Xref: utzoo comp.robotics:867 sci.electronics:20202
Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!zaphod.mps.ohio-state.edu!think.com!snorkelwacker.mit.edu!stanford.edu!msi.umn.edu!sctc.com!smith
From: smith@sctc.com (Rick Smith)
Newsgroups: comp.robotics,sci.electronics
Subject: Re: Sensing a known location to reference off of
Message-ID: <1991May15.150813.26223@sctc.com>
Date: 15 May 91 15:08:13 GMT
References: <1991May15.005403.28812@src.honeywell.com>
Organization: SCTC
Lines: 26

gcary@SRC.Honeywell.COM (Greg Cary) writes:

>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.

I'd vote for 2) primarily because a set of reliable LED emitter/detector
pairs will keep working under a broader range of conditions than 
a group of microswitches. I just imagine a microswitch getting jostled
a bit so that it doesn't contact any more (tho' I guess a LED can get
unreliable if it gets too dusty).

Naturally, the perfect thing is to put a pair for sensing each slot
in your library, thought that's probably too expensive. But don't
skimp - have one at each end of your rack, and add a few in between
so as to minimize the degree to which you trust positioning to the
stepper motor. You might want to always find the closest LED to the
slot and then step from there.

Have fun.

Rick.
smith@sctc.com    Arden Hills, Minnesota