Abstract:
Energy efficient circuitry is provided for rapid transfer of charge to and from a reactive load while avoiding excessive peak currents and significant resistive energy dissipation. For example, circuitry of the invention provides for rapid actuation of a piezoelectric mass flow valve actuator while significantly reducing electrical input power and power dissipation requirements. The invention also features circuitry for recovering a substantial portion of the energy delivered to the reactive load while still permitting rapid cycling of the load drive circuit. Controlling the activation interval of the drive circuitry provides for incremental actuation or positioning of the reactive load.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates generally to electrical control of a reactive load, and more particularly to high speed, low loss control of a piezoelectric actuator or other electromechanical positioning device.  
         [0003]     2. Brief Description of the Prior Art  
         [0004]     A piezoelectric linear actuator is a nano-positioning device that deforms its shape in response to a stimulus of electrical charge. Piezoelectric actuators are used to operate mechanical valves that regulate the flow of materials in such diverse applications as automobile fuel injectors, hydraulic servovalves, and ink jet printer nozzles. Piezoelectric linear actuators are particularly well suited for use in mass flow controller products, such as those employed in connection with plasma processing equipment, because the actuators can be operated for billions of cycles with virtually no loss in performance. Compared to other electromechanical actuators such as solenoids or stepper motors of comparable size and cost, piezoelectric actuators are capable of providing significantly greater actuating forces per unit of input energy provided, along with exceptional positional accuracy.  
         [0005]     In gas delivery applications, particularly those involving high-speed processes, the time required to actuate a control valve can directly affect the performance of a mass flow controller. A typical mass flow controller product stimulates a piezoelectric actuator by transferring charge from a power source to the actuator load using a first-order circuit arrangement. In this approach, a resistive element controls the peak current value (i.e., initial charge transfer) into the capacitive actuator load and participates in an exponential decay rate time constant. The current waveform resulting from a first-order drive circuit is an initially large peak current, followed by a long exponential decay period during which most of the charge is transferred to the actuator. Charge transfer rate can be increased by increasing the peak current. The speed of charge transfer is limited, however, by peak current stresses placed on the internal bond wires connecting the drive circuit to the piezoelectric actuator, as well as by practical considerations of the cost, size, and electrical isolation of high-current switches and other components. As a result, contemporary mass flow controller devices tend to have relatively slow actuation times on the order of several hundred milliseconds. For many process applications, these actuation times impose an undesirable lower limit on the open time of the flow path, or upper limit on the repetition rate at which the controller may be operated.  
         [0006]     The amount of electrical power consumed and dissipated by a mass flow controller may also limit its suitability for many process applications. In stimulating a piezoelectric actuator with a first-order drive circuit, half of the energy provided by the power source is always dissipated in the series resistive element. Moreover, the energy used to charge the piezoelectric actuator is typically not recovered, thus requiring dissipation of that stored energy when the actuator is discharged. As a result, power consumption and dissipation needs of a high-speed mass flow controller may become unacceptably high in a system environment where power and heat sinking resources are allocated sparingly.  
         [0007]     U.S. Pat. No. 6,320,297 describes a circuit for controlling a piezoelectric actuator with reduced electrical losses. The capacitive actuator load is charged from a capacitor bank through a load switch and a series reactance coil. To discharge the actuator, a discharge switch is closed, causing reverse current to flow from the actuator through the reactance coil. When the actuator voltage has dropped to the residual voltage of the capacitor bank, a residual discharge switch is closed, causing additional reverse current to flow in the reactance coil. The residual discharge switch must then be opened at the moment when the current in the reactance coil is at a maximum in order to release the residual energy previously stored in the actuator load back into the capacitor bank.  
         [0008]     As an alternative to in-line mass flow control, a mass flow diverter may be employed in applications requiring higher speed control of minute feed gas quantities. In this approach, a pneumatically actuated valve is located on a gas stream conduit venting continuously from a source. To inject a quantity of gas into a process, the valve is driven rapidly to a position that diverts the stream into the process environment, and then returned rapidly to the venting position. Use of high-speed pneumatics to drive the diverter valve allows for short actuation times, on the order of tens of milliseconds, and therefore greater control over the delivered gas quantity. In addition to the added cost and complexity of this approach, however, a significant disadvantage is that the vented material often cannot be recovered due to contamination concerns. For many processes, particular in the manufacture of semiconductor devices, this can result in significant waste of expensive feed gas materials along with an attendant need for scrubbing or abating greater quantities of the gases downstream of the process.  
