Newsgroups: comp.robotics
Path: brunix!news.Brown.EDU!noc.near.net!howland.reston.ans.net!math.ohio-state.edu!cs.utexas.edu!uunet!world!jonas
From: jonas@world.std.com (Jonas R Klein)
Subject: Re: Arm Position
Message-ID: <CC1JK9.256@world.std.com>
Organization: The World Public Access UNIX, Brookline, MA
References: <1993Aug18.165654.153@merlin.dev.cdx.mot.com> <1993Aug20.041720.26802@cs.cornell.edu>
Date: Fri, 20 Aug 1993 04:38:33 GMT
Lines: 32


>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
>to determine the robot's initial position completely from scratch.  I would suggest, however,

How about putting the limit switches on the tendons?  While the
pulleys driing them may go around a few times, the tendons will move a
fixed linear distance.  A limit switch can detectd something attached
to the tendons, or the movement can be coupled back into some absolute
position encoder such as a linear potentiometer. (or optical encoder,
or rotary encoder attached througha few gears to limit the travel to
less thatn 360 deg.)  If the tendon travel is too large for a linear
pot, how about using a "black & tackle" arangement to scale it down?

-- 
+Jonas Klein
 jonas@world.std.com
