Abstract:
A micromotor comprising: a piezoelectric element including a common electrode and a plurality of other electrodes formed thereon and including at least a first a and a second electrode group, each group including at least one electrode, wherein the piezoelectric element causes motion in a first direction when a voltage is applied between the first electrode group and the common electrode, and wherein the piezoelectric element causes motion in a second direction when a voltage is applied between the second electrode group and the common electrode; a voltage source that electrifies the common electrode; and at least two switches separately connected between the first and second electrode groups and a low voltage, said switches being activatable to connect one of said first and second electrode groups to the low voltage to cause selective motion in the first or second directions.

Description:
RELATED APPLICATIONS 
     The present application is a U.S. national application of PCT/IL98/00519, filed 25 Oct. 1998. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to systems for driving piezoelectric actuators and piezoelectric motors and more particularly to the electronic circuitry of such drivers. 
     BACKGROUND OF THE INVENTION 
     Piezoelectric materials are characterized in that when they are subjected to electrical fields they can be made to deflect; i.e., produce mechanical motion. Also when mechanical stress is applied to piezoelectric elements so hat they undergo displacement they generate electrical signals. These characteristics are the reason that piezoelectric materials are useful in applications ranging from sensors to mechanical motors. Examples of piezoelectric elements being used as motors is shown for example in U.S. Pat. Nos. 5,714,833 and 5,777,423; which patents are assigned to the assignee of the invention of this application. Piezoelectric motors are characterized by their mechanical simplicity. They have very few parts, no separate moving parts, and there are no critical mechanical components such as gears, shafts, bearings etc. Consequently, piezoelectric motors are relatively inexpensive and highly reliable. 
     A piezoelectric element represents an electrical reactive load (mainly capacative) that requires an AC signal of substantial voltage amplitude to cause the mechanical displacement. The required voltage amplitude is generally in the range of a few hundred volts (RMS). For effectively operating the piezoelectric element as a motor or actuator a drive circuit applies voltage thereto of a specific frequency with low harmonic distortion. For the best results, i.e. maximum displacement, the specific frequency should equal the mechanical resonant frequency of the piezoelectric element. A simplified circuit that could be used to drive a piezoelectric motor is shown in FIG. 1A, while such a motor is illustrated in FIG.  1 B. This method required two sources and two matching (high-Q) resonant circuits (not shown). 
     A preferred manner of actuating bidirectional operation of the piezoelectric element provides one side of the element with a pair of electrodes that are connected to a voltage source, such as a switch mode inverter. An opposite side of the piezoelectric element has a single common electrode. The common electrode is grounded or connected to ground by passive or active elements so that current from each of the pair of electrodes flows in opposite directions to cause bidirectional displacement of the piezoelectric element. 
     The drive circuits for piezoelectric motors basically provide AC voltages to the electrodes according to the directional movement required or desired. Typical prior art drive circuits for actuating the piezoelectric elements to provide movement in a selected direction or selected directions comprise separate inverter circuits coupled to the piezoelectric element through resonant circuitry attached to each of the elements high side electrodes. The prior art utilizes such multiple inverters to cause directional motion of the piezoelectric motors without any separate switching units. Other prior art devices utilize a single high voltage source and switching circuitry to switch the source between different electrodes. This methodology requires high voltage capable switching circuitry. 
     A major disadvantage of the prior art drivers is the complexity of the circuitry that is used. For example, a commonly used driver comprises a bridge type inverter circuit (see FIG. 2) that requires six power switches, and two high Q resonant circuits. Other prior art drivers require only one high Q resonant circuit The reduction in the number of high Q resonant circuits is accomplished by placing the high Q resonant circuit in series with the common electrode. However, six power switches are still used in that prior art device (see EP 0712 170 A1), which application is assigned to the assignee of this application. In FIG. 7 of that device, four switches are used for displacement directional control with two switches for each of the selectable directions. The high Q resonant circuits are sensitive to the accuracy of the circuit component and to the frequency of the applied voltage. As a practical solution for overcoming this sensitively, in the prior art the Q of the drivers was often lowered and the input voltage from the prior art drivers was raised Extra DC/DC converters were required to enable operation with lower input voltages. Consequently, in the prior art, while the piezoelectric motors are simple the driver circuitry has until now been complex, especially when driving piezoelectric motors in two directions. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of some preferred embodiments of the present invention to provide drivers for piezoelectric elements that are reliable, simple and of low cost. 
