Abstract:
Apparatus and methods for exciting a piezoelectric device. A control device receives a first control signal excites the piezoelectric device at about at least one predetermined electrical resonant frequency of the piezoelectric device as a function of the first control signal.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates generally to piezoelectric devices, and more particularly to heating piezoelectric devices through an excitation signal.  
         BACKGROUND  
         [0002]    Piezoelectric devices, such as piezoelectric actuators, generally consist of a piezoelectric material that deforms when an electric field, e.g., a driving field, is applied across it. Additional materials may be bonded with the piezoelectric material, such as metallic layers that act as electrodes, insulating materials to prevent current from flowing between particular areas of the device, and adhesives to bond the various layers together.  
           [0003]    In simplified terms, piezoelectric materials are comprised of many dipole unit cells. FIG. 1 symbolically depicts a unit cell  10  of a piezoelectric materia. When an electric field E 1  is applied in the direction shown, the unit cell grows in the y axis and shrinks in the x axis, in essence becoming tall and thin. Conversely, when an electric field E 2  is applied in the direction shown, the unit cell shrinks along the y axis and grows along the x axis, in essence, becoming short and fat.  
           [0004]    As the unit cell  10  becomes colder, the piezoelectric effect, i.e., response to the application of an electric field, decreases. Thus, for a given magnitude of an electric field, the unit cell will not grow/shrink as much as it did when it was warmer. As a practical matter, for the same electric field applied, this decreases the stroke of the piezoelectric device. For example, for some piezoelectric materials, a 35% loss of stroke was found when the temperature changed from 25 degrees Celsius to −40 degrees Celsius.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention provides apparatus and methods for exciting a piezoelectric device. A control device receives a first control signal excites the piezoelectric device at about at least one predetermined electrical resonant frequency of the piezoelectric device as a function of the first control signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 symbolically depicts a unit cell  10  of a piezoelectric material.  
         [0007]    [0007]FIG. 2 shows an apparatus  20  for exciting a piezoelectric device according to one embodiment of the invention.  
         [0008]    [0008]FIG. 3 shows a polarization estimating circuit according to one embodiment of the invention.  
         [0009]    [0009]FIG. 4 is a graph of a typical piezoelectric material  32  impedance according to one embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0010]    [0010]FIG. 2 shows an apparatus  20  for exciting a piezoelectric device according to one embodiment of the invention. The apparatus  20  may be used to heat the piezoelectric device. A temperature determining device, such as a temperature sensor  22  or other device described below, may determine or detect a temperature of the piezoelectric device, such as a piezoelectric sensor or piezoelectric actuator  24 , or a temperature proximate to the piezoelectric device, and transmits a temperature signal (“TEMP”) indicative of that temperature, i.e., a value indicative of the approximate temperature.  
         [0011]    In embodiments of the invention, the temperature sensor  22  may be omitted. In some embodiments, the temperature signal TEMP may be determined from an estimated ferroelectric polarization of the piezoelectric device. More particularly, instead of the temperatures sensor  22 , the temperature determining device may include a polarization estimating circuit  40  (FIG. 3) that is coupled with the piezoelectric actuator  24  by ways known to those skilled in the art, as will be evident from the discussion below.  
         [0012]    For a predetermined duration of time, the polarization estimating circuit  40  determines the change in applied voltage ΔV to the piezoelectric actuator  24  by ways known to those skilled in the art, such as with a comparator circuit  42 , for example. During that same general duration of time, the polarization estimating circuit determines the change in charge ΔQ on the piezoelectric actuator  24 , such as by a current integrating circuit  44 , for example, coupled with the piezoelectric actuator  24  by ways known to those skilled in the art.  
         [0013]    From the values for the changes in voltage ΔV and charge ΔQ, an equivalent capacitance CE of the actuator  24  may be determined generally by the following equation, where the equivalent capacitance CE is dependent on the physical construction of the actuator  24  and on the temperature proximate the operating environment of the actuator  24 :  
         
       CE=ΔQ/ΔV  
     
         [0014]    Typically, more accurate estimations of the equivalent capacitance CE are obtained with values for the change in voltage ΔV that are a significant portion (&gt;50%) of the full voltage range of the piezoelectric actuator  24 . Smaller percentages may also be used, although accuracy in the equivalent capacitance CE calculation may suffer. The voltage vs. charge hysteresis curve of the piezoelectric actuator  24  will typically determine the minimum value of ΔV that may be used in this calculation without a significant loss of accuracy.  
         [0015]    The polarization estimating circuit  40  may include an empirical map or data structure  46  that is operable to receive the equivalent capacitance CE as an input to the data structure  46 . The empirical map or data structure  46  is typically derived from the ferroelectric polarization hysteresis curves of the piezoelectric actuator  24  to generate the temperature signal TEMP.  
