Electrical Slip Rings

Posted by: Ron Walker Oct 31 2007, 06:20 PM
Someone one said that if you went to a car dealer and bought all the parts needed to build a typical $25K car, it would cost you $250K and you would have to put it together.

Same for a planetarium. It uses a lot of specialized parts and to get commercial versions would probably cost more then the half million or so a new professional planetarium projector costs. This realization brings out the experimenter in all of us and the mention of $2K slip rings makes my pocketbook ache.

I've had a lot of ideas of how to make a lot of parts so I desided that it might be worth a bit of experimentation to see if any of these ideas would actually work. Since we never ever have enough circuits to power everything we want to move around on our planetariums, slip rings would be something that I could use on any exsisting projector or for those of you building from scratch.

I have collected a ton of "good junk" and had a look in my boxes and found materials worthy of experiment.

There are two types of slip rings. Those that lay flat and have concentric circles and those that stack one on top of each other and fit around a shaft. Each has their place depending on the space available and the number of circuits needed.

While I will be experimenting with both types, I have on hand some surplus printed circuit board which I believe will make very good slip rings of the first type. Now those of you that have worked with PC boards will no doubt mention the use of photo resist and etching chemicals, but I really wanted to make these without all the mess and hostile chemicals.

The surplus boards were approximately 5X8 inches and cost about $3 each.



Since slip rings are round I just laid out a square and found a center which I cut with a hack saw. I also laid out 1/4 inch thick slip rings with 1/8 inch spaces between them.



Then I made a simple adapter to hold the PC board on a small lathe and just "cut" the copper foil out between the rings.

Posted by: charles jones Nov 1 2007, 01:42 AM
Hi Ron

Fantastic! This was my idea too except that I thought it might need thicker material and was planning to buy copper sheet and glue it to a plate, then cut the rings on a lathe as you have done.

Of course, I haven't been able to do anything while I am here.

When I get home, the first thing is I have ordered a small lathe to begin making parts.

Charles

Posted by: Ron Walker Nov 1 2007, 12:20 PM
QUOTE(mrgare5050 @ Nov 1 2007, 03:19 AM) *
ummm. can someone please, for the untechnical people here duct taping up flashlights... define slip ring? what is its basic function etc? i need help! thanks smile.gif gare

Basically it is a device to allow the transfer of current from one side to another of a moving mechanical device.

When people look at any planetarium projector the usually marvel at the precision gearing and the fantastic projectors which are part of the device. Most don't realize that it is a marvel of electrical engineering as well.

Let's look at the mother of all planetarium projectors, the Zeiss II, and see how we get power to the Sun projector as an example. Basically we have a supply wire and a ground wire that carries current from the control console to the machine itself. Now since the entire projector rotates around several axis you can see that the wires would get twisted together and eventually break if there were not some way to transfer the power past these axises. For that one simple Sun projector there are four different sets of slip rings the power must pass to reach the bulb for this one projector.

The Latitude Motion.
The Daily Motion.
The Processional Motion.
The Annual Motion.

Basically the copper rings above are half of the slip ring device. Let us say that it is attached to the central support core of the planetarium projector. Whatever projector that is attached to a central core on bearings must be able to rotate to allow projection of daily motion. Let us attach a star ball to the top shaft and a Sun projector to the bottom shaft. A motor will then turn this entire apparatus which will then project daily motion. Since the bulbs for both the stars and the Sun are now also revolving with the central core, the wires supplying power would soon tangle and eventually break unless you ran the motor in reverse as many revolutions as you went forward. This would be a bit of a pain and most would forget after a while and damage would eventually happen.

Now we will attach to the central rotating core a set of sliders (not yet built in the device above) that will turn with the central core and each one will make contact with one of the copper rings. Thus as the projector moves through the day, the slider will just slide along the copper ring and power for the projector bulbs will be continuously supplied through the ring. The projector could continue to run through day after day and there would be no tangling of the wires.

You don't need it on your copper cylinder because the cylinder rotates around the bulb which is on a fixed stalk that comes out the back of the projector. If you look at the main support for your Spitz A-1 you will see slip rings on the rod that holds the star dodecahedron on one end and the coordinates projector on the other.

