'Hollow bolts' are available, which have built-in strain gages or other
means to measure deformation. They are as expensive as strain gages and
have the same poblem (sensitive amplifier required). A capacitive 'hollow
bolt' sensor is a 'homebrew' type of sensor where the sensing element is a
capacitor formed between the bottom of the bolt (which is not drilled
through) and a conical piston that is mounted insulated inside the bolt
hole and does not touch the bottom of the drilled hole. It is sensed in an
AC bridge. The bolt construction is done such that the tip part and the
conical piston mount are far removed from the threads so the thread/bolt
tightening has no influence on the read value. If you have machining
capability you can make your own relatively easily. Capacitive readout
using a PIC can be done with a RC delay method (with C being the sensor)
which avoids A/D conversion. I do not know if you can reach 1% with this
type of conversion. That would be 'cheap' by me anyway ;-).

As you have discovered, there are no 'cheap' force measuring devices
available. One hack would be to use one of those $50 digital readout
micrometers and mount it on the hollow bolt or some other part to measure
displacement directly, using a prolonged measuring stick, and connect the
optional serial data output from the micrometer to the PIC or another
computer. This would give an advantage in that it has a direct readout in
place, for adjustment etc. If you have control over the modulus of the
part it measures, then you can make it read out directly in lb by
adjusting the modulus and the position where you measure the displacement.
50uM readouts are available, if that's 1% of 200 lb then 200lb => 5000um =
5mm. That's a lot of displacement depending on where it happens. I am not
a mech. eng. but I think I'd use a compression spring for this. I don't
know what compression springs do with temperature variation and aging
though.

hope it helps,

Peter

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