Patent Publication Number: US-2004046484-A1

Title: Interface electronics for piezoelectric devices

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S)  
     [0001] The present utility patent application claims priority of U.S. Provisional Patent Application, Serial No. 60/408,177 filed Sep. 4, 2002, subject matter of which is incorporated herewith by reference. 
    
    
     
       FIELD OF THE INVENTION  
       [0002] The present invention relates generally to a piezoelectric device and method, and more particularly, to methods and interface electronic circuits to optimize performance and reliability of a piezoelectric device.  
       BACKGROUND OF THE INVENTION  
       [0003] Piezoelectric materials are used in a variety of sensors and actuators. Piezoelectric materials convert mechanical energy to electrical energy and vice versa. For instance, if pressure is applied to a piezoelectric crystal, an electrical signal is generated in proportion thereby producing the function of a sensor. Generation of an electrical signal in response to an applied force or pressure is known as the “primary piezoelectric effect”. Similarly, if an electrical signal is applied to a piezoelectric crystal, it will expand in proportion as an actuator. Geometric deformation (expansion or contraction) in response to an applied electric signal is known as the “secondary piezoelectric effect”. Whether operated as a sensor or actuator, electrically conductive electrodes are appropriately placed on the piezoelectric crystal for collection or application of the electrical signal, respectively. Therefore, a piezoelectric sensor or actuator nominally includes a) a portion of piezoelectric material, and b) electrically-conductive electrodes suitably arranged to transfer electrical energy between the piezoelectric material and an electronic circuit.  
       [0004] Many piezoelectric materials undergo a process called “poling” to align the molecules and activate the piezoelectric properties. Poling generally involves the application of a large electric field, sometimes accompanied by high temperatures. After poling, a piezoelectric material can be used according to the primary or secondary piezoelectric effects to build sensors or actuators.  
       [0005] Piezoelectric materials have been utilized to create a variety of simple sensors and actuators. Examples of sensors include vibration sensors, microphones, and ultrasonic sensors. Examples of actuators include ultrasonic transmitters and linear positioning devices. However, in most of these examples, bulk piezoelectric material is machined and assembled in a coarse manner to achieve low-complexity devices.  
       [0006] The properties of piezoelectric materials can degrade over time through a variety of mechanisms. Piezoelectric materials have a tendency to “fatigue”. Fatigue occurs with repeated mechanical loading, during which the molecular alignment established at poling begins to degrade. Secondly, piezoelectric materials also “age”. Aging is a natural relaxation of the molecular alignment that occurs over time in the piezoelectric material, further degrading the piezoelectric properties. Lastly, “depoling” can occur in the piezoelectric material if an electric field is applied opposite to the original poling field. As the name implies, a depoling field tends to partially reverse the direction of molecular alignment, degrading the piezoelectric properties. It is very costly or impossible to perform the poling process a second time on bulk piezoelectric materials. Therefore, applications using sensors and actuators constructed from bulk piezoelectric materials require stringent limitations to prevent or reduce the extent of piezoelectric property degradation.  
       [0007] Unlike conventional bulk piezoelectric materials, thin film piezoelectrics can be integrated in a semiconductor process to create sensors with improved functionality and performance. Thin films of piezoelectric material are typically less than about 10 microns in thickness. Despite the advantages of thin film piezoelectrics, the effects of fatigue, aging, and depoling can also occur. In some cases, the degradation mechanisms may even be exaggerated with thin film piezoelectric materials.  
       [0008] Therefore, there is a need for specialized electronics and methods for interfacing with piezoelectric devices, particularly in thin film format.  
       SUMMARY OF THE INVENTION  
       [0009] To solve the above and the other problems, the present invention provides a method of improving properties of a piezoelectric device by applying a suitable electrical field bias during operation. The present invention further provides a specific electronic circuitry that simultaneously applies an electrical field bias to a piezoelectric sensor while extracting electrical signals created by the piezoelectric sensor. The present invention further provides a specific electronic circuitry that simultaneously applies an electrical field bias to a piezoelectric actuator while applying actuation electrical signals to the actuator. Application of an electric field bias to the piezoelectric creates a static electric field within the piezoelectric material. The static electric field is oriented to reinforce a polarization direction within the piezoelectric material to reduce effects of fatigue, aging, and depoling while increasing electromechanical efficiency. The present invention thereby improves overall performance and reliability of piezoelectric devices.  
