Abstract:
The present invention provides a piezoelectric ultrasonic motor driver which can perform self-oscillation, adjust the rotational direction of the motor, and be easy to manufacture. The motor includes a metal body having a desired shape; a plurality of piezoelectric plates attached to surfaces of the metal body, contracted and expanded to rotate the metal body; a self-oscillation unit for oscillating at an electromechanical frequency of the piezoelectric plates an electrical signal; and a delay unit for delaying an oscillation signal of the self-oscillation unit in phase by 90 or −90 degrees according to a desired rotational direction.

Description:
RELATED APPLICATION  
       [0001]     The present application is based on, and claims priority from, Korean Application Number 2005-16043, filed Feb. 25, 2005, the disclosure of which is incorporated by reference herein in its entirety.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a piezoelectric ultrasonic motor that includes a metal body and a plurality of piezoelectric plates attached to the metal body and converts simple vibrations of the piezoelectric plates into linear movement, and more particularly to a piezoelectric ultrasonic motor drive circuit that performs four-phase driving of the motor through self-oscillation, achieving a high efficiency, high output operation at any time, regardless of environmental factors.  
         [0004]     2. Description of the Related Art  
         [0005]     Piezoelectric ultrasonic motors convert simple vibration of piezoelectric ceramic elements, which contract and expand when current is applied thereto, into linear movement so as to serve as rotary motors. Piezoelectric ultrasonic motors are small and light weight. Because they use ultrasonic power, piezoelectric ultrasonic motors cause no noise and little electromagnetic interference. Due to these advantages, piezoelectric ultrasonic motors are typically used in lens drive units of expensive cameras.  
         [0006]     Various types of piezoelectric ultrasonic motors have been proposed.  FIG. 1  shows an example piezoelectric ultrasonic motor. As shown in  FIG. 1 , the piezoelectric ultrasonic motor basically comprises a metal body  1  serving as a stator and a plurality of piezoelectric plates  2  to  5  attached to surfaces of the metal body  1 . The piezoelectric palates  2  to  5  contract and expand if voltage is applied to the piezoelectric palates  2  to  5  through electrodes  2   a  to  5   a . If a voltage of the same frequency as the electromechanical resonant frequency of each of the piezoelectric plates  2  to  5  is applied to each of the piezoelectric plates  2  to  5 , the amplitude of contraction and expansion movement of each of the piezoelectric plates  2  to  5  is maximized.  
         [0007]     The plurality of piezoelectric plates  2  to  5  having these characteristics are longitudinally attached to the metal body  1 . The number of the piezoelectric plates  2  to  5  varies depending on the shape of the metal body  1  and the used driving scheme. If voltage signals out of phase with each other by certain degrees are applied to the piezoelectric plates  2  to  5 , the piezoelectric plates  2  to  5  contract and expand to cause a bending deformation of the metal body  1 , thereby rotating the central axis of the metal body  1 .  
         [0008]     Thus, the piezoelectric ultrasonic motor needs a drive circuit for applying suitable drive signals to the plurality of piezoelectric plates  2  to  5 .  
         [0009]      FIG. 2  shows a drive circuit of a rod-shaped piezoelectric ultrasonic motor according to a two-phase driving method proposed in U.S. Pat. No. 2,439,499. A transformer  6  transforms a 60 Hz power supply voltage and applies the transformed voltage to piezoelectric plates  2  and  3 . A phase shift circuit composed of an inductor  7  and a capacitor  8  shifts the phase of an output voltage of the transformer  6  by 90 degrees and then applies the phase-shifted voltage to piezoelectric plates  4  and  5 .  
         [0010]     In the drive circuit of  FIG. 2 , the frequency of drive signal applied to the piezoelectric plates  2  to  5  may vary according to changes in the power supply voltage received from the outside. As described above, the piezoelectric ultrasonic motor can obtain the maximum efficiency and output only when receiving a drive voltage having the same frequency as the electromechanical resonant frequency of the piezoelectric plates  2  and  5 . However, the conventional drive circuit of  FIG. 2  has a problem in that mass productivity and driving efficiency are reduced if external environmental factors or frequency differences between products cause changes in the resonant frequency.  