     It is an object of some preferred embodiments of the present invention is to provide drivers for piezoelectric elements that operate the piezoelectric elements as actuators or motors bi-directionally with a minimum of components. 
     It is an object of some preferred embodiments of the invention to provide bi-directional drivers for piezoelectric motors or actuators in which a single high voltage unswitched drive circuit is used for both direction of motion. 
     It is an object of some preferred embodiments of the present invention to provide bi-directional drivers for piezoelectric motors or actuators in which the direction of motion is changed by switching circuitry located at a low voltage connection of the control circuitry. 
     It is an object of some preferred embodiments of the present invention to provide drivers for piezoelectric elements that utilize soft switching so as to minimize switching losses and increase the overall efficiency of the piezoelectric driver unit and to improve electromagnetic compatability. 
     It is an object of some preferred embodiments of the present invention to provide a piezoelectric driver operated from a low DC voltage source without additional complexity. 
     It is an object of some preferred embodiments of the present invention to provide a discrete bidirectional switching circuit used in conjunction with an AC voltage provided for operating a piezoelectric motor or other AC load. 
     Some prior art driver circuits for driving piezoelectric motor firm a DC voltage source, integrally, combined switching and inverter circuitry. That is the inverters did the switching of the inverter output to the piezoelectric element&#39;s electrodes without any discrete switching circuitry. The present invention separates the inverter and switching portions of the driver circuitry by providing discrete switching circuitry. Surprisingly, the result is a circuit for driving the piezoelectric element bidirectionally of decreased complexity using fewer components and operating more reliably. 
     In accordance with one aspect of the invention, a driver circuit for driving a piezoelectric type motor comprises an inverter circuit for providing an oscillating voltage to cause a mechanical displacement of a piezoelectric element and a separate switching arrangement for selecting the direction of said displacement. The driver circuit in the invention of this application delivers the driving voltages to the piezoelectric element at the mechanical resonant frequency of said piezoelectric element. 
     In a preferred embodiment of the invention a discrete switch arrangement includes high frequency switches for selectively applying voltages across said piezoelectric element to cause mechanical displacement of the piezoelectric element in a selected direction or selected directions. In preferred embodiments of the invention, the switches are on a low voltage side of the connection to the piezoelectric element. 
     There is thus provided, in accordance with a preferred embodiment of the invention, a micromotor comprising: 
     a piezoelectric element including a common electrode and a plurality of other electrodes formed thereon and including at least a first and a second electrode group, each group including at least one electrode, where the piezoelectric element causes motion in a first direction when a voltage is applied between the first electrode group and the common electrode and wherein the piezoelectric element causes motion in a second direction when a voltage is applied between the second electrode group and the common electrode; 
     an voltage source that electrifies the common electrode; and 
     at least two switches separately connected between the first and second electrode groups and a low voltage, said switches being activatable to connect one of said first and second electrode groups to the low voltage to cause selective motion in the first or second directions. 
     Preferably, the low voltage is substantially ground. Preferably, the applied voltage is an AC voltage. 
     In a preferred embodiment of the invention, where the piezoelectric element comprises a rectangular piezoelectric plate having fist and second faces, the common electrode is formed on the first face and the first and second groups of electrodes are formed on the second face. Preferably, the first and second groups of electrodes each comprises two electrodes situated in opposite quadrants of the second face. 
     Preferably, the micromotor comprises a motive surface and motion is induced in a surface of a workpiece pressed against the motive surface when the piezoelectric element is electrified as aforesaid. 
     There is further provided, in accordance with a preferred embodiment of the invention, a micromotor comprising: 
     an ultrasonically vibrating element; and 
     a drive circuit comprising: 
     an oscillating voltage source connected to and electrifying at least one electrode of said ultrasonically vibrating element to cause a mechanical displacement of a portion thereof; and 
     a discrete switch arrangement attached to at least one additional electrode of said ultrasonically vibrating element to which said oscillating voltage is not connected which switch arrangement selects the direction of said displacement. 
