Shock Sensor Question

[Bernd Felsche]

Brian Dessent tapped away at the keyboard with:

> Steve.Flanagan at VerizonWireless.com wrote:

> > How exactly does a strain gauge work?  Does the output voltage change
> > depending on how hard you pull on it?

> It measures strain.  :-)  Seriously, take any elastic material and apply
> a force (stress) and it reacts by deforming (strain.)  For solids,
> stress is proportional to strain and the constant that relates them is
> called the modulus of elasticity.  A strain gauge is usually a thin
> piece of conductive material that changes its resistance with strain. 
> Used alone, it's pretty worthless.  What's so neat about it is you can
> bond it to whatever structure you're interested in and it will measure
> the amount of strain, and by doing some calculations you can relate this
> to the stress in the structure, which in turn will tell you when/if it
> will fail.  For example, you could bond a strain gauge to a steel i-beam
> in a skyscraper to see how much it flexes in the wind to know if your
> building is going to fall over in a hurricane.  Anyway, what I was
> talking about would be to buy a small strain gauge, epoxy it to part of
> the coilspring, build/buy a signal conditioner (simple instrumentation
> op-amp circuit, the full scale signal is probably 20mV or so), and then
> datalog the resulting signal.   It would be proportional to deflection
> of the coil spring if you choose the right location for the strain
> gauge.  Go to www.omega.com and find their section on strain gauges,
> they should have lots of background and application hints.

Depending on the spring's mass, you'll also see oscillations over
its length following a sudden change of external load. It's a
distributed mass-spring oscillation. A low-pass filter should
eliminate most of that signal content.

> Bernd's idea of mounting a camera on the car with a view of the
> suspension and then analyzing it with a computer is looking more and
> more attractive.  The hard part would be finding room to mount it where
> it could record a picture of the suspension.

Pointing alongside the vehicle, using mirrors or stakes for the
reference points if they're not directly visible. If you use a
pair of mirrors and mount the camera pointing downwards, you can see
both front and back axles. Some sort of fluid mount (as on the head
of most camera tripods) prevents much of the vibration being
transferred to the camera.

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