       [0010] Applying an electrical field bias to the piezoelectric device offers the following key advantages:  
       [0011] Response—Application of an electric field bias maximizes the molecular polarization within the piezoelectric, thereby maximizing the piezoelectric effect exhibited by the material and the electromechanical efficiency.  
       [0012] Reliability—With a suitably applied electrical field bias, properties of a piezoelectric device are more stable over long time periods. Effects of fatigue, aging, and depoling are reduced.  
       [0013] Accuracy—Application of a voltage bias maintains the polarization level within the piezoelectric, preventing the degradation that naturally occurs over time in piezoelectric materials. Response of the piezoelectric is thereby more stable over time and over a wider range of temperatures.  
       [0014] Cost—Conventional piezoelectric devices must be frequently recalibrated as their properties degrade over time. By reducing the degradation, recalibration is not required. The time and expense of sending parts out for calibration can be reduced or eliminated.  
       [0015] The above advantages are inherent to the present invention and enable novel configurations and unique features that increase the overall device and system performance.  
       [0016] These and other features and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, wherein it is shown and described illustrative embodiments of the invention, including best modes contemplated for carrying out the invention. As it will be realized, the invention is capable of modifications in various obvious aspects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not restrictive. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0017]FIG. 1 is a circuit schematic diagram of a conventional arrangement of a piezoelectric sensor.  
     [0018]FIG. 2 is a circuit schematic diagram of one embodiment of a single-ended piezoelectric sensor in accordance with the principles of the present invention.  
     [0019]FIG. 3 is a circuit schematic diagram of another embodiment of a single-ended piezoelectric sensor in accordance with the principles of the present invention.  
     [0020]FIG. 4 is a circuit schematic diagram of one embodiment of a differential piezoelectric sensor in accordance with the principles of the present invention.  
     [0021]FIG. 5 is a circuit schematic diagram of a preferred embodiment of a single-ended piezoelectric sensor in accordance with the principles of the present invention.  
     [0022]FIG. 6 is a circuit schematic diagram of another preferred embodiment of a singleended piezoelectric sensor in accordance with the principles of the present invention.  
     [0023]FIG. 7 is a circuit schematic diagram of yet another preferred embodiment of a singleended piezoelectric sensor in accordance with the principles of the present invention.  
     [0024]FIG. 8 is a circuit schematic diagram of still another preferred embodiment of a singleended piezoelectric sensor in accordance with the principles of the present invention.  
     [0025]FIG. 9 is a circuit schematic diagram of a preferred embodiment of a differential piezoelectric sensor in accordance with the principles of the present invention.  
     [0026]FIG. 10 is a circuit schematic diagram of a conventional arrangement of a piezoelectric actuator device.  
     [0027]FIG. 11 is a circuit schematic diagram of one embodiment of a piezoelectric actuator in accordance with the principles of the present invention.  
     [0028]FIG. 12 is a circuit schematic diagram of another embodiment of a piezoelectric actuator in accordance with the principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0029] The present invention provides a method for improving the performance of sensors and actuators that use piezoelectric materials, wherein an electric field bias is applied to the piezoelectric material. The present invention also provides a plurality of circuit embodiments for applying the electric field bias while simultaneously retaining the desired operation of a piezoelectric device.  
     [0030] A conventional arrangement for piezoelectric sensors is shown in FIG. 1 and involves a direct connection between a piezoelectric sensor element  1 ′ and an electronic signal amplifier  3 ′. The piezoelectric sensor element  1 ′ generates a first piezoelectric signal in response to physical stimulus such as force, acceleration, or pressure. The first piezoelectric signal is connected between circuit terminals  5 ′ and  7 ′. The circuit terminals  5 ′ and  7 ′ are also connected to input terminals of the electronic signal amplifier  3 ′ that generates an electronic signal output  9 ′. The electronic signal amplifier  3 ′ may generate the electronic signal output  9 ′ in proportion to the voltage difference at the inputs connected to the circuit terminals  5 ′ and  7 ′. That is, the electronic signal output  9 ′ may be proportional to the first piezoelectric signal.  
     [0031]FIG. 2 illustrates one embodiment of a single-ended piezoelectric sensor in accordance with the principles of the present invention. In FIG. 2, a piezoelectric sensor element  1  is connected to an electronic signal amplifier  3  and further connected to an electric field bias generator  11 . Similar to the arrangement of FIG. 1, the piezoelectric sensor element  1  generates a first piezoelectric signal in response to physical stimulus such as force, acceleration, or pressure and the electronic signal amplifier  3  generates an electronic signal output  9  in proportion. In accordance with the present invention, the electric field bias generator  11  applies an electric field bias signal to the piezoelectric sensor element  1  during normal operation to reduce the effects of fatigue, aging, and depoling while increasing the electromechanical efficiency.  
     [0032] Another embodiment of the single-ended piezoelectric sensor in accordance with the principles of the present invention is shown in FIG. 3. In FIG. 3, a piezoelectric sensor element  25  is distributed into equivalent circuit elements comprising an equivalent piezoelectric sensor capacitance  15  and a piezoelectric signal  17 . The piezoelectric signal  17  is generated by the piezoelectric sensor element  25  in response to physical stimulus such as force, acceleration, or pressure. The electric field bias generator in FIG. 3 is accomplished by connecting the piezoelectric sensor element  25  to a voltage bias  19  through a bias resistor  21 . The result is that over a long period of time the average voltage bias at a first circuit terminal  13  is equal to the voltage bias  19 . Over shorter periods of time, the voltage bias at the first circuit terminal  13  is equal to the piezoelectric signal  17  generated by the piezoelectric sensor element  25 .  
     [0033] Also shown in FIG. 3, the voltage bias  19  is also connected to a second circuit terminal  23 . The inputs of electronic signal amplifier  3  are connected to the first circuit terminal  13  and the second circuit terminal  23 . The electronic signal amplifier  3  generates the electrical signal output  9  in proportion to the voltage difference between the first circuit terminal  13  and the second circuit terminal  23 .  
     [0034] In FIG. 3, the value of the bias resistor  21  is set depending on the application requirements. For example, if the application requires that physical stimulus with a frequency greater than 1000 Hz (1000 cycles per second) appear at the electrical signal output  9  and the piezoelectric sensor capacitance  15  is 330 pico-Farads, then the bias resistor  21  will be selected to be greater than about 480,000 ohms. If the required frequency response is lower, then the bias resistor  21  must be greater, and if the required frequency response is higher, then the bias resistor  21  can be smaller. In summary, the value of the bias resistor  21  in conjunction with the piezoelectric sensor capacitance  15  determines the frequency range over which the electrical signal output  9  is proportional to the electrical signal  17 .  
     [0035] One embodiment of a differential piezoelectric sensor in accordance with the principles of the present invention is shown in FIG. 4. In FIG. 4, a first piezoelectric sensor element  25  is distributed into equivalent circuit elements comprising an equivalent piezoelectric sensor capacitance  15  and a first piezoelectric signal  17 . A second piezoelectric sensor element  27  is distributed into equivalent circuit elements comprising an equivalent piezoelectric sensor capacitance  35  and a second piezoelectric signal  37 . The first piezoelectric signal  17  is generated by the first piezoelectric sensor element  25  in response to physical stimulus such as force, acceleration, or pressure. Similarly, the second piezoelectric signal  37  is generated by the second piezoelectric element  27  in response to physical stimulus such as force, acceleration, or pressure. The electric field bias generator in FIG. 4 is accomplished by connecting the first and second piezoelectric sensor elements  25  and  27  to a voltage bias  19  through bias resistors  31  and  33 . The result is that over a long period of time the average voltage bias at a first circuit terminal  13  and a second circuit terminal  29  is equal to the voltage bias  19 . Over shorter periods of time, the voltage bias at the first circuit terminal  13  is equal to the first piezoelectric signal  17  generated by the first piezoelectric sensor element  25 . Similarly, over shorter periods of time, the voltage bias at the second circuit terminal  29  is equal to the second piezoelectric signal  37  generated by the second piezoelectric sensor element  27 . The inputs of electronic signal amplifier  3  are connected to the first circuit terminal  13  and the second circuit terminal  29 . The electronic signal amplifier  3  generates an electrical signal output  9  in proportion to the voltage difference between first circuit terminal  13  and second circuit terminal  29 .  
     [0036] In FIG. 4, the values of the bias resistors  31  and  33  are set depending on the application requirements. For example, if the application requires that physical stimulus with a frequency greater than 1000 Hz (1000 cycles per second) appear at the electrical signal output  9  and the piezoelectric sensor capacitances  15  and  35  are each 330 pico-Farads, then the bias resistors  31  and  33  will each be selected to be greater than about 480,000 ohms. If the required frequency response is lower, then the bias resistors  31  and  33  must be greater than about 480,000 ohms; and if the required frequency response is higher, then the bias resistors  31  and  33  can be smaller than about 480,000 ohms. In summary, the values of the bias resistors  31  and  33  in conjunction with the piezoelectric sensor capacitances  15  and  35  determine the frequency range over which the electrical signal output  9  is proportional to the difference between the first piezoelectric signal  17  and the second piezoelectric signal  37 .  
     [0037] A preferred embodiment of the present invention for a single-ended piezoelectric sensor is shown in FIG. 5. In FIG. 5, the piezoelectric sensor element  25  is distributed into equivalent circuit elements comprising the piezoelectric sensor capacitance  15  and the piezoelectric signal  17 . The piezoelectric signal  17  is generated by the piezoelectric sensor element  25  in response to physical stimulus such as force, acceleration, or pressure. The electric field bias generator in FIG. 5 is accomplished by connecting the piezoelectric sensor element  25  to the input of an operational amplifier  43  and a feedback network comprising a feedback capacitor  47  and a feedback resistor  45 . The voltage bias  19  is connected to a second input of the operational amplifier  43 . During a normal operation, the two inputs of the operational amplifier  43  at circuit terminals  39  and  41  are at approximately the same voltage level. It is appreciated that details of the operational amplifier  43  are understood by a person skilled in the field. The operational amplifier  43  in conjunction with the feedback capacitor  47  and the feedback resistor  45  create a charge amplifier that generates an electrical signal output  49  in proportion to the piezoelectric signal  17 . The constant of proportionality between the electrical signal output  49  and the piezoelectric signal  17  is approximately equal to the ratio of the values of the equivalent piezoelectric sensor capacitance  15  and the feedback capacitor  47 . The frequency response of the charge amplifier is determined by the pole of the feedback capacitor  47  and the feedback resistor  45 . Over a long period of time the average electrical signal output  49  is equal to the voltage bias  19 . Over shorter periods of time, the electrical signal output  49  is proportional to the piezoelectric signal  17  generated by the piezoelectric sensor element  25 .  
