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I am at the stage of designing a system that match impedance of the power output (half-bridge) to a piezo transducer operating at 1MHz (Max 60W of power). I calculated the piezo parameters from the impedance measurements and compared the model impedance graph with the measured impedance graph (the graphs are similar).

Most scientific sources propose a matching system made in such a way that it consists of a series inductor and a capacitance parallel to the piezo. Since the piezo already has such a capacity (C1 in my diagram), the coil itself is enough. The authors propose to select the coil so that the resonant frequency of the coil from the matching system and C1 is the same as the piezo mechanical frequency represented by the L1, C2 and R1 branches (In the work A High-Tolerance Matching Method against Load Fluctuation for Ultrasonic Transducers, the authors propose that the resonance of coil with the piezo capacitance should be the square root of the 2 of piezo resonance frequency). However, after selecting such a coil, the resonant frequency of the system changes for some unknown reason.

The questions I can't find answers to are:

  1. Is tracking this new frequency (in the case below 919kHz instead of 1.020MHz) the optimal strategy for delivering maximum power to the piezo?
  2. Isn't providing a signal at a new frequency bad because it is not the mechanical frequency of the system?
  3. If so, why is this method so popular? Are there better methods?

Simulations:

Piezo Piezo + Impedance matching Comparison
enter image description here enter image description here enter image description here

Scientific articles:

  • A high-tolerance matching method against load fluctuation for ultrasonic transducers
  • Review of the design of power ultrasonic generator for piezoelectric transducer
  • Design and construction of a prototype of an ultrasonic applicator for ultrasonic soldering, cutting and drilling
  • Effect of the Matching Circuit on the Electromechanical Characteristics of Sandwiched Piezoelectric Transducers
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  • \$\begingroup\$ Do you want to maximize power into the unloaded piezo, or do you want, say, maximum displacement when the piezo is coupled to a load?? \$\endgroup\$
    – Fred
    Commented Apr 18 at 18:28
  • \$\begingroup\$ My goal is to maximize the vibration amplitude when the piezo is loaded, and to minimize the power oscillating between the driver and transducer (reactive power). The efficiency of the system is best at resonance, which is why I use a resonance tracking system. My doubt is that using the coil as a matching system (reducing the phase in the resonance and acting as a harmonic filter) shifts the electrical resonance seen by the driver. In this case, the system tracks a frequency that in no way coincides with the frequency of mechanical vibrations. \$\endgroup\$
    – piotr
    Commented Apr 19 at 6:11
  • \$\begingroup\$ To what source impedance are you trying to match the transducer? The schematic only shows a current source. At the moment the additional 7.3uH inductor just moves the parallel resonance of the transducer a little up in frequency. \$\endgroup\$
    – Raonoke
    Commented Apr 30 at 18:09
  • \$\begingroup\$ My source impedance is very low, in order of milliohms - output of half bridge. By matching, I mean eliminating capacitance at mechanical resonance and limit current at higher harmonics. In the schematic there is a current source, because that is an easy trick to calculate impedance of the system - when current source have value of 1A, Volts are the same as impedance - 5V mean that the impedance is 5 Ohm. \$\endgroup\$
    – piotr
    Commented May 8 at 7:22

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To your first question: I think tracking a new frequency even being different from the mechanical resonance frequency, can still be an optimal strategy for delivering maximum power to the piezo.

Achieving impedance matching between the driver and the piezo is crucial for efficient power transfer. So tracking a new frequency that enhances impedance matching and boosts your power transfer efficiency can be an optimal approach.

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