Brushless DC motor for spinning GG setup?

[Chris Leong]

Hi all
Anybody tried a reasonably high end brushless DC motor to spin a CD style GG for a DOF adaptor yet? I was playing around with the idea because I like the idea of the drive spindle being on bearings and also the relative precision with which these motors are made, compared with the Walkman motor that's spinning my present GG setup.

I was just about to buy one from my local airplane model hobby shop when the guys there said that I'd have to get a set of controlling electronics to work the motor.

Of course, their controller modules were all radio controlled, so I passed for the moment, in favor of more research.

I was wondering if anybody who has gone before has located a more suitable controller circuit for our uses.

Thanks!
Cheers
Chris

[Bob Hart]

It would be a workable option but you might get a bit of high-frequency noise, probably nothing worth getting concerned about.

I had a local make up a single controller for two outrunner motors to drive two gyros for a compact stabiliser for airbourne work in real aircraft.

The motor low and high speeds can be and have to be preset as for the flying models on each power-up but they work the same otherwise. This might be a bit tricky for somebody not familiar with radio control scale aircraft and a bit more complicated than flicking a simple switch.

I chose them for the stabiliser because they exerted the least electromagnetic field for size/power this side of AC motors.

A gadget made with a pair of Ryobi vac conventional motors will deviate a magnetic compass more than 5 degrees at about two feet before it is even powered up, which is not acceptable in an aircraft.

[Chris Leong]

So Bob...

what would you recommend as an upgrade from the motor/CD mount found in a regular CD player?

I like the prop collet idea off the model planes. I can cut my own CD sized plastic disk with a 1'/8" center hole pretty accurately, then grind/sand one side. That would fix the large center hole (and getting larger) and the eccentric spinups.

It's the slippy, sloppy motor that I'm trying to cure at the moment.
Trying to clean up my back focus tolerances...
Cheers
Chris

[Bob Hart]

Chris.

Even if you get a collet mounted hub, your disk will still have to be mounted on a centre in a lathe and the outer rim skimmed for there to be any hope of it running true.

The centre hole is made first then the outer edge of the disk cut, either from a raw panel or trimmed down from a rough cut.

Radial run-out is probably little more than a noise problem with a conventional CD player hub and CD-R disk if the disk plane is kept very close to the front bearing of the motor. It is still not desirable and picture quality will suffer.

However, the scale aircraft motors carry the prop hubs considerably far outboard of the bearing. Any radial run-out will convert itself to run-out of the disk plane because of the leverage any out-of-balance state would have over the motor mounting system.

Furthurmore, the scale aircraft motors are generally mounted from the rear, not the front like the CD motors which means even more mechanical leverage on the mounting structure by an out-of-balance state in the disk.

I was recently advised by a P+S Technik rep that any more than one hundredth of a millimetre run-out of the groundglass along the optical centre axis is unsatisfactory. - my paraphrase of his comment so don't take my comment as an accurate reflection of his comment.


Here are some manifestations of the run-out problem ---

Overall loss of image sharpness due to the apparent thickness of the groundglass layer being greater as far as the camera's viewpoint is concerned. Brightness is unaffected.

Some phasing of sharpness may be observed if disk RPM is close to frame rate. Autofocus, may be sent insane trying to keep up, (manual settings only if possible).

There is a cyclic refractive effect which moves the image minutely across the camera view. There is a cyclic movement of the projected image itself. Both affect sharpness

These also appear as an interlace artifact where alternate lines become very apparent. This artifact also occurs if there is actual vibration of the unit on front of the camera and a physical shift off the optical centre axis occurs, most apparent with longer lens like 50mm and 85mm.

These moving artifacts may send the steadyshot and auto gain functions of your camera insane trying to keep up.

[Chris Leong]

Bob
Thanks for your input.

I have a small lathe and a flycutter and would be using them to true the disk.

Good point on the collet run=out. I was thinking of machining the entire thing down to reduce the disc's distance from the front bearing but can see that even if I did that, the distance from disc to front bearing would still be at least double that of the existing setup.

I experienced some of the run-out problem s you mention when I inadvertantly left my FX7's Steadyshot mode on . It caused a rippling across the image that was very weird indeed. Took me a few panic-filled moments before I nailed the cause and turned the Steadyshot off.

So maybe just rework the CD motor and bearing to make it a little more solid?

Thanks again!
Cheers
Chris

[Bob Hart]

Chris.

When you mount your motor, mount it by its front on the rear of your motor support panel the with the shaft coming through the mount panel. This positions the disk mass as close to the support structure as possible.

You have to lever the hub off the motor to do this and the hub is easily broken if you are not careful.

Don't lever against the rim of the hub but use two small screwdrivers from either side against the shank near the motor bearing.

You can afford to trim the shank down to about 7mm without adverse effects to bring the disk mass closer to the bearing.

Leave a little bit of flex in the mount plate. Easiest is simply to make it as wide or wider than the disk at the anchor points onto the case of frame and let the wide span do the flexing.

Anchor points very close to the motor do not have leverage over the disk mass for run-out but do have leverage over the disk mass for gyroscopic effect. To be flexible enough they would have to be rubber which is too hard to control and adjust.

The purpose of this layout is to allow the gyroscopic force of an otherwise true and balanced disk to pull itself into alignment when the disk to hub or hub to motor junctions are not dead true.

I found a piece of 4mm polypropolene whiteboard panel to work well and it is also not as acoustically live as metals or stiff plastic so runs quieter.

This clip might give you a bit of an idea what I did.

http://www.youtube.com/watch?v=CnweOEEqlRY

My arrangement is not a good use of available space.