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

[Joe Knapka]

Thanks to John and Keith for your replies...

John Mills <johnmills at speakeasy.net> writes:

> First, I wouldn't expect any home-made slip ring assembly to last long,
> expecially at 300 RPM. Go looking for surplus materials. (I used to find
> such at Servo-Systems [http://www.servo-systems.com]; they only list
> stepping motors on their current web page, but it might be worth a call
> (440-779-2780) to see if they have any suggestions. (It's been a while 
> since I was in this business!!) In a _far_ earlier life I haunted Eli 
> Heffron and Sons in Cambridge, MA [http://www.eli.com] - they seem to sell 
> primarily computers now, but again a call (617-547-4005) might turn up a 
> lead.
> 
> Second, Keith is dead on-target about serializing the data. The fewer
> contacts, the less noise and maintenance by some large exponential factor.  
> National Semiconductor used to build i.c.s that would 'image' a number of
> parallel lines (bidirectional IIRC, and they could be daisy-chained for
> more numerous signals) over a few serial connections (one signal-pair each
> direction, plus a clock or two, probably). If you only need 8 signals
> (plus a couple of On/Off or hand-shake liness), a pair of ordinary
> U[S]ARTs might be fine. You can really take that 'S' seriously and use
> them in synchronous mode, pretty transparently.
> 
> You can assign some microcontroller [think PIC?] to handle the signals and
> use some simple byte protocol over its serial I/O.

That is a really good idea, which hadn't occurred to me. In fact,
pretty much everything the PC software is doing could as well be done
by an MCU mounted on the rotor. It would have to be a very compact MCU
package, though, since the rotor is only about 1.5" in diameter; maybe
something like a STAMP 1 would be suitable. Such an arrangement would
necessitate only intermittent instruction from the PC (perhaps
once/minute instead of several hundred times per second). Undoubtedly
simpler to implement and more reliable. Looks like 2005 is going to be
a year of getting familiar with hardware and MCUs -- cool!

BTW in case it isn't obvious, my hardware-fu is not very good at
the moment. I'm working my way through the Horowitz & Hill book
"The Art of Electronics", but it's pretty slow going. Building a
serial interface from discrete components would be beyond me
at this point, though I'm eager to learn.

> You probably[!!] need some filtering/debouncing/conditioning if you go
> through spinning mechanical contacts. Modulation, filtering, or digital
> shift/sum registers can do this, or use latches with large hysteresis on
> their inputs. (See someone's design manual on linear and/or interface
> i.c.s.) The parallel/serial/parallel components usually make some
> provision for this; if you code up some kind of 'bit-banger' port [think
> PIC?], the filtering can also be coded-in.

I looked at some liquid-interface sliprings, from Mercotac, and at
some more traditional slip ring assemblies; but they're all way
too big. The shaft I need to mount the slip ring on is less than
1cm in diameter, but there don't seem to be standard slip rings
with less than a one-inch or so bore :-(  Maybe I'm looking in
the wrong places.

> You may find some optical couplers -- that would reduce or eliminate the
> bounce and wear problems. I'ld give this a _serious_ shot [maybe
> Digi-Key??]. Then you only have to get power through the rings, and DC
> power is [relatively] easy to filter.

The notion of an axis-mounted IR interface is appealing, if I'm
going to have an MCU available to decode the IR data. And as you
say, getting DC onto the rotor is not difficult. This project
is based on a child's toy that already supplies DC to some
fairly stupid electronics on the rotor, so adding some smarts
in the form of an MCU and some kind of serial interface (ideally
wireless) would be WAY more straightforward than my current
approach.

> Bottom line: if it has to last, the solution is probably non-trivial.

Luckily, this project is entirely for my own amusement and education
:-) If I can get it to work well for five minutes, I'll feel a great
sense of accomplishment.

Thanks very much, you have already inspired some more productive
thinking on my part.

-- 
"Meet the new boss / Same as the old boss..." -- The Who
... Oh well, at least there's 2008.
--
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