(Fwd) Re: ULTRASONIC MOTION DETECTORS

["Robert Morrison" <morrison@pasco.com>]

Hi All,
Some really interesting behavior can be attributed to Ultrasonic 
motion detectors. Most sensors use the Polariod ranging device. The 
ultrasound is emitted in a 15 degree cone from the front of the 
device. If you tip the sensor about 7 degrees off center of the 
target, or off level with the ground in this case, you can prevent 
stray returns. If you are setting up your sensor opposite a very 
reflective surface, like linoleum, you can create a standing wave. 
This can cause all kinds of strange results. In general, it's good to 
remember that the sensor favors the strongest return rather than the 
closest target
 1.5 cents worth,
 Robert


> LUDWIK and Richard,
>
> I'd like to know if you get any additional information about this. I have
> had a similar problem with the motion sensor attached to a CBL pointing
> upward towards a mass on a spring with a styrofoam plate taped to the
> underside: Good results with times of 4 or more seconds of data collection,
> bad results (the sine wave interupted with erratic data) with times less
> than 4s. I thought possibly I had made a programing error (I modified one
> of the Vernier programs) and was telling the CBL or the detector to take
> data faster than was possible (but I was too lazy to go back and check).
> Although the CBL Sonic channel can take a minimum sample time of 0.008s
> there is a note in the CBL System Guidebook that says the actual minimum
> sample time depends on the detector (and this is not given in the sensor
> info sheet).
>
> My solution so far (until I have a chance to look at it) has been to tell
> students to use 4 or more seconds...
>
> kyle
>
> At 8:46 AM 12/5/97, LUDWIK KOWALSKI 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.
>
> -----------------------------------------------------
>   kyle forinash   812-941-2390
>   forinas@indiana.edu
>   Natural Science Division
>   Indiana University Southeast
>   New Albany, IN    47150
>   http://Physics.ius.indiana.edu/
> -----------------------------------------------------
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