Design of ultrasonic power based on a transducer admittance characteristic frequency-tracking method

Resonant-frequency drift caused by load variation and thermal effects is a major source of instability in ultrasonic-machining systems, leading to reduced electro-acoustic efficiency and fluctuating output power. Conventional phase-locked-loop (PLL) tracking schemes often suffer from controller–parameter sensitivity and frequency jitter under continuously drifting resonance, while impedance-scanning methods are accurate but time-consuming. To address these issues, this study proposes an ultrasonic power supply design and resonance-tracking method based on the admittance characteristics of the piezoelectric transducer under operating conditions. A voltage–current sampling circuit is developed to measure the magnitudes and phase differences at the transducer terminals in real time; the corresponding admittance information is then reconstructed, and the series resonant frequency is estimated through a two-step frequency transformation without full-range scanning. Experiments were conducted using a 100 W, 40 kHz transducer: the resonant frequency obtained by the proposed method was 41 320 Hz compared with 41 262 Hz as measured by an impedance analyzer, yielding a frequency deviation of 58 Hz and a relative error of 0.14 %. The results demonstrate that the proposed method can rapidly locate and lock the resonance frequency using only two frequency transformations, providing accurate in situ resonance estimation and improved output power stability for ultrasonic-machining power supplies.