Shock Sensor Question

[Steve.Flanagan at VerizonWireless.com]

Brian,

I am not that concerned with what the shocks are doing down track, what I am
trying to capture is the exact action of the chassis at the time of launch.
Trying to put 1800HP to the ground, the most critical part of the pass is
the launch.  

Warren Johnson samples his Shocks at 1000 Hz, any drag race data logging
company will tell you a minimum of 500 Hz for the shocks.  A lot happens in
that first few seconds and thats what I want to capture.

The camera idea wont work.  First I think it would fail Tech under the
current NHRA regulations, for safety reasons.  Not sure of the wind effects
under a car going over 200mph.  And the frame rate of my cam-corder can
barely capture the tire spin on the starting line, never mind a shock
movement.

The strain gauge sounds interesting though.  Curious of how I could apply
strain gauges to measure wheelie bar Hit?  Any ideas?  The question is where
should it be attached.  Are they always attached to a coil/spring or can you
actually bond it to a metal bar and measure the bend, basically when the
wheelie bars hit the ground, the bars may flex some, this way we could tell
which bar is hitting harder.  How about calibrating this, any ideas?  The
only other concern is position of the sensor, if you bond a stain device to
the bottom of the bar assuming it will flex up, when we adjust the whellie
bars, we rotate the bar (on a bolt) to adjust in or out.  Any time the bars
are turned, the sensor will rotate.  Wont be easy to implement and make it
functional so that it allows for changing the position of the bars.

Steve

-----Original Message-----
From: Brian Dessent [mailto:brian at dessent.net]
Sent: Friday, December 14, 2001 3:57 PM
To: diy_efi at diy-efi.org
Subject: Re: Shock Sensor Question


Steve.Flanagan at VerizonWireless.com wrote:

> The non contact displacement sensor is what I had mentioned as a potential
> optical to reflective device, I was just in the wrong frequency range.  If
> we can mount a device that outputs a signal to a reflective surface and
then
> measure the time to return, we know the distance.  Sounds easy, however,
it
> would require some technology that can calculate the time for the signal
to
> travel?  How complicated is it to design a circuit that would output a
> voltage relative to a displacement?

If you do a Google search for "ultrasonic rangefinder" you'll get some
OK links.  However, the more I think about it I don't think this would
be the best way.  They probably don't have the resolution you would
need.  You said you're measuring a 4" shock travel, but what resolution
do you need?  Knowing this will help you choose the right solution.  The
ultrasonic transducers would probably only be able to give you a
measurement within an inch or two, using the stock electronics.  You
might be able to build a custom signal conditioner to get higher
precision but you could spent a long time working on such a thing.

> Linear Pots (the cheap ones) will not work at the Freqs that the shock
> moves, I have been told to record between 500 and 1000 Hz for these
> measurements.

Well, do what you wish, but that seems WAY to high to me.  The sprung
mass resonates at about 1 Hz and the unsprung mass resonates at around
10-20 Hz.  You could roughly estimate these frequencies with
sqrt(k/m)/(2*Pi), where k = spring rate and m = mass (make sure to use
consistant units.)  Vibrations above the resonant frequency are absorbed
almost completely by the tires.  See:

http://www.umtri.umich.edu/erd/roughness/every.pdf

I would measure DC (i.e. 0 Hz) to 50 Hz at most.  Since you said you are
interested in 1/4 mile times, I take this to mean you want to know
front/rear weight transfer, and this would be a very LOW frequency
signal since you're accelerating hard pretty much the whole time.

> 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.

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.

Brian

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