Abstract:
The present circuit is for a resonant, regenerative switching piezomotor drive amplifier that efficiently converts electrical energy into mechanical work through a piezoelectric actuator. The actuator driver of the present invention drives the real work-producing part of the system load over a broad range of frequencies from DC to several kHz, dramatically increasing the system power efficiency and full power bandwidth. The gains in efficiency are obtained by operating the motor/amplifier system at both electrical and mechanical resonances for the system. The amplifier&#39;s efficiency is greater than 80% when driving a 1 μF piezoelectric load with a 500 V peak-to-peak signal. The available output power is greater than 20 watts continuously from DC to 2.0 kHz.  
     The resonant, switching regenerative piezomotor drive amplifier described herein not only drives high voltage piezoelectric actuators, but will also serve equally well in any application that requires high power drive signals to be applied to a predominantly capacitive load.

Description:
REFERENCE TO RELATED APPLICATION  
       [0001]    This application is based upon and claims priority of a provisional application, Serial No. 60/213,640 filed on Jun. 23, 2000, the full disclosure of which is incorporated by reference herein. 
     
    
     
       FIELD OF INVENTION  
         [0002]    The present invention is directed toward a resonant, regenerative switching drive amplifier that efficiently converts electrical energy into mechanical work through a piezoelectric actuator, and operates at both electrical and mechanical resonances for a motor/amplifier system.  
         BACKGROUND OF THE INVENTION  
         [0003]    Piezoelectric actuators differ from electromagnetic actuators in the load they present and mode by which they do work. Piezoelectric actuators produce very large forces, but over micron displacements. Useful work can only be extracted by accumulating the small stroke of the actuator at high frequencies. Since the actuator displacements are small and at high frequencies, the inertia and compliance of the mechanical accumulator must be taken into consideration. On every stroke of the actuator, energy is delivered to the mechanical load and deposited in the spring-like compliance of the actuator. The system&#39;s mass and compliance form a mechanical resonant system, and energy not delivered to load or recovered from the system is lost as heat. This results in mechanical impedance of the actuator and load system. The portion of the load that does useful work has real impedance, and the portion of the load that stores energy in compression and momentum has an imaginary impedance. By driving the system at its natural compression and momentum has an imaginary impedance. By driving the system at its natural mechanical resonance, the imaginary component of the mechanical impedance is canceled, leaving just the real component that does useful work.  
           [0004]    Piezoelectric actuators also present a very large capacitive load. The first order electrical model for a piezoelectric actuator is a capacitor in series with a resistor. The resistor in the model represents the work-producing part of the mechanical load. Like the mechanical system, the load capacitance can be resonated to leave just the real part of the load. However, practical considerations often (1) prevent the coincidence of electrical and mechanical resonances and (2) dictate that the actuator be driven over a wide band of frequencies.  
           [0005]    It should be noted that several methods for resonant piezoelectric drivers are patented (U.S. Pat. Nos. 5,126,589, 4,109,174, and 4,767,959), but they are impractical because of difficulties associated with floating drive signals, inefficient diodes, BJT transistors, or SCRs. Diodes, BJT, and SCRs have a minimum forward voltage across their semiconductor junctions, thus they represent large V*I power losses.  
         SUMMARY OF THE INVENTION  
         [0006]    Piezoelectric actuators and motors deliver useful work at power densities an order of magnitude greater than that of their electromagnetic counterparts. With this in mind, the present circuit is for a resonant, regenerative switching piezomotor drive amplifier that efficiently converts electrical energy into mechanical work through a piezoelectric actuator.  
           [0007]    The Resonant Regenerative Switching Amplifier combines the wide bandwidth and flexibility of a linear power amplifier with the high efficiency of a driven tank circuit. In a linear amplifier, high current is repeatedly sourced and then sunk when driving a capacitive load. On each cycle, the capacitor is loaded with energy and then this energy is discarded. At low to moderate frequencies, this wasted reactive power can be substantially larger than the power delivered to the work-producing part of the load, thus causing very low system energy efficiency.  
           [0008]    The actuator driver of the present invention is able to drive the real work-producing part of the system load over a broad range of frequencies from DC to several kHz, dramatically increasing the system power efficiency and full power bandwidth. The gains in efficiency are obtained by operating (transferring and converting the energy) the motor/amplifier system at both electrical and mechanical resonances for the system. The amplifier&#39;s efficiency is greater than 80% when driving a 1 μF piezoelectric load with a 500 V peak-to-peak signal. The available output power is greater than 20 watts continuously from DC to 2.0 kHz.  
