Measurement of mechanical quantities using quartz sensors

Abstract

Quartz crystals supply a resonant frequency highly stable in time. However, temperature, strain, etc. can affect this frequency. Usually a quartz crystal is part of an oscillator and housed in order to avoid any influence of the environment. On the other hand by clamping it in the most sensitive direction it may work as a sensor for strain and thus for any mechanical quantity. There are three kinds of quartz sensors be presented: SAW delay lines, SAW resonators and BAW resonators. The measurement principle is the same for all of them. A transceiver sends RF pulses over an antenna in order to stimulate the sensor that produces a pulse response. There is enough energy to retransmit this signal for some time. In the case of the sensor being a resonator, the signal consists of a damped harmonic oscillation with the sensor's resonant frequency, whereas a SAW delay line produces several echo pulses after defined transit intervals. In either case the information on the measured quantity (strain) is contained in the signal. Considering known geometrical and material properties any interesting mechanical quantity is determinable. The advantage of this system is based on the passive operation of the sensor which only has to be coupled with an antenna but works without direct power supply or other devices. So very small and very reliable arrangements are possible. The achieved resolution comes to 0.5% with measurement rates of up to 104 per second. If two sensors are used, an effective temperature compensation can be ensured.