Design of an Oscillator Circuit for Langasite (LGS) Based Resonant Pressure - Temperature Sensor

A closed loop frequency tracking electronics circuit has been proposed for the measurement of pressure and temperature using a newly developed Langasite (LGS) piezo crystals. The ambient force and heat applied on the LGS crystal produces the shift in its resonant frequency. The proposed closed loop electronics tracks the shift in resonant frequency and measures the pressure and temperature by relating to the difference in frequency shift. The steady state oscillation condition of the resonator circuit is analytically derived and experimentally verified. The experimental setup consists of an operational amplifier based resonator circuit connected to the langasite piezo crystal placed between a double side diametric forces loading device with temperature controller bath. The diametric force applied on the LGS crystal produces the frequency shift in its fundamental mechanical vibration mode (Cmode). Similarly, the change in temperature produces the frequency shift in its second mechanical vibrational mode (Bmode). The frequency shifts are measured using two separate closed loop resonator electronics for two different measurand. Whenever the force on the LGS crystal is changed by the external unknown pressure, an additional intrinsic material property change is added to the piezo and changes the fundamental oscillating resonant frequency. The closed loop resonant circuit tracks the change in resonance frequency and vibrates the LGS with the new resonance frequency depends on the external pressure strength. Similar procedure is followed for temperature induce frequency tracking measurement also. The proposed frequency tracking electronics concept is simple and it is found that it has high sensitivity and linearity.