Re: ULTRASONIC MOTION DETECTORS

[brian whatcott <inet@intellisys.net>]

At 16:12 12/5/97 -0600, you wrote:
> 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. 
> ...  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.  
>
> Jim Riley

This description is so cogent that one immediately sees its virtue.

For a speed of sound c, ceiling to floor height of d, one sees that
for an inter pulse time t < 8d/c a false trigger is possible   4 rnd trips
                        t < 6d/c                    more possible 3 rnd trips
                        t < 4d/c                    probable. 2 rnd trips

These interpulse times correspond with pulse repetition frequency f = 1/t
in the usual way.

As a sanity check, I use   d = 10 ft.
                           c = 1120 ft/sec
to estimate the probable prf for error as 28 pulses per sec. or more...

Brian
brian whatcott <inet@intellisys.net>
Altus OK