Use of inharmonic modes of trapped-energy resonators for liquid-level sensing

The authors have been studying a new sensor device that employs a piezoelectric thickness vibrator operating in a trapped-energy mode for detecting slight variation in a liquid level. Both conventional type and backward-wave type trapped-energy resonators can produce inharmonic modes around the main resonance under specific conditions. The decay factor determining the profile of the evanescent field differs between the fundamental and inharmonic modes. Therefore, performance for the liquid level sensing is regarded as different depending on which of the two modes is employed. In this paper, some results of experiments are presented to confirm the prediction described above. Gradual variations in resonance or anti-resonance characteristics with liquid level have been observed when inharmonic modes are used.