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Recent years have seen Surface Acoustic Wave (SAW) devices used to measure small force with ultra precision. The SAW sensor includes a transmitter, which sends a bulk wave traveling along a substrate. A receiver will detect this wave, and then send the signal back to the transmitter. The frequency of this signal will depend on the distance between the transmitter and receiver. This distance, in turn, depends on the load on the load cell. Measuring the frequency from a SAW sensor is a simple and direct way to calculate small amounts of force or strain. These devices are also sometimes known as SAW transducers, perhaps a more accurate term as they convert mechanical force into electrical frequency. SAW cells are some of the most sensitive and reliable devices of their kind, and can withstand years of repeated use.
With strain gauges, extremely small changes in resistance must be measured. The Wheatstone bridge circuit is used to convert the microstrains on the gauge into a change in voltage that can be read by an analog-to-digital converter. Typically set up with four resistors, a Wheatstone bridge will register changes in resistance among the four and produce a fluctuation in voltage. The strain gauge is used as a replacement for one or more of the resistors in the bridge, and as it changes its shape and dimension, it unbalances the readings from the bridge and produces an output voltage that can be read to measure the strain itself.
The excitation voltage fed into the device has to be stable and regulated; fluctuations in the source voltage will produce inaccurate output voltages and output readings. Low-pass filters, signal averaging and differential voltage measurements are useful for suppressing noise interference in the signal. Amplifiers can also condition the strain gauge's extremely low-output signals before they are passed on to analog-to-digital converters.
Signal conditioning for small-force strain gauges should typically include:
- Onboard bridge-completion resistors, for quarter and half-bridge strain gauges
- Offset adjustments
- Scaling gain adjustments
- Temperature compensation
The offset adjustment can let users zero the output offset produced by a small bridge imbalance or a deformation of the mechanical member being measured. Gain adjustment can let users set a gain that optimizes the dynamic range of the device by providing a full-scale output under maximum load. Also, a shunt resistor can conveniently simulate an unbalance without having to apply a physical load.
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