Abstract:
A method is provided for exciting vibrations in a piezoelectric motor having a plurality of electrode sets, each set comprising at least one first electrode and at least one second electrode between which AC voltages are applied to excite vibrations in the piezoelectric motor, the method comprising: coupling an AC power source to the at least one first electrode and at least one second electrode of a first electrode set; electrically connecting the at least one first electrode to the at least one second electrode of a second set of electrodes with a non-zero impedance that is substantially less than an impedance between them resulting from stray capacitive coupling; and energizing the power source to apply an AC voltage difference between the at least one first electrode and at least one second electrode of the first set of electrodes to excite the vibrations.

Description:
RELATED APPLICATIONS 
     The present application is a U.S. national application of PCT/IL99/00520, filed Sep. 30, 1999. 
     FIELD OF THE INVENTION 
     The invention relates to piezoelectric motors and in particular to methods for powering piezoelectric motors using resonant circuits. 
     BACKGROUND OF THE INVENTION 
     Generally, a piezoelectric micromotor is driven with a high voltage AC driving circuit that applies an alternating polarity voltage difference between at least one first electrode and at least one second electrode comprised in the piezoelectric micromotor. The frequency of the AC voltage difference applied to the electrodes is close to a desired frequency of vibration of the piezoelectric motor. To assure proper operation of the motor, the power supply is electrically matched to electrical characteristics of the motor so that power is efficiently transmitted to the motor at the desired frequency of vibration. The at least one first electrode, hereinafter referred to as a first “driving electrode”, and at least one second electrode, hereinafter referred to as a second “driving electrode”, define a “driving set” of electrodes of the piezoelectric motor. 
     Often a piezoelectric motor comprises more than one driving set of first and second driving electrodes. Different driving sets of first and second driving electrodes are electrified to excite different desired vibration modes in the piezoelectric motor. Electrodes that are electrified by direct connection to a driving circuit while exciting a desired vibration mode are said to be active electrodes and a driving set to which the electrodes belong is said to be an active driving set. Electrodes that are not electrified by direct connection to the driving circuit while exciting a particular vibration mode and the driving sets to which they belong are said to be passive. Passive electrodes are either floating or grounded. 
     Transmission of power to a desired vibration mode of the piezoelectric motor is generally sensitive to changes in stray capacitance between passive electrodes and ground and changes in capacitance between conducting wires, hereinafter referred to as “driving lines” that connect the driving circuit to the piezoelectric motor. Hereinafter, stray capacitance to ground and capacitance between driving lines are referred to generically as stray capacitance. Changes in stray capacitance generate mismatches between desired resonant vibration frequencies of the motor and frequencies at which power is efficiently transmitted from the driving circuit to the motor. These mismatches can substantially degrade the performance of the piezoelectric motor. 
     In particular changes in stray capacitance are caused by changes in the lengths the driving lines used to connect the driving circuit to the motor. For example, assume that the driving circuit is matched to a resonant frequency of the piezoelectric motor and that the driving circuit is connected to the piezoelectric motor by driving lines two meters long. If it is required to increase the length of the driving lines to six meters, the increased capacitance between the driving lines changes the resonant frequency of the load that the driving circuit drives and generates a mismatch between the driving circuit and the piezoelectric motor. 
     SUMMARY OF THE INVENTION 
     An aspect of some preferred embodiments of the present invention relates to providing a piezoelectric motor whose operation is less susceptible than is the operation of prior art piezoelectric motors to the effects of changes in stray capacitance, and in particular to changes in stray capacitance caused by changes in lengths of driving lines that connect the piezoelectric motor to a driving circuit. 
     In preferred embodiments of the present invention, first and second driving electrodes of a passive driving set of electrodes in the piezoelectric motor are connected in parallel with an impedance substantially smaller than impedance between them resulting from stray capacitive coupling. As a result, mismatches between a frequency at which the driving circuit supplies power to the piezoelectric motor and a desired resonant vibration frequency of the motor caused by changes in stray capacitance, are substantialy moderated. 