         [0011]     To overcome this problem, studies have been done to implement a self-oscillation function in the drive circuit so as to apply drive signals of the same frequency as the electromechanical resonant frequency, regardless of environmental factors and frequency differences between products.  FIG. 3  shows an improved drive circuit having such a function.  
         [0012]     As shown in  FIG. 3 , the conventional improved drive circuit comprises a feedback resistor Rf and an inverter INV, which are connected in parallel to a piezoelectric ultrasonic motor  30 , and capacitors CL 1  and CL 2  which are connected between the ground and the piezoelectric ultrasonic motor  30 . RC oscillation is performed according to capacitances of the capacitors CL 1  and CL 2 , a capacitance caused by the piezoelectric ultrasonic motor  30 , and a resistance of the feedback resistor Rf. That is, the oscillation frequency is determined based on the capacitances of the capacitors CL 1  and CL 2 , the capacitance of the piezoelectric ultrasonic motor  30 , and the resistance of the feedback resistor Rf. By suitably controlling the capacitances of the capacitors CL 1  and CL 2  and the resistance of the feedback resistor Rf, it is possible to make the oscillation frequency equal to the electromechanical resonant frequency of the piezoelectric ultrasonic motor  30 .  
         [0013]     If the oscillation frequency is set equal to the electromechanical resonant frequency of the piezoelectric ultrasonic motor  30  in such a manner, drive signals applied to the piezoelectric motor  30  always have the same frequency as the electromechanical resonant frequency, regardless of environmental factors or differences between parts, so that it is possible to obtain the maximum efficiency and output at any time.  
         [0014]     The inverter INV inverts the phases of drive signals to be applied to two piezoelectric plates  32  and  33  or  32  and  34  provided on both sides of the piezoelectric ultrasonic motor  30 . This allows drive signals out of phase by 180 degrees to be applied to the two piezoelectric plates  32  and  33  or  32  and  34 .  
         [0015]     To enable the piezoelectric ultrasonic motor  30  to adjust the rotational direction, the common piezoelectric plate  32  is provided on one surface of a metal body  31 , and the two piezoelectric plates  33  and  34  are provided respectively on two divided sections of another surface of the metal body  31 , which is opposite to the one surface. The common piezoelectric plate  32  is connected to one end of the inverter INV and the piezoelectric plates  33  and  34  are connected to the other end of the inverter INV through a switch SW.  
         [0016]     If the switch SW is controlled to connect the output of the inverter INV to the upper piezoelectric plate  33 , the common piezoelectric plate  32  and the upper piezoelectric plate  33  contract and expand to cause a bending deformation in the metal body  31 , thereby rotating the metal body  31  clockwise. On the other hand, if the switch SW is controlled to connect the output of the inverter INV to the lower piezoelectric plate  34 , the common piezoelectric plate  32  and the lower piezoelectric plate  34  contract and expand to cause a bending deformation in the metal body  31 , thereby rotating the metal body  31  counterclockwise.  
         [0017]     The piezoelectric ultrasonic motor drive circuit of  FIG. 3  uses a two-phase driving scheme as with that of  FIG. 2 , and can prevent changes in the frequency through self-oscillation. In the conventional drive circuit configured as described above, it is necessary to ground the metal body  31  of the piezoelectric ultrasonic motor  30 . However, since the piezoelectric plates  32 ,  33 , and  34  are attached to the metal body  31 , it is difficult to form an electrode for grounding the metal body  31 . In addition, since the structure of the piezoelectric ultrasonic motor has been altered to allow changes in the rotational direction of the motor, it is difficult to change the rotational direction of a piezoelectric ultrasonic motor having a structure different from that shown in  FIG. 3 .  
       SUMMARY OF THE INVENTION  
       [0018]     Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a piezoelectric ultrasonic motor driver which is easy to manufacture and also can perform self-oscillation, can adjust the rotational direction of the motor, and can achieve a high efficiency, high output operation at any time, regardless of environmental factors.  
         [0019]     In accordance with the present invention, the above and other objects can be accomplished by the provision of a piezoelectric ultrasonic motor driver comprising: a piezoelectric ultrasonic motor including a metal body having a desired shape, and a plurality of piezoelectric plates attached to surfaces of the metal body, wherein the piezoelectric plates contracts and expands to rotate the metal body if electrical signals are applied to the piezoelectric plates; a self-oscillation unit for oscillating at an electromechanical resonant frequency of the piezoelectric plates and allowing a frequency of an electrical signal applied to one of the plurality of piezoelectric plates to be maintained at the electromechanical resonant frequency of the piezoelectric plates; and a delay unit for delaying an oscillation signal of the self-oscillation unit in phase by 90 or −90 degrees according to a desired rotational direction, and applying the delayed signal to another of the plurality of piezoelectric plates.  
