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Pasco's Motion Sensor II (
http://store.pasco.com/pascostore/showdetl.cfm?&DID=9&Product_ID=54465&Detail=1)
has a wide and narrow field switch.  I don't know how narrow narrow is, but
it definitely makes a difference in picking up stray reflections.  They
might still not be able to discriminate two object only a few cm apart, but
I wonder if you could set up some sort of device to block the beam into a
very narrow swath, or at least to keep the two beams separate (an acrylic
plate between the tracks, for example).  Both Pasco and Vernier detectors
are sturdy and reliable systems.  They do produce an audible clicking as the
pulses are sent out.  They are very accurate in the range of 0.15 to 10
meters and their interface can be set up to show a digital meter instead of
a graph.  If you want  a continuous speed readout, they are probably worth
experimenting with - somebody around you there in Bezerkley probably has
some - ask the Physics 101 profs if nobody at Lawrence Hall has one.  If you
want only one reading or a couple of readings, I would go back to the tried
and true photogates - no moving parts, exact control of location, etc.

Dave Smith

On 4/12/07, Allan Ayres <[log in to unmask]> wrote:
>
> ISEN-ASTC-L is a service of the Association of Science-Technology Centers
> Incorporated, a worldwide network of science museums and related
> institutions.
>
> *****************************************************************************
>
> Phil wrote:
>
> >We're trying to find a way to measure the speed of something like sand
> >as it flows down an inclined plane.  We were thinking of maybe trying a
> >radar gun at the bottom of the plane pointed up at the sand, but we're
> >kind of concerned about it...well, not working.  Does anyone have any
> >experience with this type of thing, or have any suggestions?
>
> We're also looking for speed-sensors right now.  In our case, we'd
> like to gauge the speeds of steel balls rolling in tracks that are
> right next to each other, so something that seems to be a feature the
> vendors like to tout (wide-angle sensitivity) is actually a bug for
> us.  :-)
>
> Plus, the cheap and easily available ones seem to be calibrated for
> 2-100 mph or so (the speed of golf balls, baseballs, go-carts, cars,
> etc.), and our speeds would be significantly less than that.
>
> Does anyone have any experience with Vernier's motion detectors?
>
> http://www.vernier.com/probes/motion.html
>
> They seem like total overkill for just getting a single number --
> speed -- as opposed to graphing position and calculating acceleration
> and everything else you can do with them, and they don't look
> particularly promising in terms of angular discrimination (i.e.
> returning the speed of just one ball).  But, compared to some of the
> industrial sensors, they're actually fairly inexpensive.
>
> Hmmmmm.
>
> Thanks for any insight,
> --Allan
>
>
>
> --
> ~~
>
> Allan Ayres
> Exhibit Developer
> Lawrence Hall of Science
> University of California, Berkeley
> Berkeley, CA 94720-5200
> [log in to unmask]
> 510-642-1254
>
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-- 
David L. Smith
Allentown, PA
[log in to unmask]

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