     [0038] Another preferred embodiment of the present invention for a single-ended piezoelectric sensor is shown in FIG. 6. In FIG. 6, the piezoelectric sensor element  25  is distributed into equivalent circuit elements comprising the piezoelectric sensor capacitance  15  and the first piezoelectric signal  17 . The piezoelectric signal  17  is generated by the piezoelectric sensor element  25  in response to physical stimulus such as force, acceleration, or pressure. The electric field bias generator in FIG. 6 is accomplished by connecting the piezoelectric sensor element  25  at a circuit terminal  41  to the input of the operational amplifier  43  and the feedback network comprising the feedback capacitor  47  and the feedback resistor  45 . The piezoelectric sensor element  25  is further connected to a voltage bias  19  while a second input of the operational amplifier  43  is connected to a reference voltage at the circuit terminal  7 . During a normal operation, the two inputs of the operational amplifier  43  at circuit terminals  7  and  41  are at approximately the same voltage level. It is appreciated that details of the operational amplifier  43  are understood by a person skilled in the field. The operational amplifier  43  in conjunction with the feedback capacitor  47  and the feedback resistor  45  create a charge amplifier that generates the electrical signal output  49  in proportion to the piezoelectric signal  17 . The constant of proportionality between the electrical signal output  49  and piezoelectric signal  17  is approximately equal to the ratio of the values of the equivalent piezoelectric sensor capacitance  15  and the feedback capacitor  47 . The frequency response of the charge amplifier is determined by the pole of the feedback capacitor  47  and the feedback resistor  45 . Over a long period of time the average electrical output signal  49  is equal to the reference voltage at the circuit terminal  7 . Over shorter periods of time, the electrical signal output  49  is proportional to the piezoelectric signal  17  generated by the piezoelectric sensor element  25 .  
     [0039] There are many ways to implement signal amplifiers in consistent with the circuit arrangement shown in FIG. 6. Still another preferred embodiment of the present invention for a single-ended piezoelectric sensor is shown in FIG. 7. In FIG. 7, the feedback resistor  45  in FIG. 6 is replaced with a field effect transistor  51 . In FIG. 7, the piezoelectric sensor element  25  is distributed into equivalent circuit elements comprising the piezoelectric sensor capacitance  15  and the piezoelectric signal  17 . The piezoelectric signal  17  is generated by the piezoelectric sensor element  25  in response to physical stimulus such as force, acceleration, or pressure. The electric field bias generator in FIG. 7 is accomplished by connecting the piezoelectric sensor element  25  at a circuit terminal  41  to the input of the operational amplifier  43  and the feedback network comprising the feedback capacitor  47  and the feedback transistor  51 . The piezoelectric sensor element  25  is further connected to the voltage bias  19  while the second input of the operational amplifier  43  is connected to a reference voltage at circuit terminal  7 . During a normal operation, the two inputs of the operational amplifier  43  at the circuit terminals  7  and  41  are at approximately the same voltage level. It is appreciated that details of the operational amplifier  43  are understood by a person skilled in the field. The operational amplifier  43  in conjunction with the feedback capacitor  47  and the feedback transistor  51  create a charge amplifier that generates the electrical signal output  49  in proportion to the piezoelectric signal  17 . The constant of proportionality between the electrical signal output  49  and the piezoelectric signal  17  is approximately equal to the ratio of the values of the equivalent piezoelectric sensor capacitance  15  and the feedback capacitor  47 . The frequency response of the charge amplifier is determined by the pole of the feedback capacitor  47  and the feedback transistor  51 . Over a long period of time the average electrical signal output  49  is equal to the reference voltage at the circuit terminal  7 . Over shorter periods of time, the electrical signal output  49  is proportional to the piezoelectric signal  17  generated by the piezoelectric sensor element  25 .  
     [0040] In FIG. 7, the feedback transistor  51  is selected and operated with an equivalent resistance value that achieves the desired frequency response. The voltage at a control terminal  57  on the feedback transistor  51  sets the equivalent resistance. One method for generating the voltage at the control terminal  57  is to force an electrical current  55  to flow through a bias transistor  53  from a voltage supply  61 . The bias transistor  53  is configured with its control terminal  59  connected to the control terminal  57  of the feedback transistor  51 .  
