A new project for Charles Jones

Posted by: charles jones Jan 23 2016, 04:00 PM
Back for another round! This should be fun!!

I have always dreamed of building a two star globe projector. The reason for splitting up the Spitz analogs: I want to dramatically reduce the distance between the star globe and the latitude axis because I will have a small dome for projection. The star globes need to be as close as possible for this application.

One benefit is that the analog mirrors will be closer to center of the projector; there will no step angle to adjust the planet to it's proper position. In addition, I can save space by putting the analogs on each side where the ecliptic is at the highest point above the equator. Also, to save space I want to eliminate the planet motors and drive the analogs with one motor similar to what you described several years ago. Then I plan to use a universal joint and rod mounted in the hollow diurnal axle to connect both sets of analogs.

I believe I can reduce the length of the planet cages to about 5-1/2" on each end. The southern cage would have the Moon, Mercury and Venus. This way the analogs would be proper as on the A3P. No alteration would be needed, especially to the complex moon projector. The northern cage would have the Sun, Mars, Saturn, Jupiter (the planets with the least inclination). The star globes need to have 2"-3" below the hemisphere line to allow for the artificial horizon. However this can be accomplished by attaching a "cylinder" with a smaller diameter than the globe. The planet projectors themselves would occupy the space just below each star globe-three on each. The moon projector would be located, as it is on the A3P, next to the moon analogs.

Again, to reduce the length of the projector, I would need to eliminate the Spitz slip rings on the diurnal axis, opting for "pancake" rings which I would have to make. They are available commercially but at a premium.

Finally, precision would have to be driven by two sets of synchronous motors (or stepper motors would be better) mounted in the same area as the analogs. A drive rod from the motor, along side the planet cage, to a bevel gear and finally a worm gear attached to the cylinder at the bottom of the star globe to transmit the rotation. So far I haven't been able to locate a worm gear from a surplus source and they could put the projector over budget.

Again to save space, the axle connecting the star globe will be very short. The actual star globe will be supported by a 12" diameter, very precise, lazy susan ring. The drive gears for precession are to be mounted inside the ring. I've just purchased them.



So that's the plan. I will make a mock-up from plywood and temporarily mount the analogs and various motors, etc., to make sure there is room for everything. Here's a rough of what it should look like--very similar to the little GOTO Mercury projector.

Posted by: Ron Walker Jan 23 2016, 06:09 PM
I must think about this for a bit.

I see no reason why the analogs cannot be operated in reverse. The analogs all assume the planet orbits are circular and when I run them backward for resetting they run just as smoothly as in the forward direction. Also the inclination of the orbits should be no problem, just remount the analog 180 degrees from the original position to maintain the original angle. If you use the original mounting plate you would have analogs on both sides. Timing belts are great as you can have them go around corners. The big problem is how to get them through the central core. Zeiss had a great idea in that they have a hole about a foot in diameter through the central core that allowed many things from drive shafts to wires to pass through. The Minolta does much the same thing but on a smaller scale.

Posted by: charles jones Jan 24 2016, 11:00 AM
QUOTE(Ron Walker @ Jan 23 2016, 05:09 PM) *
I must think about this for a bit.

Timing belts are great as you can have them go around corners. The big problem is how to get them through the central core. Zeiss had a great idea in that they have a hole about a foot in diameter through the central core that allowed many things from drive shafts to wires to pass through. The Minolta does much the same thing but on a smaller scale.


My plan is to use one pulley in the belt loop that drives a universal joint at 23.5 deg. That puts the drive from the planets in line with the diurnal axis. An attached gear drives another gear which rotates a small diameter shaft running through the diurnal motion axle and connects in much the same way to the planets on the other side of the central core. Not sure if backlash in each "universal joint" will be a problem.

Posted by: Ron Walker Jan 24 2016, 08:25 PM
No backlash in any universal joint that I have used. Same goes for timing belts which is why I like them so much.

Posted by: Ron Walker Jan 31 2016, 11:35 AM
QUOTE(charles jones @ Jan 24 2016, 07:48 PM) *
Here is a simplified drawing of the "gear train" to drive the analogs in both the northern and southern parts of the projector. Not shown are the mounting plates and 23.5 deg wedges that will support to planet cages and the analogs.

Another option is to put the universal joint at the end of the 3/16" rod if there is room for it.

Finally, does anyone know if a narrow timing belt can be TWISTED so that the two pulleys are offset by 23.5 degrees?

Unless you have some surplus universal joints (they tend to be expensive when purchased new) I would consider just using a couple of timing belt idler gears to make the 23.5 degree turn. That would also take up less space.

Posted by: Ron Walker Feb 2 2016, 12:37 PM
QUOTE(charles jones @ Feb 1 2016, 06:18 PM) *
First to answer Gary, it has been years since I've done a planetarium show. That was when I was in high school and college for my science classes and astronomy club we formed. So, it was before the day of video tape.

Ron, I found on line Robotmarketplace.com and servocity.com. One of them has fairly inexpensive universal joints. But these may not be as precise as required. BTW these two websites are great sources of materials. You can buy components, bolt them together and add gears, motors, etc. If you want to build some kind of auxiliary projector, all materials you may need is there.

But, I want to experiment with twisting the timing belts as you've suggested and I have thought about. If that will work, it is a much better solution.

I have begun ordering various components. (I think you went through this). And I have decided to get the slip rings made first so I know what amount of space they will take up. I hope this works: Laminating a copper sheet to fiberglass, then cutting the rings. I have a friend with a CNC that could easily cut away the copper making the rings. I have from surplus, motor brushes that will work for the "brushes" on the slip rings.

I remember you talking about a thin copper clad electronics board and cutting away the thin copper on a lathe or drill press. How did that work out?


I remember making devices some half century ago when I worked at the U of C that had both timing belts that did a 90 degree turn and there was never a problem. This is where I learned how great timing belts could be. When I used the A3P under my dome for part of the Christmas Star show the movements of the planets ran extremely smoothly. Also the small DC motor had beautiful variable speed. Also back in Chicago I made several of the slip ring devices out of copper clad circuit board (using the typical acid bath to remove the material) and used the contacts from old relays as the sweeper arms. They were still going strong after a year and movement of the sweeper arms was both slow and fast depending on what the system was doing at the time. I used this because I only had about 1/4 inch to make it happen. What I was most surprised about was that there was virtually no ware on the copper PC board. The copper was polished bright and to the finger tip I could feel a slight depression but could not see a depression. Considering how slow a planetarium runs, I doubt that there would be much ware at all.