Patent Publication Number: US-2005116583-A1

Title: Device and method for driving ultrasonic actuator

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application claims benefit of Japanese Application Nos. 2003-397938 filed on Nov. 27, 2003 and 2004-224501 filed in Japan on Jul. 30, 2004, the contents of which are incorporated by this reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention generally relates to ultrasonic-actuator drive devices and ultrasonic-actuator drive methods and, more specifically, to an ultrasonic-actuator drive device and an ultrasonic-actuator drive method for generating a driving force by supplying a drive signal having a frequency voltage to, for example, a stacked ultrasonic transducer of an ultrasonic actuator.  
      2. Description of the Related Art  
      In recent years, attention has been given to ultrasonic actuators as motors substituting electromagnetic motors.  
      Such an ultrasonic actuator is typically controlled and driven by an actuator drive device. The actuator drive device supplies a drive signal having a frequency voltage to the ultrasonic transducer of the ultrasonic actuator to produce ultrasonic elliptical vibration at the ultrasonic transducer, thereby performing control such that the ultrasonic transducer or a driven portion that is in contact with the ultrasonic transducer provides a driving force.  
      One example of known technology for a driving method for such an ultrasonic-actuator drive device is an ultrasonic-motor drive method disclosed in Japanese Unexamined Patent Application Publication No. 63-56178.  FIG. 21  shows an example of the configuration of an ultrasonic-actuator drive circuit for implementing the known ultrasonic-motor drive method.  
      As shown in  FIG. 21 , a known ultrasonic-actuator drive device includes an ultrasonic-actuator drive circuit  100  and an ultrasonic actuator  101 , the driving of which is controlled by the ultrasonic-actuator drive circuit  100 .  
      The ultrasonic-actuator drive circuit  100  includes an oscillator circuit  102 , a power-amplifier circuit  103 , a current detection circuit  104 , a phase-difference detection circuit  105 , a phase-difference condition determination circuit  106 , and a frequency control circuit  107 . The ultrasonic actuator  101  is connected to the power-amplifier circuit  103  via the current detection circuit  104 .  
      The oscillator circuit  102  generates an alternating signal  102   a  having a frequency defined by a frequency control signal  107   a  output from the frequency control circuit  107 , which is described below, and supplies the alternating signal  102   a  to the power-amplifier circuit  103 .  
      The power-amplifier circuit  103  amplifies the alternating signal  102   a  and supplies a resulting drive voltage signal  103   a  to the current detection circuit  104  and the phase-difference detection circuit  105 .  
      The current detection circuit  104  detects current flowing when the drive voltage signal  103   a  is supplied to the ultrasonic actuator  101 , and supplies a drive-current detection signal  104   a , which indicates the result of the detection, to the phase-difference detection circuit  105 .  
      The phase-difference detection circuit  105  detects a phase difference between the drive voltage signal  103   a  and the drive-current detection signal  104   a  and supplies a phase-difference detection signal  105   a , which indicates the result of the detection, to the phase-difference condition determination circuit  106 .  
      When the supplied phase-difference detection signal  105   a  reaches a predetermined value, the phase-difference condition determination circuit  106  supplies a phase-difference condition signal  106   a  to the frequency control circuit  107 .  
      The frequency control circuit  107  serves as controlling means for controlling the entire ultrasonic-actuator drive circuit  100 . Thus, the frequency control circuit  107  supplies a frequency control signal  107   a  to the oscillator circuit  102  such that the alternating signal  102   a  is swept from a higher frequency to a lower frequency, thereby controlling an oscillation operation of the oscillator circuit  102 .  
      In the ultrasonic-actuator drive circuit  100  having the above-described configuration, the frequency control circuit  107  performs control so as to change the frequency control signal  107   a  such that the frequency of the alternating signal  102   a  is swept until the phase-difference condition signal  106   a  to be output from the phase-difference condition determination circuit  106  is output and so as to stop the sweeping when the phase-difference detection signal  105   a  reaches a predetermined value. That is, the frequency control circuit  107  can perform control so as to provide the drive voltage signal  103   a  having a frequency at which the phase difference between the drive current and the drive voltage reaches a predetermined value. Thus, the ultrasonic-actuator drive circuit  100  allows driving at a frequency that is in a certain relationship with the resonant frequency of the ultrasonic-actuator drive circuit  100 .  
     SUMMARY OF THE INVENTION  
      In brief, the present invention provides a method for driving an ultrasonic actuator by supplying an alternating signal to an ultrasonic transducer in which piezoelectric plates and internal electrodes are alternately stacked. The method includes detecting a frequency at which a phase difference between a voltage and current of the alternating signal is in a predetermined state, from a frequency range in which an amplitude ratio between the voltage and the current of the alternating signal is more than or equal to a predetermined value; and setting a driving frequency to the detected frequency.  
      In brief, the present invention provides a device for driving an ultrasonic actuator by supplying an alternating signal to an ultrasonic transducer in which piezoelectric plates and internal electrodes are alternately stacked. The device includes a drive circuit for generating the alternating signal, an amplitude detection circuit for detecting an amplitude ratio between a voltage and current of the alternating signal, a phase-difference detection circuit for detecting a phase difference between the voltage and the current of the alternating signal, and a control circuit for setting a frequency of the alternating signal in accordance with the amplitude ratio and the phase difference. The control circuit detects a frequency at which the phase difference is in a predetermined state from a frequency range in which the amplitude ratio is more than or equal to a predetermined value, and sets a driving frequency to the detected frequency. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a block diagram showing the entire configuration of an ultrasonic-actuator drive device for realizing an ultrasonic-actuator drive method according to a first embodiment of the present invention;  
       FIG. 2A  is a front view of an ultrasonic actuator used for the ultrasonic-actuator drive device according to the first embodiment;  
       FIG. 2B  is a side view of the ultrasonic actuator used for the ultrasonic-actuator drive device according to the first embodiment;  
       FIG. 3  is a front view showing a first modification of the ultrasonic actuator in the first embodiment;  
       FIG. 4  is an exploded perspective view of a piezoelectric layered product in which piezoelectric plates are stacked in a Y-axis direction;  
       FIG. 5  is an exploded perspective view of a piezoelectric layered product in which piezoelectric plates are stacked in a Z-axis direction;  
       FIG. 6  is an exploded perspective view of a piezoelectric layered product in which piezoelectric plates are stacked in an X-axis direction;  
       FIG. 7  is a front view showing a second modification of the ultrasonic actuator;  
       FIG. 8A  is a graph showing the characteristic of velocity versus frequency of the ultrasonic-actuator driven by the ultrasonic-actuator drive method of the first embodiment;  
       FIG. 8B  is a graph showing the characteristic of voltage-current phase difference versus frequency of the ultrasonic-actuator driven by the ultrasonic-actuator drive method of the first embodiment;  
       FIG. 8C  is a graph showing the characteristic of drive-current amplitude versus frequency of the ultrasonic-actuator driven by the ultrasonic-actuator drive method of the first embodiment;  
       FIG. 8D  is a graph showing the characteristic of phase-difference detection signal versus frequency of the ultrasonic-actuator driven by the ultrasonic-actuator drive method of the first embodiment;  
       FIG. 9A  is a graph showing the characteristic of drive-current amplitude versus frequency to describe a method for detecting the lower-limit frequency of a frequency range that defines a drive frequency and to describe a method for detecting frequency-range detection method for a frequency control circuit;  
       FIG. 9B  is a graph showing the characteristic of drive-current amplitude versus frequency to describe a method for detecting the upper-limit frequency of a frequency range;  
       FIG. 9C  is a graph showing the characteristic of phase-difference detection signal versus frequency to describe a frequency range based on the lower-limit frequency and the upper-limit frequency;  
       FIG. 10A  is a graph showing the characteristic of voltage-current phase difference versus frequency in the initial stage of detection to describe a method for detecting a frequency in the vicinity of a resonant frequency in accordance with a detection result from the frequency-range detection circuit shown in  FIG. 1 ;  
       FIG. 10B  is a graph showing the characteristic of voltage-current phase difference versus frequency in the process of detection to describe a method for detecting a frequency in the vicinity of a resonant frequency in accordance with a detection result from the frequency-range detection circuit shown in  FIG. 1 ;  
       FIG. 11  is a flow chart showing an example of a resonant-frequency detection processing routine controlled by the frequency control circuit shown in  FIG. 1 ;  
       FIG. 12  is a block diagram showing the entire configuration of an ultrasonic-actuator drive device for realizing an ultrasonic-actuator drive method according to a second embodiment of the present invention;  
       FIG. 13A  is a graph showing the characteristic of drive-current amplitude versus frequency to describe a frequency range settable by an oscillator circuit and to describe a lower-limit frequency detection method performed by the frequency control circuit;  
       FIG. 13B  is a graph showing the characteristic of drive-current amplitude versus frequency to describe a method for detecting the lower-limit frequency of a frequency range to describe a lower-limit frequency detection method performed by the frequency control circuit;  
       FIG. 14  is a flow chart showing an example of a lower-limit frequency detection processing routine controlled by the frequency control circuit show in  FIG. 12 ;  
       FIG. 15  is a flow chart showing an operational flow of the entire frequency control circuit when the ultrasonic-actuator drive method of the present invention is executed;  
       FIG. 16  is a block diagram showing the entire configuration of an ultrasonic-actuator drive device for realizing an ultrasonic-actuator drive method according to a third embodiment of the present invention;  
       FIG. 17A  is a front view of an ultrasonic actuator used for the ultrasonic-actuator drive device in the third embodiment;  
       FIG. 17B  is a side view of the ultrasonic actuator used for the ultrasonic-actuator drive device in the third embodiment;  
       FIG. 18  is a side view showing a first modification of the ultrasonic actuator in the third embodiment;  
       FIG. 19  is a front view showing a second modification of the ultrasonic actuator in the third embodiment;  
       FIG. 20A  s a graph sowing the characteristic of drive-current amplitude versus frequency to describe the ultrasonic-actuator drive method of the third embodiment;  
       FIG. 20B  is a graph showing the characteristic of frequency-setting signal versus frequency to describe the ultrasonic-actuator drive method of the third embodiment;  
       FIG. 20C  is a graph showing the characteristic of phase-difference detection signal versus frequency to describe the ultrasonic-actuator drive method of the third embodiment and to describe a frequency range in which setting is disabled;  
       FIG. 21  is a block diagram showing an example of the configuration of a known ultrasonic-actuator drive circuit;  
       FIG. 22A  illustrates the characteristic of displacement versus frequency for a longitudinal primary vibration mode of the ultrasonic transducer in the present invention;  
       FIG. 22B  illustrates the characteristic of displacement versus frequency for a flexural secondary vibration mode of the ultrasonic transducer in the present invention; and  
       FIG. 23  is a graph illustrating the characteristic of velocity versus frequency of the ultrasonic transducer in the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Embodiments of the present invention will be described below with reference to the accompanying drawings.  
     First Embodiment  
      FIGS.  1  to  11  show a first embodiment of an ultrasonic-actuator drive method according to the present invention.  FIG. 1  is a block diagram showing the entire structure of an ultrasonic-actuator drive device for realizing the ultrasonic-actuator drive method.  FIGS. 2A and 2B  show an example of the structure of an ultrasonic actuator used for the ultrasonic-actuator drive device according to this embodiment,  FIG. 2A  being a top view and  FIG. 2B  being a side view thereof.  FIG. 3  is a front view showing a first modification of the ultrasonic actuator. FIGS.  4  to  6  show examples of a piezoelectric layered product of the ultrasonic actuator. Specifically,  FIG. 4  is an exploded perspective view of a piezoelectric layered product in which piezoelectric plates are stacked in a Y-axis direction,  FIG. 5  is an exploded perspective view of a piezoelectric layered product in which piezoelectric plates are stacked in a Z-axis direction, and  FIG. 6  is an exploded perspective view of a piezoelectric layered product in which piezoelectric plates are stacked in an X-axis direction.  
       FIG. 7  is a front view showing a second modification of the ultrasonic actuator.  FIGS. 8A  to  8 D are graphs showing characteristics of the ultrasonic-actuator driven by the ultrasonic-actuator drive method of this embodiment. Specifically,  FIG. 8A  is a graph showing the characteristic of velocity versus frequency,  FIG. 8B  is a graph showing the characteristic of voltage-current phase difference versus frequency,  FIG. 8C  is a graph showing the characteristic of drive-current amplitude versus frequency, and  FIG. 8D  is a graph showing the characteristic of phase-difference detection signal versus frequency.  
       FIGS. 9A  to  11  illustrate the ultrasonic-actuator drive method of this embodiment. Specifically,  FIGS. 9A  to  9 C are graphs for describing a method for detecting a frequency range that defines a drive frequency for a frequency control circuit.  FIG. 9A  is a graph showing the characteristic of drive-current amplitude versus frequency to describe a method for detecting the lower-limit frequency of a frequency range.  FIG. 9B  is a graph showing the characteristic of drive-current amplitude versus frequency to describe a method for detecting the upper-limit frequency of a frequency range.  FIG. 9C  is a graph showing the characteristic of phase-difference detection signal versus frequency to describe a frequency range based on the lower-limit frequency and the upper-limit frequency.  
       FIGS. 10A and 10B  are graphs for illustrating a method for detecting a frequency in the vicinity of a resonant frequency in accordance with a detection result from a frequency-range detection circuit.  FIG. 10A  is a graph showing the characteristic of voltage-current phase-difference versus frequency and  FIG. 10B  is a graph showing the characteristic of voltage-current phase difference versus frequency.  FIG. 11  is a flow chart showing an example of a resonant-frequency detection processing routine controlled by the frequency control circuit;  
      As shown in  FIG. 1 , an ultrasonic-actuator drive device of this embodiment includes an ultrasonic-actuator drive circuit  1  and an ultrasonic actuator  2 , the driving of which is controlled by the ultrasonic-actuator drive circuit  1 .  
      The ultrasonic-actuator drive circuit  1  serves as a circuit for driving the ultrasonic actuator  2 , and includes an oscillator circuit  3 , a power-amplifier circuit  4 , a current detection circuit  5 , a phase-difference detection circuit  6 , a current-amplitude detection circuit  7 , a comparator circuit  8 , a frequency-range detection circuit  9 , a mode control circuit  10 , and a frequency control circuit  11 . The ultrasonic actuator  2  is connected to the power-amplifier circuit  4  via the current detection circuit  5 .  
      The structure of the ultrasonic actuator  2  will now be described.  
      The ultrasonic-actuator drive device of this embodiment includes, for example, the ultrasonic actuator  2  shown in  FIG. 2A . As shown in  FIGS. 2A and 2B , the ultrasonic actuator  2  includes an ultrasonic transducer  2 A, a driven portion  2 B, external electrodes  12 , and friction members  13 . The ultrasonic transducer  2 A has a piezoelectric layered product having a rectangular prism shape. The driven portion  2 B is disposed so as to be in contact with the piezoelectric layered product of the ultrasonic transducer  2 A with the friction members  13  therebetween. The external electrodes  12  are provided on two opposite side surfaces of the piezoelectric layered product of the ultrasonic transducer  2 A, with two external electrodes  12  on each one of the side surfaces. The friction members  13  are bonded to, for example, two spots of the bottom surface of the piezoelectric layered product of the ultrasonic transducer  2 A. Predetermined pressuring means (not shown) applies pressure to the ultrasonic transducer  2 A.  
      With this ultrasonic transducer  2 A, when pressure applied to the ultrasonic transducer  2 A varies, a displacement-to-frequency characteristic of the ultrasonic transducer  2 A also varies. That is, as shown in  FIGS. 22A and 22B , as the pressure increases in the order of 0 kgf, 1 kgf, and 2 kgf, the overall displacement decreases and the displacement-to-frequency characteristic shifts toward higher frequencies. The displacement-frequency characteristics between a longitudinal primary vibration mode and a flexural secondary vibration mode are different from each other in the degree of the shifting toward higher frequencies. In this embodiment, the aspect ratio (i.e., the length-to-width ratio) of the rectangular-prism piezoelectric layered product is set to a predetermined value so that the resonant frequency of the longitudinal primary vibration mode and the resonant frequency of the flexural secondary vibration mode match each other under a predetermined pressure.  
      As shown in  FIG. 4 , the piezoelectric layered product of the ultrasonic transducer  2 A is integrally formed by stacking thin rectangular piezoelectric plates  2   c , which have been subjected to internal-electrode processing, in a Y-axis direction (i.e., the depth direction of the ultrasonic transducer  2 A, the depth direction being perpendicular to the vibration direction of the ultrasonic transducer  2 A).  
      The external electrodes  12  located at the right hand side in  FIG. 2A  are attached to internal-electrode exposing portions (not shown) extracted from the right side surface of the piezoelectric layered product of the ultrasonic transducer  2 A to thereby provide two electrical terminals (terminals A+ and A−), i.e., terminals A (phase A). The external electrodes  12  located at the left hand side in  FIG. 2A  are attached to internal-electrode exposing portions (not shown) extracted from the left side surface of the piezoelectric layered product of the ultrasonic transducer  2 A to thereby provide two electrical terminals (terminals B+ and B−), i.e., terminals B (phase B). In this case, the terminals A− and B− are configured to serve as ground for the phases A and B, respectively, and thus may be configured to be at the same electrical potential using a lead line or the like.  
      Lead lines, which are not shown, are connected to the corresponding external electrodes  12  by soldering or the like, and are also connected to the current detection circuit  5 .  
      The friction members  13  are provided at respective positions that correspond to belly portions of flexural vibration generated at the bottom surface of the piezoelectric layered product, so as to be in contact with the driven portion  2 B.  
      In this exemplary structure, it is desirable that the ultrasonic transducer  2 A has, for example, a longitudinal dimension of 5 to 20 mm. It is also desirable that pressure applied when the ultrasonic actuator  2 , including the ultrasonic transducer  2 A and the driven portion  2 B, is constructed is, for example, 0.1 to 3 kgf.  
      The above-described exemplary structure can provide an ultrasonic actuator  2  that is preferably driven in an effective manner. The use of the ultrasonic transducer  2 A having the above-described structure makes it possible to reduce component count and also to reduce variations in individual products. Further, when the drive device is designed such that the Q-value of the ultrasonic transducer  2 A is constant, the resonant frequency of the longitudinal primary vibration mode and the resonant frequency of the flexural secondary vibration mode match each other under a predetermine pressure. This makes it possible to effectively execute the resonant-frequency detection processing routine described above.  
      In this exemplary structure, although the external electrodes  12  of the ultrasonic transducer  2 A are arranged on two opposite side surfaces in the longitudinal direction of the piezoelectric layered product so as to define outer surfaces of the piezoelectric layered product, the present invention is not limited thereto. As in a first modification shown in  FIG. 3 , the external electrodes  12  may be extracted from side surfaces so as to be formed at surfaces of the piezoelectric layered product. Alternatively, the external electrodes  12  may be arranged at positions corresponding to reverse surfaces of the piezoelectric layered product.  
      Although the piezoelectric layered product of the ultrasonic transducer  2 A has been described in this embodiment as having its stacking direction in the Y-axis direction, the present invention is not limited thereto. For example, as shown in  FIG. 5 , a first piezoelectric layered product  2   a , which is a substantially-upper-half portion of the piezoelectric layered product of the ultrasonic transducer  2 A, and a second piezoelectric layered product  2   b , which is a substantially-lower-half portion of the piezoelectric layered product of the ultrasonic transducer  2 A, may be stacked in the Z-axis direction (i.e., the vertical direction, which is perpendicular to the driving direction of the ultrasonic transducer  2 A) with an insulating piezoelectric sheet  2   d  interposed therebetween. Further, as shown in  FIG. 6 , a first piezoelectric layered product  2   a , which is a substantially-left-half portion of the piezoelectric layered product of the ultrasonic transducer  2 A, and a second piezoelectric layered product  2   b , which is a substantially-right-half portion of the piezoelectric layered product of the ultrasonic transducer  2 A, may be stacked in the X-axis direction (i.e., the horizontal direction, which is parallel to the driving direction of the ultrasonic transducer  2 A) with an insulating piezoelectric sheet  2   d  interposed therebetween.  
      In addition, although the ultrasonic actuators  2  of the first embodiment and the first modification have been described as having a structure in which the piezoelectric structure is integrally constructed with the insulating layer (not shown) interposed therebetween, the present invention is not limited thereto. For example, the ultrasonic actuator  2  may be configured as an ultrasonic actuator  2 C of a second modification shown in  FIG. 7 . The ultrasonic transducer of the ultrasonic actuator  2 C has a base elastic body  18  having a rectangular prism shape, at least two stacked piezoelectric elements  17 A, holding elastic bodies  17 B, friction members  13 , and a contact portion  19  that serves as a driven portion. The stacked piezoelectric elements  17 A are secured to the base elastic body  18  so as to be parallel to each other in the longitudinal direction thereof. The holding elastic bodies  17 B press and sandwich the stacked piezoelectric elements  17 A with respect to the base elastic body  18 . The friction members  13  are provided at positions corresponding to belly portions of flexural vibration generated at a surface of the base elastic body  18 , so as to be in contact with the contact portion  19 .  
       FIGS. 8A  to  8 D show characteristics of the above-described ultrasonic actuator  2 . In a velocity-frequency characteristic shown in  FIG. 8A , when the ultrasonic actuator  2  is driven at a frequency f in a frequency range lower than a resonant frequency (a portion indicated by a dotted line shown in  FIG. 8A ), the velocity decreases sharply. Conversely, when the ultrasonic actuator  2  is driven at a frequency f in a frequency range higher than the resonant frequency, the velocity decreases gradually and decreases sharply at a certain point.  
      In the ultrasonic actuator  2 , the velocity-frequency characteristic is hardly changed depending on the sweep direction of the frequency and almost no hysteresis phenomenon occurs. Thus, as shown in  FIG. 23 , the ultrasonic actuator  2  exhibits almost no difference between the velocity-frequency characteristic obtained by sweeping the frequency from the higher side of the resonant frequency toward the lower side and the velocity-frequency characteristic obtained by sweeping the frequency from the lower side of the resonant frequency toward the higher side.  
      In conjunction with the velocity-frequency characteristic, the ultrasonic actuator  2  has the characteristic of voltage-current phase-difference versus frequency, as shown in  FIG. 8B . That is, the characteristic of voltage-current phase difference versus frequency displays a significant change in the phase difference in the vicinity of the resonant frequency, and also does not depend on the sweep direction of the frequency. Further, with the ultrasonic actuator  2 , in the drive-current amplitude versus frequency characteristic shown in  FIG. 8C , current that is sufficient for detecting a phase difference flows in the vicinity of the resonant frequency. However, the current amplitude decreases when the frequency is away from the resonant frequency, thereby making it difficult to accurately detect the phase difference, as shown in  FIG. 8D .  
      Accordingly, with respect to the ultrasonic transducer  2 A of the ultrasonic actuator  2  having the above-described characteristics, the ultrasonic-actuator drive circuit  1  in the ultrasonic-actuator drive device of this embodiment can drive the ultrasonic actuator  2  with high drive efficiency, by accurately performing phase detection and reliably supplying an alternating signal having a frequency in the vicinity of the resonant frequency.  
      Next, the configuration of the ultrasonic-actuator drive circuit  1  of this embodiment will be described with reference to  FIG. 1 .  
      As shown in  FIG. 1 , the oscillator circuit  3  included in the ultrasonic-actuator drive circuit  1  generates an alternating signal  3   a  having a frequency defined by a frequency control signal  11   a  output from the frequency control circuit  11 , and supplies the generated alternating signal  3   a  to the power-amplifier circuit  4 .  
      The power-amplifier circuit  4  amplifies the alternating signal  3   a  and outputs and supplies an amplified drive voltage signal  4   a  to the ultrasonic actuator  2  via the current detection circuit  5 . In accordance with the supplied drive voltage signal  4   a , the ultrasonic actuator  2  is driven.  
      The current detection circuit  5  detects current flowing when the drive voltage signal  4   a  is supplied to the ultrasonic actuator  2 , and outputs and supplies a drive-current detection signal  5   a , which indicates the detection current, to the phase-difference detection circuit  6  and the current-amplitude detection circuit  7 .  
      The phase-difference detection circuit  6  detects a phase difference between the drive voltage signal  4   a  and the drive-current detection signal  5   a  and outputs and supplies a phase-difference detection signal  6   a  to the frequency control circuit  11 .  
      The current-amplitude detection circuit  7  detects the amplitude of the drive-current detection signal  5   a  and supplies an amplitude result signal  7   a , which indicates the detected amplitude, to one input end of the comparator circuit  8 . A drive-current threshold signal  8   a   1 , which serves as a reference for comparison processing, is supplied to the other input end of the comparator circuit  8  from drive-current threshold-signal generating means (not shown) connected to an input terminal  8 A.  
      The comparator circuit  8  outputs and supplies a current-amplitude condition signal  8   a  to the frequency-range detection circuit  9 , when the amplitude result signal  7   a  from the current-amplitude detection circuit  7  exceeds the drive-current threshold signal  8   a   1  having a predetermined value.  
      While an upward-sweep control signal  10   a  output from the mode control circuit  10  described below is input, the frequency-range detection circuit  9  outputs, as a lower-limit frequency signal  9   b , a frequency control signal at a point when the input of the current-amplitude condition signal  8   a  is started. While a downward-sweep control signal  10   b  output from the mode control circuit  10  is input, the frequency-range detection circuit  9  outputs, as an upper-limit frequency signal  9   a , a frequency control signal at a point when the input of the current-amplitude condition signal  8   a  is started. The frequency-range detection circuit  9  supplies the upper-limit frequency signal  9   a  and the lower-limit frequency signal  9   b  to the mode control circuit  10  and the frequency control circuit  11 .  
      The mode control circuit  10  and the frequency control circuit  11  serve as controlling means (i.e., a control circuit) for controlling the entire ultrasonic-actuator drive circuit  1  of this embodiment).  
      The mode control circuit  10  outputs the upward-sweep control signal  10   a  before the driving of the ultrasonic actuator  2  is started. After the lower-limit frequency signal  9   b  is output from the frequency-range detection circuit  9 , the mode control circuit  10  stops the output of the upward-sweep control signal  10   a  and outputs the downward-sweep control signal  10   b . After the upper-limit frequency signal  9   a  is output from the frequency-range detection circuit  9 , the mode control circuit  10  stops the output of the downward-sweep control signal  10   b  and outputs a frequency-tracking control signal  10   c . The mode control circuit  10  supplies the upward-sweep control signal  10   a , the downward-sweep control signal  10   b , and the frequency-tracking control signal  10   c  to the frequency control circuit  11 .  
      While the upward-sweep control signal  10   a  is input, the frequency control circuit  11  changes the frequency control signal  11   a  such that the frequency of the alternating signal  3   a  varies from low to high. While the downward-sweep control signal  10   b  is input, the frequency control circuit  11  changes the frequency control signal  11   a  such that the frequency of the alternating signal  3   a  varies from high to low. While the frequency-tracking control signal  10   c  is input, the frequency control circuit  11  detects a frequency at which the amount of change in the phase-difference detection signal  6   a  relative to the frequency is a maximum, from a frequency range defined by the upper-limit frequency signal  9   a  and the lower-limit frequency signal  9   b . After completing the detection of the frequency, the frequency control circuit  11  performs control such that the drive frequency is set to the detected frequency.  
      Although the current-amplitude detection circuit  7  is configured to detect the amplitude of the drive-current detection signal  5   a , another configuration may be employed. For example, the current-amplitude detection circuit  7  may be configured to detect the ratio of the amplitude of the drive voltage signal  4   a  to the amplitude of the drive-current detection signal  5   a  or may be configured to derive the admittance of the ultrasonic actuator  2 .  
      Although the phase-difference detection circuit  6  is configured to output a phase difference between the drive voltage signal  4   a  and the drive-current detection signal  5   a , the phase-difference detection circuit  6  may be configured to output a phase difference between the alternating signal  3   a  and the drive-current detection signal  5   a.    
      The operation of the ultrasonic-actuator drive circuit  1  of this embodiment will now be described with reference to  FIGS. 9A  to  9 C and  15 .  FIG. 15  is a flow chart showing an operational flow of the entire frequency control circuit  11  when an ultrasonic-actuator drive method of the present invention is executed.  
      In the ultrasonic-actuator drive circuit  1  having the above-described configuration, in step S 100 , the frequency control circuit  11  performs control so as to sweep the frequency of the drive voltage signal  4   a  from low to high in the early stage of driving, as shown in  FIG. 9A , and so as to detect the lower limit of a frequency range in which the drive-current amplitude (i.e., the amplitude result signal  7   a ) exceeds a value defined by the drive-current threshold signal  8   a   1 .  
