Newsgroups: comp.robotics
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From: nivek@cs.cmu.edu (Kevin Dowling)
Subject: Re: Wires thru joints
Message-ID: <1992Jul09.174414.192091@cs.cmu.edu>
Date: Thu, 09 Jul 92 17:44:14 GMT
Organization: Robotics Institute, Carnegie Mellon
Nntp-Posting-Host: scythe.frc.ri.cmu.edu
References: <13308@mindlink.bc.ca>
Reply-To: nivek@cs.cmu.edu
Keywords: slip rings, rotating joints
Lines: 69

This is an old issue in robot design that won`t go away. I.e. it
has not been resolved to everyone`s satisfaction.

Communication and power are the primary things you want to send through
a rotating joint. To minimize wiring it is possible to use the power system as 
a carrier for information much like the X-10 control systems used for home appliance and light control but you still need to allow at least one line to rotate continuously.
[Don`t use the robot mechanisms as ground or return path - current though bearings
can destroy them]

Slip rings can be expensive. They can add complexity to the mechanics and
have implications to componentry if you need to add muxing of signals.

We've used a number of slip ring assemblies
built for the nuclear vehicles, the CMU Ambler, the Locomotion Emulator and others.
Tethers are a common application of slip rings where you need to wind and
unwind lengths of cable. You can double up cables on a spool to avoid this or
use clever counterwind schemes to minimize total cable lengths.

I used 6 slip rings on the Ambler, one for each leg, to allow the continuous
eggbeater rotation. However, even with 20 or so rings per leg it wasn`t enough
to carry all signals. The force sensors alone use a large number of signals.

As a result we developed some high speed multiplexors (time-sliced) for sending
multiple signals over a single ring. We can send significant power over the rings
as well for powering the Ambler. After a couple of years a commercial product
was switched in for the homebrew system (albeit slower). The rings have
been working pretty well considering the application.

Part of the design process is to evaluate where the system is split at the
sliprings. In the Ambler for example the motor amplifiers are on the legs but
the motion control boards (in the VME) cage are on the body. This can greatly
effect the number of signals across the rototating interface and affect the
overall design.

---Standard slip rings can supply bandwidth capabilities to 70MHz and power
needs to many watts. DC's a bit trickier due to arcing but careful brush design
can alleviate even this problem for some applications.

---Rotary transformers, as mentioned in a previous post have been around for
some time. Again, they can supply both power and data. Galileo, the deep space craft,
is using such a system for the spun and despun sections of the craft. RT are considered
more reliable and lower noise than slip rings. It can also provide data rates
in the megabit range.

--There are slip rings that allow fiber optics and even fluids simultaneously.

--Another possibility is to do what radar dishes do which is to send power
and information through a rotating waveguide. Conversion efficiencies are high.

--Fibers can also supply power as well as information through p-v cells to
convert the light intensity to power at up to 40% (not depending on solar
spectrum!). Pretty neat idea.

--There is an old article I have from Scientific American's Amateur Scientist
column that discusses this topic and shows a neat mechanical way to have non-twisting
wires through a rotating interface. Unfortunately although I have the article
here the date is not on it.


						nivek

---

aka: Kevin Dowling			Carnegie Mellon University
tel: (412) 268-8830			The Robotics Institute
adr: nivek@rover.ri.cmu.edu		Pittsburgh, PA 15213



