Driving techniques for high power PZT transducer arrays

Thesis Presented for the Degree of Magister Technologiae in the Department of Electrical Engineering Cape Peninsula University of Technology 2006Because of the nature of piezoelectric ceramics and the physical construction pf high power piezoelectric transducers, such devices are inherently non-linear and become unpredictable when driven at high power. To drive an ultrasonic transducer or an array thereof efficiently, specific resonant points are used. These poin~s are characterised by the devices' mechanical modes of oscillation. At high electrical power levels, the resonance points of PZT transducers vary. The movement of the resonances points in the frequency domain, coupled with the transducers high Q, is severe enough to seriously hamper the devices' efficiency. The problem is specifically apparent when multiple transducer arrays are driven at power. The electrical fluctuations and interactions of the characteristics of separate transducers cause arrays to be driven efficiently at a single resonance point. To efficiently drive an array of PZT transducers it is necessary to employ a .suitable technique. Although several methods exist in the literature, each is designed for a specific configuration of transducers and dedicated matching circuitry. The fundamental flaw in most methods is that they are conceived with the assumption all PZT transducers are identical and can be driven as such. Inherent nonlinearities caused by poling and construction methods, result in each transducer to be slightly different causing a superposition of resonance frequencies for each transducer array. Existing methods cannot be used to efficiently drive generic transducer arrays and a novel approach has been adopted to accommodate transducer nonlinearities. This novel approach can be described as a culmination of two driving techniques and has been named, Swept Frequency Dwelling (SFD). This thesis examines five different driving techniques and quantifies their effectiveness by means of experimental evaluation proficiencies. The driving techniques are grouped into two categories - straight driving techniques and frequency sweeping techniques - which are compared and evaluated. In conclusion, a novel method for driving ultrasonic transducer arrays was established with the aim of eliminating some detrimental effects of other driving techniques, while exploiting some of their positive attributes and was found to be effective