Slip Rings and Domed Observatory -- Continuing

[W. eWatson]

Well, things are really starting to move with my attempt to put a
control system in my 10.5' dia dome. There's no doubt,finally, this is
going to happen in the next few months. The control system will allow me
to move the shutter and rotate the dome via my PC to allow my telescope
to point to the open sky where I want to say photograph something. There
are two 1/2 HP motors. One controls the shutter position, and the other
the rotation.

Before the installation occurs, some weeks from now, I would like to
know what the better understand the slip ring mechanism is that is
likely useful in this installation. It supposedly provides continuous
power and would be located around the skirt of the obs. The skirt is the
wall area below the dome. I think it consists of three copper tubes that
follow the perimeter of the skirt.

It's easy to see how it would provide power for the rotation motor,
since it is attached to the top of the skirt and its gear meshes into a
track along the bottom of the dome. However, what I don't get is how it
can provide power to the shutter motor at the top of the dome.

Presently, a power cord comes from the skirt up to it, which at times
can wrap itself around the telescope. That has to be eliminated. It
makes no sense that a cable would come from the ring to it. My guess is
that there must be wire brushes that somehow connect power to the
shutter motor between the ring and the shutter motor. Can someone square
me in how this really works? BTW, would putting the ring mechanism in
require replacing my 110v motors with 220v motors?

 

[Jamie]

Well, things are really starting to move with my attempt to put a
control system in my 10.5' dia dome. There's no doubt,finally, this is
going to happen in the next few months. The control system will allow me
to move the shutter and rotate the dome via my PC to allow my telescope
to point to the open sky where I want to say photograph something. There
are two 1/2 HP motors. One controls the shutter position, and the other
the rotation.

Before the installation occurs, some weeks from now, I would like to
know what the better understand the slip ring mechanism is that is
likely useful in this installation. It supposedly provides continuous
power and would be located around the skirt of the obs. The skirt is the
wall area below the dome. I think it consists of three copper tubes that
follow the perimeter of the skirt.

It's easy to see how it would provide power for the rotation motor,
since it is attached to the top of the skirt and its gear meshes into a
track along the bottom of the dome. However, what I don't get is how it
can provide power to the shutter motor at the top of the dome.

Presently, a power cord comes from the skirt up to it, which at times
can wrap itself around the telescope. That has to be eliminated. It
makes no sense that a cable would come from the ring to it. My guess is
that there must be wire brushes that somehow connect power to the
shutter motor between the ring and the shutter motor. Can someone square
me in how this really works? BTW, would putting the ring mechanism in
require replacing my 110v motors with 220v motors?

I am trying to envision why this would be a problem?

In any case, you can have multiple slip rings to get around obstacles
like gears and bearings on the mass the rings maybe mounted to. The idea
is to put a channel (slot) under the gear/bearings and pass the wires
under to the upper set of rings. from there, you can extra power with
out any problems? Another set of brushes are then used.

But still, I thought the whole obs turns on the post and why should the
shutter not be turning with it? In which case, the slip rings on the
skirt as you say, will be stationary and the brush assemblies will be
mounted on the base of the obs that turns.

Jamie

 

[W. eWatson]

Based on this one post of yours, I can see absolutely no reason why a
slip-ring system would require a change from 110 to 220. (unless, of
course, you also changed to "stuff" that needs 220 instead of 110)

Nor do I. It came up in a talk with the installer, but was probably
unrelated to slip rings. I'll stick with 110.

If I were doing what you seem to be describing, I'd set things up like
so:

On the skirt side, I'd install a slip-ring for each power line (I'm
assuming you're talking about the standard "hot, neutral, ground" trio)
plus one ring each for any control signals or feedback lines that might
be involved.

On the dome side, I'd install one brush for each power, control, and
feedback line. Obviously, they have to be placed so that they make
contact with the correct slip-ring on the skirt.

I'll leave these details to the installer.

From there, it's a simple matter to run a wire from each of the brushes
to someplace convenient - *ON THE INSIDE OF THE DOME* - and terminate
them all in a junction box mounted there.

Ah, so brushes are involved. I have no idea how this would be done to
cross from the skirt to the dome. This comes close to showing them and
the rings in a professional obs.
<http://www.astronomical.com/products/dome-control>. See the photo on
the right about 1/2 way down. One can see the rings under the shelf. I
don't see any brushes.

This shows a use of rings in a photo a little big dowt on the right.
Those are big brushes! See the legend under the photo.

Once you've got the junction box wired to the brushes and attached to
the inside of the dome, you run cabling (Also secured to the inner
surface of the dome, of course) from the junction box to wherever the
power, signal, or feedback wires need to be, totally ignoring the fact
that the brushes and slip-rings exist - As far as the dome-mounted stuff
is concerned, everything comes from/goes to the junction box - where it
comes from or goes beyond the junction box is unimportant - Maybe gnomes
with buckets put it there and take it away. Maybe it's a solid wire.
Maybe it's carrier pigeons - The stuff mounted on the dome just plain
doesn't care.

