RAM  22 , the slope is calculated, and the calculated slope is used as the evaluation value to be compared with the predetermined threshold value. As shown in the experimental results to be described later, if the piezoelectric element  2  is deteriorated, the resistance value R 3  continues to decrease with a certain degree of inclination from the state where it is not deteriorated (R 3 &gt;1×103 (MΩ)). Therefore, using the change of the representative value (slope), it is possible to determine the presence or absence of deterioration of the piezoelectric element  2 . 
     Note that a known method may be used for calculating the tendency of change in representative value. For example, the slope may be obtained from two calculated consecutive representative values. Further, a regression line (slope) may be obtained by applying the least squares method or the like to the calculated three or more representative values. 
     Further, the representative value or the tendency of the representative value may be one of the determination conditions, thereby, it is possible to detect the deterioration of the piezoelectric element more accurately. 
     In the above description, the case where the resistance values R 1  and R 2  of the first resistor  11  and the second resistor  12  connected in series to the piezoelectric element  2  are set as R 1 =10 kΩ and R 2 =39 kΩ has been described, but the present invention is not limited thereto. R 1  and R 2  can be selected in accordance with the input range of the measuring device for measuring the voltage across the first resistor  11  (the region of the resistance value to be measured), and may be sufficiently smaller than the resistance value R 3  of the piezoelectric element  2 , to an extend that R 1  and R 2  may be ignored when calculating the resistance value of the piezoelectric element  2 . 
     In the above description, the case of calculating the resistance value (Ri) of the piezoelectric element  2  from the respective measured data V 1   i , according to Ri=V 0 /(V 1   i /R 1 ) (V 0  being the output voltage of the power supply unit  4 ) has been described, but the present invention is not limited thereto. Referring to  FIG.  2   , using the measured value V 1  of the voltage across the first resistor  11 , since V 0 =(R 1 +R 2 +R 3 )×I=(R 1 +R 2 +R 3 )×(V 1 /R 1 ), R 3 =V 0 /(V 1 /R 1 )−R 1 −R 2 ). Therefore, the resistance value (Ri) of the piezoelectric element  2  may be calculated according to Ri=V 0 /(V 1   i /R 1 )−R 1 −R 2  from the measured data V 1   i . In this case, the values of R 1  and R 2  may not satisfy R 1 &lt;&lt;R 3 , R 2 &lt;&lt;R 3 . 
     In the above description, the case of providing the deterioration detection circuit  15  on the electrical wiring between the output end of the negative electrode side of the power supply unit  4  and the piezoelectric element  2  has been described, but the present invention is not limited thereto. The deterioration detection circuit may be provided on the electrical wiring between the output end of the positive electrode side of the power supply unit  4  and the piezoelectric element  2 . Also in this case, the first resistor  11 , the second resistor  12  and the piezoelectric element  2  are connected in series. Note that in order to realize the same voltage division as the circuit shown in  FIG.  1   , the second resistor  12  is preferably provided closer to the positive electrode side of the power supply unit  4  than the first resistor  11 . 
     Further, as shown in  FIG.  6   , the first resistor  11  and the second resistor  12  may be disposed on both sides of the piezoelectric element  2 . Further, the second resistor  12  may not be provided depending on the measuring device used for measuring the voltage across the first resistor  11 . 
     Example 1 
     The experimental results are shown below to show the effectiveness of the present invention. 
     In the pressure type flow rate control device having the configuration shown in  FIG.  1   , the voltage across the first resistor  11  (the voltage between the first terminal  13  and the second terminal  14 ) was measured in a state where a DC voltage of 140V was supplied to the piezoelectric element  2 , and the average value thereof was calculated. The resistance values of the first resistor S 1  and the second resistor  12  were 10 kΩ and 39 kΩ respectively. In the voltage measurement, a tester (digital multimeter  289  manufactured by FLUKE Corporation) and a data logger (mobile recorder MV200 manufactured by Yokogawa Electric Corporation) were used. 
     In each of the three pressure type flow rate control devices of the same type using the same piezoelectric elements (ID 1  to ID 3 ), after opening and closing the control valve 3 million times, the voltages were measured using the above two types of measuring devices. The measured voltages of the first resistor  11 , and the resistance values of the piezoelectric element  2  calculated therefrom are shown in Table 1. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Measuring 
                   
