Continuous adaptation of the mechanical resonance frequency of power ultrasonic transducers by switching electrical circuits

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One of the biggest challenges in the production of power ultrasonic transducers is to ensure that the resonant frequency is within a sufficiently small tolerance range. In order to achieve a large vibration amplitude, ultrasonic systems are operated close to or even in resonance. The natural frequency depends on many influencing factors, which can only be controlled permanently to a limited extent. These include ambient conditions, relaxation behavior, degradation of the piezoelectric properties and the process. The typical answer to this problem is to adjust the operating frequency to the changed resonance frequency, which is often a sufficient solution. It becomes a problem, however, if several ultrasonic transducers are mechanically coupled, then an exact tuning is necessary to avoid beating. The solution to operate only one of the transducers resonantly and to "drag along" the others has limitations in power utilization, so the system has to be oversized. With this research project a completely new way will be taken. The piezoelectric coupling shall be used to adjust the resonance, i.e. the frequency at which the highest amplitude per excitation is reached. Thus, the system shall be adapted to the "desired frequency" and not the frequency to the system. Our preliminary work shows that the appropriate electrical circuitry offers this potential. However, so far only the basic feasibility has been shown and the topic has not yet been investigated systematically. In particular, there is no consideration of the interaction with the load of the ultrasonic oscillators. Three methods are to be investigated to change the resonance frequency. (1) Impedance resonance Tuning on Tuning Piezos (ITTP): In addition to the piezo elements used for excitation, "tuning" piezo elements are integrated into the system which are wired with a synthetic impedance. With this impedance, negative capacitances can be realized and thus a large potential useful frequency range can be achieved. (2) Switched resonance Tuning on Driving Piezos (STDP): Since power ultrasound systems are typically operated with switching amplifiers, there are switching pauses in which the terminals of the piezo elements can be short-circuited or isolated. Due to the piezoelectric coupling, the piezo elements exhibit significantly (20%) different stiffnesses in both states. With the ratio the average stiffness can be adjusted. (3) Switched Impedance resonance Tuning on Driving Piezos (SITDP): In the switching pauses, a synthetic impedance is connected to the terminals, thus significantly increasing the insertion range of the STDP method.As a result, it is possible to actively and continuously adjust the system characteristics instead of tracking the excitation to the system.