Shock Sensor Question

[Bernd Felsche]

Steve.Flanagan at VerizonWireless.com tapped away at the keyboard with:

> Anyone have a brainstorm of an inexpensive shock sensor.
> 
> Linear position sensor are on the order of $180 US per sensor, as
> you can see this gets expensive  for 2 and maybe 4 sensors (front
> and back).
> 
> I had thought of using a rotary pot, with a  pulley, return
> spring, you get the idea, but from speaking with someone who
> mentioned they tried something similar, they said the rotary pots
> do not react fast enough for the frequencies that the shock moves.
> Instead of the Rotary Pot, how about interfacing an optical sensor
> from a PC mouse in its place,  does anyone have an idea if the
> mouse optical sensor could be made to react fast enough?

It could react fast enough (given appropriate "gearing") but there's
no absolute count available and no recognizable position from each
pulse. So you have to keep count. All the time. Any loss in count
results in "virtual sag"; i.e. the controller is going to believe
that the displacement is different to the actual physical
displacement. Lost pulses could be avoided by Gray-coding the wheel
and having multiple pickups.

> I have been thinking of maybe a different optical approach, but I
> do not know the easiest way to do this.  Maybe a pulsed light /
> laser, to a reflector, measure the round trip time, speed of light
> should handle any frequencies that a shock can move at.  

The speed of signal processing is more likely to be the problem.
(vis. Lidar) You're looking at a bandwidth of several hundred Hertz.

> Do some of you engineers out there have any good ideas that we
> could tackle to try to build a cheap way to measure shock position
> on a Drag race car.

I guess that rules out the laser-reflector.

If it weren't for the noise levels of a drag race car, you could use
acoustic resonance of the tube. This changes with the location of
the piston within the tube.

A microwave might be short enough to resolve the displacement as
accurately as you need; the 100 GHz signal might attract HARM. :-)

None of these methods seems any cheaper than the linear transducer. 
Maybe you can get away with a shorter (cheaper) transducer by
mounting it in parallel on the same linkage, but with less overall
movement.

Without seeing the suspension layout, it's difficult to say if you
could use a set of rotary encoders instead. They tend to be less
costly.
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