Piezoelectric actuator system

A piezoelectric actuator has at least two piezoelectric actuator elements connected in series with each other mechanically and electrically and form a half bridge in an amplifier bridge circuit having a further half bridge formed by two series connected electronic switches which are operated or clocked by a control circuit such as a pulse modulator circuit for periodically energizing the piezoelectric actuators in push-pull fashion. A choke (22) is connected between the junction point (JP1) of the two piezoelectric actuators and the junction point (JP2) between the two electronic switches (23, 24) for assuring a loss free reverse charging of the two piezoelectric actuators functioning as electrical capacitors in the energizing bridge circuit (21). The choke 22 functions as an energy storage and the stored energy is used in the push-pull charging of the capacitors (19, 20) formed by the piezoelectric elements.

PRIORITY CLAIM

This application is based on and claims the priority under 35 U.S.C. 119 of German Patent Application 199 61 068.1, filed on Dec. 17, 1999, the entire disclosure of which is incorporated herein by reference.

1. Field of the Invention

The invention relates to a piezoelectric actuator system comprising at least two piezoelectric actuators mechanically arranged in series for producing an output motion or power.

2. Background Information

Piezoelectric actuators have the advantage of a high actuating precision and a fast reaction. Such actuators are components with a high electrical capacity whereby only part of the electrical energy supplied to the actuators is converted to mechanical energy. A large part of this energy is stored in the piezoelectric actuator functioning as a capacitor.

In a dynamic, repetitious or continuos operation of a piezoelectric actuator, considerable electrical power in the form of apparent power flows through the actuator. This apparent electrical power has to be supplied by the driver circuit of the actuator. In the case of periodic driving or energizing the piezoelectric actuator is electrically alternately charged and discharged, whereby electrical energy is cyclically supplied to and withdrawn from the piezoelectric actuator. In known driver circuits for periodic or repetitious driving of the piezoelectric actuator, the stored electrical energy is dissipated during the discharge phase or cycle in the driver or control circuit which has for example an ohmic resistance for the dissipating.

The German Patent Publication DE 197 39 594 C1 describes the use of driver circuits for actuator systems in which energy is dissipated during the discharge cycle. To achieve higher output forces and/or thermal compensation, two piezoelectric actuators are mechanically arranged in series and clamped against each other. In the known system the piezoelectric actuators are symmetrically driven in push-pull fashion so that with the stroke remaining the same, the output forces of the two piezoelectric actuators are added together. Thermal expansions of the piezoelectric actuators compensate each other in the known arrangement and a bias spring commonly used in such piezoelectric actuators is not required.

European Patent Publication EP 0 676 036 B1 describes trimorphic bending piezoelectric actuators with an energy dissipating driver circuit. A trimorphic bending actuator comprises a carrier or base plate with piezoelectric elements glued to both sides of the base plate which functions as one electrode. Both piezoelectric elements form electrical capacitors which are electrically driven reciprocally or in push-pull fashion.

OBJECTS OF THE INVENTION

In view of the above it is the aim of the invention to achieve the following objects singly or in combination:

to create a power-saving piezoelectric actuator system which operates periodically or continuously and is driven or energized in push-pull fashion;

to store and use energy, that is conventionally dissipated, in the reverse charge phase of twin piezoelectric actuators; and

to minimize the recharging or reverse charging energy in a piezoelectric actuator having at least two actuator elements.

SUMMARY OF THE INVENTION

According to the invention, there is provided a piezoelectric actuator system characterized by an amplifier bridge circuit including a first piezoelectric actuator forming a first quarter bridge branch, a second piezoelectric actuator forming a second quarter bridge branch, said first and second quarter bridge branches being connected in series with each other to form a first half bridge having a first junction point between said first and second piezoelectric actuators, a first electronic switch forming a third quarter bridge branch, a second electronic C 1 switch forming a fourth quarter bridge branch, said third and fourth quarter bridge branches being connected in series with each other to form a second half bridge having a second junction point between said first and second electronic switches, said first and second half bridges being connected in parallel with each other to form a full bridge including a third junction point between said first and second half bridges to form a bridge power input and a normally grounded fourth junction point between said first and second half bridges opposite said power input, a choke connected to said first and second junction points, a power supply (U ) connected to said third junction point forming said bridge power input, and a control circuit connected to said first and second electronic switches for opening and closing said electronic switches to energize said first and second piezoelectric actuator's.