         [0009]     It would be desirable to provide circuitry capable of rapidly actuating a piezoelectric or other reactive load without driving excessively large peak currents through the elements and connections of the circuitry. It would be further desirable to provide for rapid actuation of a reactive load while minimizing input power and heat sinking requirements. It would be further desirable to provide for partial actuation of a reactive load, such as a positioning device, in order to achieve incremental positioning of the load.  
       SUMMARY OF THE INVENTION  
       [0010]     This invention provides energy efficient circuitry for rapid transfer of charge to and from a reactive load. The invention generally comprises a resonant drive circuit for rapidly transferring charge to a reactive load while avoiding excessive peak currents and significant resistive energy dissipation. The invention also features circuitry for recovering a substantial portion of the energy delivered to the reactive load while still permitting rapid cycling of the load drive circuit.  
         [0011]     In one embodiment of the invention, a resonant drive circuit comprises a piezoelectric actuator and a series inductive element for rapid delivery of charge from a voltage source to the capacitive actuator load. A semiconductor switch controls the application of voltage to the circuit from a voltage source, and a series diode rectifies the resonant current waveform of the circuit for positive charge transfer to the actuator. The resonant drive circuit is optimized for rapid transfer of charge from the voltage source to the piezoelectric actuator with minimal peak currents. The switched circuit arrangement also allows the piezoelectric actuator to be driven affirmatively to a known voltage reference while naturally locating and managing zero-voltage crossings within the circuit for effective charging and discharging of the actuator device.  
         [0012]     In one embodiment of the invention, an energy conserving discharge circuit is provided in addition to a resonant drive circuit. A semiconductor switch creates a single-stage resonant discharge path from the piezoelectric actuator through the inductive element of a resonant drive circuit. The discharge circuit further comprises nonlinear elements to ensure rapid and positive transfer of charge from the actuator. Energy recovered by the discharge circuit is returned to the power source of the actuator drive circuit. The resonant discharge circuit is optimized so as to recover and recycle a substantial portion of the energy delivered to the reactive load without unduly limiting the cycling frequency of the load drive circuit.  
         [0013]     The invention provides for increased speed and energy efficiency of circuitry used to control operation of reactive loads. For example, the circuitry of the invention permits the speed at which a piezoelectric linear actuator can operate a mechanical valve to be increased several fold while significantly reducing electrical input power and power dissipation requirements. Proper selection of the resonant elements of the drive circuit ensures that charge is transferred to the reactive load within actuation time limits and peak current limits of the circuit. Transferring an electrical charge using resonant drive circuitry of the invention provides for nearly all of the source energy to be delivered to the reactive load, allowing for further reductions in electrical power requirements.  
         [0014]     In a further embodiment of the invention, the activation period of a semiconductor switch controls the actuation increment of a piezoelectric device or other reactive load. A resonant drive circuit comprises a piezoelectric actuator and a series inductive element together with nonlinear devices that ensure rapid and positive delivery of charge from a voltage source to the actuator load. Control of the activation (“on”) time of the semiconductor switch results in a variable quantity of electrical charge delivered through the resonant circuit, thus providing for incremental actuation of the piezoelectric load.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  illustrates a mass flow controller device in accordance with an embodiment of the invention.  
         [0016]      FIG. 2  illustrates circuitry for driving a reactive load in accordance with one embodiment of the invention.  
         [0017]      FIG. 3  illustrates a charging waveform response in accordance with one embodiment of the invention.  
         [0018]      FIG. 4  illustrates incremental charging waveform responses in accordance with alternative embodiments of the invention.  
         [0019]      FIG. 5  illustrates circuitry for driving a reactive load in accordance with an alternative embodiment of the invention.  