     [0041] Still another preferred embodiment of the present invention for a single-ended piezoelectric sensor is shown in FIG. 8. In FIG. 8, the feedback resistor  45  of FIG. 6 is replaced with a switched capacitor network comprising a switched capacitor  63  and switches  65 ,  67 ,  69 , and  71 . In FIG. 8, the piezoelectric sensor element  25  is distributed into equivalent circuit elements comprising the piezoelectric sensor capacitance  15  and the piezoelectric signal  17 . The piezoelectric signal  17  is generated by the piezoelectric sensor element  25  in response to physical stimulus such as force, acceleration, or pressure. The electric field bias generator in FIG. 8 is accomplished by connecting the piezoelectric sensor element  25  at the circuit terminal  41  to the input of the operational amplifier  43  and the feedback network comprising the feedback capacitor  47  and the switched capacitor network  63 ,  65 ,  67 ,  69 , and  71 . The piezoelectric sensor element  25  is further connected to the voltage bias  19  while the second input of the operational amplifier  43  is connected to the reference voltage at the circuit terminal  7 . During a normal operation, the two inputs of the operational amplifier  43  at the circuit terminals  7  and  41  are at approximately the same voltage level. It is appreciated that details of the operational amplifier  43  are understood by a person skilled in the field. The operational amplifier  43  in conjunction with the feedback capacitor  47  and the switched capacitor network  63 ,  65 ,  67 ,  69 , and  71  create a charge amplifier that generates the electrical signal output  49  in proportion to the piezoelectric signal  17 . The constant of proportionality between the electrical signal output  49  and the piezoelectric signal  17  is approximately equal to the ratio of the values of the equivalent piezoelectric sensor capacitance  15  and the feedback capacitor  47 . The frequency response of the charge amplifier is determined by the pole of the feedback capacitor  47  and the switched capacitor network  63 ,  65 ,  67 ,  69 , and  71 . Over a long period of time the average electrical signal output  49  is equal to the reference voltage at the circuit terminal  7 . Over shorter periods of time, the electrical signal output  49  is proportional to the piezoelectric signal  17  generated by the piezoelectric sensor element  25 .  
     [0042] In FIG. 8, the switched capacitor network  63 ,  65 ,  67 ,  69 , and  71  is selected and operated with an equivalent resistance value that achieves the desired frequency response. The equivalent resistance of the switched capacitor network is determined by the value of the switched capacitor  63  and the frequency at which the switches are operated. It is appreciated that the details of switched capacitor networks are understood by a person skilled in the field and there are several known ways to achieve an equivalent resistor. In FIG. 8, the switches  65  and  67  operate in unison while the switches  69  and  71  operate in unison. When the switches  65  and  67  are open, the switches  69  and  71  are closed. When the switches  65  and  67  are closed, the switches  69  and  71  are open. Opening and closing of the switches  65 ,  67 ,  69 , and  71  cycles continuously at a clock frequency. The equivalent resistance of the switched capacitor network  63 ,  65 ,  67 ,  69 , and  71  is determined by the clock frequency and the value of the switched capacitor  63 . Higher equivalent resistance is achieved with lower values for the switched capacitor  63  or lower clock frequency. Lower equivalent resistance is achieved with higher values for the switched capacitor  63  or higher clock frequency.  
     [0043] A preferred embodiment of the present invention for a differential piezoelectric sensor is shown in FIG. 9. In FIG. 9, a first piezoelectric sensor element  25  is distributed into equivalent circuit elements comprising an equivalent piezoelectric sensor capacitance  15  and the first piezoelectric signal  17 . A second piezoelectric sensor element  27  is distributed into equivalent circuit elements comprising an equivalent piezoelectric sensor capacitance  35  and a second piezoelectric signal  37 . The first piezoelectric signal  17  is generated by the first piezoelectric sensor element  25  in response to physical stimulus such as force, acceleration, or pressure. The second piezoelectric signal  37  is generated by the second piezoelectric element  27  in response to physical stimulus such as force, acceleration, or pressure. The electric field bias generator in FIG. 9 is accomplished by connecting the piezoelectric sensor elements  25  and  27  at circuit terminals  40  and  42  to the inputs of a differential operational amplifier  44 . The differential operational amplifier  44  has an inverting input at the circuit terminal  40 , a noninverting input at the circuit terminal  42 , a non-inverting output at a circuit terminal  50 , and an inverting output at a circuit terminal  52 . A