Advice please on DIY alt/azm drive.

[chrisp9au (Chris)]

I have a DIY alt/azm mount which works just fine as it is.
All movements are buttery smooth, I'm very happy with it.

So of course, I want to modify it!

I've managed to temporarily setup an azimuth drive just for slewing to objects, not tracking.
The altitude slewing can happen when, and if, I get the azimuth right.

The azimuth drive consists of a Valco 404.684 36V geared motor, made in Germany, which looks rather like a windscreen wiper motor.
I've made a hexagonal shaft for it from a large allen key that was sitting in the shed.
With a timing belt setup, it runs very nicely from a 12V 7.5Ah gel battery, providing a very satisfactory smooth gentle slewing speed in azimuth.
At 12V the motor runs at about 12 RPM, which turns the scope 360° in a bit over 2 minutes; I'm happy with that, what's the rush after all.

The next bit, electronics, is where I need some advice!

I would like to be able to vary the speed down for fine adjustments in azimuth at the eyepiece, and ultimately in altitude also.

I envisage a handset with momentary switches for forward and reverse slewing, and a speed adjustment potentiometer(?) to reduce speed when at the eyepiece.
I think I could manage that, momentary switches that reverse the wiring for change of direction, plus the potentiometer.

Is it that simple?

If I get this right, the altitude movement will be pretty much the same I guess.

Any thoughts or advice would be appreciated!

Cheers

Chris

[astro_nutt]

Hi Chris.
I use a set of Tamaya motorised gearboxes for both axis drives at the ratio of 4301:1 at 3v. Try using a few 1/2 watt 390 ohms resistors to each motor via a dtdp, (double throw, double pole) toggle switch which are available from DickSmith or Jaycar.
Cheers!

[bojan]

Hi Chris,

Originally Posted by chrisp9au

I envisage a handset with momentary switches for forward and reverse slewing, and a speed adjustment potentiometer(?) to reduce speed when at the eyepiece.
I think I could manage that, momentary switches that reverse the wiring for change of direction, plus the potentiometer.

Is it that simple?

Yes, but.. there will be power dissipation issue if you use those motors (any resistor in series with motor may get quite hot, depends on current/voltage... so you may end up with a need to introduce relays and to cool potentiometeres ( or to use wire-wound variety, not easy to find these days).
Astro-nutt's advice may be better - use a small motor with gearbox, just for adjustment/correction of position while you are observing. The coupling with your dob shafts may be frictional (with rubber wheels, for example).

However, very soon you will want more so I think you should go straight with stepper motors - later it will be much easier to upgrade to GoTo system (actually you are almost already there with steppers).

Recently I did some evaluation work on stepper driver chips, those Pololu drivers (or their clone, StepStick) are cheap, and work really well (threads are in this section, http://www.iceinspace.com.au/forum/s...ad.php?t=90164, http://www.iceinspace.com.au/forum/s...ad.php?t=91039).
The cost is ~$15 for driver module (Pololu or Stepstick), ~$10 for stepper motor, and couple of bucks for other stuff (inverter gates, connectors, wires, protoboard..). Multiply this by 2x for two axis control.
And what's more, they are compatible with both bartels scope.exe and Maciel's SoundStepper sofware (for this you have to invest ~$10-more for sound modules, some more connectors, switches, box...) and voila, you have full blown GoTo system if you add Cartes du Ciel or Stellarium to that mix.
The only (small) drawback is, you have to carry your laptop with your dob. But when you consider all the capabilities you are getting...

[alistairsam]

Whilst steppers are an ideal solution, and I'll second those modules as I've used them, one issue I found was gear reduction.
Not many are available with gearheads, and if they are, they're very expensive.
I haven't found any cheap high reduction gearheads or gearboxes.
It depends on the torque required in your setup.
If it is high, you'll need high reduction ratio , if low you may not be able to use 1/16 or 1/32 micro stepping to get slew rates you need as torque would be lower

Small Dc motors are readily available with high reduction gearheads and you could use a pwm motor speed controller that's around $20 odd on eBay
So one pot to set your speed, and a dt switch for reversible drive
Also you need something small to drive your altitude else you could have balance issues that could cause stiction in alt and increase load in az

Very difficult to control the speed of your existing motor as current would be pretty high and as Bojan mentioned, resistor in series is not recommended

I'd suggest using ball bearings on both drives, reduce weight as much as possible, then look at low rpm dc motors on ebay or other suppliers as well as multichannel pwm motor speed controllers. They can control more than one motor.

[dannat (Daniel)]

Chris this example uses a much smaller reduction then you're after but it at least has a circuit diagram for you to look at

[chrisp9au (Chris)]

Thanks guys,

Must confess I'm not thinking of 'goto' down the track, that's a level of complexity that I don't really need.

Having slept on the concept for a couple more nights, I realise that I'm not even really concerned with slewing to objects, as the mount moves very smoothly by hand.

I am interested in motorising the small fine movements when centering an object and in 'hand tracking' whilst observing. For that I reckon I can put something together.

With another small low rpm motor I found that a potentiometer does provide the speed variation I'm looking for. So this week I'll try it out.

Thanks again guys,

Chris