压电换能器锁相环频率跟踪的失效分析与解决

为提高碳纤维复合材料超声振动钻孔过程中压电换能器频率跟踪的精度及可靠性,以压电换能器等效电路为基础,分析了目前锁相环频率跟踪系统存在的失锁、误跟踪、死锁和跟踪误差大等4种失效形式的原因,指出抵消静态电容是解决跟踪失效的必由之路.据此提出一种基于静态电容补偿法的锁相环频率跟踪技术,该技术可使换能器对锁相环呈现理想的LCR串联谐振特性,有效解决了跟踪失效问题.实验表明:通过该技术,换能器机械谐振频率的跟踪精度可优于-20~10Hz,避免了误跟踪和死锁,跟踪带宽可达6 kHz.

Analysis and Solution for Frequency Tracking Degradation of Phase-Locked Loop of Piezoelectric Transducers

To improve the precision and reliability for the piezoelectric transducer’s mechanical resonance frequency tracking during the process of the carbon-fiber composites’ ultrasonic drilling, based on the equivalent circuit of a piezoelectric transducer, four kinds of tracking performance degradations of the traditional PLL (Phase Locked Loop) frequency tracking system were theoretically analyzed, which are lost for frequency lock, mis-tracking, dead lock and large tracking error. Furthermore, the law of the static capacitance and the dynamic resistance acting on frequency tracking were discussed. Then it was pointed out that to counteract the static capacitance was the only way to solve the problems above. Accordingly, a PLL frequency tracking technique based on the compensation for the static capacitance was presented. Compared with the traditional PLL technique, the transducer could be equivalent to an ideal LCR serial resonance circuit through this method, and as a result the performance degradations were effectively avoided. This frequency tracking technique takes advantages in terms of tracking bandwidth and precision. Experiments carried out show that the transducer’s mechanical resonance frequency shifting with the load can be exactly tracked using this technology, whose precision is less inferior than +10 Hz/-20 Hz. The possibility to mis-tracking and dead lock is minimized, and the tracking range is up to 6 kHz.