# Position Measurement

Discussion in 'Electronic Design' started by [email protected], Feb 6, 2007.

1. ### Guest

The problem at hand is a relatively accurate (1/8") measurement of a
cart's position for introductory level physics course. The sampling
rate would be in the area of 5 s/sec. The measurement will go up to
about 4 feet (no more than 5). After doing some googling, it seems
that the best options I have are:

--String Pot
--Laser Measurement
--Ultrasonic?

The string pot seems like the best approach, however, may be out of my
budget. I'm fairly electronically savvy and was wondering if it was
possible to build one of these myself. I figured just getting the
distance translated into a voltage would be sufficient for porting
into the data acquisition setup I have available. I saw Spaceage
Controls Series M Position Transducer looked pretty good, but I'm not
sure over how much one of these might cost.

2. ### Stanislaw FlattoGuest

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

3. ### Guest

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

4. ### Rich GriseGuest

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

5. ### jureGuest

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.

6. ### Tim WilliamsGuest

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

7. ### John FieldsGuest

---
Me too.
---
---
Doesn't have to be...

Why not just use a mouse and a spring-loaded non-rotating plunger to
keep it in contact with the floor when the table's being lifted and
moved?

That way you'd have both X and Y (so you could correct for any
lateral movement) and if you sent the mouse's output via IR or RF
the portability issues go away.

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