In transducers used for force, acceleration, and temperature measurements, the length L is modified by the measurands. In crystalline quartz devices, the fractional change in SAW velocity is small compared with the surface strain caused by a change in L.
We can therefore expect a decrease in oscillator frequency when L is increased by direct axial tension or by an increase in temperature.So, in transducers that measure force, the temperature must be maintained constant. Figure 5.27 shows a dual oscillator configuration where the temperature effects are reduced and the force sensitivity is doubled. For a force acting downward, the A1 oscillator increases its frequency and the A2 oscillator decreases its frequency.
|Figure 5.27 Dual SAW-oscillator force transducer. (a) Oscillators on opposite faces of quartz substrate achieve temperature compensation. (b) Change in oscillator frequency as a function of the force. Mixer output shows almost double the sensitivity.|
The difference in frequency is a measure of the force.