The solution according to the invention provides the advantage that the electrical energy stored in the piezoelectric actuators is not dissipated in the driver circuit formed by the electronic switches and a control circuit of the actuator system. Instead the energy is recovered and used again with the aid of the choke. Furthermore, the above-mentioned advantages of an automatic thermal compensation are also achieved in the piezoelectric driver system according to the invention.

DETAILED DESCRIPTION OF PREFERRED EXAMPLE EMBODIMENTS AND OF THE BEST MODE OF THE INVENTION

FIG. 1 shows an actuator system according to the invention with two piezoelectric actuators 1 and 2 , each formed by a respective piezoelectric stack 3 , 3 held in a respective articulated frame 4 , 4 . The frame 4 comprises a support plate 5 and an output member such as a plate 6 positioned opposite each other. The Ago frame 4 further comprises four connector rods 4 A, 4 B, 4 C and 4 D articulated to the base plate 5 and to the output plate 6 . The frame 4 also has a first end plate 4 E and a second end plate 4 F. The four connector rods are also articulated to these first and second end plates 4 E and 4 F. The base plate 5 is rigidly secured to a support frame 7 . The output plate 6 is movably guided in a guide slot 8 of the support frame 7 , whereby a power output pin 8 A or the like secured to the output plate 6 is movably guided in the guide slot 8 of the support frame 7 . The second frame 4 comprises identical elements 4 A, 4 B, 4 C, 4 D, 4 E, 4 F and a fixed base plate 5 . The output plate 6 is provided in common for both frames 4 and 4 . These elements of the frame 4 are assembled in the same way as described above with reference to the frame 4 .

Each of the frames 4 , 4 for holding the two piezoelectric stacks 3 , 3 is provided with an adjustment screw 11 , 11 passing through a respective threaded hole in the corresponding end plate 4 F and 4 F and bearing against pressure plates P, P respectively. These pressure plates are preferably electrically insulated from the respective stack. The screws 11 , 11 are used for calibrating the respective piezoelectric stack 3 , 3 to a zero position when the stack is not energized. In a properly calibrated actuator, the output pin 8 A is centered in the guide slot 8 , for example.

For energizing the piezoelectric stacks 3 , 3 each stack has two electrodes. The stack 3 has electrodes E 1 and E 2 . The stack 3 has electrodes E 3 and E 4 . The electrodes E 2 and E 3 are electrically interconnected to form, for example the junction point JP 1 in FIG. 3 . Electrode E 1 will be connected to junction point JP 3 and electrode E 4 will be connected to junction point JP 4 in FIG. 3 , for example as will be described in more detail below.

When an electrical energizing voltage is applied to the electrodes E 1 and E 3 on the one hand and to electrode E 1 or E 4 on the other hand, the piezoelectric stacks 3 , 3 expand or contract in push-pull fashion in the direction of the arrow 9 . This movement of the piezoelectric stacks is transmitted to the articulated frames 4 , 4 holding said piezoelectric stacks 3 , 3 , whereby the support plates 5 , 5 and the output plate 6 of said articulated frames move away from each other or toward each other in the direction of the arrow 10 . The extent of the motion of the output pin 8 A is amplified relative to the motion of the piezoelectric stacks 3 , 3 by the articulated frame structures 4 and 4 in which the piezoelectric actuators 1 and 2 are mechanically arranged in series and held in the support frame 7 provided for both articulated frames 4 , 4 . For this purpose the support plates 5 , 5 of the piezoelectric actuators 1 and 2 are mounted to opposite ends of the support frame 7 . The centrally positioned output plate 6 acts in response to the motions of both piezoelectric stacks in push-pull fashion, thus providing a common output for both stacks 3 , 3 of the actuator system whereby the pin 8 A carries out its working stroke in the guide slot 8 of the support frame 7 . The piezoelectric actuators 1 and 2 are driven electrically in opposition or in push-pull fashion so that one stack pulls while the other stack pushes and vice versa. As a result the output pin 8 A departs from the zero position which is adjustable by the calibration screws 11 , 11 as described above.

FIG. 2 shows another suitable mechanical serial arrangement 12 of two piezoelectric actuators 13 and 14 according to the invention. Two piezoelectric plates form the actuators 13 and 14 which are energized electrically by drive voltages having opposing polarities as shown in FIG. 2 to sustain a push-pull operation as in a known trimorphic bending actuator. The two piezoelectric plate actuators 13 , 14 of the actuator 12 are arranged one on each side of a metal carrier plate that forms a central electrode 15 connected in an electrically conductive manner to the piezoelectric plate actuators 13 , 14 for electrically driving the two plates in push-pull fashion. The two piezoelectric plate actuators 13 and 14 are connected through respective electrodes E 1 and E 2 to respective energy supply electrical conductors 16 , 17 . The control electrode 15 is connected to a supply conductor 18 .