     In some preferred embodiments of the present invention the first and second driving electrodes of the passive driving set are connected by a capacitor. The capacitance of the capacitor is preferably substantially larger than the capacitance generated by any stray capacitive coupling of the first and second electrodes. The connected capacitor, hereinafter referred to as a “moderating capacitor”, is preferably permanently connected between the first and second driving electrodes and is connected between them when they are active and when they are passive. The capacitance of a moderating capacitor while preferably substantially larger than the capacitance of any stray capacitive coupling of its driving set of electrodes, is preferably chosen small enough so that sufficient power reaches the piezoelectric motor when the driving set is electrified by an appropriate AC power supply to excite vibrations in the piezoelectric motor. 
     In some preferred embodiments of the present invention the impedance between the first and second driving electrodes is reduced to substantially zero by short-circuiting the electrodes for a non-active driving set. The short-circuit is removed when the electrodes are active and used to excite a desired vibration in the motor. 
     There is therefore provided in accordance with a preferred embodiment of the present invention a method for exciting vibrations in a piezoelectric motor having a plurality of electrode sets, each set comprising at least one first electrode and at least one second electrode between which AC voltages are applied to excite vibrations in the piezoelectric motor, the method comprising; coupling an AC driving circuit to the at least one first electrode and at least one second electrode of a first electrode set; electrically connecting the at least one first electrode to the at least one second electrode of a second set of electrodes with a non-zero impedance that is substantially less than an impedance between them resulting from stray capacitive coupling; and energizing the driving circuit to apply an AC voltage difference between the at least one first electrode and at least one second electrode of the first set of electrodes to excite the vibrations. 
     Preferably eclectically connecting the at least one first electrode to the at least one second electrode of the second set of electrodes comprises connecting them with a first capacitor having a capacitance substantially larger than a capacitance between them resulting from stray capacitive coupling. 
     Preferably connecting a first capacitor comprises closing a switch, which switch is operable to be open or closed to respectively disconnect the first capacitor from the electrodes and connect the first capacitor to the electrodes. 
     Coupling an AC driving circuit to the at least one first electrode and at least one second electrode of the first electrode set preferably comprises opening a switch, which switch is operable to be open or closed to respectively disconnect a second capacitor from between the electrodes and connect the second capacitor between the electrodes. 
     In some preferred embodiments of the present invention the method comprises connecting the at least one first electrode to the at least one second electrode of the first electrode set with a second capacitor having a capacitance substantially larger than a capacitance between them resulting from stray capacitive coupling. 
     Preferably, the first and second capacitors are connected permanently between their respective at least one first and at least one second electrodes. 
     Additionally or alternatively, the first and second capacitors preferably have substantially the same capacitance. 
     There is further provided in accordance with a preferred embodiment of the present invention, a method for exciting vibrations in a piezoelectric motor having a plurality of electrode sets, each set comprising at least one first electrode and at least one second electrode between which AC voltages are applied to excite vibrations in the piezoelectric motor, the method comprising; coupling a resonant AC driving circuit to the at least one first electrode and at least one second electrode of a first electrode set; short-circuiting the at least one first electrode to the at least one second electrode of a second electrode set; and energizing the resonant AC driving circuit to apply an AC voltage difference between the at least one first electrode and at least one second electrode of the first set of electrodes to excite the vibrations. 
     Preferably, short-circuiting the electrodes comprises closing a switch operable to be open and closed to respectively disconnect from between the electrodes and connect between the electrodes a substantially zero impedance. 
     Preferably, coupling a resonant AC driving circuit to the at least one first electrode and at least one second electrode of the first electrode set comprises opening a switch, which switch is operable to be open and closed to respectively disconnect from between the electrodes and connect between them a substantially zero impedance. 
     There is further provided in accordance with a preferred embodiment of the present invention, a piezoelectric motor comprising: a plurality of sets of electrodes, each set comprising at least one first electrode and at least one second electrode between which AC voltages are applied to excite vibration modes in the piezoelectric motor; and a capacitor for each set of electrodes that connects the at least one first electrode to the at least one second electrode of the set of electrodes wherein the capacitor has a capacitance substantially larger than a stray capacitance between the at least one first and at least one second electrode. 
     Preferably the capacitor for each set of electrodes has a capacitance substantially greater than the capacitance between the at least one first electrode and the at least one second electrode of the set of electrodes. 
     Additionally or alternatively, the capacitor for each set of electrodes is preferably permanently connected between the at least one first electrode and at least one second electrode of the set of electrodes. 
