Piezoactuator and drive circuit therefor

Abstract

A piezoactuator has a diaphragm, and the diaphragm has flat piezoelectric elements that oscillate in a longitudinal oscillation mode and a sinusoidal oscillation mode. A first electrode for detecting oscillation in the longitudinal oscillation mode, and a second electrode for detecting the amplitude of oscillation in the sinusoidal oscillation mode, are disposed on the surface of the diaphragm. When the piezoactuator is driven with a drive signal, the phase difference of a first detection signal output from the first electrode and a second detection signal output from the second electrode is detected. The frequency at which the detected phase difference becomes the maximum phase difference is then obtained, and a drive signal of a matching frequency is applied to the piezoelectric elements.

1. A drive circuit for a piezoactuator, comprising:
 at least one piezoelectric element having a first oscillation mode and a second oscillation mode, the at least one piezoelectric element being adapted to oscillate when an AC signal is applied to it, the second oscillation mode having a different oscillation direction than that of the first oscillation mode;  
 a driver for applying an AC drive voltage signal to the at least one piezoelectric element; and  
 a frequency control unit for detecting a first electrical signal from the at least one piezoelectric element indicative of oscillation in the first oscillation mode, for detecting a second electrical signal from the at least one piezoelectric element indicative of oscillation in the second oscillation mode, and for controlling the frequency of the AC drive voltage signal to optimize the phase difference between the first and second electrical signals for a particular operating condition of the at least one piezoelectric element, such that the phase difference is substantially maximized, the frequency control unit further comprising: 
   a phase difference detection circuit for detecting the phase difference between the first and second electrical signals;    
   a time differentiating circuit for determining a time differential of the phase difference detected by the phase difference detection circuit; and    
   a frequency adjusting circuit for increasing the frequency of the AC drive voltage signal when the time differential is positive, and decreasing the frequency of the AC drive voltage signal when the time differential is negative.    
 
 
     
     
       2. A drive circuit for a piezoactuator as described in  claim 1 , wherein the frequency control unit comprises a circuit for controlling the frequency of the AC drive voltage signal so that the phase difference between the first and second electrical signals is substantially maximized. 
     
     
       3. A drive circuit for a piezoactuator as described in  claim 2 , wherein the frequency control unit comprises:
 a phase difference detection circuit for detecting the phase difference between the first and second electrical signals;    a time differentiating circuit for determining a time differential of the phase difference detected by the phase difference detection circuit; and    a frequency adjusting circuit for increasing the frequency of the AC drive voltage signal when the time differential is positive, and decreasing the frequency of the AC drive voltage signal when the time differential negative.    
     
     
       4. A drive circuit for a piezoactuator as described in claim  2    1 , further comprising a voltage-controlled oscillator for supplying an output signal to the driver, wherein the frequency control unit controls the frequency of the AC drive voltage signal by increasing or decreasing a control voltage applied to the voltage-controlled oscillator. 
     
     
       5. A drive circuit for a piezoactuator as described in  claim 4 , wherein the frequency control unit comprises a memory for storing the voltage level of the control voltage when the frequency of the AC drive voltage signal is controlled to maximize the phase difference, wherein the frequency control unit determines an initial level of the control voltage based on the voltage level of the control voltage stored in memory when frequency control of the AC drive voltage signal is initiated and then increases or decreases the control voltage accordingly. 
     
     
       6. A drive circuit for a piezoactuator as described in  claim 1  , comprising:
   at least one piezoelectric element having a first oscillation mode and a second oscillation mode, the at least one piezoelectric element being adapted to oscillate when an AC signal is applied to it, the second oscillation mode having a different oscillation direction than that of the first oscillation mode;    
   a driver for applying an AC drive voltage signal to the at least one piezoelectric element; and    
   a frequency control unit for detecting a first electrical signal from the at least one piezoelectric element indicative of oscillation in the first oscillation mode, for detecting a second electrical signal from the at least one piezoelectric element indicative of oscillation in the second oscillation mode, and for controlling the frequency of the AC drive voltage signal to optimize the phase difference between the first and second electrical signals for a particular operating condition of the at least one piezoelectric element,  wherein the frequency control unit comprises a circuit for controlling the frequency of the AC drive voltage signal so that the phase difference substantially corresponds to a reference phase difference, wherein the frequency control unit further comprises: 
   a drive evaluator adapted to determine if drive of the piezoactuator satisfies a particular performance characteristic; and        an initial reference phase difference adjustor adapted to reduce the reference phase difference so that the piezoactuator satisfies the particular performance characteristic when the drive evaluator determines that the piezoactuator does not satisfy the particular performance characteristic, and to increase the reference phase difference when the drive evaluator determines that the piezoactuator satisfies the particular performance characteristic.      
 
