Voltage from a Piezoelectric Film Sensor


#1

A quick question from a newbie…

I’m trying to measure force using a piezo film sensor (https://www.digikey.com/product-detail/en/te-connectivity-measurement-specialties/1-1002910-0/356-1134-ND/2704828)

When I connect each of the two leads to a voltmeter and press on the material, I would expect to see some level of voltage as an output, but I see absolutely nothing. I have checked the function of the voltmeter (measured the voltage from an AA battery) and it seems to be working.

Am I missing something here? I’m a Chemical Engineer and software guy, but know very little about electronics.

Thanks in advance,

Pete


#2

@pedavison

Do you just have the sensor or do you have this hooked up in a circuit. One thing many people miss when they first work with a piezo sensor is that you will have an AC voltage coming off the sensor and not a DC. You also want to have a resistor in the circuit. Per the datasheet you need a min of 1 Mohm though preference is 10 Mohm. You are also looking at a frequency that can be hard to catch depending on the voltmeter. It may be easier to see with an Oscope.
Let me know if this helps or if you have further questions.
-Robert


#3

Another thing to keep in mind is that pieozelectric sensors detect changes in force, not necessarily constant force. If you press on the sensor and keep pressing, applying steady constant pressure, the sensor would pick up the original force of the push as you’re deforming the sensor, but it wouldn’t necessarily properly detect the continuous pressure applied afterwards to hold the sensor in its deformed state.

I don’t know if that’s what you’re trying to do, and I’d try Robert’s suggestions as well, but I’ve talked to folks looking to use piezo as a load cell or similar continuous force sensor, and the limits of piezoelectric technology have come up. Piezo only does its thing when it’s actively in the process of bending; holding it bent doesn’t generate any electricity and thus also doesn’t generate any sensor signals.


#4

The piezo sensor is a small capacitor. A normal voltmeter will immediately short it out!

Either you must bend the sensor quickly (whack it with a pencil,) or you must provide a buffer amplifier with an extremely high impedance (a simple op-amp chip can do this.)

For example, if your small sensor has 0.001uF capacitance, then your voltmeter’s 10Meg impedance will discharge any voltage within a few hundredths of a second. (Ten megohms, it’s almost like a dead short!)

To see any signal, you can’t just bend the sensor, since the voltmeter discharges it far faster than it charges. Instead you have to really whack it, so you create some hundredth-second motions. (Rather than a DVM meter, an oscilloscope would work much better for seeing the signal peak.)

If you want to see DC changes, the buffer-amp can be the common FET-input op-amp TL071CP. Power it with a couple 9V batteries, apply the piezo voltage to pin-3 pos input and common, and connect pin-2 neg input to the output pin-6 (for gain=1.)

The input resistance of TL071 is plenty: 10^12 ohms, that’s a thousand gig-ohms. Every time you bend the piezo film, this op amp will sloooowly discharge the film over twenty or forty minutes, rather than twenty or forty milliseconds.