Position Measurement

It may not be so simple. By your definitions you want to measure

1/(5x12x8) units = 1/480 ~ 0.2% of full scale. To do it properly your transducer should be at least certified five times more accurate than the reading obtained.

Good hunting

Stanislaw

A long-long time ago in a galaxy far-far away my physics teacher used the rotary encoder hacked off a mouse for this.

By "string pot", do you mean a long "string" of resistance wire, with the wiper on the cart, or just a string that turns a pulley on a pot?

I can't imagine either being outside the budget for a class - you'd have the kids help build the thing, and a precision pot can probably be had for less than $20.00.

Getting that kind of precision with ultrasonics might be a project in itself, albeit not for beginners. :-)

Good Luck! Rich

you don't specify the actual experiment you have in mind, but here I offer a Copernican twist.

place optical interrupters at different locations, and this way you know the time when your object was at a specific place. Then you need to time tag the transitions on a computer.

this would apply, for example, to the case of balls running down a groove..

Jure Z.

I don't think that's what he wants. Not simple enough and universal enough.

What the physics department uses at my school is the ultrasonic detectors. They plug into an USB recorder thing and the lab's laptops run some software that graphs things. Appears to be all commercial stuff, and likely quite a chunk of change too (no wonder all the gear in the department is 30+ years old :^) ).

I imagine laser rangefinders would work, too.

For home-built, on-the-cheap and easy to interface, I like the suggestion of a rotary encoder. You'd want it either spring loaded (which will affect measurements!) or looped on a pulley to maintain tension in the string, and of course, oiled for a minimum of friction (ball bearings are a must). Unfortunately this is as fixed (or moreso) as the resistance wire Rich mentioned, as far as portability and setup.

Tim

-- Deep Fryer: a very philosophical monk. Website:

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Me too.