      Subsequently, in step S 101 , the frequency control circuit  11  performs control so as to sweep the frequency of the drive voltage signal  4   a  from high to low, as shown in  FIG. 9B , and so as to detect the upper limit of a frequency range in which the drive-current amplitude (i.e., the amplitude result signal  7   a ) exceeds a value defined by the drive-current threshold signal  8   a   1 .  
      It is preferable in this case that the drive-current threshold signal  8   a   1  is set to a value corresponding to a minimum current amplitude value that allows for accurate determination of the phase difference between the drive voltage signal  4   a  and the drive-current detection signal  5   a.    
      In step S 102 , as shown in  FIG. 9C , the frequency control circuit  11  performs control so as to detect a frequency at which the amount of change in the phase-difference detection signal  6   a  relative to the frequency is a maximum, from between the detected upper limit and the lower limit of a frequency range L 3 . After completing the detection, in step S 103 , the frequency control circuit  11  performs control so as to set the drive frequency to the detected frequency.  
      Next, a method for detecting a frequency at which the amount of change in the phase-difference detection signal  6   a  relative to the frequency is a maximum (i.e., a detection processing method in step S 102  shown in  FIG. 15 ) will now described with reference to  FIGS. 10A, 10B , and  11 .  
      When the driving method for the ultrasonic-actuator drive device of this embodiment is executed, the frequency control circuit  11  starts a resonant-frequency detection processing routine shown in  FIG. 11 , so that processing in steps S 1  to S 7  is executed.  
      In step S 1 , the frequency control circuit  11  substitutes the upper value fmax of a frequency range (where fmax indicates the upper limit and fmin indicates the lower limit), including the resonant frequency detected based on the voltage-current phase difference characteristic shown in  FIG. 10A , into a frequency f 2  and substitutes the lower-limit value fmin into a frequency f 1 .  
      In processing in step S 2 , the frequency control circuit  11  determines the intermediate value (f 1 +f 2 )/2 of the frequency f 1  and the frequency f 2  and substitutes the intermediate value (f 1 +f 2 )/2 into a frequency fc.  
      Subsequently, in processing step S 3 , the frequency control circuit  11  detects a voltage-current phase difference (hereinafter referred to as a “phase difference”) corresponding to each of the frequency f 1 , the frequency f 2 , and the frequency fc. The frequency control circuit  11  then substitutes the phase difference detected at the frequency f 1 , the phase difference detected at the frequency f 2 , and the phase difference detected at the frequency fc into ph(f 1 ), ph(f 2 ), and ph(fc), respectively.  
      Subsequently, in determination processing in step S 4 , the frequency control circuit  11  compares |ph(fc)−ph(f 1 )| with |ph(f 2 )−ph(fc)|. When |ph(f 2 )−ph(fc)| is smaller, the frequency control circuit  11  replaces the value of the frequency f 2  with the value of the frequency fc in processing in step S 5  and then the process proceeds to processing in step S 7 . When |ph(fc)−ph(f 1 )| is smaller, the frequency control circuit  11  replaces the value of the frequency f 1  with the value of the frequency fc in processing in step S 6  and then the process proceeds to processing in step S 7 .  
       FIG. 10B  shows a case in which |ph(f 2 )−ph(fc)| is smaller. Thus, in the processing in step S 5 , the frequency control circuit  11  replaces the value of the frequency f 2  with the value of the frequency fc.  
      Thereafter, in determination processing in step S 7 , the frequency control circuit  11  determines whether or not the frequency f 1  is substantially equal to the frequency f 2 . In this case, when the frequency control circuit  11  determines that the relationship of f 1 ≈f 2  is not satisfied, i.e., the frequency f 1  is not equal to the frequency f 2 , the process returns to the processing in step S 2  and the processing in step S 2  is repeated.  
      On the other hand, when the frequency control circuit  11  determines that the relationship of f 1 ≈f 2  is satisfied in the determination processing in step S 7 , the frequency control circuit  11  recognizes that the relationship of f 1 ≈f 2  is satisfied and also sets the value of a frequency at this point as a value in the vicinity of a resonant frequency which is preferable for driving the ultrasonic transducer  2 A. The frequency control circuit  11  then ends this resonant-frequency detection processing routine.  
      Thus, the frequency control circuit  11  repeatedly executes the processing in step S 2  to the processing in step S 6  until the relationship of f 1 ≈f 2  is satisfied, which makes it possible to detect a frequency in the vicinity of the resonant frequency.  
      Thus, the above described operation in this embodiment can ensure that the driving frequency is set to a frequency in the vicinity of the resonant frequency while avoiding a range in which the phase difference between the drive voltage and the drive current cannot be accurately determined due to a small drive current. This arrangement, therefore, allows the ultrasonic actuator  2  to be driven with high driving efficiency.  
      In this embodiment, although the frequency control circuit  11  performs control so as to perform the downward sweep after performing the upward sweep, the frequency control circuit  11  may perform control so as to perform the downward sweep first and then perform the upward sweep. Such an arrangement can provide the same advantages as the above-described embodiment.  
      Although the description in the above-described embodiment has been given of a case in which the entire ultrasonic-actuator drive circuit  1  is constituted by circuits, the present invention is not limited thereto. For example, a microcomputer or the like can be used to constitute the ultrasonic-actuator drive circuit  1  with software. In such a case, for example, a configuration in which the frequency control circuit  11 , the frequency-range detection circuit  9 , and the mode control circuit  10  are replaced with software may be employed.  
     Second Embodiment  
      FIGS.  12  to  15  show a second embodiment of the ultrasonic-actuator drive method of the present invention. Specifically,  FIG. 12  is a block diagram showing the entire configuration of an ultrasonic-actuator drive device for realizing the ultrasonic-actuator drive method.  FIGS. 13A, 13B ,  14 , and  15  illustrate the ultrasonic-actuator drive method of the second embodiment. More specifically,  FIGS. 13A and 13B  are graphs for describing a lower-limit frequency detection method for a frequency control circuit.  FIG. 13A  is a graph showing the characteristic of drive-current amplitude versus frequency to describe a frequency range that can be set by an oscillator circuit and  FIG. 13B  is a graph showing the characteristic of drive-current amplitude versus frequency to describe a method for detecting the lower-limit frequency of the frequency range.  FIG. 14  is a flow chart showing an example of controlling of a lower-limit frequency detection processing routine performed by a frequency control circuit.  FIG. 15  is a flow chart showing an operational flow of the entire frequency control circuit when the ultrasonic-actuator drive method of the present invention is executed.  FIG. 15  is also common to both the first and second embodiments. In  FIG. 12 , the same elements as those in the first embodiment are denoted with the same reference numerals, and thus the descriptions thereof are omitted and only different elements and portions will be described.  
      The ultrasonic-actuator drive device of this embodiment includes an ultrasonic-actuator drive circuit  1 A and an ultrasonic actuator  2 . This ultrasonic actuator  2  is analogous to that in the first embodiment. In the ultrasonic-actuator drive circuit  1 A, the frequency-range detection circuit  9  in the first embodiment is eliminated.  
      As shown in  FIG. 12 , in the ultrasonic-actuator drive circuit  1 A, an oscillator circuit  3  generates an alternating signal  3   a  having a frequency defined by a frequency control signal  11   a  output from a frequency control circuit  11 A, and supplies the generated alternating signal  3   a  to a power-amplifier circuit  4 .  