                   
                   
                   
               
               
                 apparatus 
                 ID 
                 1 
                 2 
                 3 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Tester 
                 Average value of measured 
                 4.5 
                 1.0 
                 6.4 
               
               
                   
                 voltages[mV] 
               
               
                   
                 Fluctuation of average value 
                 2.8~6.1 
                 −2.0~4.0 
                 3.8~9.0 
               
               
                   
                 of measured voltages[mV] 
               
               
                   
                 Calculated resistance 
                 333 
                 1500 
                 234 
               
               
                   
                 value[MΩ] 
               
               
                 Data 
                 Average value of measured 
                 3.84 
                 4.32 
                 5.04 
               
               
                 logger 
                 voltages[mV] 
               
               
                   
                 Calculated resistance 
                 391 
                 347 
                 297 
               
               
                   
                 value[MΩ] 
               
               
                   
               
            
           
         
       
     
     The measurement conditions according to the data logger were the measurement range ±20 mV and the sampling period of 2 seconds. The measurement voltage average value (mV) is the average value of the measurement data for 100 seconds.  FIG.  5    shows the voltage values of 100 seconds measured using the data logger. As can be seen from  FIG.  5   , the fluctuation range of the measured voltages by the data logger was about 5 mV. 
     Example 2 
     Regarding the pressure type flow rate control devices using different types of piezoelectric elements, similarly to Example 1, the resistance values of the piezoelectric element used in the respective pressure type flow rate control device were calculated by measuring the voltage across the first resistor  11 . The results are shown in  FIG.  7   . 
       FIG.  7    schematically shows the changes in the resistance values calculated from the measured values for each piezoelectric element. In  FIG.  7   , the vertical axis represents the resistance value of the piezoelectric element (calculated value), the horizontal axis represents the driving time of the control valve. As the driving time increases, the number of the opening and closing times of the control valve (the number of the applying times of the voltage to the piezoelectric element) increases. The solid lines are the measurement results of the control valves equipped with the moisture adsorbent, and the dashed lines are the measurement results of the control valves without the moisture adsorbent. As can be seen from the graph of  FIG.  7   , although the insulation resistance value of the piezoelectric element varies depending on the types, when the driving time becomes longer and deterioration occurs, the resistance value of any piezoelectric element decreases from the initial insulation resistance (around 1×1010Ω). The timing at which the insulation resistance value of the piezoelectric element starts to decrease varies depending on the presence or absence of the moisture adsorbent. It can be seen that, by providing the moisture adsorbent, the start of the decrease of the insulation resistance value is delayed, that is, the deterioration of the piezoelectric element is delayed. 
     As mentioned above, although the present invention was described by describing the embodiments, the above-mentioned embodiments are only examples, the present invention is not limited to the above-mentioned embodiments and can be variously modified and executed. 
     REFERENCE SIGNS LIST 
     
         
           1  Driving device 
           2  Piezoelectric element 
           3  Control unit 
           4  Power supply unit 
           5  First connector 
           6  Second connector 
           7  First positive terminal 
           8  First negative terminal 
           9  Second positive terminal 
           10  Second negative terminal 
           11  First resistor 
           12  Second resistor 
           13  First terminal 
           14  Second terminal 
           15  Deterioration detection circuit 
           20  CPU 
           21  ROM 
           22  RAM 
           23  I/O unit 
           24  Bus 
           25  Measuring unit 
           26  Information presentation unit 
           100  Valve body 
           110  Piezoelectric element 
           102  Diaphragm valve element 
           115  Connector 
           123  Support tube