Stepper Motor System / Advice - Page 5

alistairsam · #81 22-03-2011, 12:00 PM

Quote:

Originally Posted by Bassnut

That would be a logical way. My stupid Pyxis focuser (supposedly one of the best) homes on everypower up, increadibly slow, noisy and timesome. The Optec does too, dont understand that.

other option is to keep the homing as an optional feature you can invoke on demand.
so the mcu remembers the last position, but if you notice that the position you want it to go to is not correct, you can do an init routine on demand with a switch. that'd be the best of both i guess.

Bassnut (Fred) · #82 22-03-2011, 12:07 PM

Quote:

Originally Posted by alistairsam

other option is to keep the homing as an optional feature you can invoke on demand.
so the mcu remembers the last position, but if you notice that the position you want it to go to is not correct, you can do an init routine on demand with a switch. that'd be the best of both i guess.

Yes, and home on demand, perfect.

A home switch of 0.2 - 0.5mm accurracy isnt nearly good enough IMO, but I could be wrong.

alistairsam · #83 26-03-2011, 08:47 PM

Quote:

Originally Posted by hikerbob

alistairsam I'm thinking of focusser control. If I'm understanding it right the 223 could be instructed to drive the focusser in to a stall (fully in) to establish it's position and then could position the focusser exactly at a nominated point. Perhaps temp compensation could be built in as well as focus points for camera and different eyepieces.

Bob

Hi Bob,

any progress with the stall detection? or have you thought of a limit switch? either way, let us know how you go with the 223.
how exactly would temp compensation work?

hikerbob (Bob) · #84 26-03-2011, 09:32 PM

alistairsam the first challenge is to get a chip soldered neatly to a board

I've been distracted by my play with Bluetooth so that's off to the side for a bit.

As far as I can tell stall detection will be a matter of working out the right settings (rotate a motor externally and take some measurements. Might be some trial and error.

I've looked into opto limit switches on two scopes and don't like the mechanics of fitting them. The idea's I've some u[p with so far need to have some kind of fitting on the focusser tube as well as the sensor on the body of the focusser. Both look awkward.

I've not reached the stage yet in my astrophotography yet where temp compensation is likely to make a difference but have seen mention of it in discussions about focussing. Some material at http://www.engineeringtoolbox.com/th...pes-d_283.html and other places that give some formula's which should be possible to use. It may not be worth the bother but the idea is interesting.

My thinking is an temp sensor connected to controlling micro and minor tweaks to the step position based on changes to ambient temp. If you are at focus at 15 degrees and the temp drops 5 degree's then the focusser would need to move by some number of steps.

Bob

hikerbob (Bob) · #85 26-03-2011, 09:53 PM

alistairsamI just did a quick play with Excel and the formula listed on that page. Assuming a path of 1.5 Meters, mild steel and a temp shift of 10 degree's C and assuming that I've done the maths right the change in length is 0.195 mm.

That's a basic check assuming that the only factor is the change in the length of the steel tube which is probably not a good assumption but it's a start.

Anybody got any comments on the impacts of thermal contraction/expansion on focus for a 12" Skywatcher primary?

Again not a practical issue for me yet but I enjoy the learning.

Bob

alistairsam · #86 26-03-2011, 10:07 PM

Hi Bob,

is that the total contraction in your OTA tube?
interesting, will be good to see a comparison of different material including aluminium pipes if it is a truss dob.
but can you visually or through imags, detect change in focus of a star if shift is by 0.2mm?

hikerbob (Bob) · #87 27-03-2011, 10:08 AM

alistairsam I don't have my spreadsheet with me but the page linked to earlier includes a link to a page with the coefficient of expansion for a lot of materials. Steel was 13 x 10(-6), aluminium was 22 x 10(-6) I think. Plate glass 9 and pyrex 5.5 (I think). Diamond is the material we should be working with, from memory it was at 1 x 10(-6).

Bob

hikerbob (Bob) · #88 16-05-2011, 12:52 PM

If anybody else is playing with the TMC22# IC's and an Arduino I've found what looks like a useful library at http://translate.google.com.au/trans...%26prmd%3Divns

The code comments, the PI doco etc are in German and I need to do some translation work to understand it better. Once I get some of taht to a usable format I'll try and post some relevant material.

I've not tested any of that code yet, my own attempts to communicate with the chip have been less than inspiring but a skim of the functions and procedures looks promising.

Bob