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I want to make an inflatable seat that is electronically controlled. I am specifically looking to be able to estimate the level of inflation of an airbag using small electronic sensors. The airbag is to be filled using a DC pump, and a solenoid will be used to control exhaust.

I have used the term "level of inflation" because I am using and airbag that somebody is sitting on, so the air will be getting compressed by the persons weight and movement. This will obviously cause variations in pressure and volume. In addition, I would like to use this measurement/information to inflate the airbags to the same levels at a later date. However, I won't necessarily be starting with a completely deflated bag every time I inflate the bag to this measured level.

I hope this makes some sense - apologies if it's too convoluted. I'm happy to answer further questions.

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  • $\begingroup$ You could adapt the mechanism used in an automatic sphygmomanometer (blood pressure cuff). $\endgroup$ – Dave Tweed Jul 30 '16 at 11:54
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I asked a few professors about similar hyperelastic deformation, and basically empirical testing is an imperative. If you take into account hysteresis (assuming testing of the product), strain history, etc, modeling in ANSYS or abaqus fe-safe is a personally advised necessity if you're creating a product for market. Localized displacement and strain energy density are also often dismissed as metals have a predictable stress strain relation. What I'm getting at is I can't really give you my ideas as that'd be foolish, but there are ways to obtain system values that you'd need for your feedback loop to be effective. Or go simple and use localized pressure and hope it works out!

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Some sort of strain gauge on the surface of the airbag would probably be your best bet.

And then you can make the following relations to get to the 'inflation level of the airbag':

strain gauge reading -> airbag's surface curvature -> airbag inflation

Of course there are a few steps in between those I mentioned above, and some fairly difficult math will be required to relate the external curvature to the inflation level of the airbag. But that is the general idea.

Take a look at this image to get an idea of how a strain gauge works and see if it looks like something you could use.
strain gauge description

Picture from strain gauge wikipedia page.

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  • $\begingroup$ I'm not sure how well that would work. It would be measuring something weird. For example if the person sitting on it were to compress one part of the bag, the rest could expand while that one part bunches up under the weight. It would pretty much measure pressure, but even that would probably be wildly inaccurate. You could get slightly more accurate by using more strain gauges, but even then you're going to wind up measuring local deformations without any good way to monitor volume. $\endgroup$ – JMac Nov 29 '16 at 19:01
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Really pressure is the metric that you are interested in as this will determine how hard or soft it feels to sit on.

Although this may indeed vary you don't have to rely on instantaneous pressure and it is entirely possible to base your settings on an average over some period of time and there are all sorts of control systems which can cope with quite large perturbations and sensor noise.

Indeed there are existing similar applications, for example tyre pressure sensors in F1 cars which can be very accurate despite large variations in external load and changes in temperatures. Similarly there are systems for off road vehicles which allow tyre pressures to be adjusted on the fly.

The physical design of the system can also help to damp variations, for example by filling the cushion with open cell foam.

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