Newsgroups: comp.robotics
Path: brunix!news.Brown.EDU!qt.cs.utexas.edu!cs.utexas.edu!bcm!aio!news
From: WELLS <WELLS@CTSD2.JSC.NASA.GOV>
Subject: Re: Touch sensor ideas
Message-ID: <1993Feb10.174610.21999@aio.jsc.nasa.gov>
X-Xxdate: Wed, 10 Feb 93 17:46:45 GMT
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References: <1993Feb9.112158.1131@pericles.robots.ox.ac.uk>
Date: Wed, 10 Feb 1993 17:46:10 GMT
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In article <1993Feb9.112158.1131@pericles.robots.ox.ac.uk> Nairac, 
alex@robots.ox.ac.uk writes:
>	I need a sensor able to detect physical collisions all round a
>small (600*600mm) square AGV; this sensor is to be used as a fail-safe
>device, so that the vehicle's motors are cut off before someone breaks
>a leg... It therefore needs to be extremely reliable.

We were faced with the same problem. Try a closed air bladder whose 
pressure is sensed (differential to small control bladder) for signs of 
contact. Our bladder is made of a vinyl-like plastic, cylindrical in 
cross section (about 3 in. dia), laced up in nylon fabric, and wraps 
around the bumper of a similarly sized robot we use to deliver mail. It 
reliably triggers the stop of this multi-hundred lb robot in an instant 
with the slightest contact. It provides us with an added safety layer 
needed when we found that the robot's sonar system was sometimes blind 
to collision with a moving object (walking human) while the robot is 
also moving. The bladder system has been operating routinely without 
failure (or even maintenance I think) in our office halls for about a 
year...just now passed by my office. Added tip: The mail delivery robot 
also plays music when in motion because one of the employees on our 
floor is blind. She affectionately named our robot Jasper.

For cheap, instead of a sensitive pressure transducer, you might try 
using one of the low pressure switches found in most clothes washing 
machines for cutting off the water fill. Custom make your bladder to the 
shape you need (round cross-section will be most sensitive) from vinyl 
material like that found in kiddie swimming pools. Cover it with 
material like the tough light nylon fabric in camping tents. Should be 
just as or more reliable than ours.