Posted by: mrgare5050 Nov 2 2007, 04:29 AM
ok i can see how the tangling wire problem would be solved by a circular 'track' thats powered with current, but why you need more than one i cannot visualize - what exactly is 'slipping' ? gare

Posted by: charles jones Nov 3 2007, 02:16 AM
Hi Ken -
I definitely like the spring on the "wiper" arm to keep it in contact. I was thinking of a piece of springy brass for an arm, but a small spring is a much better idea.

Hi Gare - You need a separate slip ring for each circuit (actually you might need two if the voltages are different from circuit to circuit).

For me I am looking at one ring for each of the following:

1 - Ground 3.7 voltage
1 - Star globe lamps
1 - Ecliptic projector
1 - Planet projectors

1 - Ground - 28 v or 120 v. DC annual motion
1 - One winding of the annual motion
1 - one for the reverse direction of the annual motion

1 - 120 v. ground for precession motion
1- 120 v. for one direction/precession motion motor
1 - 120 v. for reverse direction/precession motion

There is a possibility that I'd need 2 or 3 more if I decide to go with a different lamp with a different voltage for the star globe than I'd be using in the planet projectors.

I figure I can run some of wires through a hollow daily motion shaft eliminating duplicate slip rings for the same purpose on the northern and southern assemblies. So, I might only need 5 rings on each assembly. Example: Wiring to the northern assembly through slip rings for star globe, coordinates projector and planets can go through the turning shaft and supply electrical circuits to the southern assembly without another set of slip rings.

The reason I need to use the flat type of concentrate rings as Ron has shown, is I need to take up as little space as possible along the length of the daily motion shaft. The rings that are stacked (one above each other) would take up several inches of height along the shaft resulting in pushing the two star globes further apart.

Posted by: Ken Miller Nov 3 2007, 06:33 AM
The contact "slips" as it moves around the ring. The contacts can move as the rings stay stationary, or the rings can move as the contacts stay stationary, either way that you want to do it.

Motor brushes also have springs to maintain pressure against the rotor. The Spitz model A has a slip contact very similar to the way I did mine. I think my real inspiration was the "bed of pins" contacts used on PC board testers. Anyway, Ive seen this done dozens of times in different applications.

Posted by: Ron Walker Nov 3 2007, 02:20 PM
QUOTE(charles jones @ Nov 3 2007, 02:16 AM) *
Hi Ken -
I definitely like the spring on the "wiper" arm to keep it in contact. I was thinking of a piece of springy brass for an arm, but a small spring is a much better idea.

Hi Gare - You need a separate slip ring for each circuit (actually you might need two if the voltages are different from circuit to circuit).

For me I am looking at one ring for each of the following:

1 - Ground 3.7 voltage
1 - Star globe lamps
1 - Ecliptic projector
1 - Planet projectors

1 - Ground - 28 v or 120 v. DC annual motion
1 - One winding of the annual motion
1 - one for the reverse direction of the annual motion

1 - 120 v. ground for precession motion
1- 120 v. for one direction/precession motion motor
1 - 120 v. for reverse direction/precession motion

There is a possibility that I'd need 2 or 3 more if I decide to go with a different lamp with a different voltage for the star globe than I'd be using in the planet projectors.

I figure I can run some of wires through a hollow daily motion shaft eliminating duplicate slip rings for the same purpose on the northern and southern assemblies. So, I might only need 5 rings on each assembly. Example: Wiring to the northern assembly through slip rings for star globe, coordinates projector and planets can go through the turning shaft and supply electrical circuits to the southern assembly without another set of slip rings.

The reason I need to use the flat type of concentrate rings as Ron has shown, is I need to take up as little space as possible along the length of the daily motion shaft. The rings that are stacked (one above each other) would take up several inches of height along the shaft resulting in pushing the two star globes further apart.

Every time I start thinking of different projectors I find that you can never have enough slip rings. If you got the 6X6 inch boards from Allied Radio you could easily cut seven slip rings on each of two. One on either side of the central core. That would give you 14 rings which would be a good start indeed.