Optimizing piezoelectric crystal preload in ultrasonic transducers

Designers of industrial ultrasonic transducers commonly use the integral piezoelectric crystals as a force sensor for setting preload. This usually involves a press fixture and charge amplifier for the measurement of voltage versus force. Although this is a very convenient method, it may be inaccurate since the governing piezoelectric properties can vary ±20% from lot-to-lot. Other preload schemes employ a force gauge during pressing, or use some experimental metric via testing of the individual crystals prior to assembly; this can include Bode plots of crystals, or the use of a universal materials testing machine. This research quantifies the performance of ultrasonic transducers based on preload, with the goal of improving the aging stability of impedance and current versus displacement gain. Inadequate preload can result in dynamic gapping at interfaces, manifesting as higher impedance and prolonged aging affects. Excessive preload can result in pronounced depoling with unstable impedance and aging. This investigation focuses solely on the common Navy Type III, PZT8 piezoelectric material. Several metrics are investigated such as Bode phase window (individual crystals and transducer), capacitance and electromechanical coupling factor. The experimental and theoretical research methods include Bode plots, equivalent circuits, scanning laser vibrometer, materials testing machine and finite element analysis correlation.