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Re: ULTRASONIC MOTION DETECTORS



I have had the same experience and have an explanation which seems to work
for me. If you point the motion detector at the floor the pulsed sound will
bounce back and forth between floor and ceiling. The detector determines
distance by measuring the time between the issuance of a pulse and the first
echo that comes back to it, ignoring all other echos. Under the right
sample rate and ceiling to floor height you can get a multiply reflected
echo from a previous pulse returning to the detector before the echo from
the current pulse. This is exacerbated by highly reflecting surfaces such
as a linoleum floor and acoustical tile ceiling. Changing the sample rate
allows the reflections to die off between samples so that the first echo to
return is the desired one. Putting carpet on the floor dampens the first
reflection so that subsequent reflections are not large enough to be detected.

To test this theory I have placed a piece of plywood beneath the motion
detector and at an angle to the floor. The pulsed sound which strikes the
plywood is directed away from the ceiling and there is no false echo. One
ought to be able to calculate for a given ceiling to floor height what pulse
rate allows this multiply reflected echo to be a problem. I haven't done
this yet but intend to.

At 08:46 AM 12/5/97 -0400, you wrote:
The air resistance data will be posted shortly.

We discovered a very strange behavior of motion detectors. Facing our
linoleum floor, and collecting data at the rate of 10 samples per second,
the distance to the floor (3 m) was correctly measured. But it was not
correct at higher collection rates (for example 2 m instead of 3 at 20
samples per second). We called Vernier and they suggested that we cover
the floor with a piece of carpet. This cured the situation. How can this
strange effect be explained? The f of the ultrasonic waves is 40 KHz.

Ludwik Kowalski and Richard Hodson
P.S.
Previously posted air resistance data should be ignored; they were
collected without using the carpet.

Jim Riley
Department of Physics
Drury College
Springfield Missouri 65802
(417) 873 7233
e-mail: jriley@lib.drury.edu
fax: (417) 873 7432