     In some preferred embodiments of the present invention., the piezoelectric motor comprises a switch for each set of electrodes operable to be open and closed to respectively disconnect the capacitor from between the electrodes and connect the capacitor between the electrodes. 
     Additionally or alternatively all the capacitors preferably have substantially a same capacitance. 
     There is further provided in accordance with a preferred embodiment of the present invention, a piezoelectric motor comprising: a plurality of sets of electrodes, each set comprising at least one first electrode and at least one second electrode between which AC voltages are applied to excite vibration modes in the piezoelectric motor; and a switch between the first and second at least one electrode, which switch is operable to be open and closed to respectively disconnect from between the electrodes and connect between the electrodes a substantially zero impedance. 
    
    
     BRIEF DESCRIPTION OF FIGURES 
     The invention will be more clearly understood by reference to the following description of preferred embodiments thereof read in conjunction with the figures attached hereto. In the figures, identical structures, elements or parts which appear in more than one figure are generally labeled with the same numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are chosen for convenience and clarity of presentation and are not necessarily shown to scale. The figures are listed below. 
     FIG. 1A schematically shows a piezoelectric motor being driven by an AC driving circuit in accordance with prior art: 
     FIG. 1B is a diagram of the piezoelectric motor and the driving circuit, shown in FIG. 1A, in which the piezoelectric motor is replaced by electrical components that characterize its electrical functioning; 
     FIG. 2A schematically shows a piezoelectric motor being driven by an AC driving circuit, in accordance with a preferred embodiment of the present invention; 
     FIG. 2B is a diagram of the piezoelectric motor and the driving circuit shown in FIG. 2A, in which the piezoelectric motor is replaced by electrical components that characterize its electrical functioning; 
     FIG. 3A schematically shows a piezoelectric motor being driven by a resonant driving circuit, in accordance with prior art; 
     FIG. 3B is a diagram of the piezoelectric motor and the resonant driving circuit shown in FIG. 3A, in which the piezoelectric motor is replaced by electrical components that characterize its electrical functioning; 
     FIG. 4A schematically shows a piezoelectric motor being driven by a resonant driving circuit, in accordance with a preferred embodiment of the present invention; and 
     FIG. 4B shows a diagram of a circuit showing the electrical connections between the piezoelectric motor and the resonant driving circuit shown in FIG. 4A, in which the piezoelectric motor is replaced by electrical components that characterize its electrical functioning. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1A schematically shows a piezoelectric motor  20  being powered by an AC driving circuit  22 , in accordance with the prior art. 
     Piezoelectric motor  20  is of a type described in U.S. Pat. No. 5,453,653, to Zumeris et al, the disclosure which is incorporated herein by reference. Piezoelectric motor  20  comprises a thin rectangular ceramic piezoelectric crystal  24  having a lop planar surface  26  and a bottom planar surface  28 . Bottom planar surface  28  is normally hidden in the perspective of FIG.  1 A and is shown in ghost lines. Four quadrant electrodes  31 ,  32 ,  33  and  34  are located in a symmetric checkerboard pattern on top face surface  26 . A single large electrode  36 , shown in ghost lines, is located on bottom surface  28 . Preferably, a friction nub  40  on an edge surface  42  is used for coupling vibrations of piezoelectric motor  20  to a moveable element. 
     Vibrations in piezoelectric motor  20  are used to generate clockwise and counter clockwise elliptical vibrations in friction nub  40 . These clockwise and counterclockwise elliptical vibrations of friction nub  40  are useable to move a moveable element (not shown), to which friction nub  40  is pressed, in either direction along double arrowhead !in-e  41 . 
     Diagonally opposite quadrant electrodes  31  and  33  are electrically connected together to form an electrode referred to hereinafter as “diagonal driving electrode  31 - 33 ”. Similarly, diagonally opposite electrodes  32  and  34  are connected together to form a diagonal driving electrode  32 - 34 . When an AC voltage is applied between diagonal driving electrode  31 - 33  and large electrode  36 , piezoelectric friction nub  40  vibrates clockwise. When an AC voltage difference is applied between diagonal driving electrode  32 - 34  and large electrode  36 , friction nub  40  vibrates counterclockwise. 