     
     
       7. A drive circuit for a piezoactuator as described in  claim 6 , wherein the frequency control unit comprises:
 a phase difference detection circuit for detecting the phase difference between the first and second electrical signals;  
 a comparator for comparing the phase difference detected by the phase difference detection circuit with the reference phase difference; and  
 a frequency adjusting circuit for increasing or decreasing the frequency of the AC drive voltage signal based on the comparison result obtained by the comparator.  
 
     
     
       8. A drive circuit for a piezoactuator as described in  claim 7 , wherein the frequency control unit further comprises voltage-controlled oscillator for supplying an output signal to the driver; and herein the frequency adjusting circuit comprises a voltage adjusting circuit for increasing or decreasing the control voltage applied to the voltage-controlled oscillator based on the comparison result obtained by the comparator. 
     
     
       9. A drive circuit for a piezoactuator as described in  claim 6 , wherein the frequency control unit comprises:
 a drive evaluator adapted to determine if drive of the piezoactuator satisfies a particular performance characteristic; and    an initial reference phase difference adjustor adapted to reduce the reference phase difference so that the piezoactuator satisfies the particular performance characteristic when the drive evaluator determines that the piezoactuator does not satisfy the particular performance characteristic, and to increase the reference phase difference when the drive evaluator determines that the piezoactuator satisfies the particular performance characteristic.    
     
     
       10. A drive circuit for a piezoactuator as described in claim  9    6 , wherein, when it is determined that the reference phase difference at which the piezoactuator drive satisfies the particular performance characteristic is substantially the same for a predetermined consecutive number of times, the initial reference phase difference adjustor is controlled to not increase nor decrease the reference phase difference for a pre-specified period of time. 
     
     
       11. A drive circuit for a piezoactuator as described in claim  9    6 , wherein the frequency control unit comprises a frequency counter for measuring the frequency of the AC drive voltage signal, and wherein the drive evaluator determines whether or not the piezoactuator satisfies the particular performance characteristic based on whether or not the frequency measured by the frequency counter is within a predetermined range. 
     
     
       12. A drive circuit for a piezoactuatoras described in  claim 6  , comprising:
   at least one piezoelectric element having a first oscillation mode and a second oscillation mode, the at least one piezoelectric element being adapted to oscillate when an AC signal is applied to it, the second oscillation mode having a different oscillation direction than that of the first oscillation mode;    
   a driver for applying an AC drive voltage signal to the at least one piezoelectric element; and    
   a frequency control unit for detecting a first electrical signal from the at least one piezoelectric element indicative of oscillation in the first oscillation mode, for detecting a second electrical signal from the at least one piezoelectric element indicative of oscillation in the second oscillation mode, and for controlling the frequency of the AC drive voltage signal to optimize the phase difference between the first and second electrical signals for a particular operating condition of the at least one piezoelectric element, wherein the frequency control unit comprises a circuit for controlling the frequency of the AC drive voltage signal so that the phase difference substantially corresponds to a reference phase difference,  wherein the frequency control unit further comprises:  
 means for obtaining, each time the piezoactuator is driven, an indication of a change in the phase difference between the first and second electrical signals from a previous drive operation of the piezoactuator; and  
 means for increasing or decreasing the reference phase difference according to the change in the phase difference.  
 
     
     
       13. A method for controlling a drive circuit having at least one oscillatible piezoelectric element of a piezoactuator, the method comprising the steps of:
 applying an AC drive voltage signal to the at least one piezoelectric element;  
 outputting an output signal having a frequency corresponding to a frequency of a control voltage;  
 receiving a first electrical signal from the at least one piezoelectric element indicative of oscillation in a first oscillation mode and receiving a second electrical signal from the at least one piezoelectric element in indicative of oscillation in a second oscillation mode, the second oscillation mode having an oscillation direction different from that of the first oscillation mode;  
 detecting a phase different between the first and second electrical signals; and  
 optimizing the oscillation frequency of the output signal based on the detected phase difference, comprising increasing the oscillation frequency if a time differential of the detected phase difference is greater than zero, and decreasing the oscillation frequency if the time differential is less than zero, the increasing or decreasing being performed until the time differential of the phase difference over a pre- specified period of time is within a specified range.    
 
