stepper motors and ASCOM controllers

[troypiggo]

I was reading through David's SharpSky stepper focuser controller thread (http://www.iceinspace.com.au/forum/showthread.php?t=91023) the other day, and watching some of the videos on his site. Got me to wondering about the actual stepper motors moreso than the controller.

I have recently bought a Mount Hub Pro. It has a built-in stepper controller that is supposed to be ASCOM compliant with Moonlite and Robofocus motors. I bought it for the power, USB, and dew control features in one box, not so much for the focus.

But seeing David's video, I'm wondering if all I need is a cheap $15 (or thereabouts) 12V stepper motor and could make my own for use with the MHP. He makes it look so easy. Am I dreaming? Where would I start with figuring out the wiring connections between the MHP and the motor? I believe it's a DB9 type connector.

[Moon]

You can get a robo motor only from Peter Tan for $890 HKD.
Jase posted a link a while back which you might find interesting. LINK (http://www.iceinspace.com.au/forum/showpost.php?p=772926&postcount=25)
These are cool because they have the gearbox all in one unit.

As for wiring a stepper directly, I guess you just need to check the wiring diagram. It shouldn't be to difficult I expect.
James

[Moon]

I should add the MHP does an excellent job controlling the robo motor. No more hassels getting the silly USB<>Serial driver working.

[troypiggo]

Thanks mate. Based on Jase's post and link, I'm beginning to see that there are stepper motors and there are stepper motors. Will have to research a bit more. He was supporting much more weight than I intend to. I've only got a QSI583 hanging off mine. So am wondering how important it is to pay $95 or whatever it is for the "super" motor.

[Nico13]

Hey Troy Bojan is someone you should talk to also as he is into stepper control in a big way.
I'm just about to start a stepper focus controller using a geared stepper but just using a hand paddle to adjust it so I don't bump the scope while adjusting focus.
Still using an SLR for imaging you see, but he Bojan that is will have some info for you I'd reckon.

[troypiggo]

Thanks mate. Might point him in this direction :poke:

[Nico13]

Yeh just send him a PM with a link to the thread and ask him to have a look at it to see what he thinks, won't do any harm. :thumbsup:

[bojan]

Hi guys,
Have a look at this thread:
http://www.iceinspace.com.au/forum/showthread.php?t=90164
The Pololu drver is ~$15 per motor.
Motors from ebay are in the range of $5 ~ $15 (and steppers are just steppers - anything above ~20 (unless it's high torque, high current and/or high power.. but we don't need any of those here for driving telescopes) is rip off. Especially when the steppers can be found in a variety of old equipment (old 5.25" floppy drives, printers, photocopiers.. )

Or, try ebay, and search for "long motors" for example (manufacturer in China - I am using their motors (400steps/rev, $10 each) in my EQ6, with Pololu drivers, also from Ebay).
Or search for "NEMA17" or "1.8° stepper" or "0.9° stepper" or similar (this is the standardised size of the stepper motor).
Hope this helps ;)

[bojan]

I just had a look at user manual http://www.hitecastro.co.uk/support/MHP/MHP.pdf

Yes, it is easy... this thing can control standard unipolar stepper (with 5 or 6 wires), the connection is shown on page 10.

[dtrewren]

Hi Guys,

Thanks for the PM invite Troy, thought I would may a few comments if that's ok.

I designed my SharpSky digital focuser around the same motor technology that ML & Robo use that is 4 phase unipolar motors. The motors I use have five wires some unipolar motors have more wires for flexibility but once configured for 4 phase operation end up with five connections. You essentially end up with a common and then one wire per phase. The stepping sequence is very straight forward, basically a walking one or walking pair if two coils are to be driven at once. The big advantage of permanent magnet unipolar motors is that you only need a very simple driver (I used a Darlington driver IC) whereas bipolar motors require a much more sophisticated (and expensive) driver.

I use one of two motors both supplied from China. Both motors are very cheap one at about £4 and the other at about £7. Both motors have built in gear boxes and with my controller produce ~2048 steps per revolution but are capable of twice that. The second of the two motors really had grunt with a pull in torque of >147mN.m, The reason these motors are so cheap is that they are used in just about every air-conditioning unit on the planet are are made in the millions.