         [0020]     The piezoelectric ultrasonic motor driver according to the present invention changes the rotational direction by changing the phase delays of drive signals applied to the piezoelectric plates through the delay unit. The piezoelectric ultrasonic motor driver can be applied to any piezoelectric ultrasonic motor structure.  
         [0021]     Preferably, the plurality of piezoelectric plates may include first and second piezoelectric plates disposed at 90 degrees to each other, and may also include first and second piezoelectric plates opposing each other and third and fourth piezoelectric plates opposing each other, the first to fourth piezoelectric plates being provided on the surfaces of the metal body, so that the piezoelectric ultrasonic motor can be driven in two-phase and four-phase driving schemes.  
         [0022]     In the case where the plurality of piezoelectric plates include first and second piezoelectric plates disposed at 90 degrees to each other, the self-oscillation unit preferably comprises a first capacitor provided between the first piezoelectric plate and the ground; a second capacitor provided between the metal body of the piezoelectric ultrasonic motor and the ground; a feedback resistor provided between a node between the first capacitor and the first piezoelectric plate and a node between the second capacitor and the metal body; and a first inverter connected in parallel to the feedback resistor. In this case, the delay unit preferably comprises a switch for allowing the oscillation signal of the self-oscillation unit to be output to one of the two selective terminals according to a requested rotational direction; and first and second delayers connected respectively to the two selective terminals of the switch, wherein the first and second delayers delay a voltage signal received through the switch in phase by 90 and −90 degrees, respectively, and provide the delayed signal to the second piezoelectric plate.  
         [0023]     In the case where the plurality of piezoelectric plates include first and second piezoelectric plates opposing each other and third and fourth piezoelectric plates opposing each other, the first to fourth piezoelectric plates being provided on the surfaces of the metal body, the self-oscillation unit preferably may comprise a first capacitor, one end thereof being connected to both the first and second piezoelectric plates and the other end being grounded; a second capacitor provided between the metal body of the piezoelectric ultrasonic motor and the ground; a feedback resistor provided between a node between the first capacitor and the first and second piezoelectric plates and a node between the second capacitor and the metal body; and a first inverter connected in parallel to the feedback resistor. In this case, the self-oscillation unit may also comprise a first capacitor provided between the first piezoelectric plate and the ground; a second capacitor provided between the second piezoelectric plate and the ground; a feedback resistor provided between a node between the first capacitor and the first piezoelectric plate and a node between the second capacitor and the second piezoelectric plate; and a first inverter connected in parallel to the feedback resistor. In this case, the delay unit comprises a switch for allowing the oscillation signal of the self-oscillation unit to be output to one of the two selective terminals according to a requested rotational direction; and first and second delayers connected respectively to the two selective terminals of the switch, wherein the first and second delayers delay a voltage signal received through the switch in phase by 90 and −90 degrees, respectively, and provide the delayed signal to both the third and fourth piezoelectric plates. Alternatively, the delay unit comprises a switch for allowing the oscillation signal of the self-oscillation unit to be output to one of the two selective terminals according to a requested rotational direction; first and second delayers connected respectively to the two selective terminals of the switch, wherein the first and second delayers delay a voltage signal received through the switch in phase by 90 and −90 degrees, respectively, and provide the delayed signal to the third piezoelectric plate; and a second inverter for delaying an electrical signal transferred from the first or second delayer to the third piezoelectric plate in phase by 180 degrees and providing the delayed electrical signal to the fourth piezoelectric plate. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0025]      FIG. 1  is a perspective view illustrating a basic structure of a conventional piezoelectric ultrasonic motor;  
         [0026]      FIG. 2  is a circuit diagram of a conventional piezoelectric ultrasonic motor driver;  
         [0027]      FIG. 3  is a circuit diagram of another conventional piezoelectric ultrasonic motor driver;  
         [0028]      FIG. 4  is a circuit diagram of a piezoelectric ultrasonic motor driver according to a first embodiment of the present invention;  
         [0029]      FIGS. 5   a  to  5   d  illustrates the relationship between drive signals and the direction of polarization of piezoelectric plates provided in a piezoelectric ultrasonic motor according to the first embodiment of the present invention; and  
         [0030]      FIG. 6  is a circuit diagram of a piezoelectric ultrasonic motor driver according to another embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]     A piezoelectric ultrasonic motor drive circuit according to the present invention will now be described in detail with reference to the accompanying drawings.  
