Xref: utzoo comp.robotics:871 sci.electronics:20213
Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!ncar!gatech!prism!jm59
From: jm59@prism.gatech.EDU (MILLS,JOHN M.)
Newsgroups: comp.robotics,sci.electronics
Subject: Re: Sensing a known location to reference off of
Message-ID: <29100@hydra.gatech.EDU>
Date: 15 May 91 21:37:16 GMT
References: <1991May15.005403.28812@src.honeywell.com> <1991May15.203301.18003@mcs.anl.gov>
Followup-To: comp.robotics
Organization: Georgia Institute of Technology
Lines: 47

In article <1991May15.203301.18003@mcs.anl.gov> winans@sirius.mcs.anl.gov (John Winans) writes:
>In article <1991May15.005403.28812@src.honeywell.com> 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.

If you can't trust the steppers, you can't repeat your positioning.  I've
seen the following method used for machine tool "home" indexing:
  (1) one detector is interrupted by the carriage [coarse],
  (2) second detector is interrupted by the rotational angle of the
      lead screw (if you have one) [medium], and
  (3) force the stepper coils to a predetermined excitation pattern [fine]
      (the use of fractional or partly redundant excitation patterns is
      sometimes used for high resolution, called "microstepping").

If the system can stop quickly enough, you may be able to logically gate
the signals from (1) and (2), rather than sensing them separately.

If that won't do it, you need another design.  If the drive has appreciable
backlash (mechanical looseness), always approach from the same direction:
overstep and back up, if needed.

This scheme will work with switches or optical interrupters, but the optical
approach is preferred.  Darlington or hysteresis-type buffers can
be used with the opticals, but probably won't be needed (except for
possible level shifting).

For optical interrupters, I suggest Digi-Key (Thief River
Falls, MN 56701-0677, or 1-800-DIGI-KEY  (1-800-344-4539), or your local
Hewlett-Packard electronics distributor (Newark, etc.), but have you
checked with Radio Shack?  (These things look like a little plastic block
with two posts sticking up.)

Digi-Key has a great catalog of ic's and piece-parts.

Have fun!

Regards --jmm--

-- 
MILLS,JOHN M.
Georgia Institute of Technology, Atlanta Georgia, 30332
uucp:	  ...!{decvax,hplabs,ncar,purdue,rutgers}!gatech!prism!jm59
Internet: jm59@prism.gatech.edu