The motors do exhibit some backlash as they have gear boxes. This is not really an issue as I compensate for this in software and there is a parameter on the driver where the user can tweak the backlash compensation to suit their setup.

The SharpSky design uses a D-type connector and is pin compatible with ML/Robo and a couple of guys are using the design which such motors. Personally before shelling out lots on an expensive ML motor I would get hold of one/both of the motors listed below and give them I go. I fully expect that they will be more than adequate for Astro needs. Loads of imagers are using SharpSky with the smaller of the two motors and have reported great performance. If you really have a lot of kit to move maybe the larger motor would be better.

If you want any information regarding wiring or stepping sequences please ask away.

Smaller motor : 28BJY-48
Larger motor : 35BYJ-412B

Cheers & clear skies,

Dave

[dtrewren]

Hi Guys,

Further to my last post and with regard to the Mount Hub Pro.

If the Mount Hub Pro is designed to drive ML/Robo motors the wiring for the motor is shown in the SharpSky build PDF document as SharpSky is also ML/Robo pin compatible. See section 11 of the document and this shows the wiring of the 9 way D-type and the colour coding of the two motors I listed in my previous post. If you wire these motors as shown and then use a pin to pin D-type straight through lead the motor will work just fine.

The SharpSky PDF is available on : www.dt-space.co.uk/SharpSky_Kit

Cheers & clear skies,

Dave

[troypiggo]

Thanks for chiming in, bojan. Had a read of that thread, but most of it is quite technical and beyond my electronics knowledge. If I understand it correctly, and using lay terms, you're using a USB sound card to control a stepper motor. Read like you were having issues finding the right card/board?

Presumably with the MHP the hard part is done for me, and just need the right motor and wiring?

[troypiggo]

Thanks so much for confirming that. I think this is worth me having a crack at.

[troypiggo]

Do you get the impression that the MHP can handle the bipolar motors? Would these be zero backlash?

[bojan]

That was at the beginning, no issues here any more :thumbsup:
and the software (SoundStepper, Maciel from Brasil is the author) supports Ascom and whatever you may ever want.

Yes, with MHP there shouldn't be any problems - small unipolar motor (5 or 6 wires) will work (as far as hardware is concerned...)
However I don't know anything about software side of it.

[troypiggo]

I see. Thanks. This unipolar/bipolar thing is confusing me. David's comment about backlash. Do the bipolar ones have this issue too? Trying to get a feel for cost/benefits of cheapies vs expensive motors.

[MrB]

Troy, the backlash is due to the gearbox only.
Bipolar/Unipolar steppers without gearheads do not exhibit this problem.

Oils aint oils and equally, steppers aint steppers.
Like almost everything else, some steppers are better than others with respect to positional accuracy and repeatability.
http://www.euclidres.com/apps/stepper_motor/stepper.html
It may be worth checking the datasheets for any steppers you are considering, if available.

[troypiggo]

Thanks mate. Been doing some more research and that just clicked that its the gears. Checking out that link now.

[MrB]

No worries.
BTW, the accuracy of steppers is mainly due to the quality of the 'teeth' on the rotor and stator. Cheap stepper will be made from punched sheet steel. The steppers I have salvaged over the years from old floppy/harddrives had machined or ground teeth.
However I'm guessing the accuracy of the motor would not be such an issue with these gearheaded motors. Probably don't even need to microstep them if the gear reduction/steps per rev is high enough.

[bojan]

Bipolar vs unipolar..
Perhaps those links will help to clarify things:
http://en.wikipedia.org/wiki/Stepper_motor
http://www.solarbotics.net/library/pdflib/pdf/motorbas.pdf

Expensive and cheap... this depends more on manufacturer/distributor, less on the actual quality.
If stepper has ball bearings, and in general looks solid it is not bad, but it may be cheap. Even hybrid, NEMA17, 0.9° s/rev are ~$10 or so (from Long motors in China, for example).
Data sheet is a good indication on what you are dealing with.

