Abstract:
An apparatus for electrically stimulating a smart material includes a controllable power source for charging the smart material and/or switching circuitry for discharging the smart material. The controllable power source includes a regulated DC to DC converter having controllable drive circuitry associated therewith. The drive circuitry can be self-oscillating through associated feedback means. The switching circuitry can be responsive to one or more control signals.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of provisional patent application Ser. Nos. 60/408,277 and 60/408,468, both filed on Sep. 5, 2002, which is incorporated by reference herein. This application is related to U.S. patent application Ser. No. 10/630,065 filed Jul. 30, 2003 for an Apparatus and Method for Charging and Discharging a Capacitor to a Predetermined Setpoint. 

   FIELD OF THE INVENTION 
   The present invention relates to electronic methods and circuits for controlling general-purpose, smart material-based actuators. 
   BACKGROUND OF THE INVENTION 
   Actuator technologies are being developed for a wide range of applications. One example includes a mechanically-leveraged smart material actuator that changes shape in response to electrical stimulus. Since this shape change is generally effectuated predominantly along a single axis, such actuators can be used to perform work on associated mechanical systems including a lever in combination with some main support structure. Changes in axial displacement are magnified by the lever to create an actuator with a useful amount of displacement and force. This displacement and force is useful for general-purpose industrial valves, beverage dispensers, compressors or pumps, brakes, door locks, electric relays, circuit breakers, and most applications employing a solenoid-type actuator. Smart materials, however, piezoelectric specifically, can require hundreds of volts to actuate and cause displacement. This type of voltage may not be readily available and may have to be derived from a lower voltage as one would find with a battery. 
   Another characteristic of piezoelectric materials is that the materials are capacitive in nature. Moreover, a single actuator is often controlled using two separate signals: a main supply and a ground using watts of energy during the moment of actuation. 
   SUMMARY OF THE INVENTION 
   The present invention provides a simple, low-power, and cost-effective means to drive mechanically-leveraged smart material actuator including a specialized power source operatively connected to switching circuitry. 
   The specialized power source of the present invention includes a controllable power source, a regulated direct current (DC to DC) converter, to apply a known voltage potential across a smart material and thereby convert a control voltage to a level suitable for the smart material. Thus, according to the present invention, the control and main supply signals are combined into one conductor. This permits the proposed invention to be retrofit into present control systems, directly replacing existing actuators. 
   The present invention further includes a smart material actuator coupled to one or both of a controllable power source for charging the smart material actuator and switching circuitry for discharging the smart material actuator. According to a first embodiment of the invention, the controllable power source is a regulated DC to DC converter that includes a transformer having primary and secondary windings. The primary winding of the transformer, in turn, is coupled to controllable drive circuitry for generating drive signals 180° out of phase with one another. To this end, the controllable power source operates in a binary manner: either supplying a known stimulating voltage potential across the smart material, or shorting across the smart material. According to an embodiment of the present invention, the drive circuitry of the controllable power source can further include feedback means such that the circuitry is self-oscillating. The feedback means can further include push-pull circuitry as well as an auxiliary winding associated with the transformer. The push-pull circuitry can further include a pair of negative-positive-negative (NPN) transistors. 
   A rectifier may further be associated with the secondary winding of the transformer for generating a DC voltage from an alternating current (AC) signal associated with the secondary winding. Noise reduction circuitry can also be coupled to the secondary winding of the transformer for filtering noise that may be generated by the controllable drive circuitry. 
   An apparatus for driving a smart material actuator according to the present invention thus includes a controllable power source for charging the smart material actuator and switching circuitry coupled between the controllable power source and the smart material actuator such that the switching circuitry discharges the smart material actuator upon removal of a power source. The rate of the discharge of the smart material actuator is determined by the impedance of the switching circuitry whereas the rate of charge of the controllable power source is determined by the impedance of the controllable power source. 
   Other applications of the present invention will become apparent to those skilled in the art when the following description of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The description herein makes reference to the accompanying drawings wherein like reference numerals refer to like parts throughout the several views, and wherein: 
       FIG. 1  is an electronic schematic of a controllable power source according to the present invention; 
       FIG. 2  is an electronic schematic of a first embodiment of switching circuitry according to the present invention; 
       FIG. 3  is an electronic schematic of a second embodiment of switching circuitry according to the present invention; 
       FIG. 4  is an electronic schematic of an apparatus for driving a smart material actuator implementing the controllable power source of FIG.  1  and the switching circuitry of  FIG. 2 ; and 
       FIG. 5  is an electronic schematic of an apparatus for driving a smart material actuator implementing the switching circuitry of FIG.  3  and the DC to DC converter of FIG.  1 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  is an electronic schematic diagram illustrating a controllable power source  10 , where a voltage source, or input voltage,  12  of known potential is connected to a reverse protection diode  14  coupled in series with a bead inductor  16 . The bead inductor  16  acts as a filter to remove noise generated by the collector of an NPN transistor  18  such that it does not reach the voltage source  12 . The NPN transistor  18  and an NPN transistor  20  from a push-pull driver for a transformer  22 . Resistors  24 ,  26 ,  28 ,  30  form resistive voltage dividers and set the basic bias points for the NPN transistors  18 ,  20 . The transformer  22  is wound not only with primary and secondary windings  22 A and  22 B but also an auxiliary winding  22 C. The auxiliary winding  22 C on the transformer  22 , resistors  32 ,  34 ,  28 , and capacitors  36 ,  38  form feedback means for creating oscillation on the bases of the NPN transistors  18 ,  20 . The oscillation is 180 degrees out of phase between the two NPN transistors  18 ,  20 , thus forming a self-oscillating push-pull transformer driver. The secondary winding  22 B of transformer  22  is connected to a rectifier in the form of a diode  40 , which is connected to a bead inductor  42  and a capacitive load  44 , in this case a piezoelectric smart material actuator. The bead inductor  42  acts as a filter to remove noise generated by the oscillation of the circuit and feeds the capacitive load  44 . A Zener diode  46  acts as feedback means through a current limiting resistor  48 . When the Zener voltage is exceeded a transistor  50  is turned on, causing the base of the transistor  20  to be grounded and stopping the self-oscillating mechanism. 