         [0032]      FIG. 4  shows a preferred embodiment of a piezoelectric ultrasonic motor driver according to the present invention. As shown in  FIG. 4 , the piezoelectric ultrasonic motor driver comprises a piezoelectric ultrasonic motor  40 , a self-oscillation unit  46 , and a delay unit  47 . The piezoelectric ultrasonic motor  40  includes a metal body  41  and a plurality of piezoelectric plates  42  to  45  attached to outer surfaces of the metal body  41 . The self-oscillation unit oscillates at the electromechanical resonant frequency of the piezoelectric plates  42  to  45  so that the frequency of electrical signals applied to the piezoelectric plates is maintained at the electromechanical resonant frequency of the piezoelectric plates. The delay unit  47  delays an oscillation signal of the self-oscillation unit  46 , which is applied to one of the piezoelectric plates, by 90 or −90 degrees according to a desired rotational direction (i.e., clockwise or counterclockwise) and applies the delayed signal to another of the piezoelectric plates.  
         [0033]     Although the piezoelectric ultrasonic motor  40  includes the four piezoelectric plates  42  to  45  in this embodiment, the number of piezoelectric plates may vary depending on the shape of the metal body  41  and the used driving scheme as described above.  
         [0034]     Circuitry of the self-oscillation unit  46  and the delay unit  47  may vary depending on whether a four-phase driving scheme or a two-phase driving scheme is employed. The circuitry shown in  FIG. 4  is an example implemented using a 4-phase driving scheme.  
         [0035]     In the case where the four-phase driving scheme is used, the self-oscillation unit  46  comprises first and second capacitors CL 1  and CL 2 , a feedback resistor Rf, and a first inverter INV 1 . The first and second piezoelectric plates  42  and  43  of the piezoelectric ultrasonic motor  40 , which oppose each other, are grounded through the first and second capacitors CL 1  and CL 2 . One end of the feedback resistor Rf is connected to the first piezoelectric plate  42  of the motor  40  and the other end is connected to the second piezoelectric plate  43  through the resistor R. The first inverter INV 1  is connected in parallel to the feedback resistor Rf.  
         [0036]     The delay unit  47  comprises a switch  48 , first and second delayers  49  and  50 , and a second inverter INV 2 . The switch  48  has a common contact, connected to the output of the first inverter INV 1 , and two selective terminals. The first and second delayers  49  and  50  are connected respectively to the two selective terminals of the switch  48 . Each output of the first and second delayers  49  and  50  is connected to the third piezoelectric plate  44  of the piezoelectric ultrasonic motor  40 . The first and second delayers  49  and  50  delay the phases of input signals by 90 and −90 degrees, respectively. The second inverter INV 2  inverts the phases of electrical signals transferred from the first and second delayers  49  and  50  to the third piezoelectric plate  44 , and provides the inverted signals to the fourth piezoelectric plate  45 .  
         [0037]     In the embodiment of  FIG. 4 , there is no need to separately ground the metal body  41 , and it is also possible to change the rotational direction simply using the switch  48  and the first and second delayers  49  and  50  without changing the structure of the piezoelectric plates  41  and  45 .  
         [0038]     The operation of the piezoelectric ultrasonic motor driver according to the present invention will now be described with reference to  FIG. 4 .  
         [0039]     The self-oscillation unit  46  performs RF oscillation according to the capacitances of the capacitors CL 1  and CL 2  and the piezoelectric ultrasonic motor  30  and the resistance of the feedback resistor Rf. Since the capacitance of the piezoelectric ultrasonic motor  40  is fixed, it is possible to obtain the same oscillation frequency as the electro-mechanical resonant frequency of the piezoelectric ultrasonic motor  40  by adjusting the resistance of the feedback resistor Rf and the capacitances of the first and second capacitors CL 1  and CL 2 .  