The solution according to the invention is not limited to the mechanical arrangements described above. Other alternative mechanical arrangements of two or more piezoelectric actuators are suitable for the purposes according to the invention as long as the arrangements of the piezoelectric actuators are energizable in push-pull fashion.

FIG. 3 shows a schematic electrical circuit diagram according to the invention. The piezoelectric actuator system is constructed as a push-pull system comprising a clocked amplifier bridge 21 wherein the two electrically and mechanically serially connected piezoelectric actuators 1 and 2 of FIG. 1 or 13 and 14 of FIG. 2 form a first half bridge with a first junction point JP 1 of the bridge circuit 21 . In FIG. 3 the piezoelectric actuators are shown as capacitors 19 and 20 because the actuators function as capacitors in the bridge circuit 21 .

The second half bridge is formed by two potential reversing electronic switches 23 , 24 providing a second bridge junction point JP 2 . According to the invention, a choke 22 is connected between the junction points JP 1 and JP 2 of the first and second half bridges of the amplifier bridge circuit 21 . The piezoelectric actuators are energized by an external supply voltage U connected to a third junction point JP 3 . One end of each of the two half bridges is also connected to the third junction point JP 3 . The fourth bridge junction point connects the opposite ends of the two half bridges to ground. The two electronic switches 23 and 24 are so controlled that one switch is closed while the other is open and vice versa for repeatedly changing the potential of the energizing voltage at the terminals of the piezoelectric actuators 19 and 20 to operate these actuators in push-pull fashion. A control circuit 25 for controlling the operation of the electronic switches 23 , 24 is preferably a pulse width modulator circuit PWM which in turn is controlled in closed loop fashion by a closed loop control circuit 26 of the actuator system.

According to the invention a choke 22 is connected between a first junction point JP 1 and a second junction point JP 2 . The first junction point JP 1 is formed between the two piezoelectric actuators shown as capacitors 19 and 20 . The second junction point JP 2 is formed between the two polarity reversing electronic switches 23 , 24 . The third junction point JP 3 interconnects the supply voltage terminal U , one terminal of the electronic switch 23 and one terminal of the actuator 19 . The fourth junction point JP 4 interconnects ground with one terminal of the electronic switch 24 and one terminal of the actuator 20 .

The pulse-width modulator switch control circuit 25 has one input corrected to a rated input voltage U R . A control input of the control circuit 25 receives an output feedback control signal from a status closed loop control circuit 26 which acquires and processes the current value I L and the voltage actual values U L at the choke 22 . For this purpose the choke 22 is preferably the primary winding of a transformer which has a secondary winding 22 one end of which is connected to one input terminal of the closed loop control circuit 26 and the other end of the secondary winding 22 is connected to ground. Another control input of the circuit 26 is connected to the first junction point JP 1 .

In the push-pull system described above the capacitors 19 and 20 are recharged during frequent operation by means of the clocked amplifier bridge 21 . In this arrangement, the supply voltage U is applied to the capacitors 19 and 20 but with repeatedly reversing polarities as controlled by the switches 23 , 24 .

The supply voltage U is maintained by a voltage source (not shown) and a grounded blocking capacitor 27 . As mentioned the choke 22 is connected between the output of the half bridge with the two electronic switches 23 , 24 and the first junction point: JP 1 of the two piezoelectric actuators 19 , 20 in the other half bridge of the bridge amplifier circuit 21 . The choke 22 serves as an energy storage thereby assuring a substantially loss-free recharging of the capacitors 19 and 20 in push-pull fashion by means of potential equalization between the control connection or junction point JP 1 and the supply voltage U during a clocked continuous operation. With switch 23 open and switch 24 closed as shown the potential at the junction point JP 1 is Ua.

In this way it is possible to reverse charge the capacitors 19 and 20 substantially without any external energy supply. In this arrangement, the choke 22 functions as an intermediate energy storage device. Only the losses in the capacitors and the external dissipation need to be covered or provided by the voltage supply U . Due to the fact that the reverse recharging process of the capacitors according to the invention takes place almost without an external energy supply, the electrical power requirement for driving the piezoelectric actuators 1 and 2 has been considerably reduced according to the invention as compared to hitherto known solutions.

Incidentally, the diodes D 1 and D 2 connected in parallel to the switch 23 , 24 respectively function as protection diodes to prevent current flow in a wrong direction.