Help testing Pressure Sensors

Question

I have a few ME600 Metallux sensors that I harvested from some discarded products. I want to build a project with these but would like to verify they work before getting too far. From the datasheet and my understanding of how the sensor works, I have come up with the following simple test setup. I have connected my power supply to the Vref and Gnd pins and my multimeter to the S+ and S- pins.

Surprisingly when I apply pressure to the sensor I don't get any voltage (V or mV) reading. Even on the one sensor I know is good.

After banging my head against this for a while i started trying random things, and discovered that I could measure a resistance change when applying pressure, but only if the sensor was powered on.

Questions:

Aren't these types of sensors supposed to generate a voltage output?

Aren't you not supposed to measure resistance when a circuit is powered?

Shouldn't the resistance change be measurable without the sensor powered?

What is the proper way to check these sensors?

ME600 DataSheet

Answer 1

The S- and S+ outputs are differential, meaning that you need to connect these to a differential amplifier.

There should be a tiny change seen using a multimeter, but not all multimeters may be able to measure it. You need something more sensitive than that in practice - the differential amplifier.

Now an op-amp can be used as a differential amplifier, but many are not optimized for this. Best way forward would probably be to study some current differential amplifier device datasheets and look at their example circuits.

Aren't these types of sensors supposed to generate a voltage output?

Yes, but the difference may be very small. The "sensors" inside this device are organized into a "wheatstone bridge" configuration. Researching this term can give additional information and example circuits. The applied voltage is distributed across the bridge, evenly for no pressure, and unevenly depending on the level of pressure.

Aren't you not supposed to measure resistance when a circuit is powered?

Generally this is correct. You can not measure resistance, accurately anyways, because the device supplied power will interfere with the meter's supplied (tiny) power. It just may be that your particular meter functions well as a differential amplifier in this non-standard role.

Shouldn't the resistance change be measurable without the sensor powered?

Yes, but it may be a far smaller change than anticipated. Hence, why an amplifier is needed.

What is the proper way to check these sensors?

Connect them to a differential amplifier, and set the gain to something reasonable, say 10 or 100 to start with.