Motorising a DIY dome

[ChrisM]

I'm contemplating motorising my DIY dome and have started searching for a suitable drive motor/gearbox. I plan to apply pressure to the final drive wheel to bring it up under the steel ring on the base of the dome, and hope that this method will provide enough friction to drive reliably.

Has anyone else found a suitable motor/gearbox combination, or would it be best to use a motor and pulleys? I would also be interested in a variable speed drive so that I could ramp up/down the speed, and use an appropriately low speed for tracking purposes, and a faster speed for moving the dome around to line up with a new target object.

Any thoughts welcome.

Chris

[kinetic (Steve)]

Chris,
hi fellow Garlitz geodome owner
I'm interested in your outcome as I'm in the same situation.
I'm ready to convert the base-ring to a 360 degree drive and have it
synchronised to the Bartel drive via it's interface.
The spanner in the works is finding a suitable mechanical drive that
will not slip I have two identical motor/gearboxes ready for the
job to share half of the load each side of the dome.

Barry from Canberra might have some suggestions.(bloodhound on IIS)
I believe his is still manual but he has been through a few prototype
designs and is happy with his present drive idea, last time I checked.

Steve

[AussieSky (Greg)]

Contact Brett ('bert' on this forum) for some advice. He is into the kind of automation stuff you are talking about and has all kinds of good solutions.

Greg

[ChrisM]

Steve, do you know the specs of your drives? ie. rpm & torque? I have calculated that I need a torque of around 3 - 5 Nm although incorporating a margin above that range is probably advisable. And I think a speed of around 50 - 60 rpm for the output shaft should translate into an acceptable dome speed. On the internet I found the CMG geardrive "sub-fractional" horsepower units which employ single phase 240 Vac motors.

To get traction, I am thinking that a soft rubber wheel might provide the necessary "grip" on the steel ring, but there may be other solutions.


Thanks for the tip Greg re Brett.

Chris

[bert (Brett)]

Hi Chris,

Check out

http://www.dppobservatory.net/DomeAu...DomeDriver.htm

As a suggestion stick to 12/24v rather than 240v, to change the speeds as you mentioned earlier at 240v requires an inverter module (read expensive), necessitating the use of a three phase motor. Unless the dome you have is nearly impossible to move by hand, a small 12 or 24v will do the trick, and if you need to use the power of a 240v motor, a friction drive will more than likely slip.

Brett

[davewaldo]

Would windscreen wiper motors work? I know it sounds dodgy but they have a lot of grunt for their small size, have low rpm, 12v or 24v and can be found cheap. I've seen people make electric wheelchairs from wiper motors...

You could use 2-4 if necessary.....

EDIT: A quick google shows a lot of wiper motors operate around 50rpm with torque around 20nm. Sounds good. I believe with a PWM speed controller you can maintain the torque but slow the motor if needed.

[bert (Brett)]

Your spot on Dave.

Most garage door motors look like windscreen wiper motors, although many have their electrical motors wound differently, and take a look at what they can do. The trick is to find a motor with a compatible encoder built in, to give positional information for dome automation. I am currently doing some experimentation with a garage door motor that looks like a windscreen wiper, but has more torque, and gives positional info for lesvedome.

Brett

[kinetic (Steve)]

Originally Posted by ChrisM

Steve, do you know the specs of your drives? ie. rpm & torque? I have calculated that I need a torque of around 3 - 5 Nm although incorporating a margin above that range is probably advisable. And I think a speed of around 50 - 60 rpm for the output shaft should translate into an acceptable dome speed. On the internet I found the CMG geardrive "sub-fractional" horsepower units which employ single phase 240 Vac motors.

To get traction, I am thinking that a soft rubber wheel might provide the necessary "grip" on the steel ring, but there may be other solutions.


Thanks for the tip Greg re Brett.

Chris

These are the two matching DC motor/reductions that I have in
mind for sharing the load via friction roller or winch or crown gear
type arrangement.
http://www.iceinspace.com.au/forum/s...&postcount=115
They have a DC clutch, DC motor and good , strong set of final
gears.The last gear has a positional encoder on it.
It might seem perfect for the job but a problem I see is if I use
this setup in a shared meshed arrangement, it is so strong that
any problem in one drive would cause the other to keep driving
and possibly destroy the dome base-ring. It's only plywood.

