Ultra low frequency FM sensing of piezoelectric strain voltage

A new method of piezoelectric strain sensing is proposed, allowing measurements at lower frequencies than previously possible. Piezoelectric devices generally exhibit highpass characteristics when loaded, which causes difficulty in measuring at low frequencies using traditional methods. The method proposed in this article converts changes in the piezoelectric device´s strain voltage to changes in capacitance with a high impedance varactor diode. The varactor diode forms part of a feedback network in a Colpitts oscillator, converting variations in capacitance to variations in frequency. The frequency variations are then demodulated using an FM demodulator, and demodulated signals down to 15mHz were achieved. Due to the high impedance of the varactor diode at low frequencies, this method allows the measurement of ultra low frequency variations, and holds the potential for a wireless link between the actuator and a demodulating receiver.