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

功率超声换能器生产中最大的挑战之一是确保谐振频率在足够小的公差范围内。为了获得较大的振动幅值，超声系统工作在接近共振甚至共振状态。固有频率取决于许多影响因素，这些因素只能在有限的程度上永久控制。这些因素包括环境条件、弛豫行为、压电性能的退化和过程。对这个问题的典型回答是将工作频率调整到变化的谐振频率，这往往是一个足够的解决方案。然而，如果几个超声波换能器是机械耦合的，那么就需要进行精确的调谐以避免跳动。解决方案是只让其中一个换能器产生共鸣，并“拖着”其他换能器，这在功率利用上有限制，所以系统必须超大。这个研究项目将采用一条全新的道路。压电耦合应使用调节谐振，即每次激励达到最高振幅的频率。因此，系统应适应“期望频率”，而不是适应系统的频率。我们的初步工作表明，适当的电路提供了这种潜力。然而，到目前为止，仅显示了基本的可行性，尚未对该主题进行系统的研究。特别是，没有考虑超声振荡器与载荷的相互作用。研究了三种改变谐振频率的方法。(1)调谐压电（ITTP）上的阻抗共振调谐：除了用于激励的压电元件外，“调谐”压电元件被集成到与合成阻抗连接的系统中。利用这种阻抗，可以实现负电容，从而可以实现较大的潜在有用频率范围。(2)驱动压电开关共振调谐（STDP）：由于功率超声系统通常与开关放大器一起工作，因此存在开关暂停，其中压电元件的端子可能被短路或隔离。由于压电耦合，压电元件在两种状态下的刚度差异显著（20%）。利用该比值可以调节平均刚度。(3)驱动压电开关阻抗共振调谐（SITDP）：在开关暂停时，将合成阻抗连接到终端，从而显着增加了STDP方法的插入范围。因此，可以主动和连续地调整系统特性，而不是跟踪系统的励磁。

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.