   Referring now to  FIG. 2 , switching circuitry  11  for discharging a smart material actuator capacitive load  58  is shown. When a switch  52  is closed, current flows from a voltage source  54  through the switch  52  and through the bead inductor  56  for charging the capacitive load  58  representing, in this case, a piezoelectric smart material actuator. Also, current flows into a resistive voltage divider network  60  driving an NPN transistor  62  on, which turns an NPN Darlington pair  64  off. The rate of charge is determined by the impedance of the bead inductor  56 , the resistor  66  and the capacitive load  58 . When the switch  52  is opened, the current stops flowing in the capacitive load  58  and the NPN transistor  62  is turned off. This turns the NPN Darlington pair  64  on, causing current to flow through the resistor  66  for discharging the capacitive load  58 . The rate of discharge is determined by the resistor  66  and the capacitive load  58 . The resistor  68  and the base of NPN transistor  62  serve as a level translator between the switched voltage source  54  and a control signal. Therefore, the resistor  68  and the base of NPN transistor  62  do not have the same voltage levels or voltage swings. 
   Referring now to  FIG. 3 , a second embodiment of switching circuitry  111  for discharging a smart material actuator capacitive load  158  is shown. When the switch  152  is closed, current flows into the voltage divider network  160  from the source  154 , turning the NPN transistor  162  on and causing current to flow through the resistor  70 . This turns the NPN Darlington transistor pair  164  off, and the positive-negative-positive (PNP) transistor  72  on, causing current to flow through the resistor  166  for discharging the capacitive load  158 . The rate of discharge is determined by the impedance of the resistor  166  and the capacitive load  158 . When the switch  152  is open, the NPN transistor  162  turns off, allowing current to flow through the resistor  70  to the base of the PNP transistor  72 , turning the PNP transistor  72  off. The NPN Darlington pair  164  supplies current to the capacitive load  158  through the resistor  74 . The rate of charge is determined by the impedance of the resistor  74  and the capacitive load  158 . The resistor  70  and the NPN transistor  162  serve as a level translator between the voltage source  154  and a control signal generated by the closure of switch  152 , for example. Therefore, the resistor  70  and the base of NPN transistor  162  do not have to have the same voltage levels or voltage swings. 
   Referring now to  FIG. 4 , a preferred embodiment of a driver for a smart material actuator capacitive load  76  according to the present invention includes a controllable power source  10 A and switching circuitry  11 A. A switchable input voltage source  12 A is applied to the controllable power source  10 A and at the same time the switching circuitry  11 A is disabled and the capacitive load  76  is charged. When the input voltage source  12 A is removed, the controllable power source  10 A is stopped, and the switching circuitry  11 A is enabled and the capacitive load  76  is discharged. The actual impedance of the controllable power source  10 A controls the rate at which the capacitive load  76  is charged, and the impedance of the switching circuitry  11 A controls the rate which the capacitive load  76  is discharged. 
   Referring now to  FIG. 5 , a second embodiment of a driver for a smart material actuator according to the present invention includes a controllable power source  10 B and switching circuitry  111 A,  111 B,  111 C,  111 D,  111 E,  111 F. An input voltage source  12 B is applied to the controllable power source  10 B. The voltage to be switched is generated continuously. When the control signal to the switch circuits  111 A,  111 B,  111 C,  111 D,  111 E,  111 F is low, the NPN Darlington pair  164 A of each respective circuit  111 A,  111 B,  111 C,  111 D,  111 E,  111 F, but shown only in circuits  111 A and  111 F, is enabled and each respective capacitive load is charged. When the control signal is high, the PNP transistor  72 A of each respective unit  111 A,  111 B,  111 C,  111 D,  111 E,  111 F, but shown only in circuits  111 A and  111 F, is enabled and the capacitive load is discharged. 
   In the embodiment of  FIGS. 1-5 , various components were included according to the current carrying ability, voltage rating, and type of the components. Other suitable components can include Field Effect Transistor (FET) and bipolar junction transistor (BJT) small signal and power transistors, wire wound, thin film and carbon comp resistors, ceramic, tantalum and film capacitors, wound, and Low Temperature cofired ceramic (LTCC) transformers, or any combination of suitable components commonly used for high volume production. Although these materials given as examples provide excellent performance, depending on the requirements of an application use of other combinations of components can be appropriate. Likewise, the embodiment illustrates components that are commercially available. 
   While the invention has been described in conjunction with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment but, on the contrary, it is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as permitted under law.