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From: jbs@en.ecn.purdue.edu (Spaggie)
Newsgroups: sci.electronics
Subject: Re: measuring vehicle speed
Summary: Laser Interferometry...
Keywords: car speed
Message-ID: <1991Jun20.024446.18473@en.ecn.purdue.edu>
Date: 20 Jun 91 02:44:46 GMT
References: <31487@hydra.gatech.EDU> <1991Jun18.003032.18571@mulga.cs.mu.OZ.AU> <31529@hydra.gatech.EDU>
Sender: jbs@ecn.purdue.edu
Organization: Purdue University Engineering Computer Network
Lines: 35


     I have been catching the end of this posting, but I had a few
     ideas I thogh I would throw in.  While working at Allied Signal
     this last fall, I was involved in closed loop DSP control systems.
     I was also involved in using laser interferometry to measure 
     vibration on an engine (aircraft).  I too had an interest
     in determining the speed of my car, but not for ABS.  I was just
     interested in what my real speed was. (Speedos are so inaccurate
     these days at speeds about 70 MPH).  After discussing the idea with
     several people the following idea developed.

     When you shine a laser on a diffuse surface, you get what is called
     a speckle patter.  This pattern can be seen with the eye. As you move
     the laser across the surface, the pattern moves with it.  If one were
     to hook up a laser to shoot at a spot on the ground, and pick up the
     signal (amplitude) with a ccd array, then store the resultant frame in
     the dsp.  Next you would grab another frame (just a few msecs apart)
     and compare them in the DSP. (you could do a cross correlation to find
     the phase difference.. DSP is fast in that respect)  If you can tell 
     the amount the ground has moved (knowing the projected width of the 
     CCD array), and the time frame at which it occured, you can determine
     the speed relative to ground.

     Does this sound feasable?.. with a laser diode, and Analog
     Devices 2105 DSP ($10!!!), and a CCD array, it would
     no be too expensive...


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Jeff Sponaugle                      Allied Signal Aerospace, ECD
938 Roberts St                      Dept 862, Optical Engineering
South Bend, IN 46625                717 N Bendix Dr.
(219) 234-5103                      South Bend, IN 46620
Internet: jbs@ecn.purdue.edu        (219) 231-3063
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