Piezoresitive pressure sensors - Page 1
Forum section: electronics / beginners

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: December 21, 2018, 06:59:04 am
content: I have a few piezo buzzers in my kitty.And I see that If I connect a scope across the piezo, and hit the piezo it shows a spike on the scope. The piezo only shows a spike when there is a change of pressure, but I keep a constant pressure on it there is no output on the scope.So that's what makes me wonder, if the piezoelectric effect only shows up when there is a change in pressure.How do piezoresistive sensors work or generate an output on constant pressure?

author: ebastler
author_rank: Super Contributor
timestamp: December 21, 2018, 08:51:15 am
content: Quote from: ZeroResistance on December 21, 2018, 06:59:04 amDo such beasts exist? I think the static pressure sensors use other principles: Capacitive sensing mostly; not sure whether optical sensing of a membrane position is also used?Piezo sensors generate a charge when pressure is applied (changes). With careful design, the discharge time constant can be made several (many) seconds long, so "quasi-static" measurements are possible. But I am not aware of piezo sensors which can provide true static, or absolute, pressure measurements.One could of course try to add electronics which keep integrating and tracking the charge variations, to provide an absolute readout at any time. But I assume that these would suffer from drift problems over time -- unless you can assume the pressure to regularly drop back to a known baseline value.

author: CJay
author_rank: Super Contributor
timestamp: December 21, 2018, 09:03:58 am
content: Yes, piezoelectric pressure sensors do exist.There's a bunch of different technologies used for static pressure sensors but I think most 'traditional' sensors use a strain gauge type arrangement, either of the common platinum film bridge arrangement or a 'formed' element (usually piezo resistive), the other technologies (capacitive/inductive) tend to be rather more expensive.

author: ebastler
author_rank: Super Contributor
timestamp: December 21, 2018, 09:23:27 am
content: Quote from: CJay on December 21, 2018, 09:03:58 amOf course. But do piezoelectric sensors forstaticpressure exist?

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: December 21, 2018, 09:34:55 am
content: Quote from: ebastler on December 21, 2018, 09:23:27 amPlease have a look at the pressure sensors from Freescale semiconductor (Now NXP)I would think a majority of them would be based on the "piezoresistive" effect.https://www.nxp.com/products/sensors/pressure-sensors:DRSNSPRSSRThe attached block diagram is of one such sensor.It has probably sensing elements (peizo resistors) in bridge arrangement.

author: Rerouter
author_rank: Super Contributor
timestamp: December 21, 2018, 09:48:15 am
content: It works just like how piezo water level sensors work, you inject a pulse in, and measure the return pulse amplitude from the deflection, the higher the static pressure across the sensing element, the weaker the return signal. Not to mention you can tell direction of pressure by inverting the polarity of your pulse, the lower pressure side will give a different signal. I believe higher in amplitude from memory.

author: CJay
author_rank: Super Contributor
timestamp: December 21, 2018, 09:49:02 am
content: Quote from: ebastler on December 21, 2018, 09:23:27 amYes and no, piezoelectric sensors are generally only good for measuring dynamic pressure changes but I've seen sensors that claim to be piezoelectric and capable of measuring static pressure, I seem to remember it used the principle of a SAW filter, excited by an AC signal and the resonant frequency changes as the piezoelectric element is stressed but it's been a long time since I designed in pressure sensors* so I'm not sure what the current state of the art is (or if it was just marketing wank, they were for pressures far in excess of my measuring requirements)To answer the OP's question, piezo resistive sensors generally rely on the deflection of a sensing membrane, the 'stretch' in the sensor changes the balance of a bridge and outputs a differential voltage proportional to the deflection, google wheatstone bridge or one of the Honeywell/Sensym datasheets and application notes, they had some great ones that explained it.* We used clamp on piezoelectric sensors for measuring and monitoring diesel fuel injector pulses as well as other pressure sensors for fuel pressure, differential air pressure and various other automotive applications, I have probably forgotten more than I remember about them now though.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: December 21, 2018, 10:03:46 am
content: Quote from: CJay on December 21, 2018, 09:49:02 amSo again, say if the membrane flexes and the sensor generates and output, but then the sensor stays flexed, will the output still be generated in a static flexed state? As seen from the block diagram I posted above there seems to be only a temperature compensation and amplification stage after the bridge element.Quote from: Rerouter on December 21, 2018, 09:48:15 amThis is an interesting take on in, I will try to dig into the Freescale / NXP docs to see if something similar is being done.