Investigation of the variable-mass method for regulating the resonant frequency of ultrasonic transducers

To meet the diverse frequency requirements of ultrasonic systems operating under multi-material and multi-condition environments, this paper proposes a resonant frequency regulation method for ultrasonic transducers based on the variable-mass approach. By attaching an external metal ring (hereinafter referred to as a frequency-tuning ring) to the transducer and varying its axial position, the coupled effect of the ring on the transducer’s stiffness and inertia is utilized to achieve both upward and downward frequency adjustment. Contrary to the conventional understanding that “adding mass reduces frequency”, both simulation and experimental results show that when the frequency-tuning ring is installed near the displacement node of the transducer, the system frequency not only does not decrease, but instead increases. The influence of the tuning ring on the longitudinal modal frequency was calculated using the finite element method, and the equivalent mass and equivalent stiffness of the system were obtained from the kinetic energy and elastic potential energy, respectively. Based on these, a mathematical model for frequency variation was derived using the Rayleigh quotient theory. The results show that the tuning range is affected by the material, position, and geometric parameters of the tuning ring. Taking a transducer with a nominal resonant frequency of 28 kHz as the study object, a tuning range of up to 9 kHz was achieved. The simulation and experimental results are consistent. This method offers a wide tuning range, simple structure, and strong potential for practical engineering applications and large-scale implementation.