Low noise techniques applied to a piezoceramic receiver for gas coupled ultrasonic flaw detection
Piezoelectric plate transducers are commonly used for the generation and detection of ultrasonic signals and have applications in, for example, non-destructive testing and medical imaging. A rigorous theoretical investigation of thermal noise in plate transducers has been undertaken with the aim of establishing the absolute limits of receiver sensitivity in terms of both Minimum Detectable Power (MDP) and Minimum Detectable Force (MDF). The central feature of the work has been the development of two independent theories which provide identical results. One theory is based on an electrical approach which makes use of an extensively modified version of Hayward's linear systems model of the piezoelectric plate transducer, along with the well known work of Johnson and Nyquist. The other theory is based on a mechanical approach which makes use of the less well known work of Callen and Welton. Both theories indicate that only two parameters are required in order to determine the MDP and MDF of an open circuit transducer. These parameters are the transducer's characteristic acoustic impedance and its mechanical quality factor. Significantly, the thermally limited sensitivity of an open circuit receiving transducer has been shown not to be related to its electromechanical coupling efficiency or any of its electrical properties. By applying the new theories it has been possible to design an ultra low noise ultrasonic receiver with wide ranging applications. Among other things, this receiver has been used to demonstrate the viability of a robust and truly practical air-coupled Lamb wave scanner suitable for detecting defects in thin plates which can be made from almost any type of material. The complete system has sufficient sensitivity to allow rapid scanning without the requirement for transducer matching layers or electronic signal averaging.