ANALYSIS OF THE CHARACTERISTICS OF THE INDUCED VOLTAGE ACROSS A THICKNESS-MODE PIEZOELECTRIC TRANSDUCER

The transient voltage generated directly across a thickness-mode piezoelectric transducer excited by an external voltage source has been related in reference 1 to the width of the initial pulse in the received ultrasonic signal display and hence to the width of the dead zone of detection. One dimensional theory predicts that this "induced voltage" is approximately the superposition of a decaying high frequency oscillation, of frequency close to that of the radiated ultrasonic wave, and a steep exponential rise, resulting principally from the charging of the clamped capacity of the transducer by the applied transient voltage.
In this paper detailed computations of this induced voltage and of the seperate effects of several structural parameters are given for a step voltage excitation, based on Mason's one dimensional equivalent circuit model These parameters are （1） the specific acoustic impedance of the backing or loading, （2） the mechanical Q of the piezoelectric plate, （3） the thickness of the wear plate （of corundum）, and （4） the inductance of a parallel induction coil.
Theoretical computations show that:
（1） With light loading or backing, the high-frequency oscillations are large and extended; with heavy loading or backing, they are small and decay rapidly.
（2） For the same ceramic, when the mechanical Q is approximately smaller than 100, a decrease in Q will decrease the amplitudes of the high-frequency oscillations while when Q>100 approximately, varying Q does not noticeably change the high-frequency oscillations.
（3） In the case of light loading, increasing the thickness of the wear plate will raise the amplitudes of the high-frequency oscillations; in the case of heavy loading, when the specific acoustic impedance of the load matches or nearly matches that of the piezoelectric ceramic, the wear plate does not noticeably affect the induced voltage.
（4） The preliminary investigation of the effect of a parallel induction coil encourages a more thorough study of the effects of adding electrical components.
The effect of varying the specific acoustic impedance of the loading or backing, as predicted by the theory, has been experimentally verified for a PT ceramic. For a PZT ceramic, there appear in the induced voltage an additional, theoretically unpredicted, irregular low frequency oscillation component. Likely the one dimensional assumption does not hold sufficiently for this ceramic. Further investigation is needed.