Piezoelectric resonant sensors with contactless interrogation for mass-sensitive and acoustic-load detection

Piezoelectric resonator sensors that can be contactless interrogated as passive elements are proposed. The interrogation technique is based on time-gated excitation and detection phases, exploiting the sensing of the transient response of the resonator to derive both its fundamental resonant frequency and quality factor. The technique provides the advantage to be to first order independent on the interrogation distance across the operating range. The proposed system can be exploited for the measurement of physical or chemical quantities affecting the electromechanical resonant response of the sensor. In particular, PZT thick-film resonators with fundamental resonant frequencies of 5.7 MHz and 6.5 MHz have been successfully interrogated both in air and in liquid environments. Operating distances of up to 20 mm in air have been attained. The principle has been applied implementing a variable-mass resonator sensor for humidity sensing in closed volumes and a submersible resonator sensor operated in liquid environments.