Capacitive coupling offers a simple solution to wirelessly probe ultrasonic transducers. This paper investigates the theory, feasibility and optimization of such a capacitively coupled transducer system (CCTS) in the context of non-destructive evaluation (NDE) applications. The non-contact interface relies on an electric field formed between four metal plates, two of which are physically connected to the electrodes of a transducer while the other two are in a separate probing unit connected to the transmit/receive channel of the instrumentation. The complete system is modelled as an electric network with the measured impedance of a bonded piezoelectric ceramic disc representing a transducer attached to an arbitrary solid substrate. A transmission line model is developed, which is a function of the physical parameters of the capacitively coupled system such as the permittivity of the material between the plates, the size of the metal plates, and their relative position. This model provides immediate prediction of electric input impedance, pulse-echo response and the effect of plate misalignment. The model has been validated experimentally and has enabled optimization of the various parameters. It is shown that placing a tuning inductor and series resistor on the transmitting side of the circuit can significantly improve the system performance in terms of the signal to cross talk ratio. Practically, bulk wave CCTSs have been built and demonstrated for underwater and through-composite testing. It has been found that electrical conduction in the media between the plates limits their applications.
|Journal||IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control|
|Publication status||Accepted/In press - Dec 2013|