         [0040]     If an electrical signal is applied to the piezoelectric ultrasonic motor  40 , RF oscillation is performed so that a certain-level voltage signal, which is in the form of a sinusoidal wave at the oscillation frequency, is applied to the first piezoelectric plate  42 . The first inverter INV 1  inverts the phase of the voltage signal applied to the first piezoelectric plate  42  and provides the phase-inverted voltage signal to the second piezoelectric plate  43 . Accordingly, the second piezoelectric plate  43  receives a voltage signal which is out of phase with the voltage signal applied to the first piezoelectric plate  42  by 180 degrees and has the same frequency as that of the voltage signal applied to the first piezoelectric plate  42 .  
         [0041]     Then, the oscillation signal of the self-oscillation unit  46  is applied to one of the first and second delayers  49  and  50  through the switch  48  in the delay unit  47 . The first and second delayers  49  and  50  delay input signals in phase by 90 and −90 degrees, respectively.  
         [0042]     The electrical signal provided from the self-oscillation unit  46  to the delay unit  47  is delayed in phase by 90 or −90 degrees according to the selection operation of the switch  48 .  
         [0043]     For example, if the switch  48  selects one of the two selective terminals, which is connected to the first delayer  49 , a signal output from the first inverter INV 1  is input to the first delayer  49  so that the signal is delayed in phase by 90 degrees. The signal delayed in phase by 90 degrees is applied to each of the second inverter INV 2  and the third piezoelectric plate  44  of the piezoelectric ultrasonic motor  40 . The second inverter INV 2  inverts the phase of the electrical signal applied to the third piezoelectric plate  44  and provides the phase-inverted signal to the fourth piezoelectric plate  45  of the piezoelectric ultrasonic motor  40 . That is, two signals out of phase with each other by 180 degrees are applied to the third and fourth piezoelectric plates  44  and  45 , respectively.  
         [0044]     In this manner, four voltage signals, adjacent signals of which are out of phase by 90 degrees and which have the same frequency (i.e., the oscillation frequency), are sequentially applied to the first to fourth piezoelectric plates  42  to  45 . As the four-phase drive signals are applied to the piezoelectric ultrasonic motor  40 , bending deformation occurs in the metal body  41 , thereby rotating the central axis of the metal body  41 .  
         [0045]     Next, if the switch  48  selects the other of the two selective terminals, which is connected to the second delayer  50 , the oscillation signal of the self-oscillation unit  46  is delayed in phase by −90 degrees through the second delayer  50 . The signal delayed in phase by −90 degrees is input to each of the second inverter INV 2  and the third piezoelectric plate  44 . The second inverter INV 2  inverts the phase of the input signal (i.e., delays the phase by 180 degrees) and provides the phase-inverted signal to the fourth piezoelectric plate  45 . Accordingly, two signals out of phase with each other by 180 degrees are applied to the third and fourth piezoelectric plates  44  and  45 , respectively. Here, the phases of the two signals applied to the third and fourth piezoelectric plates  44  and  45  are opposite to the phases of the two signals applied thereto when the first delayer  49  is selected. Consequently, the piezoelectric ultrasonic motor  40  rotates in the opposite direction to that when the first delayer  49  is selected.  
         [0046]     The operation of the piezoelectric ultrasonic motor driver according to the present invention will now be described in more detail with reference to  FIGS. 5   a  to  5   d.    
         [0047]      FIGS. 5   a  to  5   d  illustrate drive signals that the drive circuit according to the present invention provides to the piezoelectric ultrasonic motor  40 .  FIGS. 5   a  and  5   c  show examples in which the switch  48  selects the first delayer  49 , and  FIGS. 5   b  and  5   d  show examples in which the switch  48  selects the second delayer  50 .  FIGS. 5   a  and  5   b  show examples in which the direction of polarization in the first to fourth piezoelectric plates  42  to  45  are from the center of the metal body  41  to the outside, and  FIGS. 5   c  and  5   d  show examples in which the direction of polarization in the first to fourth piezoelectric plates  42  to  45  are from the outside to the center of the metal body  41 .  