     Preferably, the ultrasonically vibrating element comprises a piezoelectric element 
     Preferably, the at least one additional electrode comprises a plurality of electrodes applied to a first face of said vibrating element; and the at least one electrode comprises a common electrode applied to a second face of said element. Preferably, the discrete switch arrangement selectively applies voltage between a first group of said plurality of electrodes and said common electrode to cause displacement in a first direction, said first group including at least one electrode. Preferably, the discrete switch arrangement selectively applies voltage between a second group of said plural of electrodes and said common electrode to cause displacement in a second direction, said second group comprising at least one electrode. 
     In a preferred embodiment of the invention, the discrete switching arrangement comprises a pair of switches connected to apply voltages across said element to drive current through said element, and controls for selectively operating said switches to select the direction of said displacement. Preferably, the switches of said discrete switching arrangement comprise solid state switches, preferably, transistorized switches and most preferably, Mosfet transistors. 
     In a preferred embodiment of the invention, the discrete switch arrangement comprises: 
     a first Mosfet connected between a first voltage and said first group of electrodes; 
     a second Mosfet connected between said first voltage and said second group of electrodes, said common electrode being connected to a second voltage, and 
     a control that selectively operates said Mosfet switches to selectively apply said first voltage to the first electrode group or to said second electrode group. 
     Preferably, the micromotor includes a source of control voltages selectively applied to the gates of said first and second Mosfet transistors for selectively switching said first or said second Mosfet transistors from the non conducting state to the conducting state. 
     Preferably, the micromotor includes a pair of diodes, one of which is connected across each said Mosfet transistor. Preferably, the diodes are connected such that they conduct DC current toward the micromotor. 
     In a preferred embodiment of the invention, the transistor is off, one end of the Mosfet is at a DC voltage equal to the peak of the oscillating voltage and the oscillating voltage is impressed across the Mosfet transistor, such that the voltage across transistor is substantially unipolar. 
     In a preferred embodiment of the invention, the source comprises an inverter, preferably, a forward-flyback inverter. 
     Preferably, the forward-flyback inverter comprises: 
     a magnetic element having a primary winding and a secondary winding, said primary winding being connected between a first inverter voltage and one side of an inverter operating switch, the other side of said inverter operating switch connected to a second inverter voltage so that when said inverter operating switch is in the conductive stage, current flows through said primary and when said inverter operates switch is in a non conductive state substantially no current flows through said primary; and 
     a control voltage source which selectively turns said inverter operating switches switch on or off, the secondary of said magnetic element being connected to said discrete switching arrangement. 
     Preferably, the inverter operating switches comprise solid state switches, preferably transistorized switches most preferably, Mosfet transistor, and further comprises circuitry that causes said inverter output to resonate at substantially the mechanical resonant frequency of said piezoelectric element. 
     In a preferred embodiment of the invention, the circuitry that causes said inverter output to resonate at substantially the mechanical resonant frequency of the piezoelectric comprises a capacitor bridging said switch and in series with the pi of the magnetic element, said capacitor operating in conjunction with the capacitance of said ultrasonically vibrating motor to turn said magnetic element to resonate at substantially the mechanical resonant frequency of the motor. 
     In a preferred embodiment of the invention, the inverter is a push-pull inverter. 
     Preferably, the push-pull inverter comprises a transformer, having a primary and a secondary; 
     a series inductor connecting a first inverter input to a mid part of the primary of said transformer, 
     the secondary of said transformer connected to said discrete switching arrangement and one side of the primary of said transformer being connected through a first push-pull inverter switch to a second inverter input, 
     the other side of said primary of said transformer being connected through a second push-pull inverter switch to said second input. 
     Preferably, the fist and second push-pull switches comprise solid states switches, more preferably, transistorized switches and most preferably, Mosfet type switches. 
