[ale] [Almost totally OT] Rotating hardware interface.

[Keith R. Watson]

At 23:50 12/1/2004 -0700, you wrote:
>This is an unusually weird question, even for ALE. However, I assume
>that the denizens of ALE can collectively answer nearly any
>conceivable practical technical question...
>
>I've got a gob of electronics mounted on a spinning rotor, which I
>need to control via a PC parallel port. (Some small green gnomes awoke
>me in the night and told me to do this; I don't ask questions.) The
>ON-topic bit is that I'm using the parapin library
><http://parapin.sourceforge.net> under Linux to write the driver;
>parapin is a very nice hunk of software.
>
>The OFF-topic bit is that interfacing the parallel port to the
>spinning gizmo presents a mechanical engineering problem that I'm
>having trouble solving effectively, and I don't know where to begin
>looking for the "right" answer. The angular velocity is on the order
>of 300 RPMs. At the moment, for proof-of-concept, I've got the I/O
>pins on the spinning thingy connected to some 22-gauge wires that just
>crudely wrap around the motor shaft, and another jury-rigged set of
>stationary brushes (also made from 22-gauge wire) that make annoyingly
>intermittent contact with those loops, and to which the parallel port
>is connected. It kinda works, but it's not very reliable, and the
>crappy brush mechanism places a noticable mechanical load on the motor
>(RPMs decline significantly when the brushes are in contact with the
>loops, even though the motor power supply is completely isolated from
>the power source driving the electronics).
>
>I bet there's some standard solution to this kind of problem that I'm
>just not aware of, which I could adapt to my needs. Any pointers
>thereto would be greatly appreciated.
>
>Thanks,
>
>-- Joe Knapka


Joe,

I worked on periscopes which have a similar problem. They use a device 
called a slip ring assembly to pass signals from the stationary part of the 
periscope to the rotating part. The signals consist of analog control 
signals, DC power, and RF signals in the low GHz range. Do a Google search 
on "slip ring assembly" and you'll get a bunch of hits.

You might consider converting the parallel data stream to serial to reduce 
the number of connections needed. Another option would be to convert the 
parallel data stream to serial infrared. Then use a hollow drive shaft and 
send the infrared signals through the shaft. Try a Google search on 
"parallel infrared converter"

You could also use RF to couple the stationary and rotation parts. Put the 
antenna on the rotating part in the center so that it's orientation remains 
constant.

happy hardware hacking,
keith


-------------

Keith R. Watson                        GTRI/ISD
Systems Support Specialist III         Georgia Tech Research Institute
keith.watson at gtri.gatech.edu           Atlanta, GA  30332-0816
404-894-0836