Stepper motor question

[garymck]

Hi,
I have an EQ6 Synscan. This uses 4 wire steppers. I am investigating options to improve the drive mechanically (belt drives). One option would require flipping the motors around which would mean that they would be rotating in the wrong direction. Can I change the direction of rotation of these 4 wire steppers just by swapping the polarity of the wires from the drive electronics? ny Gotcha's?
cheers
Gary

[bojan]

You can change the rotation direction by swapping terminals of only one phase, does not matter which one.

[garymck]

Thanks Bojan,
appreciate the quick reply...
cheers
Gary

[AlexN]

Very interested in seeing how your modification goes.. There is another member here, Bert A.k.a. "Avandonk" who has converted his EQ6 Pro to belt drives in the quest to eliminate backlash.. Quite ingenious...

[mswhin63]

I personally don't like belts as they require tensioning, direct drive is much better, but you may want to consider anti-backlash gears, maybe a match can be found for the EQ6. I don't where to start but know they exist.

[AlexN]

Agreed - Direct drive is a better option Malcolm, However I would imagine harder to implement... Tensioning the belts is a minor inconvenience... Especially if compared to the inconvenience of backlash error. Anti-backlash gears is something I will have a look at though.

[mswhin63]

Start here - http://www.smallparts.com.au/store/categories/gears/

[bojan]

Guys, it is not only backlash that is a problem (and it is not too bad per se, because it could be taken care of in the software)
Gears (straight type) have significant periodic error as well.
This is why I am planning to do similar modification.

Ideally, the gears should be helicoidal.. however they are very expensive and small parts doesn't have them (not in required sizes, that is).
For direct drive we need steppers with 1000+ steps per revolution... again very expensive.

[AlexN]

Indeed Bojan, Its also the somewhat random bumps given by transfer gears that can cause issues with your standard stepper setups. A belt reduces those.. smooth PE is easily guided out, or controlled with PEC, but random errors, being random by definition, are impossible to predict and counteract.

[bojan]

Exactly right :-)

[garymck]

Avandonk bought his Bartelised belt drive mount from me during a period of financial stress I had several years ago. I even wrote an article on it for Cloudynights

http://www.cloudynights.com/item.php?item_id=1538

I stupidly sold it, especially since the need for cash I had was over in a couple of weeks. I bought a new eq6 with synscan to replace it, and it has never worked properly on a consistent basis. Having now found a source of 47 tooth pulleys, I'm going to convert it to belt drive to get the same smooth tracking I had in the old eq6.

cheers
Gary

[AlexN]

There you go!!

Well, I'm still very interested. Although, I wonder.. Can you belt drive the mount whilst keeping the synscan control? setup, or using EQMOD... The bartel system seems a little archaic to me, even if it is very powerful, the interface reminds me too much of excel in MSDOS... :D

[garymck]

Alex,
yes you can belt drive the eq6 and keep the hand controller or eqmod. The key was to source a 47 tooth pulley to repalce the 47 tooth gear on the worm. It has taken me 3 years to find one!!!! I am currently awaiting delivery from the UK of a 12 tooth pulley, a 47 tooth pulley and a matching belt to fit to my eq6. Hopefully should be here in the next week or so. I will post when it has been converted and the results are in. As to the Bartels, I regard it as a fabulous sytem, and if I still had an observatory I would use it in an instant. The keboard interface is super quick once you know the shortcuts.

cheers
Gary

[pmrid]

I did a belt drive conversion on an LXD75 a while back and also converted an older CG5 mount to belts. The thing to watch for is that with a straight belt drive system, you are probably going to be driving your worm in the opposite direction to normal so you'll have to change the Dec and RA ratios in your handpiece from a + to a - value. The other issue had to do with maintaining the gear ratios themselves - otherwise the Dec and RA ratios have to be recalculated down to about 6 decimal places and while I was able to get the LXD75 right I never did get the CG5 to work properly.

Peter

[bojan]

I replaced the original motors and gearboxes (standard EQ6) with new ones, have a look:
http://www.iceinspace.com.au/forum/showthread.php?t=43823
My plan is to go further and to put belt pulleys inside the mount (I am still not sure how I am going to do it... the space inside is very tight indeed).
But even as it is now, I have smaller PE. The un-even jumps are still there, though (because 47-teeth transfer gears are still there, together with their imperfections).
One day, when belts are in, I hope to achieve smooth PE curve.