           [0009]    The resonant, switching regenerative piezomotor drive amplifier described herein not only drive high voltage piezoelectric actuators, but will also serve equally well in any application that requires high power drive signals to be applied to a predominantly capacitive load.  
           [0010]    An article describing the present invention, entitled DESIGN ADVANCES FOR HIGH-EFFICIENCY REGENERATIVE PIEZOELECTRIC DRIVE AMPLIFIER, Proceeding of SPIE, Smart Structures and Materials, March 2001, written by Wayne Zavis and Wayne Shanks is incorporated herein by reference. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a graph showing the efficiency of different Piezoelectric Drive systems based on the amplifier used and the frequency of an input signal;  
         [0012]    [0012]FIG. 2 is a graph showing the piecewise approximation of the mechanical resonance by the electric resonance;  
         [0013]    [0013]FIG. 3 shows a circuit diagram of a basic piezoelectric drive amplifier of the present invention;  
         [0014]    [0014]FIG. 4 shows transfer of energy from the storage capacitor to the piezoelectric element of FIG. 3;  
         [0015]    [0015]FIG. 5 shows a circuit diagram of the piezoelectric drive amplifier of the present invention incorporated into a power handling system; and  
         [0016]    [0016]FIG. 6 shows a picture of the piezoelectric motor mated with a drive amplifier. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    Referring now to the drawings, a preferred embodiment is described where like elements are designated by like elements numbers. FIG. 1 shows a chart  10  comparing the efficiency of a piezo-driver system using various drive amplifiers for a range of input frequencies. The chart  10  shows that the efficiency for a circuit using the resonant regenerative switching amplifier  12  of the present invention provides high efficiency at low frequencies. The tank driven circuit  14  and the linear amplifier circuit  16  have efficiencies which increase as the frequency increases, and the fixed-value tank circuit  18  has a narrow and limited band of frequencies where the efficiency of the circuit peaks.  
         [0018]    The wasteful reactive component of the impedance can be canceled by adding a conjugate inductance, leaving the load a pure resistance. Electrically this only occurs at one frequency, the resonant frequency of the inductor-resistor-capacitor or LRC (tank) system. The efficiency of this tank circuit can be explained by realizing that the energy stored on the capacitor is not thrown away, but transferred to the inductor and then transferred back to the capacitor every cycle. External power need only provide what is lost to mechanical work and resistive heating. The most efficient conversion of electrical energy to mechanical work will thus occur only at the narrow band of frequencies around electrical resonance. To make available a larger band of frequencies, the inductor value must be dynamically adjusted to change the resonant frequency. Since dynamically adjustable power inductors are currently impractical, the high-efficiency operation of the system is severely band limited.  
         [0019]    This problem of narrow-band operation can be overcome if temporary energy storage is accomplished not in the inductor, but another capacitor. Analogous to water tanks, energy can be resonantly transferred between two capacitors, the load piezoelectric element and a storage capacitor, without dissipating appreciable power. Unlike energy stored in an inductor, the back and forth transfer of energy in two capacitors can stop for an arbitrary period and then resume with little loss of energy. By transferring small bursts of energy at high frequencies, the voltage on the piezoelectric element can be ramped up or down in a piece-wise approximation to any arbitrary waveform, as shown in FIG. 2. In this way, the electric resonance can be made to match the mechanical resonance of the system. With this technique, the driver can operate with high efficiency at frequencies from direct current (DC) all the way to some limiting frequency below the energy-transfer resonance. Present switching technology puts this upper frequency limit at several kHz, but tradeoffs in signal distortion and power efficiency can raise or lower this upper bound.  
         [0020]    [0020]FIG. 3 shows a circuit  20  of the preferred embodiment using the resonant regenerative switching amplifier which allows high efficiency at low frequencies. The process of moving stored energy from one capacitor to the other, and vice versa is described herein, where the first capacitor is a piezoelectric element  19  having a capacitance Cx and the second capacitor is a storage capacitor  21  having a capacitance Cs. For the purposes of this description, consider that the capacitance Cs of the storage capacitor  21  starts out charged to the system&#39;s maximum potential (V max ) and the capacitance Cx of the piezoelectric element  19  is at a 0 volt potential. All potentials are always positive, and the piezoelectric element  19  and the storage capacitor  21  are equal-valued capacitors. The circuit  20  is designed to piece-wise approximate on the piezoelectric element  19  an arbitrary waveform seen at an input (V IN ) of an error amplifier  22 . At time zero, both the voltage (V cx ) on the piezoelectric element  19  and an input signal  24  start at 0 V.  