     In a prefer embodiment of the invention, the capacitance of said ultrasonic motor and the inductances of the series inductance and of said transformer match the electrical circuit to the mechanical resonance of said piezoelectric element. Preferably, the first or the second push-pull inverter switches are each selectively operated by control voltages, preferably, square wave voltages. 
     In a preferred embodiment of the invention, the push-pull inverter includes a buck section for controlling the amplitude of the voltage connected to said primary of said transformer. Preferably, the buck section includes: 
     a series buck section switch connected between the first input of the inverter and the series inductor; 
     a diode connected between an output of the buck section switch and said second input of the inverter and, 
     a control voltage operative to cause the buck section switch to conduct. 
     In a preferred embodiment of the invention, the second input is ground. Preferably, the first input is electrified with a DC voltage. 
     There is further provided, in accordance with a preferred embodiment of the invention, a method of supplying switchable AC power to a load comprising: 
     connecting a fist terminal of an AC power source to one side of the load; 
     connected a Mosfet transistor between a second terminal of the AC power source and the other side of the load; and 
     selectively supplying power to the load by applying a voltage between a gate of the Mosfet and the second AC terminal. 
     Preferably a diode is connected across the Mosfet. Preferably, the diode is connected such that it conducts current between the second terminal and the other side of the load. 
     In a preferred embodiment of the invention a capacitor is placed in series with the load, which preferably, does not include a DC blocking capacitor. 
     In accordance with a preferred embodiment of the invention, when the transistor is off, one end of the Mosfet is at a DC voltage equal to the peak voltage of the AC source and AC voltage of the AC source is impressed across the Mosfet transistor, such that the voltage across transistor is substantially unipolar. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above named and other features and objects of the present invention will be best understood from the following description of preferred embodiments of the invention taken in conjunction with the accompanying drawings, in which: 
     FIGS. 1A and 1B are a simplified schematic showing of a bi-directional piezoelectric motor and associated driver circuitry of the prior art; 
     FIG. 2 is prior art bridge circuit implementation of a bi-directional piezoelectric motor driver; 
     FIG. 3 is a basic electrical component schematic showing of an AC switch operating a piezoelectric motor; 
     FIG. 4 is a schematic showing of a discrete bi-directional switch used with a piezoelectric motor; 
     FIG. 5 is a schematic showing of a preferred driver circuit comprising a forward-flyback inverter used in conjunction with the bi-directional switch for operating the piezoelectric motor; 
     FIG. 6 is an equivalent circuit of FIG. 5 showing elements of the secondary circuitry reflected to the primary; 
     FIG. 7 is the schematic showing of the circuit of FIG. 6 when the switch Qff is conducting; 
     FIG. 8 is a schematic showing of the circuit of FIG. 6 when the switch Q ff  is non-conducting; 
     FIG. 9 is a schematic showing of a preferred driver circuit comprising a push-pull parallel resonant inverter used in conjunction with the bi-directional switch for operating the piezoelectric motor; 
     FIG. 10 is a schematic showing of the push-pull inverter of FIG. 9 including a buck section providing output voltage control; and 
     FIG. 11 is a showing of the voltage wave form at point A of FIG.  10 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGS. 1A and 1B are a simplified schematic showing of a bi-directional piezoelectric motor as used in the prior art to aid in understanding the invention. The piezoelectric motor  11 , comprises a piezoelectric crystal or ceramic element  12 , that operates responsive to electric voltage thereacross to provide mechanical displacement. 
     As shown in FIGS. 1A and 1B, to operate the motor bi-directionally, two pairs of electrodes  13  and  14  are attached to a first face  16  of element  12  and a common electrode  17  is attached to and covers substantially all of an opposite face  18  of element  12 . Common electrode  17  is grounded at  19 . A first AC voltage V 1  is applied between electrodes  14  and  17  by the operation of switch SW 1  responsive to a control signal S 1 . Voltage V 1  causes mechanical displacement of a surface pressed against an extension  9  of element  12  in a first direction. 
     Operation of switch SW 2 , responsive to a control signal S 2 , applies a second AC voltage V 2  between electrodes  13  and  17 . Voltage V 2  causes mechanical displacement of a surface pressed against extension  9  in a second direction. The piezoelectric element  12  is reactive in nature and mechanically resonates at particular frequencies in transverse directions. 
     It should be understood that when electrodes are shown, in the following drawings, in the manner of electrodes  13  and  14 , what is meant is a configuration such as that shown in FIG.  1 B. 
     FIG. 2 shows a typical prior art implementation of a bi-directional driver for a piezoelectric motor. A prior art bi-directional inverter driver as shown in FIG. 2 has a positive supply voltage Vin applied to conductor  21 . A switch shown as Q 1  is connected to conductor  21 . The switch is shown as a Mosfet type transistor with the source  22  connected to the conductor  21 . The switch Q 1  in the preferred embodiment is operated to the conducting state by a pulsed gate control signal S 1  applied to gate DR 1 . Responsive to the gate control signals the switch Q 1  applies DC voltage Vin as pulses to the drain  23  of switch Q 1 . The drain  23  is connected to conductor  24  which connects to common electrode  17  of piezoelectric motor  11 . Conductor  24  is connected to ground through switch Q 2  operated to the conducting condition in response the pulsed control signal S 2  applied on the gate DR 2  of switch Q 2 . The source  26  of switch Q 2  is connected to conductor  24  and the drain  27  of switch Q 2  is connected to ground  28  through conductor  29 . 
     Electrodes  14  of piezoelectric motor  11  is connected to the junction of a capacitor C 1  and an inductor L 1 . Capacitor C 1  and inductor L 1  form a resonant circuit tuned to the mechanical resonance frequency of the piezoelectric motor. The other side of capacitor C 1  is connected to conductor  24 . The other side of inductor L 1  is connected through conductor  30  to the junction of another pair of switches, Q 5  and Q 6 . Transistor switches Q 5  and Q 6  operate to the conductive state responsive to pulsed signals S 5  and S 6  on the gate drivers shown as DR 5  and DR 6 . The source  31  of switch Q 5  is coupled to the DC voltage Vin on conductor  21 . The is drain  32  of switch Q 5  is coupled to the source  33  of switch Q 6 . The drain  34  of switch Q 6  is coupled is to ground conductor  29 . 
     Another half bridge circuit connects electrodes  13  of the motor  11  to source voltage Vin. More particularly, the electrode  13  of the motor  11  is connected to the junction of inductor L 2  and capacitor C 2  which form a second resonant circuit. The other side of capacitor C 2  is connected to conductor  24 . The other side of inductor L 2  is connected to the junction of the drain  36  of switch Q 3  and the source  37  of switch Q 4 . The source  38  of switch Q 3  is coupled to the DC voltage Vin on conductor  21 . The drain  39  of switch Q 4  is coupled to the grounded conductor  29 . The switches Q 2 , Q 4  are operated responsive to signals S 3 , S 4 . 
     In operation, Q 1  and Q 2  act together with either Q 3  and Q 4  or Q 5  and Q 6  to apply an oscillating voltage to the resonant circuit of L 1 /C 1  or L 2 /C 2  respectively, in a manner well known in the art. Depending on which half bridge is operated together with Q 1 /Q 2 , either electrodes  13  or  14  will be energize. This allows for reversible motion of a surface pressed against extension  9 . 
     In greater detail when switches Q 1 , Q 2  and Q 5 , Q 6  are operated, an AC voltage is applied between electrodes  14  and  17  of the motor  11  at the resonant frequency set by capacitor C 1  and inductor L 1 . Alternatively, an AC voltage is applied between electrodes  13  and  17  by the operation of switches Q 1 , Q 2  and Q 3 , Q 4 . This applies an AC voltage across electrodes  13  and  17  at the resonant frequency set by inductor L 2  and capacitor C 2 . Thus, the voltage across electrodes  13  and  17  responsive to the operation of switches Q 1 -Q 4  cause the motor to operate in one direction and the operation of switches Q 1 , Q 2  and Q 5 , Q 6  cause the motor to operate in the opposite direction. This typical prior art circuitry requires two high Q resonant circuits, C 1 , L 1  and C 2 , L 2  and six switching components operated responsive to signals S 1 -S 6  applied to the gates DR 1 -DR 6 . 
     A high voltage Vin is needed across the motor if reasonable Q factors are to be used. For example, a Q factor of  5  requires the input voltage to be about ⅕ of the required motor voltage. A motor voltage of 400 volts is reasonable. This implies that the input voltage needs to be 80 volts. If lower voltages are required then additional circuitry would have to be added. 
     FIG. 3 shows an AC switch and equivalent circuit of the piezoelectric device, respectively, useful in understanding preferred embodiments of the invention. 
     In FIG. 3, L s /C s  represent the mechanical resonance of the piezoelectric devise R s  represents mechanical work and losses and C p  is the electrode capacitance. Since C p  is very large, an inductor is usually added in series with the piezoelectric device. 
     When switch SWL is closed, V s  is applied to the piezoelectric device and the motor operates. When SWL is open, capacitor C p  is clamped to the peak value of Vs via diode D s . This DC voltage is maintained so long as SWL is open. In this situation, (DC voltage across the piezoelectric element) the motor does not operate. AC Voltage appears across the diode. 
     FIG. 4 shows a preferred embodiment of the discrete bidirectional AC switch for operating with an AC voltage V s  to drive piezoelectric motor  11 . As in FIG. 1, piezoelectric motor  11  comprises electrodes  13 ,  14  on one face and electrode  17  on the opposite face of piezoelectric element  12 . While any solid state switching units can be used, in a preferred embodiment there two Mosfet switches M S1  and M S2 , each connected to electrodes  13  and  14  respectively are used as the switches. Diodes D S1  and D S2  shown as connected across the switches M S1  and M S2  are inherent in Mosfet transistors. Source  41  of switch M S1  and diode D S1  are both connected to electrodes  13  of motor  11 . Drain  42  of switch M S1  and the other side of diode D S1  are shown as being grounded at  43 . Source  44  of switch M S2  is connected to the anode of diode D S2  and to electrodes  14 . Drain  46  of switch M S2  is connected ground  43  through conductor  47 . One side of the alternating voltage source is connected to the common electrode  17  of the piezoelectric motor  11  while the other side of the voltage source is coupled to ground through conductor  47 . The operating direction of the piezoelectric motor  11  is controlled by gate voltages V gs1  and V gs2  applied to the gates of switches M S1  and M S2  respectively. When M S1  is closed (and M S2  is open), the AC voltage is applied between electrodes  17  and  13  and a surface pressed against extension  9  moves in a first direction. When M S2  is closed an M S1  is open, travel is in the reverse direction. 
     This is facilitated by the fact that when M S2  is open, it disconnects the AC current path (the AC voltage appears across the switch) from electrode  14 . The DC voltage that develops, under these conditions, between electrodes  14  and  11  does not interfere with the operation of the motor, as effected by the AC voltage between electrodes  17  and  13 . 
     Major advantages of this bidirectional switching configuration is that: 
     a) the control voltages are all referred to ground, and 
     b) only one AC source is required. 
     One reason these advantages are available is that unlike the prior art drivers, the common electrode is not referenced to ground, Rather, one of the electrode pairs  13  or  14  is grounded. This allows for a single power source to feed the common electrode and, since the individual electrodes are near ground, for the switching to be controlled with low voltages. 
     This switching scheme can be incorporated in conjunction with any inverter driver but it has special merits when the inverter configuration of this invention is used to provide the driving voltage V s . 
     A preferred embodiment of the AC source is a forward-flyback type inverter such as schematically shown in FIG.  5 . Therein a high frequency switch such as Mosfet transistor Q ff    51  is driven by square wave signal V sg  applied to the gate electrode  52 . Source  54  of switch Q ff  is connected to a primary winding  58  of a two winding magnetic element indicated at  53 . Drain  56  of switch Q ff  is coupled to ground  57 . While many types of switches can be used at Q ff , the preferred embodiment encompasses a Mosfet type transistor. The diode inherent in the Mosfet transistor is shown as D ff  coupled from drain  56  to source  54  of transistor Q ff . The other side of the primary winding  58  of the magnetic element  53  is connected to positive DC voltage Vin as shown at  59 . The switch Q ff  is bridged by a capacitance C r  connected between its source  54  and drain,  56 . Capacitance C r  forms a resonant circuit with the other reactive components. 
     The inverter is preferably connected to piezoelectric element  12  and the pair of discrete bidirectional switches through the secondary winding  61  of the two winding magnetic element  53 . The turns ratio between the primary winding  58  and the secondary winding  61  is shown as being 1:n where n in one preferred embodiment is in a range between 2 and 25. 
     One side of winding  61  is preferably connected to ground at  62 . The other side is connected to the common electrode  17  of the motor  11 . The other electrodes of the motor  11 , i.e. electrode pairs  13  and  14  are connected to high frequency switches such as switches M S1  and M S2 , in the manner described above with respect to FIG.  4 . 
     The operation of the combination of the discrete inverter and the discrete bi-directional switch, in combination, is more clearly dd with reference to the schematic of FIG.  6 . That schematic shows one of the switches open and the other closed with all secondary components (including the closed lines) being reflected to the primary . The inductance of the two element magnetic unit  53  is shown as L lkg , i.e. the leakage inductance of the magnetic element  53 . The piezoelectric element  12  of the motor  11  is represented by the impedance Z M  which is mainly capacitive in the name at the mechanical resonance frequency. It is shown as being divided by n 2  when reflected into the primary of magnetic unit  53 . 
     A square wave voltage V sq  is shown applied to the gate of switch Q ff  whose drain is connected ground and whose source is connected to inductor L m  and to motor (impedance) element Z M . Diode D ff  is Anon as is resonance capacitance C r , both bridging switch Q ff . Capacitance Cr is in series with and resonates with the inductance L m  bridged by the leakage inductance L lkg  in series with the reactance Z M /n 2 . 
     While a square wave operating voltage V sg  is shown; it should be understood that the operating voltage can assume other forms as known in the art within the scope of the invention. 
     When the switch Q ff  is conducting, as shown in FIG. 7, energy is delivered to the motor in a resonant manner in the two half cycles. When the high frequency switch Q ff  is conducting, energy is transferred from the voltage source to the motor and energy is also stored in inductance L m . A resonance circuit is formed between the leakage inductor L kj  and the motor impedance Z m . Hence the voltage wave form fed to the motor is sinusoidal. During a non conducting period of the switch Q ff , as shown is FIG. 8, the energy stored in the inductance L m  is transferred to the motor. Notice that the voltage to the motor now is of opposite polarity than the voltage that was applied during the conducting stage of switch Q ff . Reactive element L m , bridged by the series connection of L kg  and Z m /n 2  forms a resonant circuit that can shape the voltage across the motor to be sinusoidal by properly choosing the circuit parameters. Preferably, this circuit is resonant at the mechanical resonance frequency. 
     Note that when the wave form across the switch is sinusoidal the switch operates under “soft” switching conditions. In other words if switch Q ff  turns off after voltage across it reduces to zero, the switch is turned on under zero voltage conditions. Then, when the switch Q ff  is turned off, the voltage will rise across it relatively slowly due to the action of capacitor C r . Hence zero voltage switching at turn off is achieved. 
     Thus the circuit of FIG. 5, according to this preferred embodiment of the invention, may have one or more of a number of advantages, for example: 
     i) a single switch is used for implementing the inverter section, 
     ii) all signal drives are referred to one ground, 
     iii) by choosing the proper turns ratio the motor voltage can be made high even if V in  is a low DC voltage, 
     iv) the motor signals are of low harmonic content, and in addition 
     v) the total part count is very low, i.e. the inverter section is based on a single transistor and the discrete bidirectional switch has only two transistor. 
     Further the embodiment of FIG. 5 is suitable for relatively low power levels. 
     As can be well understood, the act design details for such circuitry is a function of the exact physical and electrical parameters. However, the design can be optimized for the particular characteristics of piezoelectric element utilizing design programs such as SPICE or the like. Once such a design is achieved, the components (including the non-ideal transformers used in the designs) may be designed and produced using methods well known in the art or magnetic component design. 
