I am working on a project to detect spine curvature during deadlifting using the Adafruit Piezo Ribbon Sensor(4931). When I started establishing proof of concept by taping the sensor to a pool noodle then hooking it up to a voltmeter I found that the output voltages were rarely ever stable and that they would spike as soon as I bent the noodle into tension, however the voltage would then reduce as the angle of curvature increased. This leads me to believe that the sensor is responding to the rate of change of the deformation rather than the deformation itself, is this the case? If so, is there a hardware solution that would allow me to use the same sensor?
On a separate note I was reading a resting voltage in the range of 0 - ~20mV and a peak around 150 mV when pinched. I tested the sensor in 3 different configurations, without a resistor, with a 1 megaohm resistor in parallel, and in a circuit with an LM324 opp amp(Gain: ~20). In all 3 of the configurations the output voltage way for the most part in the same range. Is this due to the internal resistance of the sensor changing too quickly? Ideally I would receive a voltage that peaks when the angle is greatest that is of a large enough magnitude that it can be read with high enough resolution by an arduino esp32 nano. Any assistance would be greatly appreciated.
Reading about this sensor on the Adafruit website it states it creates a small current pulse across the terminals when it is shifted or touched. You can place a 1M resistor across the terminals to see get the current changed to voltage. They are talking about you will see voltages spike when the piezo has vibrated. There is a wiring guide for the Arduino on the Adafuit website. I am just not sure this product is meant to do what you want it to. We can see if anyone else has any input on this as we have many followers.
The basic idea of piezoelectric devices is that they act like a bit like a ‘squishy’ capacitor, in the sense of one of those fluid-filled stress ball sort of toys that bulges when squeezed.
If one were to poke through the side of such a device with a small pipe, it may bulge momentarily when squeezed, until such time as the gel filling finished spraying out of the pipe.
Your voltmeter/resistor/op amp circuit or most anything you’d use to measure voltage across the device will (to varying degrees) behave like that pipe jammed through the wall of the squishy ball; any signal you get by bending the sensor is going to bleed off over time.
That’s essentially what’s known as an AC-coupled system, which can be nifty for measuring vibrations and such but isn’t great for slow things like position measurements. Yes a person can invest in mitigating the effects, but the time is probably better spent looking for an alternative sensor mechanism that might actually deliver a more time-stable reading.