      The power-amplifier circuit  4  amplifies the alternating signal  3   a  and outputs and supplies a drive voltage signal  4   a  to the ultrasonic actuator  2  via a current detection circuit  5 . In accordance with the supplied drive voltage signal  4   a , the ultrasonic actuator  2  is driven.  
      The current detection circuit  5  detects current flowing when the drive voltage signal  4   a  is supplied to the ultrasonic actuator  2 , and outputs and supplies a drive-current detection signal  5   a , which indicates the result of the detection, to a phase-difference detection circuit  6  and a current-amplitude detection circuit  7 .  
      The phase-difference detection circuit  6  detects a phase difference between the drive voltage signal  4   a  and the drive-current detection signal  5   a  and outputs and supplies a phase-difference detection signal  6   a , which indicates the result of the detection, to the frequency control circuit  11 A.  
      The current-amplitude detection circuit  7  detects the amplitude of the drive-current detection signal  5   a  and supplies an amplitude result signal  7   a , which indicates the detected amplitude, to one input end of a comparator circuit  8 . A drive-current threshold signal  8   a   1 , which serves as a reference for comparison processing, is supplied to the other input end of the comparator circuit  8  from drive-current threshold-signal generating means (not shown) connected to an input terminal  8 A.  
      The comparator circuit  8  outputs and supplies a current-amplitude condition signal  8   a  to the frequency control circuit  11 A, when the amplitude result signal  7   a  supplied from the current-amplitude detection circuit  7  exceeds the drive-current threshold signal  8   a   1  having a predetermined value.  
      A mode control circuit  10 A and the frequency control circuit  11 A serve as controlling means for controlling the entire ultrasonic-actuator drive circuit  1 A of this embodiment.  
      The mode control circuit  10 A outputs a lower-limit frequency detection control signal  10   e  before the driving of the ultrasonic actuator  2  is started. After an operation completion signal  11   b  is supplied from the frequency control circuit  11 A, the mode control circuit  10 A stops the output of the lower-limit frequency detection control signal  10   e  and outputs an upper-limit frequency detection control signal  10   d . After the operation completion signal  11   b  is supplied from the frequency control circuit  11 A, the mode control circuit  10 A stops the output of the upper-limit frequency detection control signal  10   d  and outputs a frequency-tracking control signal  10   f . This mode control circuit  10 A supplies the lower-limit frequency detection control signal  10   e , the upper-limit frequency detection control signal  10   d , and the frequency-tracking control signal  10   f  to the frequency control circuit  11 A.  
      In a state in which the lower-limit frequency control signal  10   e  is input, the frequency control circuit  11 A detects a lower-limit frequency at which the current-amplitude condition signal  8   a  is output, while changing the frequency in a discrete manner. In a state in which the upper-limit frequency detection control signal  10   d  is input, the frequency control circuit  11 A detects an upper-limit frequency at which the current-amplitude condition signal  8   a  is output, while changing the frequency in a discrete manner. Further, in a state in which the frequency-tracking control signal  10   f  is input, the frequency control circuit  11 A detects a frequency at which the amount of change in the phase-difference detection signal  6   a  relative to the frequency is a maximum, from a frequency range defined by the upper-limit frequency and the lower-limit frequency. After completing the frequency detection, the frequency control circuit  11 A performs control so as to set the drive frequency to the detected frequency.  
      Other configurations are analogous to those in the first embodiment.  
      The operation of the ultrasonic-actuator drive circuit  1 A of this embodiment will now be described with reference to  FIG. 15 .  
      In the ultrasonic-actuator drive circuit  1 A having the above-described configuration, in step S 100 , the frequency control circuit  11 A performs control in the early stage of driving so as to detect the lower-limit frequency (the lower-limit value) of a frequency range in which the drive-current amplitude (i.e., the amplitude result signal  7   a ) exceeds a value defined by the drive-current threshold signal  8   a   1 .  
      Subsequently, in step S 101 , the frequency control circuit  11 A performs control so as to detect the upper-limit frequency (the upper limit value) of a frequency range in which the drive-current amplitude (i.e., the amplitude result signal  7   a ) exceeds a value defined by the drive-current threshold signal  8   a   1 .  
      It is preferable in this case that the drive-current threshold signal  8   a   1  is set to a value corresponding to a minimum current amplitude value that allows for accurate determination of the phase difference between the drive voltage signal  4   a  and the drive-current detection signal  5   a.    
      In step S 102 , the frequency control circuit  11 A detects a frequency at which the amount of change in the phase-difference detection signal  6   a  relative to the frequency is a maximum, from between the detected upper limit and lower limit of a frequency range L 3  (see  FIG. 9C ).  
      Lastly, in step S 103 , the frequency control circuit  11 A performs control so as to set the drive frequency to the frequency detected in processing in step S 102 .  
      A method for detecting the lower-limit frequency in step S 100  will now be described with reference to  FIGS. 13A, 13B , and  14 .  
      When a drive method for the ultrasonic-actuator drive device in this embodiment is executed to perform the processing in step S 100  shown in  FIG. 15 , the frequency control circuit  11 A starts a lower-limit detection processing routine shown in  FIG. 14  and processing of steps S 10  to S 16  is executed.  
      In step S 10 , the frequency control circuit  11 A substitutes the lower limit fmin of a frequency range (where fmax indicates the upper limit and fmin indicates the lower limit shown in  FIG. 13A ) settable by the oscillator circuit  3  into a frequency fa.  
      Subsequently, in processing in step S 11 , the frequency control circuit  11 A substitutes an intermediate value (fmax+fmin)/2 of the upper limit fmax and the lower limit fmin into a frequency fb (see  FIG. 13B ), and the process proceeds to step S 12 .  
      Thereafter, in processing in step S 12 , the frequency control circuit  11 A substitutes an intermediate value (fa+fb)/2 of the frequency fa and the frequency fb into a frequency fcc, and the process proceeds to step S 13 .  
      Next, in determination processing in step S 13 , the frequency control circuit  11 A determines whether or not the current-amplitude condition signal  8   a  is output when driven at the frequency fcc. In this case, as shown in  FIG. 13B , when it is determined that the current-amplitude condition signal  8   a  is not output, the process proceeds to step S 15 , in which the frequency control circuit  11 A replaces the frequency fa with the frequency fcc. The process then proceeds to determination processing in step S 16 . On the other hand, when it is determined in step S 13  that the current-amplitude condition signal  8   a  is output, the process proceeds to step S 14 , in which the frequency control circuit  11 A replaces the frequency fb with the frequency fcc. The process then proceeds to determination processing in step S 16 .  
      In the determination processing in step S 16 , the frequency control circuit  11 A determines whether or not the frequency fa is substantially equal to the frequency fcc or the frequency fb is substantially equal to the frequency fcc. In this case, when the frequency control circuit  11 A determines that they are not equal to each other, i.e., the relationship of fa≈fcc or fb≈fcc is not satisfied, the process returns to the processing in step S 12  and the processing in step S 12  is repeated.  
      On the other hand, when the frequency control circuit  11 A determines that they are substantially equal to each other, i.e., the relationship of fa≈fcc or fb≈fcc is satisfied, the frequency control circuit  11 A recognizes that the relationship of fa≈fcc or fb≈fcc is satisfied and also sets the value of the frequency fcc at this point as the lower-limit frequency (the lower-limit value). The frequency control circuit  11 A then ends this lower-limit detection processing routine.  
