Shock Sensor Question

[Steve.Flanagan at VerizonWireless.com]

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?

For shock travel, we are talking about 4 (+/- 1) in Inch displacement.
Where can I get info for non contact displacement sensors?  I will do a
search, but do you have a good source?

Sounds like the 180 sensor aren't that expensive after all. 

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.  

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





-----Original Message-----
From: Brian Dessent [mailto:brian at dessent.net]
Sent: Thursday, December 13, 2001 6:53 PM
To: diy_efi at diy-efi.org
Subject: Re: Shock Sensor Question


Steve.Flanagan at VerizonWireless.com wrote:

> Don't know how you can record Digital pictures into an analog data
recorder.

He meant just put a video camera on a tripod and record the run and then
analyze it to death with a computer.  Although I don't know how you
would deal with the changing angle (assuming a fixed camera to the
side.)

> I have a multi channel 0-4V analog data recorder,  and am looking for a
> cheap
> way to monitor the position of my shocks on the launch and also down
track.
> 
> The expensive way is to use the $180-$200 dollar linear position sensors,
I
> am just
> looking for a cheaper way.  The budget wont allow for that expense when
the
> money
> is needed else where on the car.

What about using a non-contact displacement sensor under the fender that
measures distance to the ground?  You could either use one in the front
and one in the rear, or just a single unit at one fender and an
inclinometer to measure the body's angle.  The ultrasonic rangefinder is
cheap and easily available, but I don't know about its distance
resolution.  They use them all the time in robots to sense when they are
coming close to walls, objects, etc, and in this application they are
sensing in the tens of feet range.  You might be able to adapt one (with
a modified controller) to sense the distance from under the fender to
the ground.  Since the distance does not change much you don't need much
dynamic range and so this might enble you to use a high frequency
counter to get the resolution you need.  Infrared rangefinding might
also be in the realm of DIY, I don't know the details but I think most
automatic cameras use this to set focus.  (The ultimate cheap source of
ultrasonic rangefinders for hobbyists used to be Polaroid cameras which
used them for focus.)  Since money is an issue I don't think you'll be
using laser displacement measuring but it would definitely work just
fine.

What is your resolution requirement?  How much does the suspension move
and how precisely do you want to measure it?  Do you need to measure at
all four corners or would just front/back work?  What about just sensing
the angle of the chassis instead of displacement of the shocks?

Other ideas:

- Linear potentiometers that have several inches of travel can be had
for a few bucks and would work fine for a quick and dirty setup,
assuming you could figure out how to mount them.  It's a "poor man's
LVDT."  (LVDT = an inductor whose core moves in and out, changing the
inductance.  THe ones I've worked with can measure displacement down to
about a micron [0.00004 inch] and they have very little to zero
friction.)

- You could mount a strain gague either directly to part of a coil
spring or put a load cell between the coil spring and its housing.  If
you have a torsion spring this is an even more attractive idea.  This
might not be linear and it probably wouldn't relate directly to distance
but it would relate directly to the force acting on the wheel, which
might be equally useful for suspenstion tuning.  Bare (unmounted) strain
gagues are pretty cheap and you're not really interested in absolute
calibration, just repeatable relative measurements.  Omeaga sells
packages of 10 for $49.

Brian
(just brainstorming)

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