first feedback network comprising a first feedback capacitor  47  and a first feedback resistor  45  is connected between the circuit terminals  50  and  40 . A second feedback network comprising a second feedback capacitor  48  and a second feedback resistor  46  is connected between the circuit terminals  52  and  42 . It is appreciated that the differential operational amplifier  44  is understood by a person skilled in the field. The piezoelectric sensor elements  25  and  27  are further connected to a voltage bias  19 . During a normal operation, the two inputs of the differential operational amplifier  44  at the circuit terminals  40  and  42  are at approximately the same voltage level as the reference voltage level at the terminal  7 . The operational amplifier  44  in conjunction with the feedback capacitors  47  and  48  and the feedback resistors  45  and  46  create a differential charge amplifier that generates a differential signal output between the circuit terminals  50  and  52  in proportion to the difference between the first piezoelectric signal  17  and the second piezoelectric signal  37 . The constant of proportionality between the differential signal output at the terminals  50  and  52  and the difference between the first piezoelectric signal  17  and the second piezoelectric signal  37  is approximately equal to the ratio of the values of the equivalent piezoelectric sensor capacitances  15  and  35  and the feedback capacitors  47  and  48 . The frequency response of the charge amplifier is determined by the pole of the feedback capacitors  47  and  48  and the feedback resistors  45  and  46 . Over a long period of time the average electrical output signals at the terminals  50  and  52  are equal to the reference voltage at circuit terminal  7 . Over shorter periods of time, the differential electrical signal output between terminals  50  and  52  is proportional to the difference between the first piezoelectric signal  17  generated by the first piezoelectric sensor element  25  and the second piezoelectric signal  37  generated by the second piezoelectric sensor element  27 .  
     [0044] It is appreciated that the feedback resistors  45  and  46  in FIG. 9 may be replaced with the other circuit components, such as a field effect transistor or a switched capacitor network, to achieve a similar effect.  
     [0045] A conventional arrangement for piezoelectric actuators is shown in FIG. 10 and involves a direct connection between a piezoelectric actuator element  91  ′ and an electronic drive signal generator  73 ′. The piezoelectric actuator element  91  ′ generates a physical motion in response to the electronic drive signal at a terminal  75 ′.  
     [0046]FIG. 11 illustrates one embodiment of a piezoelectric actuator in accordance with the principles of the present invention. The piezoelectric actuator includes a piezoelectric actuator element  91  connected to an electronic drive signal generator  73  and further connected to a voltage bias  19 . The piezoelectric actuator element  91  generates a physical motion in response to the electronic drive signal at a terminal  75  generated by the drive signal generator  73 . The presence of the bias voltage  19  maintains an electric field bias in the piezoelectric material of the piezoelectric actuator element  91  to reduce the effects of fatigue, aging, and depoling while increasing electromechanical efficiency.  
     [0047] An alternative embodiment of the piezoelectric actuator in accordance with the principles of the present invention is shown in FIG. 12. In FIG. 12, the piezoelectric actuator element  91  is connected to a signal summing circuit  77  at a terminal  81  and a reference terminal  7 . The electronic drive signal generator  73  generates a drive signal at the terminal  75  that is also connected to a first input of the signal summing circuit  77 . The voltage bias  19  is connected at a terminal  79  to a second input of the signal summing circuit  77 . The signal summing circuit  77  generates an electrical signal output at a terminal  81  that is the sum of the signal inputs at the terminals  75  and  79 . The piezoelectric actuator element  91  generates a physical motion in response to the electronic drive signal at the terminal  75  generated by the drive signal generator  73 . The presence of the bias voltage  19  maintains an electric field bias in the piezoelectric material of the piezoelectric actuator element  91  to reduce the effects of fatigue, aging, and depoling while increasing electromechanical efficiency.  
     [0048] From the above description and drawings, it will be understood by those of ordinary skill in the art that the particular embodiments shown and described are for purposes of illustration only and are not intended to limit the scope of the present invention. Those of ordinary skill in the art will recognize that the present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. References to details of particular embodiments are not intended to limit the scope of the invention.