Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!samsung!uunet!ogicse!milton!sumax!ole!ssave
From: ssave@ole.UUCP (Shailendra Save)
Newsgroups: sci.electronics
Subject: Re: measuring vehicle speed
Message-ID: <2026@ole.UUCP>
Date: 22 Jun 91 23:02:44 GMT
References: <13571@uhccux.uhcc.Hawaii.Edu>
Organization: Seattle Silicon Corp., Bellevue, WA.
Lines: 38


 Here's an idea:

 Put under your front bumper (or near the front) an RF point source (say
 about 8MHz) pointing downwards.  Along the underside of your car (in 
 line with the source, place sensitive RF pickups, say about every
 2cm.  Now, since the car is about 2m long, you will be able to have
 a measurement of 100mph. This is how:


 L___________________________________________________I  <-underside of car
   \\    I I I I I I I I I I I I I I I I I I I I I I
     \         /
      \       /
       \     /
	\   /
	 \ /
------------------------------------------------------  <-  road surface

   The \\ in the front is the RF source.  The I I I I 's are the RF
   pickups.  Now, using a micro, you are going to have to send pulses
   of RF every millionth of a second.  Find out which of the sensors
   receives a signal.  Then over a hundredth of a second, average the
   number of the sensor.   The reason why I say millionth and
   hundredth, is that I expect that there will be some pits etc on the
   road and so you will get erroneous reading sometimes. This will be
   averaged out by taking a large number of samples.  Also, averaging
   every hundredth of a sec will make it possible to update the speedo
   fast enough to give it real-time value.

   On real bumpy roads, keep your eyes on the road! Don't look at your
   speedo. :-)

   Shailendra
   ssave@caen.engin.umich.edu
   sumax!ole.uucp!ssave

   PS: someone could actually calculate the wavelength required.