RA drive motors

[frank farrell]

Hello all.
I built a 10" Dob and now I wish to drive it in RA. I have made a RA mount. The motor is a Synchron 24v 5 watt 60cycles. Made in USA. Trouble is I can not vary the speed.
I am puzzled. I have been reading the forums about stepper and DC motors. This has fried my brain.
Does a stepper motor have the same or better torque than the Synchron motor and can it operate from 0.5rpm to above 1rpm.
I have the Synchron motor driving a gearbox which drives two pulleys. Scope is mounted above the larger pulley.
The same question will apply to DC motors.

Thankyou.

[wasyoungonce (Brendan)]

Frank, a synchronous motor will only operate (rotate) at their designated frequency, this cannot be altered, unless you can alter the frequency (have a look on the net for drive corrector or variable frequency drives (VFDs)).

Steppers and Servo motors...is an old joust. They both have properties that make them good for use and each have properties that make them better than each other in respective areas.

Steppers: pulses can be counted thus easy to track how many pulses sent and thus where the system is pointing. Steppers have more poles and can make very small steps. Steppers suffer at high speed from torque loss. Steppers are no fuss long life systems.

Servo motors (DC motors), Good torque useful for low and high speed applications, require an encoder to tell how much rotation.

Reality is that despite what motor system is chosen you will need some control of it to correct minor errors. Have a look on the net and these sites for Bartels drive and roboscope, even our own site has drive info for dobs.

[frank farrell]

Thank you Brendan. I shall check this out.

[Barrykgerdes]

Hi Frank

Driving a DOB so it will track is quite a task.

Have you mounted it on an equatorial platform or is it on a true equatorial mount? The procedure can be quite different.

The first consideration will be the azimuth bearing that will become the RA axis. Because the platform will be tilted the bearing will need side thrust control as well as vertical and because the tension of a drive belt will need to be taken into account. I once modified a DOB mount with proper thrust bearings for for the azimuth but I have a well equipped machine shop.

The next consideration is the drive method. Here the use of toothed belts can be used. I salvage these from old printers etc. The overall gearing for the belt stage will need to be as high as you can get 360:1 is a good aiming point as normally achieved by a worm drive but may not be practical in you case. My old DOB drive used a primary reduction of 60:1 with a combination of a belt stage and mechanical gears (stolen from my Meccano set)

A stepper motor drive will probably be the easiest way to go but the overall reduction will need to be at least 360 x 60 x 60 to get a reasonably smooth drive. This will mean a microstep drive on your stepper.

Once you get to this stage the Bartel's system has all the electronics designed to drive your stepper.

The drive system that you already have I suspect is designed for the US 60Hz system and it will probably use a standard small shaded pole type motor. The speed of these can be controlled within limits with a variable frequency power supply. However this method is asynchronous not synchronous

If you have photography in mind this system will be virtually useless of course and for visual use an alti/azimuth drive will be much easier to implement even with a stepper drive for both axes. That is all I used for my old DOB mount.

Barry

[frank farrell]

Thank you Barry. Your thoughts are appreciated. I understand that it is a complex task. But I am learning. I started this project a few years ago but had to put it aside due to moving houses and jobs. Just recently got the bug again. I can see that photography will be very difficult. But I should be ok with visuals.
The whole Dob is placed on a equatorial platform which is adjustable for latitude. The altitude bearing supports are then aimed south. Maybe more of a "equatorial fork"?. RA drive is achieved through the base.

Cheers
Frank

[Barrykgerdes]

Hi Frank

You should have a lot of fun making the telescope work. Just like I did over 20 years ago. I worked my DOB over and eventually turned it into a goto/tracking alt/azm system. This was before the days of cheap goto/tracking telescopes. I used a DOB 11 drive system that while it worked for tracking I really only used it as an interface to the stepper drives.

In those days computers were driven with 80286 processors that were really too slow for simple high level programing. However I wrote a program in BASIC that had all the controls to drive the steppers as a goto/tracking telescope complete with catalogues. I got the system to work but it was quite cumbersome and I came into some uncommitted money and bought a LX200 10" telescope. The DOB was put aside but still exists as it was in 1992.

If I had adapted the much later Bartel system to drive the steppers I would have had a much more practical telescope control.

For interest you can obtain the notes and program from

http://barry.sarcasmogerdes.com/obse...s/dobmount.doc

and

http://barry.sarcasmogerdes.com/obse...s/astrodrv.zip

Note: The program has a non operational display for a demo if the dob ll is not available and it has been compiled with Qb64(gl) so it will work on 64 bit computers.

Barry