     Diagonal driving electrode  31 - 33  and large electrode  36  are first and second driving electrodes respectively of one driving set of electrodes of piezoelectric motor  20 . Diagonal driving electrode  32 - 34  and large electrode  36  are a second driving set of first and second driving electrodes respectively of piezoelectric motor  20 . Large electrode  36  functions as a second driving electrode for both first and second driving sets of electrodes. When diagonal driving electrode  31 - 33  is active and electrified with respect to large electrode  36 , diagonal driving electrode  32 - 34  (and the driving set comprising diagonal driving electrode  32 - 34  and large electrode  36 ) is passive and either floating or grounded. When diagonal driving electrode  32 - 34  is active, diagonal driving electrode  31 - 33  (and the driving set comprising diagonal driving electrode  31 - 33  and large electrode  36 ) is passive and either floating or grounded. 
     AC driving circuit  22  comprises a power source  21  coupled to piezoelectric motor  20  through a power transformer  23 . An output lead  25  of power transformer  23  is connected to ground. An output lead  27  of power transformer  21  is connected by a switch  44  to one or the other of diagonal driving electrodes  31 - 33  or  32 - 34  via driving lines  46  and  48  respectively. 
     In FIG. 1A, switch  44  is shown, by way of example, connecting output lead  27  to diagonal driving electrode  32 - 34 , which is therefore an active electrode in the configuration of FIG.  1 A. Diagonal driving electrode  31 - 33 , which is a passive electrode, is shown as floating. Stray capacitive couplings that affect operation of piezoelectric motor  20  are represented by “ t stray” capacitors  50 ,  52  and  54 , shown with dashed lines, that have capacitance C 50 , C 52  and C 54  respectively. Capacitance C 50 , C 52  and C 54  are strong functions of the lengths of power lines  46  and  48  as well as of the environment of piezoelectric motor  20 . 
     FIG. 1B is a diagram of a circuit  58  showing the electrical connections between piezoelectric motor  20  and driving circuit  22 , shown in FIG. 1A, in which piezoelectric motor  20  is replaced by a circuit  60 , shown inside an elliptical border  59 . Circuit  60  represents the electrical functioning of piezoelectric motor  20  for coupling energy into a single vibration mode, e.g. a particular longitudinal or transverse vibration mode of the motor, defined by a particular resonant frequency. Circuit  60  is a standard circuit used in the art to represent coupling energy into a vibration mode of a piezoelectric motor. Circuits that model the electromechanical functioning of piezoelectric motors are discussed, for example in “Fundamentals of Piezoelectricity” by Takuro Ikeda, Oxford University Press  1990  which is incorporated herein by reference. 
     In circuit  60 , capacitance between each of diagonal driving electrodes  31 - 33  and  32 - 34  and large electrode  36  is explicitly shown. The set of driving electrodes comprising diagonal driving electrode  31 - 33  and large electrode  36  is shown as a capacitor  61 . The set of driving electrodes comprising diagonal driving electrode  32 - 34  and large electrode  36  is shown as a capacitor  62 . Both capacitors have a same capacitance, “C D ”, commonly referred to as a “damping capacitance”. Capacitors  61  and  62  are coupled to an RLC sub-circuit  64  by transformers  65  and  66  respectively. RLC sub-circuit  64  comprises a resistor “R”, capacitor “C” and inductor “L” in series. As is well known in the art, kinetic energy of piezoelectric motor  20  is represented by L 1   2  and internal energy loss in the piezoelectric motor is represented by RI 2 , where I represents current in sub-circuit  64 . Voltage across capacitor C represents elastic potential energy of piezoelectric motor  20 . Transformers  65  and  66  have primary windings  67  and  68  respectively that are connected to capacitors  61  and  62  and secondary windings  69  and  70  respectively that are connected in series with the components of RLC sub-circuit  64 . The ratio of secondary to primary windings in each transformer  65  and  66  is “n”. The resonant frequency of RLC circuit  64  is substantially the same as the frequency of the desired vibration mode. 