      Thus, the frequency control circuit  11 A repeatedly executes the processing in step S 12  to the processing in step S 16  until the relationship of fa≈fcc or fb≈fcc is satisfied, which thereby allows for high-accuracy detection of the lower-limit frequency (the lower-limit value) of a frequency range in which the drive-current amplitude (the amplitude result signal  7   a ) exceeds a value defined by the drive-current threshold signal  8   a   1 .  
      Although a case in which the frequency control circuit  11 A controls the lower-limit frequency detection processing has been described by way of example in this embodiment, the present invention is not limited thereto. Similarly, the frequency control circuit  11 A may control the upper-limit frequency detection processing in step S 101  shown in  FIG. 15 . The frequency control circuit  11 A in this embodiment controls processing for a method for detecting a frequency at which the amount of change in the phase-difference detection signal  6   a  is a maximum relative to the frequency, in the same manner as that in the first embodiment.  
      Thus, as in the first embodiment, the second embodiment can ensure that the driving frequency is set to a frequency in the vicinity of the resonant frequency while avoiding a range in which the phase difference between the drive voltage and the drive current cannot be accurately determined due to a small drive current. This arrangement, therefore, allows the ultrasonic actuator  2  to be driven with high driving efficiency.  
      Although the frequency control circuit  11 A performs control so as to perform the upper-limit frequency detection processing (i.e., the processing in step S 101  shown in FIG.  15 ) after performing the lower-limit frequency detection processing (i.e., the processing in step S 100  shown in  FIG. 5 ), the frequency control circuit  11 A may perform control so as to perform the upper-limit frequency detection processing first and then perform the lower-limit frequency detection processing. Such an arrangement can provide the same advantages as the above-described embodiment.  
      Further, although the description in the second embodiment has been given of a case in which the entire actuator drive circuit  1 A is constituted by circuits, the present invention is not limited thereto. For example, a microcomputer or the like can be used to constitute the ultrasonic-actuator drive circuit  1  with software. In such a case, a configuration in which the frequency control circuit  11 A, the mode control circuit  10 A, and so on are replaced with software may be employed.  
     Third Embodiment  
      FIGS.  16  to  19  shows a third embodiment of the ultrasonic-actuator drive method of the present invention. Specifically,  FIG. 16  is a block diagram showing the entire configuration of an ultrasonic-actuator drive device for realizing the ultrasonic-actuator drive method.  FIGS. 17A and 17B  show an example of the structure of an ultrasonic actuator used for the ultrasonic-actuator drive device according to the third embodiment,  FIG. 17A  being a front view and  FIG. 17B  being a side view.  FIG. 18  is a side view showing a first modification of the ultrasonic actuator in this embodiment and  FIG. 19  is a front view showing a second modification of the ultrasonic actuator in this embodiment.  FIGS. 20A  to  20 C are graphs for describing an ultrasonic-actuator drive method of this embodiment. Specifically,  FIG. 20A  is a graph showing the characteristic of drive-current amplitude versus frequency,  FIG. 20B  is a graph showing the characteristic of frequency setting signal versus frequency, and  FIG. 20C  is a graph showing the characteristic of phase-difference detection signal versus frequency to describe a frequency range in which setting is disabled. In FIGS.  16  to  19 , as in the first embodiment, the same elements are denoted with the same reference numerals, and thus the descriptions thereof are omitted and only different elements and portions will be described.  
      The ultrasonic-actuator drive device of this embodiment includes an ultrasonic-actuator drive circuit  1 B and an ultrasonic actuator  2 D. In the ultrasonic-actuator drive device of this embodiment, the structure of the ultrasonic actuator  2 D is different from that in the first embodiment. Further, the ultrasonic-actuator drive circuit  1 B is different from that in the first embodiment in that the frequency-range detection circuit  9  and the mode control circuit  10 , which are included in the first embodiment, are eliminated and a phase-difference condition determination circuit  14  is additionally provided.  
      As shown in  FIG. 16 , the actuator drive circuit  1 B is a circuit for driving the ultrasonic actuator  2 D, and includes an oscillator circuit  3 , a power-amplifier circuit  4 , a current detection circuit  5 , a phase-difference detection circuit  6 , a current-amplitude detection circuit  7 , comparator circuit  8 B, and a frequency control circuit  11 B, as well as the phase-difference condition determination circuit  14 . The ultrasonic actuator  2 D is connected to the power-amplifier circuit  4  via the current detection circuit  5 .  
      The structure of the ultrasonic actuator  2 D for use in this embodiment will now be described.  
      The ultrasonic-actuator drive device of this embodiment includes, for example, the ultrasonic actuator  2 D shown in  FIG. 17A . As shown in  FIGS. 17A and 17B , this ultrasonic actuator  2 D has an ultrasonic transducer  2 A that includes a piezoelectric layered product, friction members  13 , a first guide  15  and a second guide  16 . The friction members  13  are provided at at least two spots of each of the top surface and the bottom surface of the piezoelectric layered product. The first guide  0 . 15  and the second guide  16  sandwich the piezoelectric layered product while applying a predetermined pressure thereto. Predetermined pressing means (not shown), including the guides  15  and  16 , applies a predetermined pressure to the ultrasonic transducer  2 A.  
      With this ultrasonic transducer  2 A, similarly, when pressure applied to the ultrasonic transducer  2 A varies, the displacement-to-frequency characteristic of the ultrasonic transducer  2 A varies. That is, as shown in  FIGS. 22A and 22B , as the pressure increases in the order of 0 kgf, 1 kgf, and 2 kgf, the overall displacement decreases and the displacement-to-frequency characteristic shifts toward higher frequencies. The displacement-frequency characteristics between a longitudinal primary vibration mode and a flexural secondary vibration mode are different from each other in the degree of the shifting toward higher frequencies. In this embodiment, the aspect ratio of the rectangular-prism piezoelectric layered product is set to a predetermined value so that the resonant frequency in the longitudinal primary vibration mode and the resonant frequency in the flexural secondary vibration mode match each other under a predetermined pressure.  
      It is desired that the friction members  13  are provided at positions where the ultrasonic actuator  2 D can provide a highest-level output characteristic, i.e., where the ultrasonic transducer  2 A can produce ultrasonic elliptical vibration at its highest level. As indicated by the arrows shown in  FIG. 17A , elliptical vibration occurs at at least one of the friction members  13 . In general, since elliptical vibration acts as a driving source, it is preferable to arrange the friction members  13  so that the sum total of driving forces resulting from vibrations produced at all the friction members  13  does not become “0”.  
      In this exemplary configuration, it is desirable that the ultrasonic transducer  2 A has, for example, a longitudinal dimension of 5 to 20 mm. It is also desirable that pressure applied when the ultrasonic actuator  2 D, including the ultrasonic transducer  2 A and the ultrasonic guides  15  and  16 , is constructed is, for example, 30 gf to 100 gf.  
      The characteristics of the ultrasonic actuator  2 D and the stacking direction of the piezoelectric layered product of the ultrasonic transducer  2 A in this exemplary structure are substantially the same as those in the first embodiment.  
      When the ultrasonic-actuator drive circuit  1 B supplies a drive signal, which is an alternating signal, to the ultrasonic actuator  2 D of this embodiment, elliptical vibration occurs in the vicinities of the friction members  13  of the ultrasonic transducer  2 A to thereby drive the ultrasonic transducer  2 A in the horizontal direction while being guided by the first and second guides  15  and  16 .  