Presumably the power would go to a control box on the side of the done
for the shutter electronics (circuit board, relays, limit switches,
motor), and another like it on the skirt for rotation. Gnomes are not
allowed, but I've had a bird or two try to get through the open shutter.

 

[W. eWatson]

I am trying to envision why this would be a problem?

In any case, you can have multiple slip rings to get around obstacles
like gears and bearings on the mass the rings maybe mounted to. The idea
is to put a channel (slot) under the gear/bearings and pass the wires
under to the upper set of rings. from there, you can extra power with
out any problems? Another set of brushes are then used.

But still, I thought the whole obs turns on the post and why should the
shutter not be turning with it? In which case, the slip rings on the
skirt as you say, will be stationary and the brush assemblies will be
mounted on the base of the obs that turns.

Jamie

The of the obs as a silo. There is no post. The dome rolls on a track
sitting on the skirt driven by a motor.

 

[Jamie]

The of the obs as a silo. There is no post. The dome rolls on a track
sitting on the skirt driven by a motor.

Sounds like an easy job then.

I've seen induction power coupling that works very nice for that, it's
nothing more than a transformer primary at the fixed base and the
rotating part has the secondary of the transformer sitting inside the
primary section. There are no slip rings and things are done wireless
from a remote point.

Jamie

 

[Don Bruder]

Sounds like an easy job then.

I've seen induction power coupling that works very nice for that, it's
nothing more than a transformer primary at the fixed base and the
rotating part has the secondary of the transformer sitting inside the
primary section. There are no slip rings and things are done wireless
from a remote point.

Jamie

Overkill much?

There's an old saying - an elephant is a mouse built to a committee's
specifications. Your idea is a prime case of that. You're trying to
"over-tech" it beyond ridiculous for no significant gain. I suppose
you'd advocate using a sledgehammer to swat a mosquito that just landed
on your forehead, too?

A simple, reliable, purely mechanical solution in the form of slip rings
and brushes will do the job more than adequately, be ultra-low
maintenance, cost *A LOT* less, both to install and to operate, be
orders of magnitude lighter, will be reasonably weatherproof without the
need for expensive enclosures, won't generate anywhere near the amount
of heat (which equals lower operating cost due to not having the waste
of electricity being turned into heat instead of motion) that a magnetic
coupling would, and have too many other benefits compared to inductive
coupling for me to even try listing them all here - the ones I already
listed practically wrote themselves - if I were to actually sit down and
ponder the idea, I'd probably come up with dozens more.

 

[Don Bruder]

Nor do I. It came up in a talk with the installer, but was probably
unrelated to slip rings. I'll stick with 110.

I'll leave these details to the installer.
Ah, so brushes are involved. I have no idea how this would be done to
cross from the skirt to the dome. This comes close to showing them and
the rings in a professional obs.
<http://www.astronomical.com/products/dome-control>. See the photo on
the right about 1/2 way down. One can see the rings under the shelf. I
don't see any brushes.

My best guess from the picture is that the brushes are hidden by the
motor.

This shows a use of rings in a photo a little big dowt on the right.
Those are big brushes! See the legend under the photo.

And now I think I know where the 220 versus 110 question comes up - The
page you're looking at *SEEMS* to be written specifically for a 220
(240, actually, but lets not quibble) volt system. Exactly the same
concepts apply for both voltages - You've got 3 rings on the tower, each
isolated from everything else, and all 3 stationary. For a 110 system,
one rai gets connected to "hot" (110 volts relative to "ground"), one to
"neutral" (theoretically zero volts relative to ground, in practice, not
ALWAYS true), and the third to "ground" (which, for 110, is the
so-called "safety ground") For a 220 system, instead of "hot, neutral,
ground", the labeling is usually "L1, L2 and ground" (Each of "L1" and
"L2" are +110 volts relative to "ground", giving you a total working
voltage of 220 volts)

On the dome side, you've got three brushes - each one will be a piece of
spring metal (perhaps something like a strip of phosphor bronze)
pressing a block of graphite down against one of the three rails. for a
110 system, one brush will be riding on the "hot" rail, another on the
"neutral" rail, and the third on the "ground" rail. Same thing for a 220
system, but the brushes are called "L1", "L2", and "ground" respectively.

Put it all together, and the whole mess gets power - either 110 or 220 -
across the gap between the tower and the dome, with no solid wires
connecting the two pieces, leaving the dome able to rotate freely. A
wire from each brush then goes to a junction box mounted on the dome,
where anything on the dome that needs power can be connected - Notice I
said *ANYTHING* - the rotation motor, the shutter motor, electronics to
control both, a fluorescent light fixture, a socket to plug in a
blow-dryer to unfog the 'scope mirror - ANYTHING you can imagine that
might need to both have power, and rotate with the dome can be powered
out of that junction box. (up to the limits of the wiring supplying the
juice, of course - if you've only got a 50 amp circuit powering it, it
should be obvious that you're not going to be able to run 100 amps worth
of gear)

Presumably the power would go to a control box on the side of the done
for the shutter electronics (circuit board, relays, limit switches,
motor), and another like it on the skirt for rotation.

Yep, exactly.

 

[John Nagle]

It's not clear what you have, what you're buying, and what you're
building.


John Nagle