         [0048]     First, if the switch  48  selects the first delayer  49 , as shown in  FIG. 5   a , a positive sine wave signal (+sin), a positive cosine wave signal (+cos), a negative sine wave signal (−sin), and a negative cosine wave signal (−cos), adjacent signals of which are out of phase by 90 degrees, are applied respectively to the first, third, second, and fourth piezoelectric plates  42 ,  44 ,  43 , and  45  in clockwise order. On the other hand, if the switch  48  selects the second delayer  50 , as shown in  FIG. 5   b , a positive sine wave signal (+sin), a positive cosine wave signal (+cos), a negative sine wave signal (−sin), and a negative cosine wave signal (−cos), adjacent signals of which are out of phase by 90 degrees, are applied respectively to the first, fourth, second, and third piezoelectric plates  42 ,  45 ,  43 , and  44  in counterclockwise order. In this manner, the piezoelectric ultrasonic driver shown in  FIG. 4  applies the four-phase drive voltage signals, adjacent signals of which are out of phase by 90 degrees, to the four piezoelectric plates  42  to  45 , respectively, so that bending deformation occurs in the metal body  41  in the piezoelectric ultrasonic motor  40 , thereby rotating a rotor (not shown) coupled to the central axis of the metal body  41 . As the phases of drive signals applied to the third and fourth piezoelectric plates  44  and  45  are changed according to the selection of the switch  48  as shown in  FIGS. 5   a  and  5   b , the rotational direction of the piezoelectric ultrasonic motor  40  is changed to clockwise or counterclockwise.  
         [0049]     The piezoelectric ultrasonic motors  40  shown in  FIGS. 5   c  and  5   d  have the same operating principle as those of  FIGS. 5   a  and  5   b , except that the direction of polarization of  FIGS. 5   c  and  5   d  is opposite to that of  FIGS. 5   a  and  5   b . Specifically, if the switch  48  selects the first delayer  49 , as shown in  FIG. 5   c , a positive sine wave signal (+sin), a positive cosine wave signal (+cos), a negative sine wave signal (−sin), and a negative cosine wave signal (−cos) adjacent signals of which are out of phase by 90 degrees, are applied respectively to the first, third, second, and fourth piezoelectric plates  42 ,  44 ,  43 , and  45  in clockwise order. On the other hand, if the switch  48  selects the second delayer  50 , as shown in  FIG. 5   d , a positive sine wave signal (+sin), a positive cosine wave signal (+cos), a negative sine wave signal (−sin), and a negative cosine wave signal (−cos), adjacent signals of which are out of phase by 90 degrees, are applied respectively to the first, fourth, second, and third piezoelectric plates  42 ,  45 ,  43 , and  44  in counterclockwise order. As the four-phase drive voltage signals are applied to the four piezoelectric plates  42  to  45 , bending deformation occurs in the metal body  41  in the piezoelectric ultrasonic motor  40 , thereby rotating the central axis of the metal body  41 . Since the direction of polarization of the first to fourth piezoelectric plates  42  to  45  is opposite to that of  FIGS. 5   a  and  5   b , the metal body  41  is subjected to bending deformation opposite to that of  FIGS. 5   a  and  5   b  under the same condition. Thus, if drive signals in the same states as in  FIGS. 5   a  and  5   b  are applied to the first to fourth piezoelectric plates  42  to  45 , the rotational direction of the piezoelectric ultrasonic motor  40  is opposite to that of  FIGS. 5   a  and  5   b.    
         [0050]     The piezoelectric ultrasonic motor driver according to the present invention may also employ a two-phase driving scheme to drive the motor.  
         [0051]      FIG. 6  illustrates another embodiment of the piezoelectric ultrasonic motor driver according to the present invention, which uses a two-phase driving scheme.  
         [0052]     As shown in  FIG. 6 , the piezoelectric ultrasonic motor driver comprises a piezoelectric ultrasonic motor  60 , a self-oscillation unit  66 , and a delay unit  67 . The piezoelectric ultrasonic motor  60  includes a metal body  61  and a plurality of piezoelectric plates  62  to  65  attached to outer surfaces of the metal body  61 . The self-oscillation unit oscillates at the electromechanical resonant frequency of the piezoelectric plates  62  to  65  so that the frequency of electrical signals applied to the piezoelectric plates is maintained at the electromechanical resonant frequency of the piezoelectric plates. The delay unit  67  delays an oscillation signal of the self-oscillation unit  66  by 90 or −90 degrees according to a desired rotational direction and applies the delayed signal to a specific one of the piezoelectric plates.  