     
     
       14. A method as described in  claim 13 , wherein the optimizing of the oscillation frequency comprises increasing the oscillation frequency if a time differential of the detected phase difference is greater than (d/dt) 1 , where (d/dt) 1  is greater than zero, and decreasing the oscillation frequency if the time differential is less than (d/dt) 2 , where (d/dt) 2  is less than zero, the increasing or decreasing being performed until the time differential of the phase difference over a pre-specified period of time is between (d/dt) 1  and (d/dt) 2 . 
     
     
       15. A method as described in  claim 13 , wherein the optimizing of the oscillation frequency comprises increasing the oscillation frequency until the detected phase difference is greater than or equal to a reference phase difference. 
     
     
       16. A method as described in  claim 15 , further comprising a step of  for controlling a drive circuit having at least one oscillatible piezoelectric element of a piezoactuator, the method comprising the steps of:
   applying an AC drive voltage signal to the at least one piezoelectric element;    
   outputting an output signal having a frequency corresponding to a frequency of a control voltage;    
   receiving a first electrical signal from the at least one piezoelectric element indicative of oscillation in a first oscillation mode and receiving a second electrical signal from the at least one piezoelectric element indicative of oscillation in a second oscillation mode, the second oscillation mode having an oscillation direction different from that of the first oscillation mode;    
   detecting a phase difference between the first and second electrical signals;    
   optimizing the oscillation frequency of the output signal based on the detected phase difference, wherein the optimizing of the oscillation frequency comprises increasing the oscillation frequency until the detected phase difference is greater than or equal to a reference phase difference; and   
 determining if driving the piezoactuator satisfies a particular performance characteristic, and correcting the reference phase difference based on the determination.  
 
     
     
       17. A device-readable medium embodying a control program for controlling a drive circuit having at least one oscillatible piezoelectric element of a piezoactuator, the control program comprising:
 applying an AC drive voltage signal to the at least one piezoelectric element;  
 outputting an output signal of a frequency corresponding to a frequency control voltage;  
 receiving a first electrical signal from the at least one piezoelectric element indicative of oscillation in a first oscillation mode and receiving a second electrical signal from the at least one piezoelectric element indicative of oscillation in a second oscillation mode, the second oscillation mode having an oscillation direction different from that of the first oscillation mode;  
 detecting a phase difference between the first and second electrical signals; and  
 optimizing the oscillation frequency of the output signal based on the detected phase difference, comprising increasing the oscillation frequency if a time differential of the detected phase difference is greater than zero, and decreasing the oscillation frequency if the time differential is less than zero, the increasing or decreasing being performed until the time differential of the phase difference over a pre- specified period of time is within a specified range.    
 
     
     
       18. A device-readable medium as described in  claim 17 , wherein the medium comprises a physical storage device or an electromagnetic signal on which the program of instructions is carried. 
     
     
       19. A portable electronic device, comprising:
 a piezoactuator comprising at least one piezoelectric element having a first oscillation mode and a second oscillation mode, the at least one piezoelectric element being adapted to oscillate when an AC signal is applied to it, the second oscillation mode having a different oscillation direction than that of the first oscillation mode; and  
 a drive circuit comprising: 
 a driver for applying an AC drive voltage signal to the at least one piezoelectric element; and  
 
 a frequency control unit for detecting a first electrical signal from the at least one piezoelectric element indicative of oscillation in the first oscillation mode, for detecting a second electrical signal from the at least one piezoelectric element indicative of oscillation in the second oscillation mode, and for controlling the frequency of the AC drive voltage signal to optimize the phase difference between the first and second electrical signals for a particular operating condition, such that the phase difference is substantially maximized, the frequency control unit further comprising: 
   a phase difference detection circuit for detecting the phase difference between the first and second electrical signals;    
   a time differentiating circuit for determining a time differential of the phase difference detected by the phase difference detection circuit; and    
   a frequency adjusting circuit for increasing the frequency of the AC drive voltage signal when the time differential is positive, and decreasing the frequency of the AC drive voltage signal when the time differential negative.    
 
 
     
     
       20. A portable electronic device as described in  claim 19 , wherein the portable electronic device is a wristwatch comprising:
 a rotor adapted to be rotationally driven by the piezoactuator; and  
 a display mechanism linked to the rotor for displaying information related to time.  
 
     
     
       21. A portable electronic device as described in  claim 19 , wherein the portable electronic device is a contactless IC card. 
     
     
       22. A portable electronic device, comprising the drive circuit for a piezoactuator as described in  claim 6 .