Xref: utzoo comp.robotics:875 sci.electronics:20259
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From: jack@rml.UUCP (jack hagerty)
Newsgroups: comp.robotics,sci.electronics
Subject: Re: Sensing a known location to reference off of
Summary: Lots o' ways
Message-ID: <279@rml.UUCP>
Date: 17 May 91 02:06:28 GMT
References: <1991May15.005403.28812@src.honeywell.com>
Reply-To: jack@rml.UUCP (jack hagerty)
Followup-To: comp.robotics
Organization: Robotic Midwives, Ltd.
Lines: 49

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.  


Well, let's see. Off of the top of my head I can come up with these:

1) Microswitches, which you mentioned. Cheap but clunky.

2) LED Emitter/receiver pair. Expensive since you need a lot of either
	emitters or receivers (whichever you put in front of the slot)

3) A better version of 2): a break beam pair mounted on the carriage
	with tabs in front of each stop.

4) A retro-reflective unit (emitter and receiver in one housing) with
	a target (a corner cube reflector or "reflector tape") at each
	position where you want to stop.

5) Non-optical variations on 4): a inductive proximity sensor on the
	carriage and a metal post at each position. Or a capacitance
	sensor with non-metal posts, or magnetized posts with a reed
	switch or Hall effect switch on the carriage, or...

6) A linear position sensor. This is a little box with a wire that plays
	out like a tape measure. The motion is converted to rotary and
	encoded internally, but you don't have to know that. They come
	in lengths far longer than a home CD storage would require.

7) The trickest one of all: mount a bar code reader on the carriage
	(the non moving type like a wand, since the carriage is doing
	the moving) and put a label on each position. That way you 
	could encode each slot with a discrete name and know not just 
	that you got there, but who's home!

- Jack

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