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From: wli@fornax.UUCP (William Li)
Newsgroups: comp.robotics,sci.electronics
Subject: Re: Sensing a known location to reference off of
Message-ID: <2700@fornax.UUCP>
Date: 16 May 91 06:59:01 GMT
References: <1991May15.005403.28812@src.honeywell.com>
Organization: School of Computing Science, SFU, Burnaby, B.C. Canada
Lines: 39

In article <1991May15.005403.28812@src.honeywell.com>, gcary@SRC.Honeywell.COM (Greg Cary) writes:
> I am constructing a compact disc jukebox for my CD collection, and I
> would very much appreciate some advice on location sensing.
> What I need to do is determine when I am passing by a known location, so
> that I can count off the right number of stepper motor steps so that
> I end up at the right CD.  Two methods that come to my mind right away
> are 1) using a microswitch, and 2) interrupting the light between an
> LED emitter/detector pair.  By far the most important thing is that I
> get repeatable results.
> 

How many different locations do you need to locate?  If it's just one,
then a single IR pair would be the best.  If you need to locate several,
consider using an optical rotary shaft encoder, essentially just a
wheel with a lot of regularly-spaced slots and three pairs of IR tx/rx
pairs -- two to get speed and direction of rotation, one to provide
a sync pulse every complete rotation.  This is a little expensive,
though.  Alternatively, you could try using a multi-turn potentiometer
with its shaft slaved to the stepper somehow (eg, via a belt -- depends
on what your exact setup is) and use the resistance to give you an
absolute indication of position/rotation.  If you're doing linear
motion, you could use a pair of resistor rails.  For your application,
it doesn't sound as if you need ultra-high precision at all points
in your travel.  Steppers are generally very reliable and will miss
step only if you're trying to apply too much torque to them or make
them go too fast.  You might be able to get away with using a counter,
a spinning disk with one hole cut in it, and one IR tx/rx pair to
provide a sync pulse for every rotation.

Just some random thoughts.

William Li
School of Engineering Science
Simon Fraser University
Burnaby, B.C.
Canada  V5A 1S6

Tel: (604) 291-4451
Fax: (604) 291-4951