Robert Rolf writes:
>Basically you emit a fast frequency sweep (microwave range),
>and the frequency of the heterodyne result on the reflection
>gives you the range quite accurately.
>
>Radio has the advantage of not being mucked up by dirt on the
>sensor, and that it averages the height, whereas a laser would
>be ground, treetop, tree branch, leaves, etc.
>
>Laser range finders typically use phase measurement of an AM modulating
>carrier (100 MHz or so) so that they don't need extremely fast electronic coun
>ters.

	Thank you!  That partly answers a question I have had for
some time, now.  The recent discussion on this list about audible
warning signals for blind pedestrians reminded me of something
called a laser cane which was always about a year away from
revolutionizing travel for the blind (for about the last 35 years),
and one of the things it could reportedly do was detect down
drops like curbs and stairs.  That part was even back in the
early seventies so I wondered how it could use a light beam to
reliably measure the kind of close distances we are talking about.

	One problem with sending an AM signal like that is that
multiples of the wavelength are exact replicas of only one
wavelength so if you used a 150-MHZ pulse train, the pattern
would repeat every meter or so.  A product detector whose
chopping frequency was the modulation source's oscillator and
whose signal input was the photo detector would probably show
plus and minus products as one moved through a wavelength.  So,
how do you know that the wavelength you are paying attention to
is the first and not maybe the 4TH or 5TH multiple?

	A laser cane would certainly have to not get confused on
multiples or you would go crazy with false alarms and it would be
useless.

	As one who uses the definitely low-tech version of a
white cane, I can say that an electronic version would have to be
extremely more useful and reliable to beat what the traditional
cane does.  Some suggestions are that one would use a traditional
cane, but with an electronic aid attached to it that would warn
of overhanging projections and objects that are too far away to
touch practically, but are nice to know about such as
obstructions like parked cars, trash and construction items which
are apt to be transient and unusually shaped.

	Traditional white canes are not used for support so much
as a probe to tell you whether you are about to step on to a
step, go in to water or on to grass or any number of other
mechanical details about what's in front of you so electronics
might be a good addition, but it probably won't completely
supplant what the aluminum or fiberglass canes of today do.

Martin McCormick WB5AGZ  Stillwater, OK 
Systems Engineer
OSU Information Technology Department Network Operations Group
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist