Remote sensing using quartz sensors

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Abstract
Sensors based on quartz material can be used for measurement of mechanical strain. There exist two different structures due to propagation mode of the mechanical waves: bulk acoustic wave (BAW) and surface acoustic wave devices (SAW). Another way to classify the sensors is to divide them into resonators or delay-lines (latter only SAW). The sensors are passive elements so that no additional energy supply is needed. The interrogation signal is both information and energy-carrier. Furthermore the measurement system works in the MHz range. With these properties it is possible to operate the system using wireless transmission. In this way a sensor is obtained that is well suited for applications which are not easy or impossible to realize using wired systems: e.g. torque on rotating shafts can be measured at high rates (using DSP up to 10,000 measurements/second). The principle of signal processing works as follows: several periods of a rf sine-wave are received by the sensor antenna every 10 - 50 microseconds for a duration of some 100 ns. They are coupled into the sensor via the piezoelectric effect. Depending on the sensor type a damped oscillation (resonator) or a number of reflected sine-waves (delay line) can be received. In case of the resonator the resonant frequency changes with mechanical strain applied to the sensor and can be measured via radio echo transmission. The principle of signal processing for the SAW delay line is the same as that used for coherent pulse radar: a stressed sensor causes a phase shift of the received signal which is proportional to the elongation. With a maximum elongation of about 1000 ppm of the device used and a resolution of about 5 ppm provided by the signal processing, an accuracy of 0.5% is achieved. Fluctuations of temperature can be eliminated by summing the received signals of two sensors, one of which is stressed and the other compressed. By subtracting the two received and processed signals from each other the temperature can also be measured.

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