Analysis and solution for frequency tracking degradation of phase-locked loop of piezoelectric transducers

In order to improve the precision and reliability of frequency tracking of piezoelectric transducers during the ultrasonic drilling of carbon-fiber composites, four kinds of tracking degradations of the traditional phase-locked loop (PLL) system, namely the lock losing, the mistracking, the dead lock and the large tracking error, are analyzed based on the equivalent circuit of piezoelectric transducers. Then, the reasons for the degradations are explained, finding out that to counteract the static capacitance is the only way to overcoming the degradation. Accordingly, 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 transducer mechanical resonance is higher than -20~10Hz, that the mistracking and the dead lock are effectively overcome, and that the tracking bandwidth is up to 6kHz.