     Another preferred embodiment of the invention is shown in FIG. 9, in which the discrete inverter is of a push-pull configuration. The push-pull configuration is suitable for higher power levels then is the forward flyback inverter. In the push-pull arrangement of the inverter shown in FIG. 9, two switches Q p1  and Q p2  operate on the load in a push-pull manner through a resonant network. The resonant network comprises the inductance of a transform TR whose secondary is bridged by capacitor C pp . Current from the DC voltage V in  is applied to primary  66  of transformer TR through series inductor L sp . The input to the transformer is to the middle of the primary windings  66  as shown in FIG.  9 . In a preferred embodiment of the invention, the two switches Q p1  and Q p2  are Mosfet transistors. 
     One side of primary  66 , goes to the source  67  of transistor Q p1 . Drain  68  of transistor Q p1  is coupled to ground at  69 . The other side of the Primary wing  66  of transformer TR is coupled to source  72  of switch Q. Drain  73  of transistor switch Q p2  is coupled to ground. Both transistors Q p1  and Q p2  are shown as having square wave control voltages applied to their gates  74  and  76  respectively, for converting the DC voltage input to an AC voltage at the resonant frequency. Control voltages with other waveforms could be used within the scope of the invention. The secondary winding n 1  of transformer TR, has one side  77  connected to ground and the other side  78  connected to common electrode  17  of motor  11 . When the parameters of the circuit are properly chosen the signal fed to the motor  11  will be of a sinusoidal shape with low harmonic distortion. 
     The major benefit of the push-pull embodiment over the forward-flyback embodiment is the capability to handle higher power levels. Furthermore, by slight modification, the output of voltage of the push-pull embodiment can be firer controlled as is illustrated in FIG.  11 . Here an additional high frequency switch Q buck  and diode D buck  have been added. These elements together with inductor L sp  form a “down” or “buck” inverter. This type of inverter is controlled by the duty cycle D on  of Q buck . Assuming a sinusoidal wave form, the voltage at point “A” in FIG. 11 will be as shown in FIG.  12 . The peak value V pk  of this wave form will be related to the motor voltage by the equation: 
     
       
         
           V 
           PK 
           =V 
           motor 
           /n. 
         
       
     
     At steady state, the average voltage across the inductor L sp  will be zero and hence the average of V pk  must be equal to the voltage at point “B” in FIG. 10 which is equal to V in *D on . Hence: 
     
       
           V   PK   =nV   in   *D   on . 
       
     
     Consequently from these equations: 
     
       
           V   motor   =n*V   in   *D   on . 
       
     
     Hence the motor voltage can be controlled by varying D on  of Q buck . This feature is particularly applicable when variable motor voltages, are called for such as for varying the speed of the motor. 
     The preferred embodiments have been desired in relation to a piezoelectric motor having a common electrode on one face of a piezoelectric ceramic and two pairs of electrodes on the other face. However, the motor may also operate more of fewer electrodes and with different configuration. Further it can be adapted to other configurations of piezoelectric motors. 
     Furthermore, a bipolar switch in accordance a preferred embodiment of the invention may be used for various other applications. In fact many AC loads may be switched such a bipolar switch. In general, it is difficult to switch an AC load with a single MOS transistor. In accordance with a preferred embodiment of the invention, a switch, such as one half of the circuit shown in FIG. 4, may be used to switch a capacitive AC load, using a single transistor. Furthermore, if the load is not capacitive a large (compared to the admittance of the load) capacitor may be placed in series with the load. For AC purposes (i.e., when the switch is closed) the capacitor has negligible effect. However, when the switch is open, the voltage across the capacitor will rise to the peak of the AC voltage switching the load of, as described above. 
     Various modifications will be apparent to and will be readily available to those skilled in the art without departing from the scope and spirit of the invention. Accordingly it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein but rather that the claims be broadly constructed. It should be understood that the verbs “comprise” and “include” and their conjugations, when used in the claims, mean “including, but not necessarily limited to.”