Newsgroups: comp.robotics
Path: brunix!news.Brown.EDU!noc.near.net!howland.reston.ans.net!spool.mu.edu!torn!news.ccs.queensu.ca!steve
From: steve@cs.ubc.ca (Steve Gillen)
Subject: Re: Arm Position
Message-ID: <CC2t7M.qr@knot.ccs.queensu.ca>
Sender: news@knot.ccs.queensu.ca (Netnews control)
Organization: Department of Physics, Queen's University at Kingston
References: <1993Aug18.165654.153@merlin.dev.cdx.mot.com> <1993Aug20.041720.26802@cs.cornell.edu> <CC1JK9.256@world.std.com>
Date: Fri, 20 Aug 1993 21:04:34 GMT
Lines: 32

In article <CC1JK9.256@world.std.com> jonas@world.std.com (Jonas R Klein) writes:
>
>>In article <1993Aug18.165654.153@merlin.dev.cdx.mot.com> johnb@sgi.dev.cdx.mot.com (John Bottoms) writes:
>>>I am interested in building an arm. It looks like this:
>>>
>>>[ascii pic deleted]
>>>So it's got 4 degrees of freedom and is a small arm used 
>>>to do pick and place.  It is driven by something using 
>>>tendons running up the arm to each joint. Since it's 
>>>small I can't put limit switches on it like you might 
>>>with a big arm. Some of the motors may need to turn more 
>>>than 360deg to get the required forced to move the arm. 
>>>
>>>Here's my problem. Now do I tell the starting position of
>>>the arm? Even if I use servos which have a zero position I
>>>won't know where the various joints are.
>>>
>>
>>	Well, given the more than 360 degree stipulation, there's not going to be any way

  I don't know if you can find an easy way to attach it but...
Bourns makes 10 turn potentiometers (the resistive element is wound as a
helix internally) .  If you can get an exact enough reading you can run 
anywhere inside of the 10 turns.
   Alternatly you could use an optical switch with a flag on the shaft of
coupling.  In extreme envirorments (such as moisture/fog/smoke/oil spray)
you could also used an inductive sensor (like they do in automotive ditributers
in place of the old points).  I think you could get adequate resolution to 
get a unique reference point.  Good Luck :)
                 Steve 
                 steve@phy-server.phy.queensu.ca