         [0053]     Circuitry of the self-oscillation unit  66  and the delay unit  67  slightly differ from that of  FIG. 4 .  
         [0054]     Specifically, the self-oscillation unit  66  comprises first and second capacitors CL 1  and CL 2 , a feedback resistor Rf, and a first inverter INV 1 . One end of the first capacitor CL 1  is connected to both the first and second piezoelectric plates  62  and  63 , and the other end is ground. The second capacitor CL 2  is connected between the ground and the metal body  61  of the piezoelectric ultrasonic motor  60 . The feedback resistor Rf is connected between a node between the first capacitor CL 1  and the first and second piezoelectric plates  62  and  63  and a node between the second capacitor CL 2  and the metal body  61 . The first inverter INV 1  is connected in parallel to the feedback resistor Rf. The delay unit  67  comprises a switch  68  and first and second delayers  69  and  70 . The switch  68  has a command contact and two selective terminals, and allows an oscillation signal from the self-oscillation unit  66  to be output to one of the two selective terminals according to a requested rotational direction. The first and second delayers  69  and  70  are connected respectively to the two selective terminals of the switch  68 . The first and second delayers  69  and  70  delay an electrical signal received through the switch  68  in phase by 90 and −90 degrees, respectively, and provides the delayed signal to the third and fourth piezoelectric plates  64  and  65 .  
         [0055]     The piezoelectric ultrasonic motor driver shown in  FIG. 6  uses a two-phase driving scheme so that the same signal is applied to the same pair of opposing piezoelectric plates ( 62  and  63 ) and ( 64  and  65 ). The direction of polarization of the first to fourth piezoelectric plates  62  to  65  differs from that of the embodiment of  FIG. 4 . Specifically, the direction of polarization of each of the first to fourth piezoelectric plates  62  to  65  is from the center of the metal body  61  to the outside or from the outside to the center of the metal body  61 , and the first and second piezoelectric plates  62  to  63  have the same direction of polarization, and the third and fourth piezoelectric plates  64  to  65  have the same direction of polarization as shown in  FIG. 6 .  
         [0056]     If electrical signals are applied to the first to fourth piezoelectric plates  62  to  65 , the piezoelectric plates  2  to  5  contract and expand to cause a bending deformation of the metal body  1 , thereby rotating the central axis of the metal body  1 . The other operations of the self-oscillation unit  66  and the delay unit  67  are the same as those of  FIG. 4 .  
         [0057]     In the embodiment of  FIG. 6 , the piezoelectric ultrasonic motor  60  may also include only two piezoelectric plates. For example, one of the two opposing piezoelectric plates  62  and  63  and one of the other two opposing piezoelectric plates  64  and  65  may be removed from the piezoelectric ultrasonic motor  60  in  FIG. 6 . In this case, the self-oscillation unit  66  and the delay unit  67  have the same configurations and operations as when the motor  60  includes all the four piezoelectric plates.  
         [0058]     As described above, the piezoelectric ultrasonic motor driver according to the present invention is unrestricted in terms of configuration or, as compared to the conventional one shown in  FIG. 3 . For example, the piezoelectric ultrasonic motor driver according to the present invention can be applied not only to an ultrasonic motor including four piezoelectric plates but also to an ultrasonic motor including two piezoelectric plates.  
         [0059]     As is apparent from the above description, a piezoelectric ultrasonic motor driver according to the present invention has the following advantages. Both four-phase and two-phase driving schemes can be applied to the piezoelectric ultrasonic motor driver. By means of self-oscillation, the frequency of signals for driving a piezoelectric ultrasonic motor can be maintained at the electromechanical resonant frequency of piezoelectric plates provided on the piezoelectric ultrasonic motor, so that the maximum efficiency and output can be achieved at any time when the motor is driven. It is also possible for the driver to change the rotational direction simply by changing the phases of electrical signals applied to the piezoelectric plates using a switch and a delay circuit provided in the driver, without changing the structure of the piezoelectric ultrasonic motor. Especially, there is no need to ground a metal body of the piezoelectric ultrasonic motor when the four-phase driving scheme is employed, thereby making it easy to manufacture the driver.  
         [0060]     Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.