Newsgroups: comp.robotics
Path: brunix!news.Brown.EDU!agate!ames!aio!l44db!mike
From: mike@l44db.jsc.nasa.gov (Mike Ross)
Subject: Re: Fast/Flexible Manipulators
Message-ID: <1993Dec1.141521.23800@aio.jsc.nasa.gov>
Sender: usenet@aio.jsc.nasa.gov (USENET News Client)
Organization: Lockheed Engineering & Sciences Co.  Houston, TX.
References: <1993Nov29.203443.12741@aio.jsc.nasa.gov> <2df5p5$f5s@info.epfl.ch> <1993Nov30.201927@IASTATE.EDU>
Date: Wed, 1 Dec 1993 14:15:21 GMT
Lines: 44

In article <1993Nov30.201927@IASTATE.EDU> bouton@IASTATE.EDU (Chad E Bouton) writes:
)
)In article <2df5p5$f5s@info.epfl.ch>, Philippe Guglielmetti
)<guglielmetti@elia.epfl.ch> writes:
)
)> If you are interested in research, you have two choices: 
)> - work on flexible robots, where a big competition exists to solve an 
)>   almost uniquely academic problem. (Japanese people do not work in this
)> area).
)
)3. What is the most prevalent type of manipulator being researched
)    for space use?  Is parallel or flexible or other?
)

The Shuttle's RMS is somewhat flexible, exhibiting about a 7 lb/in 
stiffness normal to the booms when stretched out.  Most of this
flexure comes from the gearboxes, which have very large gear ratios
(around 737:1 at the wrist, 1842:1 at the shoulder).  One footpound
exerted on the motor drive end of the shoulder gearbox will result
in a radian of deflection with the output end fixed! 

This flexibility problem has occupied many fine minds at NASA, and yes,
it's a hard problem.  One wonders whether the problem posed by the
RMS' flexibility warrants the effort required to resolve it.

Flexibility, however, also offers passive compliance.  Active
compliance in stiff manipulators requires accurate load-sensing
devices being sampled very fast, with equally fast compensating commands
being carried out by the manipulator.  Not easy. The RMS is too slow to be
able to offer that kind of compliance, since its update rate is 12.5 Hz.
Good active compliance requires 500 to 1000 Hz to react quickly to 
outside disturbances.

All maneuvers posed for two manipulators in space have only one active
arm.  The other one will have brakes on.  Obviously, the only time
both manipulators will be holding the payload will be during handoffs.

-mike

-- 
******************************** mike@l44db.jsc.nasa.gov ******
* Michael L. Ross/C33 | Lockheed Engineering & Sciences Co.   *
* Robotics Department | 2400 Nasa Rd. 1, Houston, TX 77058    *
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