Newsgroups: comp.robotics
Path: cantaloupe.srv.cs.cmu.edu!das-news2.harvard.edu!news2.near.net!news.mathworks.com!uhog.mit.edu!bloom-beacon.mit.edu!news.media.mit.edu!minsky
From: minsky@media.mit.edu (Marvin Minsky)
Subject: Re: Artifical Muscles
Message-ID: <1995Jan23.045934.4749@news.media.mit.edu>
Sender: news@news.media.mit.edu (USENET News System)
Cc: minsky
Organization: MIT Media Laboratory
References: <D2q97L.Jy7@freenet.carleton.ca> <D2sCCD.69r@freenet.carleton.ca> <3fsn8b$phi@crl12.crl.com>
Date: Mon, 23 Jan 1995 04:59:34 GMT
Lines: 38

In article <3fsn8b$phi@crl12.crl.com> dreed@crl.com (Darin L. Reed) writes:
>Tim Sallans (av574@FreeNet.Carleton.CA) wrote:
>: Really?  I didn't know this...perhaps I'm overestimating my knowledge.
>: <grin>  Then does that mean I've gotten it backwards, and the temperature
>: just catalyzes (the correct term for a non-chemical interaction?)
>: the release of mechanical energy stored while stretching the material?
>: Then that would mean that you would need some mechanism to stretch
>: the metal (a spring wouldn't work, or that would be a perpetual
>: motion machine :) )...the energy output of the metal must either come
>: from the heat or the stretching.  Which is it?  I really don't
>: know enough about this stuff, and would be interested in a quick
>: answer.
>
>Umm...Temperature isn't really a catalyst in this case (It never is 
>actually. A catalyst is some atom or molecule that aids in the reaction, 
>but is not actually consumed or changed once the reaction is completed) 
>but it does provide the activation energy needed to make the wire move. 
>You do some work while stretching the material. Say it takes .1 newtons 
>of force to stretch the wire. You add heat, and the wire contracts with 1 
>full newton of force. The difference comes from the heat being converted 
>into mechanical energy (although not all that efficiently, as some have 
>pointed out).

Well, it is clearly a heat engine that does no recycling, so it can be
no better than a simple steam engine. I have the impression that the
upper temperature is only about 100 degrees C above the lower one,
which means that the efficiency must be less than the order of 25%.
For single time use, though, as in lowering a landing gear, it might
have some virtue.  

However, I'd be surprised if it stores more energy than a rubber band
of the same weight, and it's easy to cut same with a heated wire -- so
that might be a better alternative for one time use.

Come to think of it, is there any reason to suppose that nitinol is
better than a good steel spring for one-time uses?.

