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的最大挑战之一是确保谐振频率在足够小的公差范围内。为了实现大的振动幅度，超声波系统在接近共振或甚至共振的情况下操作。固有频率取决于许多影响因素，这些因素只能在有限的范围内永久控制。这些因素包括环境条件、弛豫行为、压电性能的退化和工艺。这个问题的典型解决方案是将工作频率调整到改变的谐振频率，这通常是一个足够的解决方案。然而，如果几个超声换能器机械耦合，则需要精确调谐以避免跳动，这将成为一个问题。仅共振地操作其中一个换能器并且“沿着”其它换能器的解决方案在功率利用方面具有限制，因此系统必须是超大的。这项研究项目将采取一种全新的方式。压电耦合应用于调整谐振，即达到每次激励最高振幅的频率。因此，系统应适应于“所需频率”，而不是系统的频率。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，只有基本的可行性已经显示，该主题尚未进行系统的研究。特别地，没有考虑与超声波振荡器的负载的相互作用。要研究三种方法来改变谐振频率。(1)调谐压电元件的阻抗共振调谐（ITTP）：除了用于激励的压电元件外，“调谐”压电元件还集成到系统中，并与合成阻抗连接。利用该阻抗，可以实现负电容，并且因此可以实现大的潜在有用频率范围。(2)驱动压电元件的开关谐振调谐（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以短路或隔离。由于压电耦合，压电元件在两种状态下表现出显著（20%）不同的刚度。利用该比率，可以调整平均刚度。(3)驱动压电元件的开关阻抗谐振调谐（SITDP）：在开关暂停时，合成阻抗连接到端子，从而显著增加了STDP方法的插入范围，因此可以主动连续地调整系统特性，而不是跟踪系统的激励。

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.