     
    
     DETAILED DESCRIPTION  
       [0020]      FIG. 1  illustrates one embodiment of a mass flow controller device that incorporates features of the invention. Mass flow controller  10  comprises controller base  12  having upstream and downstream gas flow passages  14  and  16 , respectively. Gas flow between passages  14  and  16  is controlled by valve assembly  20 . When actuated, piezoelectric stack  30  transmits force through diaphragm  32  and plunger  22  to poppet  24 , which opposes the force of spring  26  to create a gas flow path through orifice plate  28 . Actuation of piezoelectric stack  30  is controlled by drive circuitry on electronics board  40 . A sensor loop  50  provides measurements of properties of the gas stream flow to control electronics on electronics board  40 .  
         [0021]      FIG. 2  illustrates actuation circuitry in accordance with one embodiment of the invention. Circuit  100  drives a piezoelectric actuator  102  depicted as a capacitive load. Circuit  100  further comprises low voltage source  104  providing DC power at voltage V 1  and step-up converter  110  that boosts V 1  to DC voltage V 2 . Charging switch  112  controls the charging of actuator  102  through series inductor  118  and diodes  120  and  124 , and discharge switch  114  controls discharging of the actuator through inductor  118  and diodes  122  and  126 .  
         [0022]     To charge the piezoelectric actuator, charging switch  112  is closed, connecting the resonant circuit comprising inductor  118  and actuator  102  to voltage V 2  provided by step-up converter  110 . A positive current waveform is initiated having a periodicity at the natural frequency of the resonant circuit formed by inductor  118  and capacitive actuator load  102 . In the case of the embodiment illustrated in  FIG. 2 , the underdamped second-order circuit arrangement results in a current flow in the form of an exponentially decaying sine function. When the voltage across the actuator load  102  reaches V 2 , diode  120  turns off and diode  124  becomes conductive to permit the remaining energy that has become stored in inductor  118  to be delivered into the actuator load  102 . Thus, when the current waveform reaches zero after the first half-cycle of the sine function, all charge has been delivered to the actuator load  102  at voltage V C . Diodes  120  and  124  rectify the current waveform in the resonant charging circuit to prevent reverse current flow from actuator  102 . During the charging cycle, discharging switch  114  remains open.  
         [0023]     Inductor  118  is selected to have an inductance that optimizes the rate of charge transfer to actuator  102 . In the case of a mass flow controller, for example, improving the actuation time of a piezoelectric valve actuator allows for greater control over the open time of the flow path and operation of the controller at a higher repetition rate. In general, a lower value of inductance results in a higher natural frequency of the resonant circuit comprising inductor  118  and actuator  102 . A circuit having a higher natural frequency delivers charge to the actuator more rapidly, but with a higher value of peak current. Proper selection of the reactive value of the series resonant elements of the circuit ensures that charge is transferred to the load within a desired actuation time limit and peak current limits of the circuit. Thus, an inductance value is chosen that optimizes the speed of charge transfer to actuator  102  while preventing the charging currents from exceeding limits imposed by electrical and physical constraints of circuit elements and connections. The optimal speed of charge transfer is also determined based upon mechanical actuation limitations that may be characteristic of the piezoelectric device itself. It will be appreciated that the function of inductor  118  may be implemented using alternative circuit elements that mimic the properties of inductors, such as negative-impedance converters or gyrators, or by making use of the inductive properties of other circuit elements, such as the leakage inductance of a transformer.  
         [0024]      FIG. 3  illustrates advantages of a charging circuit of one embodiment of the invention over a first-order charging circuit arrangement. The second-order charging circuit of the invention results in a sinusoidal current waveform that delivers a full charge to a reactive load at time T 1  with peak current I 1 . By comparison, a first-order charging circuit that delivers a full charge at time T 1  imposes a considerably higher peak current I 2  on the charging circuit. Alternatively, a first-order charging circuit that is restricted to a peak current value of I 1  requires a considerably longer time T 2  to deliver a full charge to the load.  