         [0016]    The equivalent capacitance CE effectively gives a unique slope value on the ferroelectric polarization hysteresis curves that can be correlated to the estimated temperature proximate the piezoelectric actuator  24 , as will be appreciated by those skilled in the art. The data structure  46  may be a look-up table stored in RAM or ROM, a software algorithm or a hardwired circuit that is operable to generate the temperature signal TEMP in response to the equivalent capacitance CE.  
         [0017]    A control device, e.g., a controller, such as a microcontroller or microprocessor  26 , may receive the temperature signal TEMP from the temperature determining device, e.g., temperature sensor  22 , and also receive a first control signal (“CONTROL 1 ”) indicative of a desired excitation of the piezoelectric actuator  26 . The first control signal CONTROL 1  may be generated by any of a variety of ways known to those skilled in the art. Although the piezoelectric actuator  24  is depicted as a thermally pre-stressed bender actuator, a variety of other piezoelectric actuators could also be used, such as a variety of unimorph, bimorph, and/or stacks known to those skilled in the art.  
         [0018]    The microprocessor  26  transmits a second control signal (“CONTROL 2 ”) to the piezoelectric device  24  as a function of at least one of the temperature signal TEMP and the first control signal CONTROL 1 . In one embodiment of the invention, the microprocessor  26  may transmit the second control signal CONTROL 2  when the temperature signal TEMP indicates that the temperature of the piezoelectric actuator  24  is less than a predetermined temperature and/or when the first control signal CONTROL 1  is received. The predetermined temperature may vary depending on the application of the piezoelectric actuator. Thus, in embodiments of the invention either one of the temperature signal TEMP and the first control signal CONTROL 1  may be omitted.  
         [0019]    The second control signal CONTROL 2  creates an electric field across the piezoelectric actuator  24  by any of a variety of ways known to those skilled in the art. For example, the second control signal CONTROL 2  could be a voltage or current/charge applied to a first electrode  28  of the piezoelectric actuator  24 .  
         [0020]    A second electrode  30  may be spaced some distance from the first electrode  28 , with a piezoelectric material  32 , such as PZT-5A, for example, disposed therebetween. The second electrode  30  is typically grounded, although in other embodiments it need not be, thereby creating an electrical field between the two electrodes  28 ,  30 , and across the piezoelectric material  32 . In other embodiments, a third control signal, such as the complement of the second control signal CONTROL 2  or some other voltage, current and/or charge could be applied to the second electrode  30 .  
         [0021]    In one embodiment of the invention, the second control signal CONTROL 2  may be at a frequency approximately equal to an electrical resonant frequency of the piezoelectric actuator  24 . In another embodiment of the invention, the second control signal CONTROL 2  may be at a frequency near an electrical resonant frequency, or within some predetermined range of an electrical resonant frequency. The predetermined range may depend upon the desired efficiency of heating of the piezoelectric actuator  24 , as will be described below.  
         [0022]    In one embodiment of the invention, the frequency of the second control signal CONTROL 2  may be at a first series resonance of the piezoelectric material  32 . In another embodiment of the invention, any of the other appropriate series resonant frequencies of the piezoelectric material  32  may be used. With many piezoelectric materials  32 , however, with each higher series resonant frequency, greater power is required to generate the signal, typically due to a higher impedance of the piezoelectric material  32 . This may be undesirable in some applications. Thus, for lower power usage, the first series resonant frequency may be used.  
         [0023]    In one embodiment of the invention, the frequency of the second control signal CONTROL 2  may be at a first parallel resonance of the piezoelectric material  32 . In another embodiment of the invention, any of the other appropriate parallel resonance frequencies of the piezoelectric material  32  may be used. Like the series resonant frequencies, however, with each higher parallel resonant frequency, greater power may be required to generate the second control signal CONTROL 2 . Again, this may be undesirable in some applications.  
         [0024]    By applying the second control signal CONTROL 2  at a resonant frequency of the piezoelectric material  32 , the piezoelectric material  32  is in effect, a purely (or nearly pure) resistive load. The reactant component of the piezoelectric material  32  is reduced, or minimized. This encourages power dissipation in the piezoelectric actuator  24  through heating of the piezoelectric material  32 .  
         [0025]    By applying the second control signal CONTROL 2  at a frequency close to, or within some predetermined range of the resonant frequency of the piezoelectric material  32 , the piezoelectric material  32  becomes less of a resistive load and more of a reactive load. For a given time period, the power dissipation through heating will typically be proportionally lower the more reactive the load is. Thus, less heating will be achieved for the same level of power provided to the piezoelectric actuator  24 . This, however, may be acceptable depending on the particular application and power supply being used.  
