Stage Revolve (Motor and Power Question)

[rapscaLLion]

Sorry, not sure where this should go.

I am building a 16' (diameter) stage revolve. It will consist of floor mounted wheels upon which the platform will ride. There will be a pivot at the center to keep the platform centered, but is not load bearing. The set is built on top of this platform and may require some household lightbulbs and possibly a fog machine (with DMX cable). My problem is how to run power onto the revolve? I figure running cables under the platform to the center and then up through the pivot (then back through the platform to the devices that need power) is the best and simplest way, however I am concerned about the cables getting too twisted. The revolve may complete multiple rotations in one direction before reversing. Is there any way to solve this? Or should we put a limit on how many revolutions the turntable can make in one direction?

Also, a question about motors. We are going to use a permenant magnet DC motor to drive the turntable. Which method is cheaper/easier? Belt driven via the central pivot, belt driven via the circumfrence, or by direct contact with a spinning wheel underneath? The max rotation speed is slow but variable, and precision should be within 5 - 10 degrees max.

Any suggestions?

[Van]

I'll leave the electrical to someone else, though I know there are several techniques of producing electrical "races". As to the drive portion I would suggest you use a direct drive wheel on the underside of the outside diameter of the revolve. This will give you the most torque for horsepower, trying to turn a 16' revolve from the center pivot is next to impossible , 'specially when you get the added weight of scenery on it .
Now, just to CMA it's possible that if you don't have enough wieght on the outside edge of the revolve table, that the wheel might not get enough traction. This can be allieviated byt adding a "pinch wheel" on the topside above where your motor wheel makes contact withthe underside ofthedeck. If this is not possible then you may need to either add traction tape to the underside < Which I would suggest anyway> or move the wheel to drive from the top side of the platform.
Are you using a Stepper motor ? or some sort of passive feedback system to control the stop points ? Is you motor geared ? or straight driven? that will effect how accurate the start stop points are as well.
Good Luck Tell us how it goes. I'd love to see pics. I love a well engineered theatre project!

[rapscaLLion]

Hi,

This project is in the planning phase, and will most likely be built in increments. We will likely start with just a simple speed controller, but we are designing with the eventual aim of the computer controlled motor. But I will certainly post pictures, and likely a full DIY tutorial since everyone seems to like turntables.

I am hoping to engineer this one well enough to be able to rent it out to small tradeshows and school theatres and the like.

As for the direct drive idea, I agree that direct drive on the outside is best. Underneath is of course preferable since it is hidden. Traction tape is an excellent idea that I never thought of. Also, the point about weight... I am certain this one will have enough weight to work since there will be a heavy set attached on top, it's a good point regardless. I don't like the idea of a wheel on top because it is visible. I would rather not put the wheel onto the side for fear of warping the circle. Any other ideas for providing extra traction?


Also, what motor would you recommend to use. A stepper motor could work since it is already designed to do preset movements (ie- 1 degree). But would it be: A) Strong enough, B) Smooth enough?

[Foxinabox10]

For added traction, Home Depot or stores of the sort sell an additive for paint that you just mix in and paint it on. It's like little bits of sand, but it gives the motor more to grab on to. Wouldn't be a bad idea for the top either for when it rotates with actors on it too.

[rapscaLLion]

A great idea FoxinaBox. Extra traction on top for the performers is another thing I never thought of but makes sense.

[rapscaLLion]

Well no one seems to have any input, so I just thought I'd say I've invested in a couple stagecraft books that should arrive soon that will hopefully have some tips!

[cutlunch]

I would go with the friction drive because if the revolve jams the drive wheel will start slipping. On a belted system the motor will try to keep driving until the belt snaps.

For the cables I would bring them up through the center like you suggested. But I would wind the turntable back every so often to untangle them. On a professional system they would probably have a series of contacts and brushes. These can just keep revolving a round for ever so long as the contacts are cleaned.

How are you going to get your positional information. On some systems they use shaft encoders attached to the end of the motor shaft. But they can be quite hard to program and cost a bit.

On your system I was thinking that you could use reed switches. You would have a series of reed switches mounted very close to the turntable on the non -turning platfrom. Then you mount a magnet under the lip of the turntable so it passes within millimeters of the reed switch. When the switc closes you know where you are.

What voltage is the DC motor? Do you have a background in building speed controllers? I just ask because this one will have to be pretty heavy duty. You have probably already thought of this but make sure there is a safety stop switch that disconnects the motor from the power.

I just have been looking through an old second edition copy of "Scene design and Stage lighting" Parker , Smith. They have a small section on turntables. I would take it the later editions also had this section.

I am sorry I don't know your skill level but I would probably get some outside help with this project especially with the design of the turntable and it's load bearing. Also if you want to hire it out to other venues this adds another whole sets of problems. Eg how to make it portable to get into other venues, what if the floor in another venue is not level? etc

I look forward to your journaling of this project.

[j_blinker]

http://www.mercotac.com/ has rotating connectors that I've used in revolves. I just mounted it in the center pivot. It means you can only use a single connector so it has to have enough poles in a single until. The wheels up design will make it easy to run your power under the rotating deck. The hardest part of constructing a revolve is keeping its circle true and its pivot at exactly center. The wheels all have to be 90 degress to the pivot or they will drag. And if you plan on moving this thing around a metal frame is a big must.

As far as you DMX line you could rig in into the rotating connector but the resistance might wreck havoc on your data.

[cutlunch]

I forgot in my last post to disagree with Foxinbox about putting that traction paint on the top surface. This would cause to much friction for the dancers. If the deck is made out of plywood it should have enough grip just using an ordinary paint.

j_blinker those revolving contactors look good. If their spec's can be believed it could be worth giving them a try for the DMX. Milliohms of resistance shouldn't effect it, also they are rated for frequencies upto approx 100MHz. Do you make revolves for a living? Are there any more tips you can share with us. Also what is your normal means of drive, drive control, positioning reference? I understand you may not be able to tell us due to business confidentiality but anything you can will be useful. Thanks.

[Van]

Got to go with cutlunch. No traction over the general area of the platform top, only on the wheel traction area. Dancers would kill you.
Disagree with blinker, on the metal framing. I've had a revolved tour for over 5 years that is a completely wodden design and holds up quite well, not to mention it is quite light for a 16' diameter turntable.. Oh what the heck I'll post a nasty Acad drawing of it.

Ok it's a .zip file with .pdfs and a .dwg