Abstract:

In the uhrasonic order to improve the precision and reliability of frequency tracking of piezoelectric transducers during drilling of carbon-fiber composites, four kinds of tracking degradations of the traditional phase-locked loop （PLL） system, namely the lock losing, the mis-tracking, the dead lock and the large tracking error, are ana- lyzed based on the equivalent circuit of piezoelectric transducers. Then, the reasons for the degradations are ex- plained, finding out that to counteract the static capacitance is the only way to overcoming the degradation. Accor- dingly, a PLL frequency tracking technique based on the static capacitance compensation is proposed, by which the transducer is equivalent to an ideal LCR serial resonance circuit, thus the tracking degradations being effectively solved. Experimental results show that, by using the proposed technique, the frequency tracking precision of trans- ducer mechanical resonance is higher than -20 - 10 Hz, that the mis-tracking and the dead lock are effectively overcome, and that the tracking bandwidth is up to 6 kHz.

Key words: piezoelectric transducer, phase-locked loop, frequency tracking, tracking degradation, static capacitance