         [0021]    The operation of this switching system can be considered in two categories of energy transfer, (1) the transfer of energy from the storage capacitor  21  to the piezoelectric element  19  and, (2) the transfer of energy from the piezoelectric element  19  to the storage capacitor  21 . The storage capacitor to piezoelectric element sequence is shown in FIG. 3, which increases the voltage (V cx ) on the piezoelectric element  19 . This is initiated by the error amplifier  22  when (V IN -V cx )&gt;αΔV, where ΔV is voltage step size, and α is a constant (0&lt;α&lt;1). When this condition is met, a current pre-load sequence is started in the switching controller  25  by closing a third switch  33 . A current pre-load before the actual energy transfer is needed because during the transfer of energy from storage capacitor  21  to the piezoelectric element  19 , the system is a freely oscillating inductor-capacitor (LC) system with a positive slope on V cx  and an instantaneous current present in the inductor  23 . Since these boundary conditions of voltage and current are not present in the system during its hold state, where all the energy resides on one of the two capacitors, for a given V cx  some portion of the energy in the storage capacitor  21  must be transferred into the inductor  23 . The initial conditions needed to transfer energy into piezoelectric element  19  are:  
         
       {square root}{square root over (V css   2 +V cx   2 )}− 
       V 
       cx 
       ≧V 
       cs  
     
         [0022]    In the equation above, V css  is the voltage on storage capacitor  21  before the third switch is closed, V cx  is the voltage on the piezoelectric element  19 , and V cs  is the dropping voltage on the storage capacitor  21 . When the boundary conditions are met, the transfer of energy is started by opening the third switch  33  and closing the first and second switches,  31  and  32 . Inductors  34  and  36  transformer couple piezoelectric element  19  into storage capacitor  21  through inductor  23  to form a freely oscillating inductor-capacitor (LC) system. Assuming there is sufficient energy contained in the storage capacitor  21  for the transfer, V cx  increases until (VCX-V IN )&gt;(1-α)ΔV. The transfer is terminated by the opening of the first and second switches,  31  and  32 . For most steps there will be some energy remaining in inductor  23  at the termination of the transfer. This energy is recovered through a diode  26  connected between an inductor  38  and a ground  28 . Most of the time the diode  26  is reverse biased, thus preventing the storage capacitor  21  from discharging through the inductor  38 . During the inductor energy recovery phase, the collapsing field in the common core of the inductors,  23  and  38 , forward bias the diode  26  and current flows into the storage capacitor  21 , thus recovering nearly all the unused energy. The term “nearly all” is used since there is a 0.7V forward-voltage drop in the diode  26 . This voltage, times the current through the diode  26 , constitutes a loss that results in heating of the diode  26 . The system now enters a hold phase until the next transfer event starts.  
         [0023]    The other switching event is the transfer of energy from the piezoelectric element  19  to the storage capacitor  21 . This is initiated by the error amplifier  22  when (V cx -V IN )&gt;αΔV. When this condition is met, the first switch  31  is closed and the piezoelectric element  19  starts to discharge through the inductor  34 . If the potential on the storage capacitor  21  permits, the diode  30  on the second switch  32  is&#39; forward biased; thus the piezoelectric element  19  and the storage capacitor  21  are transformer coupled through the inductor  23 . The transfer proceeds until (V IN -V cx )&gt;(1-α)ΔV, at which point the first switch  31  is opened. If V cs  is too large to allow the diode  30  on the second switch  32  to forward bias when the first switch  31  is closed, then when the first switch  31  is opened the rapidly collapsing field in the core of the inductor  36  will forward bias the diode  30  and the energy will be transferred into the storage capacitor  21 .  
         [0024]    [0024]FIG. 4 shows the timing diagram of the change in the charge on the piezoelectric element and the storage capacitor in relation to the switches  31 - 33 .  
         [0025]    It is also possible to use variable energy stepsizes (ΔV≠constant) to piecewise reconstruct the mechanical resonance or input wave shapes. For example, applications driven by lower distortion requirements must make a tradeoff between distortion and bandwidth or another parameter to use variable step size to optimize their piezomotor/amplifier system performance.  
