Sensorless control of a thermoacoustic refrigerator

A method for commanding the frequency of the signal that drives an electrodynamic sound source to maintain the coupled thermoacoustic resonator at its resonance frequency is described. The method uses only the current and voltage applied to the source terminals to select the operating frequency, thereby eliminating the need for pressure and displacement sensors used in previous phase-locked-loop frequency tracking systems. An equivalent circuit model of the coupled electro-mechanical system is used to extract piston position and pressure from the measured input current and voltage waveforms. Parameters of the electro-mechanical model can be obtained from a least-squares fit of the model using measured current, voltage, position, and force over the frequency range of interest. Parameters thus obtained are in good agreement with individual physical measurements of those same parameters. Results are presented for the implementation of this controller in a large thermoacoustic chiller.