     In the configuration shown in FIG. 1B the impedance loading driving circuit  22  is the impedance seen at primary winding  68  of transformer  66  in parallel with the impedance of series connected stray capacitors  54  and  52 , the impedance of stray capacitor  50  and the impedance of capacitor  62 . Let the impedance seen at primary winding  68  be represented by “Z 0 ”. Energy is most efficiently coupled from driving circuit  22  to the particular vibration mode of piezoelectric motor  20  that is represented by circuit  60  at a frequency for which Z 0  is real. i.e. at the resonant frequency of Z 0 . 
     Z 0  is equal to Z 1 /n 2  where Z 1  is the impedance across secondary winding  70  of transformer  66 . Z 1  is determined by R, L and C in series with the impedance seen at secondary winding  69  of transformer  63 . This impedance is equal to n 2  times the impedance across primary winding  67  of transformer  65 , which is substantially equal to n 2 /iω(C D +C 52 ), where “i” is the imaginary “i” and ω is a frequency of a voltage across secondary winding  69 . In other words, Z 1  is determined by R, L and C in series with a capacitor that replaces transformer  65  in sub-circuit  64  and that has a capacitance (ignoring effects of stray capacitor  54 ) substantially equal to (C D +C 52 )/n 2 . The resonant frequency of Z 1 , and therefore of Z 0  is thus a function of C 52 . As a result, a change in C 52 , such as a change caused by a change in lengths of driving lines  46  and/or  48 , changes the resonant electrical frequency of piezoelectric motor  20  and shifts the resonant electrical frequency away from a desired vibration frequency of the motor. A shift away from the resonant desired frequency degrades the performance of the piezoelectric motor. 
     FIG. 2A schematically shows piezoelectric motor  20  being driven by driving circuit  22  in accordance with a preferred embodiment of the present invention. As in FIG. 1A, in FIG. 2A driving circuit  22  is shown, by way of example, connected to diagonal driving electrode  32 - 34  through switch  44 . 
     In accordance with a preferred embodiment of the present invention diagonal driving electrode  31 - 32  is coupled to large electrode  36  by a moderating capacitor  71 . Similarly, diagonal driving electrode  32 - 34  is connected to large electrode  36  by a moderating capacitor  72 . Each time switch  44  switches driving circuit  22  to one or the other of diagonal driving electrodes  31 - 32  and  32 - 34  the driving circuit is also connected to the moderating capacitor that connects the respective diagonal driving electrode to large electrode  36 . Preferably, the capacitance of moderating capacitors  71  and  72  are equal. Preferably, the capacitance of moderating capacitors  71  and  72  is substantially larger than the capacitance of stray capacitors coupled to driving electrodes  31 - 33  and  32 - 34  and large electrode  36 . Let the capacitance of moderating capacitors  71  and  72  be represented by C M . 
     FIG. 2B shows a diagram of a circuit  74  showing the electrical connections between piezoelectric motor  20  and driving circuit  22 , shown in FIG. 2A, in which piezoelectric motor  20  is replaced by circuit  60  comprising sub-circuit  64 . 
     As in FIG. 1B, in FIG. 2B capacitance between diagonal driving electrodes  31 - 33  and  32 - 34  and large electrode  36  is represented by capacitors  61  and  62  respectively. Moderating capacitors  71  and  72  are in parallel respectively with capacitors  61  and  62 . The effect of moderating capacitors  71  and  72  is therefore to increase the effective capacitance and therefore decrease the impedance between diagonal driving electrodes  31 - 33  and  32 - 34  respectively, and large electrode  36 . 
     Circuit  74  is analyzed similarly to prior art circuit  58 . The resonant frequency for transmission of energy to piezoelectric motor  20  is the resonant frequency of sub-circuit  64  with transformer  65  replaced by a capacitor having a capacitance (C D +C M +C 52 )/n 2 . The impedance of this capacitor is n 2 /iω(C D +C M +C 52 ), which for values of CM substantially larger than C 52 , is relatively independent of changes in C 52 . Typically C 52 , other stray capacitance and C D , have values of about a nanofarad (nf). Preferably, C M  has a value equal to about 5 nf. Typically, n is on the order of  1 - 10 . 
