Flexural ultrasonic transducers are a widely available type of ultrasonic sensor used for flow measurement, proximity, and industrial metrology applications. The flexural ultrasonic transducer is commonly operated in one of the axisymmetric modes of vibration in the low-kilohertz range, under 50 kHz, but there is an increasing demand for higher frequency operation, towards 300 kHz. At present, there are no reports of the measurement of high-frequency vibrations using flexural ultrasonic transducers. This research reports on the measurement of high-frequency vibration in flexural ultrasonic transducers, utilizing electrical impedance and phase measurement, laser Doppler vibrometry, and response spectrum analysis through the adoption of two flexural ultrasonic transducers in a transmitreceive configuration. The outcomes of this research demonstrate the ability of flexural ultrasonic transducers to measure highfrequency ultrasound in air, vital for industrial metrology.

Flexural ultrasonic transducers are capable of high electro-mechanical coupling efficiencies for the generation or detection of ultrasound in fluids. They are the most common type of ultrasonic sensor, commonly used in parking sensors, because the devices are efficient, robust, and inexpensive. The simplest design consists of a piezoelectric disc, bonded to the inner surface of a metal cap, the face of which provides a vibrating membrane for the generation or detection of ultrasonic waves in fluids. Experimental measurements demonstrate that during the excitation of the piezoelectric element by an electrical voltage, there are three characteristic regions, where the frequency of the emitted ultrasonic wave changes during the excitation, steady-state, and the final decay process. A simple mechanical analogue model is capable of describing this behaviour. V C 2017 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license

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The flexural ultrasonic transducer is a unimorph device which typically comprises a piezoelectric ceramic bonded to a metallic membrane. It is widely applied in industrial applications for metrology and proximity sensing. However, the electromechanical and dynamic characteristics of this class of transducer have only recently been reported, and the influence of different excitation levels on dynamic nonlinearity remains unclear. Dynamic nonlinearity in high-power piezoelectric ultrasonic transducers is familiar, where the performance or dynamic stability of the transducer can significantly reduce under high amplitudes of excitation. Nonlinearity can manifest as measurable phenomena such as resonance frequency drift, influenced by thermomechanical phenomena or structural constraints. There is relatively little reported science of the dynamic nonlinearity in the vibration response of flexural ultrasonic transducers. This study examines the vibration responses of four flexural ultrasonic transducers, showing the existence of dynamic nonlinearity for increases in excitation voltage. An analytical solution of the governing equations of motion for the flexural ultrasonic transducer is presented which complements the experimental investigation, and suggests a close relationship between material properties and nonlinearity. This research demonstrates a detailed dynamic characterization of the flexural ultrasonic transducer, showing the potential for the optimization of dynamic performance in industrial measurement applications.

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Recent studies of the electro-mechanical behavior of flexural ultrasonic transducers have shown that their response can be considered as three distinct characteristic regions, the first building towards a steady state, followed by oscillation at the driving frequency in the steady state, before an exponential decay from the steady state at the transducer's dominant resonance frequency, once the driving force is removed. Despite the widespread industrial use of these transducers as ultrasonic proximity sensors, there is little published information on their vibration characteristics under different operating conditions. Flexual transducers are composed of a piezoelectric ceramic disc bonded to the inner surface of a metallic cap, the membrane of which bends in response to the high-frequency ceramic vibrations of the ceramic. Piezoelectric devices can be subject to nonlinear behavior, but there is no reported detail of the nonlinearity in flexural transducers. Experimental investigation through laser Doppler vibrometry shows strong nonlinearity in the vibration response, where resonance frequency reduces with increasing vibration amplitude.