         [0025]     To discharge the piezoelectric actuator, charging switch  112  depicted in  FIG. 2  is opened and discharging switch  114  is closed, causing reverse current to flow from load  102  through series inductor  118 . Due again to the low-loss second-order circuit arrangement of the discharge path, the current waveform is an underdamped sine function. When the voltage across the actuator load  102  reaches zero, diode  126  becomes conductive to prevent further current flow from the load that would drive the actuator voltage negative, while permitting the energy that has become stored in inductor  118  to discharge into energy storage element  106  of low voltage source  104 . Series diode  122  rectifies the current waveform in the resonant discharging circuit to prevent forward current flow back into actuator  102 .  
         [0026]     The efficiency and rate of energy discharge from inductor  118  to storage element  106  is determined by ohmic losses present in the discharge path. Ideally, energy in inductor  118  is discharged rapidly so as to readily permit the inductor to participate in a next charging cycle of actuator  102 . Non-ideal behaviors of the elements of the discharge circuit, however, may result in a cumulative circuit resistance that dissipates energy and lengthens the discharge time constant of the circuit, thereby impeding transfer of the energy in inductor  118 . In this case, energy recovery may be improved and the discharge time shortened by terminating the discharge path into a bias voltage. In the embodiment illustrated in  FIG. 2 , storage element  106  is disposed within low voltage source  104  that provides a constant DC voltage V 1 . As a result, when discharging switch  144  is closed to discharge actuator load  102 , the reduced potential between the charged actuator voltage and V 1  causes current through inductor  118  to build at a reduced rate. When the actuator voltage falls to V 1 , current through inductor  118  begins to decrease. When the actuator is completely discharged, parallel diode  126  becomes conductive, allowing the energy stored in inductor  118  to discharge into energy storage element  106  of low voltage source  104 . Because the energy storage element is biased at voltage V 1 , however, the current present in inductor  118  rapidly decays to zero, as compared to the longer time that would be required for the current to decay to ground potential. Thus, providing bias voltage V 1  in the discharge path allows inductor  118  to be discharged rapidly, which in turn permits rapid cycling between charging and discharging of actuator  102 . The presence of bias voltage V 1  also serves to lower the peak value of currents that develop during the discharging cycle, thus reducing ohmic dissipation in the discharge circuit and increasing energy recovery.  
         [0027]     In the embodiment of  FIG. 2 , charging and discharging switches  112  and  114  are illustrated as MOSFET semiconductor devices activated by driving circuits  116 . Due to their rapid activation times and low “on” resistance, the use of MOSFET devices as switches is advantageous in many applications. It will be understood, however, that switching devices of the invention may alternatively include other semiconductor devices such as IGBTs or thyristors, relays, or any switching devices appropriate to the circuitry and application of the invention. It will also be appreciated that while charging switch  112  provides a step function of voltage to the resonant charging circuit depicted in  FIG. 2 , the switching devices of the invention may also be operated so as to provide a voltage ramp, for example, or any input voltage or current waveform appropriate to the particular application.  
         [0028]      FIG. 2  further illustrates how the invention may be practiced without the need for precise timing of the activation and deactivation times of switching devices that control the charging and discharging functions of the driving circuitry. In the embodiment shown in  FIG. 2 , each of switches  112  and  114  is closed for a time interval sufficient to complete the respective charging or discharging operation controlled by the switch, and need only be opened at some time prior to the initiation of the next half-cycle controlled by the opposing switch.  
         [0029]     In the embodiment of  FIG. 2 , a second-order resonant circuit arrangement results in a sinusoidal current waveform that delivers charge to the reactive load. Embodiments of the invention also include circuitry that produces current waveforms having harmonic components of a fundamental resonant frequency for improved charge transfer. For example, a current waveform comprising the third harmonic of a resonant frequency in addition to fundamental frequency itself more closely approximates a square wave or step function, which for a given maximum current value yields an even more rapid transfer of charge from a voltage source to a load.  
