Where to start prototyping with straingauges?

Can anyone recommend a reasonably-priced prototyping kit for straingauges or something similar, and/or a good document on how to actually design around these beasts? What I'm trying to build is essentially a box with a handle on each end, which can measure the force applied inward or outward on those ends. Measurement range would probably about +/- 40 pounds, though I'd be happier if it could tolerate (but not necessarily measure) higher forces just in case someone does something stupid. The measurement doesn't have to be on a smooth curve but does need to be continuous and repeatable so I can calibrate it reasonably accurately. If you think of it as a kind of game controller, where you're squeezing it together or pulling it apart to varying degrees to make the game respond, that's fairly close to what I need. Doesn't necessarily have to be straingauge, but needs to be something with essentially that behavior -- I really want to measure the pressure/tension applied with imperceptable motion (for fast response). And yes, I do need both directions. Unfortunately all the documentation I've found on straingauges talks about the electronic aspects. and says very little about the practical mechanical aspects of working with them. And I haven't yet seen anything resembling a reasonably priced experimenter's/prototyping kit from any of the manufacturers. Yeah, I suppose I could cannibalize an electronic scale for the first prototype... but I'd still like to learn the proper approach and find an experimenter-friendly source of parts.

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orksecurity (author) 8 years ago
Instructables is nudging me to select a best answer. Tough, because Sean gave me some excellent pointers specifically toward resolving the question I posed, but I _may_ actually wind up going with resistive for this particular project and if so Steve's pointer might wind up being more directly applicable. I think the conversational thread with Sean gets the edge, in depth and detail, but I really want to thank both of you; you've been a huge help, and if the project comes together as planned I'll make sure to credit you both.
orksecurity (author)  orksecurity8 years ago
(Tempting to select this as best so the first thing folks see is the credit for both of you, but I don't want to assign myself a best. Instructables really needs to allow Best Answers, plural.)
orksecurity (author) 8 years ago
Plan is a microcontroller doing initial encoding of the A/D (and tracking a bank of other controls), with data out via simple serial protocol.
SPotted resistive strain gauges on a UK site (rapid electronics) which work out at around 7 USD each
orksecurity (author) 8 years ago
Slightly moving piece: The idea would be that you could sandwich a member attached to one side between parts attached to the other side, with a pressure sensor on each, and subtract one pressure from the other. I was thinking that it would be hard to do that without having some slop in the system, but maybe not. Hmmm. If we can get the right range of measurements, and sufficient durability, then this might indeed work. Requested docs from the suppliers you've mentioned; we'll see whether they're willing to send me hardcopy/cd or if they tell me to download (which I'm willing to consider, but for references I'd rather have paper I can bookmark and scribble on and so on.)
Are you hung up on resistance straingauges ?
Why not use piezo based gauges instead, or simple conductive foam pressure sensors, suitably arranged or Honeywell force sensors
How stiff do you want this thing to be ? If you can allow some compliance while the force is applied, you have even optical methods that could measure force -a pair of fibres perhaps, with the coupling allowed to flex.
orksecurity (author)  steveastrouk8 years ago
Not hung up on technology; anything which gives me the appropriate measurement range is worth considering. Some compliance is OK, as long as the thing isn't going to fail or rattle when direction of pressure/tension is reversed fairly suddenly. (As I said, thinking of it as a game controller is pretty close to what it will be subjected to.) (Sorry about being cagy about the end-goal -- it's nothing highly secret, just something which I'd like to have a working demo of before I publish it.)
orksecurity (author) 8 years ago
Hm. To get bidirectional measurements out of that I'd either have to have a spring maintaining mid-range pressure, or I'd need one on either side of the (slightly) moving piece. But that's doable. And as you said, the broader range of response (so it can be amplified directly rather than needing to be part of a bridge) does simplify the electronics. Worth investigating...
orksecurity (author) 8 years ago
$49 isn't unreasonable if I have a design likely to work... though that is up into the "may be cheaper to cannibalize a scale" range. Hitting a manufacturer for a set of handbooks is a good idea; just wasn't sure who would actually have something useful. Thanks for the pointer!