Steve

[rally]

Steve,

Use a circuit breaker or fuse !

Why use two anyway ?

[ChrisM]

Thanks Dave Steve & Brett for your suggestions. I like the windscreen wiper motor idea, especially as they should be cheap. Using two - spaced 180 degrees apart - to provide a couple rather than a moment, might produce smoother turning, but I suspect that I would try just one initially.

Brett, what are the pros and cons of having the positional encoder on the drive motor/gear rather than on the dome itself. The latter would surely give absolute position, but would obviously need some sort of coded scale (gray scale?) around the entire 11.3 metre circumference. I guess that if the drive system did not ever slip, it would be simpler to have it on the drive itself.

Thanks too for the link to the Lesve dome driver site. I had not heard of this one previously and note that the PCBs should be available through Jaycar.

Chris

[kinetic (Steve)]

Originally Posted by ChrisM

Thanks Dave Steve & Brett for your suggestions. I like the windscreen wiper motor idea, especially as they should be cheap. Using two - spaced 180 degrees apart - to provide a couple rather than a moment, might produce smoother turning, but I suspect that I would try just one initially.

Brett, what are the pros and cons of having the positional encoder on the drive motor/gear rather than on the dome itself. The latter would surely give absolute position, but would obviously need some sort of coded scale (gray scale?) around the entire 11.3 metre circumference. I guess that if the drive system did not ever slip, it would be simpler to have it on the drive itself.

Thanks too for the link to the Lesve dome driver site. I had not heard of this one previously and note that the PCBs should be available through Jaycar.

Chris

Chris, you might want to have a look through this thread.
http://www.iceinspace.com.au/forum/s...light=velleman
A bit about the Vellerman kit in here and LESVE
Edit: oops, sorry Chris, you came in at post#7

Rally,
as for having two versus one drive....only really me visualising
what sort of problems I might have.

Two for a friction roller situation based on one might slip.
Two for a geared / meshed situation because I can picture one
trying to drive the dome off centre and a lot of pressure on my
centreing wheels. (I have 5 centreing wheels)
Two would eliminate most of the tendency for the dome to want to
drive itself out of mesh off centre.
I only know this because of years of hand pushing the dome from
the inside at one push point and seeing the transfer of that force
straight on to the centreing wheels. Probably panicking for nothing
but I think Chris would be in a similar situation too with his Garlitz
Geo.

Steve

[bert (Brett)]

Chris, I buy the k8055 from ocean controls in melbourne pre-assembled, I have been happy with their service.

You can use Gray code scaling which is preferrable to using a on/ off encoder. The gray code does provide some error correction from overrun after the motor has stopped but the dome inertia moves it further. Using the k8055 board you can use the gray code or a simple on/off transition, remembering the overrun errors will be reset every time the dome passes the home switch. Most dome manufacturers do not use gray code, rather the simple on/off transition and rely on the home switch to reset errors that have accumulated over time.

Brett

[ChrisM]

Originally Posted by bert

Chris, I buy the k8055 from ocean controls in melbourne pre-assembled, I have been happy with their service.

You can use Gray code scaling which is preferrable to using a on/ off encoder. The gray code does provide some error correction from overrun after the motor has stopped but the dome inertia moves it further. Using the k8055 board you can use the gray code or a simple on/off transition, remembering the overrun errors will be reset every time the dome passes the home switch. Most dome manufacturers do not use gray code, rather the simple on/off transition and rely on the home switch to reset errors that have accumulated over time.

Brett

Thanks Brett and Steve, I'll have to find out more on these boards. At present, I'll concentrate on finding a suitable drive arrangement, but keep in mind the need to be able to control it automatically and/or remotely in the future. The Arduino boards mentioned in another thread also sound good with a few more I/O.

Steve, I agree entirely with your assessment of the potential benefits of using two drives rather than just one. I have just three centering wheels and I'm sure that some of the torque effort is wasted on lateral thrust. My dome "wanders" just a little, and whilst it generally rotates nicely, it occasionally "binds" a bit; this may be a temperature effect. I used a spring balance to measure the force required to start it rotating (8 kg = 80 N), and then to sustain rotation (4 kg = 40 N). I reckon that that's not too bad for nearly quarter of a tonne!

Chris