The figure schematically depicts an oscillator circuit for driving a piezoelectric transducer to excite vibrations in a mechanical structure. The circuit was designed and built to satisfy application-specific requirements to drive a selected one of 16 such transducers at a regulated amplitude and frequency chosen to optimize the amount of work performed by the transducer and to compensate for both (1) temporal variations of the resonance frequency and damping time of each transducer and (2) initially unknown differences among the resonance frequencies and damping times of different transducers. In other words, the circuit is designed to adjust itself to optimize the performance of whichever transducer is selected at any given time. The basic design concept may be adaptable to other applications that involve the use of piezoelectric transducers in ultrasonic cleaners and other apparatuses in which high-frequency mechanical drives are utilized.
This circuit includes three resistor-capacitor networks that, together with the selected piezoelectric transducer, constitute a band-pass filter having a peak response at a frequency of about 2 kHz, which is approximately the resonance frequency of the piezoelectric transducers. Gain for generating oscillations is provided by a power hybrid operational amplifier (U1). A junction field-effect transistor (Q1) in combination with a resistor (R4) is used as a voltage-variable resistor to control the magnitude of the oscillation. The voltage-variable resistor is part of a feedback control loop: Part of the output of the oscillator is rectified and filtered for use as a slow negative feedback to the gate of Q1 to keep the output amplitude constant. The response of this control loop is much slower than 2 kHz and, therefore, does not introduce significant distortion of the oscillator output, which is a fairly clean sine wave.
The positive AC feedback needed to sustain oscillations is derived from sampling the current through the piezoelectric transducer. This positive AC feedback, in combination with the slow feedback to the voltage-variable resistors, causes the overall loop gain to be just large enough to keep the oscillator running.
The positive feedback loop includes two 16-channel multiplexers, which are not shown in the figure. One multiplexer is used to select the desired piezoelectric transducer. The other multiplexer, which is provided for use in the event that there are significant differences among the damping times of the 16 piezoelectric transducers, facilitates changing the value of one of the resistors in the positive-feedback loop to accommodate the damping time of the selected transducer.
The amplitude of the oscillator output is controlled by use of an externally generated potential, between –5 and +5 VDC, applied via Zener diode D3 and resistor R5 to the gate of Q1: +5 VDC corresponds to an output amplitude of 25 V peak to peak; –5 VDC corresponds to an output amplitude of 9 V peak to peak.
Prior to the development of this circuit, it was common practice to excite vibrational piezoelectric transducers by use of “bang-bang” oscillators, the outputs of which contain significant proportions of harmonics. The harmonics contribute to stress and waste of power in heating the transducers. The near-sine-wave output of this circuit has much lower harmonic content and, therefore, imposes less stress on the transducers and enables them to operate at lower temperature.
Previously, it was also common practice to control the drive amplitude of oscillation by using an additional regulator circuit to control the supply potential. In this circuit, the supply potential is not varied and the amplitude of oscillation is controlled by use of a DC control potential as described above, eliminating the need for the additional regulator circuit.