Dome rotation upgrade

[KathyNS]

Ever since we moved, taking my observatory with us, my dome rotation has been dodgy. Something shifted or warped in the move, and the dome skirt, which never wanted to be round in the first place, seems to be less round than before. Part of it drags on the support ring. It is not a major drag: pulling the dome around by hand with a spring scale, I hardly notice the extra friction, but the stepper motor driving it is operating close to its torque capacity and the rotation stalls. Which of course ends the session when the guide scope loses its vision.

I have been thinking of re-installing the original "infinite torque" DC motor that came with the dome. But reconfiguring the control system for it would be a pain, since everything is set up for a stepper now. So I was pleasantly surprised to find the the vendor from whom I got the original stepper motor, now carries a much more powerful one. And it is still within the capacity of my controller.

I'll be going from 2.9 Nm of torque to 23.5 Nm! More power!!

Because of the higher gear ratio, I will have to clock the motor at a higher rate, and I will have to program longer wait times into my sequences. But I will be getting back to reliable operation.

[bobharmony]

A nice solution without a lot of re-work. Good to hear you will be back in normal operation!

Bob

[JayTee]

A little more than 8 times more torque. I hope your dome doesn't start spinning like a flying saucer. Although that might be fun to watch!

Yay, for the upgrade.

Cheers,

[Gordon]

Glad you found a solution!!

[KathyNS]

My new stepper motor arrived in the mail yesterday. It is about twice the size of the old one, rather an impressive upgrade. It is comparable in size to the original DC motor, which will make mounting it easier than I expected.

I dug out the original motor and its mounting bracket to compare to the new motor. The drive shaft is the same size (1/2") as the original motor, so attaching the drive gear will be trivial. The gear is currently mounted upside-down, with an after-market hub on it for the small stepper. I will just flip it over right-side-up again, and it will fit right onto the new motor using its original hub.

I spent some time today liberating the mounting bracket from the original motor. The mounting screws were seized, and an overnight soak in WD-40 didn't loosen them. The hex heads stripped when I tried to remove the bolts. So I ground the heads off and then was able to remove them. The bolt spacing on the new motor is 33mm, compared to 29mm on the old one. That meant drilling/filing out the bolt holes in the bracket. That is now done, so as soon as I can go to the hardware store and replace the bolts, it will be ready for mounting.

I will reprogram the driver hardware for the higher current draw of the new motor, and adjust the timing in the Adruino. Because of the higher gear ratio, the dome will rotate at 1/3 the speed if I clock the motor at the same speed. Right now, my worst case for an emergency return to Home is about 35 seconds. Without reprogramming, that would increase to 105 seconds, which is a bit much if it is starting to rain. I will see how fast I can reliably clock it without disrupting other functions.

We have some crappy weather coming, so I am not expecting to use the dome in the next several days. By the time we get good observing weather again, the dome may be fully functional again.

[JayTee]

It's so nice when an upgrade installation turns out to be easier than you thought it would. I've got my fingers crossed that Murphy doesn't creep in!

[KathyNS]

I've got my fingers crossed that ****** doesn't creep in!

Shh! Don't say his name or he will appear.

[KathyNS]

I repainted the original mounting bracket (it got scratched grinding off the bolt heads) and went to the hardware store for new M5x.8 bolts for the motor. Today, I mounted the motor to the adjustable part of the bracket, and mounted that to the stationary bracket. The dome drive gear is a perfect fit for the motor shaft, so the whole thing looks very much like the original installation, but with the geared stepper instead of the old DC motor.

I have to do some reprogramming of the Arduino, to account for the new dome circumference (measured in motor steps), and to include a new rotation speed and acceleration profile. The motor does drive the dome, but it still isn't quite sure where it is going at the moment. But I am optimistic that it can power through any friction.

Here is a size comparison, showing the new motor installed, with the previous one to the right, leaning against it. The size difference is obvious.