     The effect of connecting moderating capacitor  71  across the primary winding  67 , in accordance with a preferred embodiment of the present invention, is therefore to replace transformer  65  in sub-circuit  67  with a capacitive “replacement” impedance that is substantially independent of capacitance C D  and C 52 . Furthermore, this “replacement” impedance is much smaller than the impedance of capacitor C in sub-circuit  64 . Capacitor C typically has a value of about 10 picofarads and for ω=50,000 typical impedance having a magnitude equal to 2×10 6  ohms. On the other hand, assuming that n is 3, ω=50,000, that C D  =C 52 = 1  nf and that C M =5 nf, the replacement impedance has a magnitude of about 2.5×10 4  ohms. As a result, the resonant frequency of sub-circuit  64  is determined substantially by components R, L and C of sub-circuit  64  and C M . The resonant frequency is relatively stable and substantially independent of changes in C 52 . 
     In addition, moderating capacitor  71  effectively grounds stray capacitor  54 . As a result, stray capacitor  54  is connected across power leads  25  and  27  of driving circuit  22  and is in parallel with the impedance piezoelectric motor  20 . While stray capacitor  54  drains current from driving circuit  22  it does not directly affect the resonant frequency and quality of operation of piezoelectric motor  20 . 
     Therefore, as a result of the addition of moderating capacitors  71  and  72 , in accordance with a preferred embodiment of the present invention, the performance of piezoelectric motor  20  is relatively immune to changes in stray capacitance. Circuit  74 , in accordance with a preferred embodiment of the present invention, provides a more reliable and predictable operation of piezoelectric motor  20  than does prior art circuit  58 . Changes in the environment of piezoelectric motor  20  or in the lengths of driving lines that connect the motor to driving circuit  20  do not significantly change or disrupt the performance of the motor. 
     It should be noted, that whereas moderating capacitors  71  and  72  are shown permanently connected between large electrode  36  and diagonal driving electrodes  31 - 33  and  32 - 34  respectively, in some preferred embodiments of the present invention they may be connected to their respective electrodes via switches. Moderating capacitors  71  and  72  are disconnected and connected between their respective electrodes as needed by opening and closing the switches. In some preferred embodiments of the present invention a single moderating capacitor is used. The single moderating capacitor is switched so as to connect large electrode  36  to that diagonal driving electrode of diagonal driving electrodes  31 - 33  or  32 - 34  which is passive. 
     In some configurations, piezoelectric motors are driven with resonant circuits. As in the case of driving a piezoelectric motor using an AC power supply and transformer according to prior art, changes in stray capacitance adversely affect the performance of a piezoelectric motor driven by a resonant circuit in accordance with prior art. FIG. 3 schematically shows piezoelectric motor  20  being driven with a resonant driving circuit  80  according to prior art. 
     Resonant driving circuit  80  comprises an AC power supply  21  having a grounded lead  82  and a power lead  84 , an inductor  86  and a capacitor  88 . Inductor  86 , connects grounded lead  82  to large electrode  36 . Power lead  84  of AC power supply  21  is connected through switch  44  to either diagonal driving electrode  31 - 33  or diagonal driving electrode  32 - 34 . Capacitor  88  connects nodes  90  and  92  together. The magnitudes of the inductance of inductor  86  and the capacitance of capacitor  88  are determined so that resonant circuit  80  and piezoelectric motor  20  are matched to form a circuit having a resonant frequency near to a desired resonant vibration frequency of piezoelectric motor  20 . In FIG. 3A, as in FIGS. 1A-2B, switch  44  is shown by way of example connecting power supply  22  to diagonal driving electrode  32 - 34 . Diagonal driving electrode  31 - 33  is floating. 
     The performance of piezoelectric motor  20  driven by resonant driving circuit  80  is affected by the same stray capacitance, represented by capacitors  50 ,  52  and  54 , as is the performance of piezoelectric motor  20  driven by AC driving circuit  22  shown in FIGS. 1A-2B. In addition the performance of piezoelectric motor  20  driven by resonant driving circuit  80  is also affected by stray capacitance represented by capacitors  94  and  96 . 
     FIG. 3B shows a diagram of a circuit  100  showing the electrical connections between piezoelectric motor  20  and resonant driving circuit  80 , shown in FIG. 3A, in which piezoelectric motor  20  is replaced by circuit  60  comprising sub-circuit  64 . From circuit  100  it is seen that the resonant frequency and quality of operation of piezoelectric motor  20  are sensitive to changes in the capacitance of stray capacitors  52 ,  54 ,  94  and  96 . Stray capacitor  50  is in parallel with the impedance piezoelectric motor  20  and while it drains current from power supply  21  it does not directly affect the resonant frequency and quality of operation of piezoelectric motor  20 . 