         [0030]     In an alternative embodiment of the invention, charging circuitry provides for incremental delivery of energy to a reactive load. Referring to the circuit architecture of  FIG. 1 , the drive circuit  116  of charging switch  112  in this alternative embodiment is configured to allow charting switch  112  to be closed for an arbitrary time interval. To charge the piezoelectric actuator, charging switch  112  is closed, which connects the resonant circuit comprising inductor  118  and actuator  102  to voltage V 2  provided by voltage converter  110 . A positive current waveform is initiated having a periodicity at the natural frequency of the resonant circuit formed by inductor  118  and capacitive actuator load  102 . Before the current waveform reaches zero, however, at which point full charge would be delivered to load  102 , switch  112  is opened. Parallel diode  124  becomes conductive, allowing energy stored in inductor  118  to be delivered to load  102 . When the freewheeling current waveform through parallel diode  124  and inductor  118  reaches zero, an incremental charge has been delivered to load  102  resulting in a voltage across the load that varies depending upon the actuation time of switch  112 . As shown in  FIG. 4 , for example, if switch  112  is closed at time T 1 , T 2 , or T 3 , corresponding voltage V 1 , V 2 , or V 3 , respectively, will appear across the load. Series diode  120  prevents reverse current flow from actuator  102 . In this way, incremental positioning of a piezoelectric actuator is accomplished by controlling the activation time of charging switch  112 . Depending upon the voltage available from voltage converter  110 , an incrementally actuated piezoelectric actuator may be further incrementally actuated by additional activation of switch  112 , or the actuator load may be discharged through a discharging circuit and reactuated to a new position by activation of switch  112  for a correspondingly different time interval.  
         [0031]     In one embodiment of the invention, a mass flow controller device comprises a gas valve controlled by a piezoelectric actuator that is capable of being incrementally actuated. One or more properties of the gas stream are sensed and used to control the position of the valve to regulate the gas stream to a desired mass flow set point.  
         [0032]     A drive circuit arrangement in accordance with an alternative embodiment of the invention is illustrated in  FIG. 5 . Circuit  200  controls charging and discharging of a piezoelectric actuator  202  through switches  212  and discharge switch  214 . Voltage source  204  provides DC power at voltage V 1  for charging of storage capacitor  230  through switch  232  and charging inductor  234 . To charge actuator  202 , charging switch  212  is closed, connecting the resonant circuit comprising first resonant inductor  218  and actuator  202  to storage capacitor  230 . First resonant inductor  218  is selected to have an inductance that optimizes the rate of charge transfer to actuator  202 , such that charge is transferred to the load within a desired actuation time limit and peak current limits of the circuit. Storage capacitor  230  is selected to have a capacitance such that energy stored therein at voltage V 1  results in a desired voltage V C  across the piezoelectric actuator  202  when charge transfer is complete. Diodes  220  and  224  rectify the current waveform in the resonant charging circuit to prevent reverse current flow from actuator  202 . To discharge the piezoelectric actuator, charging switch  212  is opened and discharging switch  214  is closed, causing reverse current to flow from load  202  through second resonant inductor  228  and into energy storage element  206  of voltage source  204 . Second resonant inductor  228  is selected to have an inductance that optimizes the discharge time constant from actuator  202 , which may be a different from the charging time constant as determined by first resonant inductor  218 . Series diode  222  rectifies the current waveform in the resonant discharging circuit to prevent forward current flow back into actuator  202 . Switch  232  is actuated as needed to recharge storage capacitor  230  in preparation for another charging cycle of load  202 .  
         [0033]     In the foregoing, embodiments of the invention have been described for charging and discharging piezoelectric actuator loads, which may include linear, tube, disk, or bimorph/polymorph actuators, as well as ultrasonic micromotors or inchworm actuators. The piezoelectric actuator may be used to operate a mechanical valve of a mass flow controller, flow shut-off device, flow pressure regulator device, or for any other purpose. Circuitry of the invention is also useful for rapidly actuating other types of reactive loads such as shape memory, electromagnetic, polymer, or magnetostrictive actuators; piezoelectric transformers, micro pumps or radial field diaphragms; solenoid or voice coil actuators; or stepper motors.  
         [0034]     Although specific structure and details of operation are illustrated and described herein, it is to be understood that these descriptions are exemplary and that alternative embodiments and equivalents may be readily made by those skilled in the art without departing from the spirit and the scope of this invention. Accordingly, the invention is intended to embrace all such alternatives and equivalents that fall within the spirit and scope of the appended claims.