Newsgroups: comp.robotics
Path: brunix!cat.cis.Brown.EDU!agate!howland.reston.ans.net!swrinde!news.dell.com!tadpole.com!uunet!timbuk.cray.com!driftwood.cray.com!kilian
From: kilian@cray.com (Alan Kilian)
Subject: Re: Servo motor conversion
Message-ID: <1994Jul19.092740.23164@driftwood.cray.com>
Lines: 69
Nntp-Posting-Host: poplar
Organization: Cray Research, Inc.
Date: 19 Jul 94 09:27:40 CDT

I have never seen a potentiometer (pot) that went all the way around
AND geve reasonable readings for the entire 360 degrees.

There are a few things you could do.

If your motor only needs to turn 10 full rotations and then goes the other 
way (Like a motor turning a threaded lead screw to move an arm) you 
might be able to use a multi-turn pot. They make ten and twenty turn pots.

If you really need to go "round and round" then you can take a normal
pot and open it up and take out the thing that keeps you from turning
it all the way around. Some pots are better than others at this.

The problem is that a pot usually looks like this inside:



                           O   <------ part of a circle of carbon
                         O   O
                        O  .--=  <---- A metal wiper
                         O | O
                         | | |
                         o o o   <---- Three metal contacts

As you can imagine when the wiper wipes across the mess at the bottom
you get messy values out of it.

For a 10,000 Okm pot you might get something like this:

1,000	The beginning
2,000
5,000	The middle
8,000
10,000 	The far end
100,000
Infinity	The wiper is touching itself (Woo Woo)
Zero		The wiper is touching the metal contact at the beginning
200		Some strange value at the beginning.
1,000	The beginning again

So you need to deal with that.

How about two of these taken apart pots rotated 180 degrees from one another
and mounted on the same shaft?

You would get good values out of at least one of them and you would have to 
know when to start paying attention to one or the other.
You would need a fast processor to sample the pots often enough to know which
one to pay attention to and when to switch.

Now, tell us more about what you want to do with this. THere might be a better
way to solve your problem.

Do you just want to get motor speed for a fast running motor or do you really
want to measure the actual location of the motor shaft?

I know you are trying to make a small robot so what is the critical dimention?
Are you trying to make a < 1cm thick robot for the BEAM olympics?

I'm sure there is a solution, we just need to find it.

Send questions to the group, not just me.

                 -Alan Kilian

-- 
 -Alan Kilian    kilian@cray.com 612.683.5499 (Work)
 "In 1969 We went to the moon and there was no air and only a bunch of
  dirt and WE LOVED IT!" -Tim Whitley talking about kids today and Apollo.