         [0026]    [0026]FIG. 4 is a graph  50  of a typical piezoelectric material  32  impedance according to one embodiment of the invention. The reactive portion of the impedance will typically be minimized, e.g., zero, where the impedance phase angle is equal to zero. The first series resonant frequency (“fSR 1 ”) will typically be at a frequency where the impedance is minimized. For example, for PZT-5A piezoelectric material, the first series resonant frequency fSR 1  will typically occur around 90-100 kHz.  
         [0027]    In many instances, when exciting the piezoelectric material  32  at a resonant frequency, the piezoelectric actuator  24  will not move. The resonant frequency may be significantly higher than the normal operating frequency of the piezoelectric material. For example, the PZT-5A piezoelectric material is typically driven to cause displacement using approximately a 55 kHz signal, while the first series resonant frequency is around 90-100 kHz. At the first series resonant frequency, the input signal to the piezoelectric actuator  24  may be changing too rapidly for the piezoelectric actuator  24  to respond mechanically. Thus, the piezoelectric actuator does not displace when it is excited at this higher frequency. This may prolong the life of the piezoelectric actuator  24  as compared to other heating techniques that cause movement by the actuator  24 .  
         [0028]    The particular waveform of the second control signal CONTROL 2  may be arbitrary, e.g., a sine wave, a square wave, a triangle wave, etc. Generally, better heating of the piezoelectric actuator  24  may be achieved by using a waveform having significant signal power at the resonant frequency.  
         [0029]    The apparatus  20  for exciting the piezoelectric actuator  24  may be separate from, or integrated into (as shown) the normal control device/circuitry for the piezoelectric actuator  24 . When integrated, typically the first control signal CONTROL 1  will have a first characteristic when excitation of the piezoelectric actuator  24  at an electrical non-resonant frequency is desired (e.g., when movement is desired), and will have a second characteristic when excitation at an electrical resonant frequency is desired (e.g., when heating is desired). This may be accomplished by a variety of ways known to those skilled in the art, such as by way of example, applying a first magnitude of voltage as the first control signal CONTROL 1  when an electrical non-resonant frequency is desired and applying a second magnitude of voltage when an electrical resonant frequency is desired.  
       INDUSTRIAL APPLICABILITY  
       [0030]    In operation, in embodiments of the invention using both the temperature signal TEMP and the first control signal CONTROL 1 , the first control signal is typically transmitted to the microprocessor  26  when heating of the piezoelectric actuator  24  is desired. This may occur during a pre-operational, initialization, or warm-up period by any of a variety of ways known to those skilled in the art.  
         [0031]    If the temperature signal TEMP indicates that the temperature of the piezoelectric device  24  is below the predetermined temperature, the second control signal CONTROL 2  may be transmitted to the piezoelectric actuator  24 . The second control signal CONTROL 2  may be at one or more of the electrical resonant frequencies of the piezoelectric material  32  in the piezoelectric actuator  24 . Thus, the piezoelectric actuator  24  will generate heat, but little or no movement. The second control signal CONTROL 2  may then continue until the piezoelectric actuator reaches a second predetermined temperature, or until some other event occurs, such as the passage of a predetermined time period.  
         [0032]    Thus, the apparatus  20  may be used to warm a piezoelectric actuator  24  that is at a cold temperature to avoid the loss of performance due to (cold) temperature effects.  
         [0033]    In embodiments of the invention where the temperature signal TEMP is omitted, the microprocessor  26  may transmit the second control signal CONTROL 2  whenever the first control signal CONTROL 1  is received.  
         [0034]    In embodiments of the invention where the first control signal CONTROL 1  is omitted, the microprocessor  26  may transmit the second control signal CONTROL 2  whenever the temperature of the piezoelectric actuator  24  is below or above the predetermined temperature. This situation will typically occur when separate control circuitry (not shown) is used to control the displacement of the piezoelectric actuator  24 . The separate control circuitry may be prevented/preempted from trying to move the piezoelectric actuator  24  while the apparatus  20  is exciting the piezoelectric actuator  24  at an electrical resonant frequency, e.g., during the heating period.  
         [0035]    The apparatus  20  may be used to heat up a piezoelectric actuator  24 . By appropriate selection of the frequency of the signal used to excite the piezoelectric actuator, mechanical movement by the actuator  24  may be minimized or eliminated. By heating the piezoelectric actuator  24  in this way, the effective operating temperature range of the piezoelectric actuator  24  may be increased. Further, when the piezoelectric actuator  24  is in a cold environment, the stroke of the piezoelectric actuator  24  may not be reduced because the actuator  24  is at a warmer temperature.  
         [0036]    From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.