Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!zaphod.mps.ohio-state.edu!casbah.acns.nwu.edu!ftpbox!mothost!motcid!svoboda
From: svoboda@motcid.UUCP (David Svoboda)
Newsgroups: sci.electronics
Subject: Re: measuring vehicle speed
Message-ID: <4746@guppie4.UUCP>
Date: 21 Jun 91 20:37:48 GMT
References: <1991Jun20.024446.18473@en.ecn.purdue.edu>
Organization: Motorola Inc., Cellular Infrastructure Div., Arlington Heights, IL
Lines: 30

From article <1991Jun20.024446.18473@en.ecn.purdue.edu>, by jbs@en.ecn.purdue.edu (Spaggie):
> 
>      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.

Outstanding.  But I could envision a problem with an inconsistant ground
surface, as well as a problem illuminating the ground sufficiently from
under the car to be picked up reliably by the receiver.  A solution might
be to actually shine the laserdiode and pickup at the top of the tire,
inside the well.  The tread surface would necessarily be moving at the
same speed as the ground (except in a skid) and the speed reading would
be independent of tire wear, pressure, etc (unlike the transmission or
driveshaft methods).  The laser/pickup package should be mounted directly
above the front tire to deal properly with turning and the up-down 
supsension motion, or above a back tire.

           Dave Svoboda, Motorola CID, RTSG, Arlington Heights, IL
               uucp => {uunet|mcdchg|att}!motcid!svoboda
              internet => svoboda@void.rtsg.mot.com
           UNIX was written so a couple of hackers could run
                           space war easier.