     In some preferred embodiments of the present invention, the affects of stray capacitance on the operation of piezoelectric motor  20  are reduced by coupling diagonal driving electrodes  31 - 32  and  32 - 34  to large electrode  36  by moderating capacitors. In the same way that moderating capacitors reduce the effects of stray capacitance on the performance of piezoelectric motor  20  when it is driven by driving circuit  22  shown in FIG. 2A, they protect the performance of piezoelectric motor  20  driven by resonant circuit  80 . 
     In some preferred embodiments of the present invention, each diagonal driving electrode  31 - 32  and  32 - 34  is connected by a switch, which when the driving electrode is passive, short-circuits the driving electrode to large electrode  36 . Short-circuiting the passive electrode to large driving electrode  36  effectively buffers the operation of piezoelectric motor  20  against changes in stray capacitance. 
     FIG. 4A schematically shows piezoelectric motor  20  being driven by driving circuit  80  in accordance with a preferred embodiment of the present invention, in which diagonal driving electrodes  31 - 33  and  32 - 34  are respectively connected to large electrode  36  by switches  101  and  102 . As in FIGS. 1A and 2A driving circuit  22  is shown, by way of example, connected to diagonal driving electrode  32 - 34  by switch  44 . Switch  101  is closed and short-circuits passive diagonal driving electrode  31 - 32  and large electrode  36 . Switch  102  is open and active driving electrode  32 - 34  and large electrode  36  are connected by capacitor  88  of driving circuit  80 . 
     FIG. 4B shows a diagram of a circuit  110 , in accordance with a preferred embodiment of the present invention, showing the electrical connections between piezoelectric motor  20  and resonant driving circuit  80 , shown in FIG.  4 A. In circuit  110  piezoelectric motor  20  is replaced by circuit  60  comprising sub-circuit  64 . Diagonal driving electrodes  31 - 33  and  32 - 34  are connected by switches  101  and  102  respectively to large electrode  36 . 
     Because passive capacitor  61  is short-circuited by switch  101 , there is never a potential difference between diagonal electrode  31 - 33  and large electrode  36  (as long as switch  101  is closed). In sub-circuit  64  transformer  65  is replaced by a short-circuit and the resonant frequency of sub-circuit  64  is therefore substantially unaffected by stray capacitance. Furthermore, capacitive coupling of diagonal electrode  31 - 33  to ground does not generate unpredictable voltage differences between diagonal electrode  31 - 33  and large electrode  36 . As a result, passive diagonal electrodes  31 - 33  cannot excite vibration modes in piezoelectric motor  20  that may unpredictably disrupt desired vibration modes excited by active driving electrodes  32 - 34  in the piezoelectric motor. 
     Whereas a switching system for neutralizing stray capacitance that short circuits passive driving electrodes has been described for a piezoelectric motor driven by a resonant driving circuit, such switching systems are not limited to piezoelectric micromotors driven by resonant driving circuits. Similar switching systems are applicable, in accordance with preferred embodiments of the present invention, to piezoelectric motors driven by other types of driving circuits. For example, diagonal electrodes  31 - 33  and  32 - 34  in piezoelectric motor  20  driven by driving circuit  22  as shown in FIG. 1A, may be connected to large electrode  36  by “shorting,” switches in the same way that they are connected to large electrode  36  in FIG.  4 A. 
     In the description and claims of the present application, each of the verbs, “comprise” “include” and “have”, and conjugates thereof, are used to indicate that the object or objects of the verb are not necessarily a complete listing of components, elements or parts of the subject or subjects of the verb. 
     The present invention has been described using detailed descriptions of preferred embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. The described preferred embodiments comprise different features, not all of which are required in all embodiments of the invention. Some embodiments of the present invention utilize only some of the features or possible combinations of the features. Variations of embodiments of the present invention that are described and embodiments of the present invention comprising different combinations of features noted in the described embodiments will occur to persons of the art. The scope of the invention is limited only by the following claims.