Can a poled PVDF sheet be used as a pressure transducer?

Question

Hello,I believe that poling a PVDF sheet gives it piezoelectric properties,which makes it useful for pressure transducer applications (acoustics,etc). However, my understanding of piezoelectric materials is thatthey generate a voltage as they are in the process of being stressedor unstressed. I have always thought of this behaviour as beingdynamic (i.e. a varying pressure will cause a varying voltage to begenerated). I would like to confirm if a poled PVDF sheet is alsouseful for measuring *static* pressure. For example, could it be usedas a sensor for an electronic weighing scales, where the pressurewould be constant during the measuring process? My currentunderstanding of energy conservation laws leads me to guess that avoltage would only be generated as the weight is being applied orremoved, and not under static pressure (else we could build an'infinite' power source by sitting a weight on top of a PVDF sheet). Iwould be grateful for any help in clarifying this point.Thanks in advance,John.

Phil Hobbs · Accepted Answer

Transducers have input variables and output variables. Assuming power is applied, a strain gauge takes a displacement and produces a voltage, with some significant drive capability due to its low impedance. A photodiode takes optical radiation and produces electric current; over a reasonable bias range (say +0.3V to -25V for a typical silicon device) the photocurrent is independent of bias voltage--a high impedance.

Piezoelectric and pyroelectric devices (PVDF is both) produce a *charge* as their output, not a current or a voltage. Of course, since the device looks like a capacitor, the charge will produce some open circuit voltage, but it produces no dc current whatever--once the charge flows away (either by leakage or through an external circuit) it's gone.

That means that piezo and pyro detectors intrinsically detect *changes* in position or temperature, not absolute values. This takes care of your conundrum, I think.

Cheers,

Phil Hobbs

dvt · Answer

There will be a decay time inherent in such a measurement. So if you want to measure pseudo-static pressure, PVDF will work. PVDF would be a bad choice for static measurements, such as weighing a rail car as it is filled.

Mark Thorson · Answer

Any polymer material will undergo cold flow (a.k.a. creep or compression set). Therefore, it is inherent in these materials that application of stress will result in a time-dependent change in their properties.

Can a poled PVDF sheet be used as a pressure transducer?

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