      Other operations are analogous to those in the first embodiment (see  FIG. 2 ).  
      The exemplary structure described above can provide an ultrasonic actuator  2 D that is preferably driven in an effective manner. The use of the ultrasonic transducer  2 A having the above-described structure makes it possible to reduce component count and also to reduce variations in individual products. Further, when the device is designed such that the Q-value of the ultrasonic transducer  2 A is constant, the resonant frequency in the longitudinal primary vibration mode and the resonant frequency in the flexural secondary vibration mode match each other under a predetermined pressure. This makes it possible to effectively execute the resonant-frequency detection processing routine described above.  
      In this embodiment, although the external electrodes  12  of the ultrasonic transducer  2 A are arranged on two opposite side surfaces in the longitudinal direction of the piezoelectric layered product so as to define outer surfaces of the piezoelectric layered product, the present invention is not limited thereto. As in a second modification shown in  FIG. 19 , the external electrodes  12  may be extracted from side surfaces so as to be formed at surfaces of the piezoelectric layered product. Alternatively, the external electrodes  12  may be arranged at positions corresponding to reverse surfaces of the piezoelectric layered product.  
      In addition, as shown in  FIG. 17A , the first and second guides  15  and  16  have been described as having a rectangular prism shape, the present invention is not limited thereto. For example, a cylindrical or a semi-cylindrical shape may be used as for a first guide  15 A and a second guide  16 A shown in a first modification shown in  FIG. 18 . In such a case, friction members  13 A which have a U shape or V shape need to be used so as to correspond to the shapes of the first guide  15 A and the second guide  16 A.  
      The configuration of the ultrasonic-actuator drive circuit  1 B of this embodiment will now be described.  
      As shown in  FIG. 16 , in the ultrasonic-actuator drive circuit  1 B, the oscillator circuit  3  generates an alternating signal  3   a  having a frequency defined by a frequency control signal  11   a  output from the frequency control circuit  11 B, and supplies the alternating signal  3   a  to the power-amplifier circuit  4 .  
      The power-amplifier circuit  4  amplifies the alternating signal  3   a  and outputs and supplies an amplified drive voltage signal  4   a  to the ultrasonic actuator  2 D via the current detection circuit  5 . In accordance with the supplied drive voltage signal  4   a , the ultrasonic actuator  2  is driven.  
      The current detection circuit  5  detects current flowing when the drive voltage signal  4   a  is supplied to the ultrasonic actuator  2 D, and outputs and supplies a drive-current detection signal  5   a , which indicates the result of the detection, to the phase-difference detection circuit  6  and the current-amplitude detection circuit  7 .  
      The phase-difference detection circuit  6  detects a phase difference between the drive voltage signal  4   a  and the drive-current detection signal  5   a  and outputs and supplies a phase-difference detection signal  6   a , which indicates the result of the detection, to the phase-difference condition determination circuit  14 .  
      The phase-difference determination circuit  14  outputs a phase-difference condition signal  14   a  when the amount of change in the phase-difference detection signal  6   a  relative to the difference exceeds a predetermined value. The phase-difference condition determination circuit  14  has, for example, a differentiating circuit and a comparator circuit which are not shown, and supplies the phase-difference condition signal  14   a  generated by those circuits to the frequency control circuit  11 B.  
      The current-amplitude detection circuit  7  detects the amplitude of the drive-current detection signal  5   a  and supplies an amplitude result signal  7   a , which indicates the detected amplitude, to one input end of the comparator circuit  8 A. A drive-current threshold signal  8   a   1 , which serves as a reference for comparison processing, is supplied to the other input end of the comparator circuit  8 A from drive-current threshold-signal generating means (not shown) connected to an input terminal  8 A.  
      The comparator circuit  8 A outputs and supplies a frequency-setting disable signal  8   b  to the frequency control circuit  11 B, when the amplitude result signal  7   a  supplied from the current-amplitude detection circuit  7  falls below the drive-current threshold signal  8   a   1  having a predetermined value.  
      The frequency control circuit  11 B serves as controlling means for controlling the entire ultrasonic-actuator drive circuit  1 B of this embodiment. The frequency control circuit  11 B sweeps the frequency and sets the driving frequency to a frequency at which the frequency-setting disable signal  8   b  is not output from the comparator circuit  8 A and the phase-difference condition signal  14   a  is output from the phase-difference condition determination circuit  14 .  
      The operation of the ultrasonic-actuator drive circuit  1 B of this embodiment will now be described with reference to  FIGS. 20A  to  20 C.  
      In the ultrasonic-actuator drive circuit  1 B having the above-described configuration, the frequency control circuit  11 B sweeps, for example, a frequency shown in  FIG. 20A  from high to low at a constant velocity. In response, the phase-difference detection signal  6   a  at this point varies in conjunction with the frequency sweep performed by the frequency control circuit  11 B. Since the velocity of the frequency sweep is constant, causing the differentiating circuit (not shown), included in the phase-difference condition determination circuit  14 , to differentiate the phase-difference detection signal  6   a  can obtain the amount of change in the phase-difference detection signal  6   a  relative to the frequency.  
      Thus, the phase-difference condition determination circuit  14  compares outputs from the differentiating circuit (not shown) using the comparator circuit (not shown), which makes it possible to determine whether or not the amount of change in the phase-difference detection signal  6   a  relative to the frequency exceeds a predetermined value (see  FIG. 20C ). The determination result, i.e., an output of the comparator circuit (not shown), is supplied to the frequency control circuit  11 B as the phase-difference condition signal  14   a . In response to the phase-difference condition signal  14   a , in an enabled period in which the frequency-setting disable signal  8   b  shown in  FIG. 20B  is at a low level, the frequency control circuit  11 B recognizes the phase-difference condition signal  14   a , stops the sweeping, and detects a frequency in the vicinity of the resonant frequency. The frequency control circuit  11 B then sets the driving frequency to the detected frequency, in the same manner as in the first embodiment.  
      In this embodiment, the frequency control circuit  11 B recognizes the frequency setting disable signal  8   b  output from the comparator circuit  8 A, in a frequency range from which the phase cannot be detected due to a small drive current, as shown in  FIG. 20C . This allows the frequency control circuit  11 B to perform control so that the drive frequency is not set to a frequency that is irrelevant to the resonant frequency.  
      Thus, according to this embodiment, the above-described operation can ensure that the driving frequency is set to a frequency in the vicinity of the resonant frequency while avoiding a range in which the phase difference between the drive voltage and the drive current cannot be accurately determined due to a small drive current. This arrangement, therefore, allows the ultrasonic actuator  2 D to be driven with high driving efficiency.  
      Although the description in this embodiment has been given of a case in which the phase-difference condition determination circuit  14  includes the differentiating circuit and the comparator circuit, the present invention is not limited thereto. The phase-difference condition determination circuit  14  may be realized with software that scans frequencies in a discrete manner, stores a phase-difference detection signal relative to the frequency, and compares the amount of change in the phase-difference detection signal relative to the frequency.  
      The configuration of any of the ultrasonic actuators in the first to third embodiments may be used for the ultrasonic-actuator drive device according to the present invention, and the configurations of the ultrasonic actuators may be used in combination as needed.  
      In this invention, it is apparent that various modification different in a wide range can be made on this basis of this invention without departing from the spirit and scope of the invention. This invention is not restricted by any specific embodiment, including the first to third embodiments described above, except being limited by the appended claims.