Hysteresis effects in high-power ultrasonic transducers

Abstract

It is well known that ultrasonic piezoelectric transducers can be difficult to drive at high power levels. Transducers usually have high Qs, which implies a narrow driving bandwidth and characteristics that may change with temperature, component wear, or variations in the load. Some method of feedback control is usually required to lock onto the optimum transducer driving frequency. Small-signal parameters are usually used in the design of both the control and the amplifier matching system. In this work we have measured the characteristics of several sandwich transducers coupled to water at significant drive levels (50–250 V) by measuring the drive current, voltage, and phase. Increasing the drive level shows a decrease in optimum operating-point frequency. In addition, both the Q and admittance at the driving point decreases. This creates a practical problem as the matching of transducers done on the basis of small-signal values will be inaccurate. Increasing the power levels also results in an abrupt transition to a lower impedance value after the onset of cavitation. Decreasing the power levels is shown to exhibit a hysteresis effect which is thought to be due to the persistence of gas bubbles in the liquid.