         [0026]    In another embodiment of the invention, low on-resistance field effect transistor (FET) switches can be used to ensure that very little energy is lost to resistive heat. With a slight addition in circuit complexity, the diodes  26  and  30  described above can be replaced with FET synchronous rectifiers that have an added bias component. These FET switches behave like ideal diodes, and thus they dissipate very little energy when they conduct current. The circuit losses may be low, but they are non-zero. In addition, the piezoelectric element  19  is dissipating energy in the form of performed and delivered mechanical work. At some point energy must be added to the system. This is accomplished by periodically charging the storage capacitor  21  to a voltage that corresponds to the largest possible energy transfer from the storage capacitor  21  to the piezoelectric element  19 . For a system with an energy step at the top of the voltage range, from 475V to 500V, the storage capacitor  19  requires approximately 160V. If the storage capacitor  19  is ever below this potential, it is quickly charged to slightly greater then 160V, thus always providing enough energy to make 25V increments all the way up to 500V. Since 160V represents the energy increment needed for one ΔV of 25V at approximately 500V, the maximum voltage on the storage capacitor  21  is 525V.  
         [0027]    [0027]FIG. 5 shows the circuitry for a power handling system using the resonant, regenerative switching piezomotor drive amplifier technology. The circuit shown minimizes all power losses while dealing with the shortcomings of available circuit components. In this circuit high voltage, high speed, N-channel MOSFETs are used.  
         [0028]    The system operates by chopping portions of the undriven inductor-capacitor (LC) resonance into discrete voltage steps at the actuator. The energy losses in the circuit come from resistive heating of the FET switches and other passive components. The FETs used have an on-resistance of 0.2 ohms and dominate the losses of the system.  
         [0029]    Total system equivalent resistance is of the order of 1 ohm. Therefore, most of the energy moving around within the system is delivered to the load with a real load resistance as low as 10 ohms. A second feature of the chosen circuit topology is the use of ground referenced N-channel MOSFETs. This feature greatly simplifies the circuit operation. None of the control voltages needs to be floated at high voltage. Highly efficient “over the counter” gate driver integrated circuits (ICs) are used, keeping the switching transition time below 200 ns.  
         [0030]    An example of an application for the resonant, regenerative switching piezomotor technology is to use the drive amplifier to power a miniature 12-beam piezomotor, shown in FIG. 6. The electromechanical performance of the motor is as follows:  
                                                       Resonant Mode:   2 nd  at approximately 900 Hz           No-load Speed:   600 RPM (10 Hz)           Stall Torque:   0.5 N-m with the drive frequency increasing               by approximately 10% at stall           Output Power:   4 Watts peak           Electric Drive:   130 Vac-pk (no DC offset) using standard               linear drive electronics           Output Current:   220 mA-pk at peak power with               56 degrees of phase shift           Motor Efficiency:   46%                      
 
         [0031]    One of the 12 bimorph motor beam elements  60 , of which eleven are shown in FIG. 6, incorporates a strain sensing structure which is used for resonance and feedback monitoring by the prototype amplifier. This sensing structure and dynamic control circuitry within the amplifier is used since the resonance of the piezomotor changes as a function of both rotational speed and output loading. Both no-load speed and stall torque increase linearly with drive voltage, when driven at resonance. The ceramic, bimorph beams  60  can safely be driven up to 300 V peak-to-peak (0.6 kV/mm, electric field break-down), which should therefore double both the no-load speed and stall torque, and quadruple the power output when driven at 300 volts. The bimorph beams  60  are located within a mass element  62 , which surrounds a driven shaft  64  and a roller clutch  66 .  
         [0032]    The present invention discloses generalized piezomotor drive electronics that efficiently operate at both electrical and mechanical resonance. The power efficiency of the Resonant Regenerative Switching Amplifier has been calculated to be greater than 80% when driving a 1 μF piezoelectric load with a 500 V peak-to-peak signal. The available output power should be greater than 20 watts continuously from DC to 2.0 kHz.  
         [0033]    Although certain presently preferred embodiments of the present invention have been specifically described herein, it will be apparent to those skilled in the art to which the invention pertains that variations and modifications of the various embodiments shown and described herein may be made without departing from the spirit and scope of the invention. For example, numerical values are illustrative rather than limiting, as are references to specific integrated circuit technology. Accordingly, it is intended that the invention be limited only to the extent required by the appended claims and the applicable rules of law.