Stepper motors

[Via]

Still waiting for my scope to arrive ? but I want to put together a stepper motor set up for electronic focusing like what has been done here... https://shortcircuitsandinfiniteloops.blogspot.com/2018/02/diy-electric-focuser-for-celestron-127.html?m=1

I'm thinking instead of having it web controlled however I'll have it connected to a separate control box with a rotary control and a switch between coarse and fine focusing.

However I've never played with stepper motors before and I'm worried about any possible damage if it tries to turn focusing knob past stop point.

It will probably be just a cheap eBay motor and I'm guessing it won't have any kind of intelligence to know what position it is in so only way I can think of is having a preset start position e.g. fully anti clockwise and then have the pi count each step to fully clockwise and then remember each step it makes and not allow that to exceed 0 or the high limit.

Biggest problem I can foresee with this is if focusing knob is moved manually then obviously it will no longer match starting position but any manual movement will be accidental and hopefully avoidable.

Can anyone else see a better way of doing this or any flaws in my idea I've not thought of? 

[blinky]

Most (all?) of the DIY focuser apps let you define start and end points.  So set 0 a little way from max in focus and then say 10,000 (depending on how many steps you have) a little way from max out focus.  That way the controller will never go past the max stops.  If you have a crayford focuser it should in general be OK anyway, with rack and pinion it would cause issues.  If you use one of the focusing apps I dont think it will matter if you have manually changed the focuser position as it will just refocus automatically.

[Stargazer33]

If you are driving your focus knob with a band from the stepper as in my build (below) then the band should just slip if you go past the stops.

[NEM]

You could use a micro-switch or slotted opto-switch at each end of the travel and get the pi to use them to limit the travel.

[Gina]

I've also used Hall switches and magnets to determine end of travel with a Pi.

[stash_old]

As Blinky says "So set 0 a little way from max in focus and then say 10,000 (depending on how many steps you have) a little way from max out focus" never had a problem using that method - simple pretty full proof way. ?

[Chriske]

Aren't the steppers in the link not a bit overkill..??  If I'm not mistaken those are Nema17 steppers.?

All the focusers I designed and printed so far will be re-designed to have them all working electronically. I was planning to use very small steppers.
Using a worm/wormwheel + extra reduction I think it would work(well, that's what I hope...)

[Gina]

I've used 28BYJ-48 tiny geared steppers for focusers very successfully.

[Via]

Cheers for all the advice.

I've got toothed cogs and belt so can't rely on slipping.

I agree with one in link does seem excessive and gone for a much smaller one.

Decided to use arduino nano as controller for few reasons; almost instant turn on, no need for anything more fancier as control will be through rotary encoder and also no idea where I put my raspberry pi zero ? but tempted to switch it to that if I can think of extras I will want to do needing that in future.

Got basics setup and working now, still waiting for scope so at a bit of an impasse now as can't order a belt as I have no idea what size I need until can mount it up. Can't work on speed / steps per click until all mounted and see what effect gearing has on it, but do think this one may be a bit too slow.

Also unsure of mounting and control box atm, not sure if I'll have just stepper mounted on scope and everything else boxed up or everything mounted on scope with just rotary encoder on a long lead into a mini box.

 

[Via]

Just wanted to share final(ish) version, the wire I got to solder to boards is terrible, amount that broke before it finished was a joke but working minus the rotary encoder switch being a bit temperamental. Switch controls amount of steps per click of rotary encoder relevant to amount of LEDs lit or off (LEDs stay on but dimmed to show power.

Very happy with it minus the fact I'm still without a scope to mount it on ?.

[Stub Mandrel]

Stepper can't do any more damage than turning too far by hand - that is, not much as long as you don't overdo it. Just overshooting should be OK, keeping it running could be a problem.