[garymck]

Bojan,

That is my problem, my overall PE is acceptable, but the errors caused by the intermediate gears make autoguiding and using medium to long focal lengths for imaging impossible. These are the errors I'm hoping the belt drive will fix. Based on my previous experience, I'm pretty sure it will.Try this source for small pulleys. They have the 47 tooth one (only source I've ever found, and then only recently) and the 2.5mm pitch may fit inside the mount. If not I'll flip the motor and put it outside (hence the question I posed about stepper rotation at the beginning of this thread....

http://www.hpcgears.com/

cost for 12 tooth/47 tooth/belt/airmail was only $65

cheers
Gary

[bojan]

Gary,
thank you for this link.
I noticed they also have helical gears - this may be the solution to the problem as well, since helical gears have much lower short-period PE (because more than one pair of teeth are engaged at the same time, while with spur gears, it is only one pair).
I am not concerned with the number of teeth too much, since I have Standard EQ6 with Darren's mod - so as long as I am sort of close to optimal (step size etc) with reduction ratio, I am OK.

[alistairsam]

Hi,

I salvaged a few steppers, helical gears from printers and fax machines after reading threads here, and wanted a control circuit for a 6 wire stepper.

one of these from a fax machine happens to have datasheets on the manufacturers website, and from that, its a 1.8deg stepper, 2A max, 50-850mNm torque. It has 6 wires. Not sure if it is unipolar or bipolar.
I've seen posts using the bartel system, but i don't want to use a laptop yet.

Just wanted to have three switches or options - slew, forward/reverse, track using micro stepping, and possibly micro stepping before tracking.

I've built a fork mount for my 8" Bintel Newtonian, so my prime objective is just tracking on the RA axis.

any suggestions, ideas?

would a servo be more appropriate if requirement is just tracking, or a PWM based speed controller for a normal DC motor.

I've seen the circuits from oatleyelectronics, but found this site to have better kits http://www.ozitronics.com/

[kinetic]

If it's any help I used a standalone dedicated IC to drive my
Bartels System focuser.
http://www.iceinspace.com.au/forum/showthread.php?t=51301

Chip is an MC3479 but another is the TA7774P
Capable of sinking about 500ma of motor current.

Circuit and details can be found on Ben Davies Scope Drive Excepts List.
Focuser page here:http://ben.davies.net/scopemanual.htm#FOCUS%20/%20FIELD%20DEROTATOR


This circuit can be driven with TTL logic to half step/ microstep
forward and reverse without a laptop or PPort.

Of course, if you want, you could go the whole hog and use
the 'archaic' (AlexN's description) complete Bartels System.:)
It is very powerful, as Gary has mentioned and, given a choice
if ever I set another permanent dome up, wouldn't hesitate to
implement it again.

Many other good ideas here on IIS. Just search using 'stepper'
and it brings up dozens.

Steve

[aokman]

has anyone taken into account the step rate of these motors also. At work we use types ranging from 3.6deg step down to 0.8deg step 3 phase with weighted dampeners and the driveline vibration is vastly better with a smaller step :)

im currently trying to incorporate one of these puppies as a focuser :)

[Bassnut]

Just quietly, why arnt belt drives used stock on (any?) mounts these days?. Seem better, logical, and cheap, so..........:shrug:

[kinetic]

I use 20 microsteps per step on the Bartels system but it is capable of
40.This is on a 400 step/rev stepper(0.8)
On siderial rate the steppers sing at a fairly high pitch, almost silently
if I have late night redneck AM radio on in the dome too.:rolleyes:
I have to actually strain to hear it.
I have the Astrosyn stepper mounted on a round washer made from
a tube of neutral cure silicone sliced up into a thin 'rubber' mount.
I have experimented with 'rubber' flywheel couplings between the
motor and gearbox but a simple coupling of medical tubing works best.
High speed ramped slews are smooth as well. I can't really complain.
But all of this was done over years of refining.
The Bartels system will work 'out of the box' with default settings on
most stepper/ gearbox arrangements but it will need some settings
custom tweaked. Things like microstep spacings, motor currents, duty
cycles. All of this takes time. And patience. But it is well worth
persevering. Most of the criticism of this fabulous (free BTW) system
comes from people who haven't even tried the solution out.

Sorry for meandering OT!:)

Steve

[bojan]

Well, timing belts are not cheap.. gears are cheaper.
Also it may be that it is simply "this thing is done this way" attitude..

In motor industry, timing belt is a norm today.
I remember when first cars with timing belts (instead of chain) appeared on the market: they were looked at with suspicion...

[aokman]

i have been playing with a few lately ranging from a 0.8deg 1.5A 3 phase which i havn't had much luck with (yet) due ot the controller chip integration. Having more luck with the 3.6deg screw drive ones i have though :) you can just controll them through a parallel port if you wish, im assuming a 6 wire is a unipolar motor with 2 wires connected straight to supply.

Belt drive would certainly be interesting, like i said we use it at work but i would have thought the load of a mount with scope would cause a bit of stretching in the belt and maybe throw the alignment out ever so slightly...