[dtrewren]

Hi Guys,

Lots of comments while I was pushing up Zzzz :)

Yes all true regarding the backlash it is due to the gear box. However, the backlash is very small and can be compensated for in software (as I do with my SharpSky design). I find even with the gearbox the repeatability is excellent. The client software I wrote has filter pre-sets and I find after hitting a pre-set I only need a very small tweak to nail the focus, takes less than a minute. I did lots of experiments moving back and forth about a focus point and found everything very stable even with the cheap motors. Obviously, you do get what you pay for but I wouldn't get too hung up on the gear box until you have tried one and they are so cheap it's worth picking one up. I think you will find the motors are more than adequate.

A few guys are also using these cheap motors with autofocus software with great success.

Cheers & clear skies,

Dave

[bojan]

I also intend to use one of those cheap, but adequate (for the purpose - focuser. It is also good enough for tracking, at short FL, up to 300mm) motors with gearbox, see here:
http://www.iceinspace.com.au/forum/showthread.php?t=88216

[troypiggo]

Ordered one of these little motors: http://www.ebay.com.au/itm/280778076149?ssPageName=STRK:MEWNX: IT&_trksid=p3984.m1497.l2649
Figure it's a cheap enough experiment, and hopefully within my capabilities.

[bojan]

Should be OK (it is unipolar), however it is relatively coarse - seems it doesn't have gearbox.
This would have been more appropriate: http://www.ebay.com.au/itm/5V-4-phase-Stepper-Motor-ULN2003-Driver-Board-f-Arduino-/280868686474?pt=AU_Components&hash=item4165140a8a
(you don't need driver board, but it may be useful later for the next project..)

[troypiggo]

D'oh. The model I ordered was one of the ones David mentioned, I think.

That one you've listed is only 5V. I was trying to keep to 12V for simplicity of power supply.

[dtrewren]

Blast, sorry about that you need the 12v version as it does pack more punch. I started using 5v motors then switched to 12v

Again sorry for the misinformation !

[troypiggo]

No Dave, you're fine. The motor I ordered was the 35BYJ-412B one you mentioned earlier in this thread as the "larger" one with built-in gearbox. It is 12V. That's right, isn't it? The eBay ad doesn't mention gearbox I think.

I was responding to the one bojan linked, which looks like it's 5V.

[bojan]

Yes, it is listed as 5V.. with coil resistance of 200Ω.
The one you ordered has 120Ω.... so it will dissipate more heat.

Voltage is not so important here, but coil current is (and power dissipation that goes with this).
Both motors will work powered with anything from 5V to 12V, as long as the coil current produces enough torque for motor to step without skipping and/or stalling.
However, because focuser doesn't have to be powered all the time, neither motor will go too hot if you switch off the power after focussing is done. And, if you want to keep the power supply ON all the time, to reduce the motor current (and dissipation with it) you can just put 150Ω ~ 250Ω, 1/2W resistors (you need 4, one for each coil) in series with the coils.
The only issue here is gearbox -present or not (and final resolution). If it is inside the package of motor you've ordered, it is probably of planetary design (because the output shaft is not offset from the centre of the package).. but on the website they have mentioned
.. which led me to the conclusion that possibly there is no gearbox (3.75 ° step is equal to 96 steps/rev).. but I may be wrong - despite those motors are used everywhere, the documentation has a lot of room for improvement (maybe everyone who use them, knows everything about them.. so they reckon, no need for docs ?)
The one I proposed has final resolution of 2096 steps/rev (driven in half-step mode) at the output (32 steps/rev at motor shaft inside, 1:64 gearbox)
Anyway, see how you go....

EDIT:
When connecting the motor to your driver, first you have to identify the common terminal with ohm-meter.. it is not necessarily the one in the middle of the connector.
It will be the one from which the rest of the terminals have close to 120 ohms.. If you measure 240 ohms, that means neither of terminals is the middle one.
After that has been done, if the rest of them are not in a right sequence, motor will vibrate around one position and refuse to move. Try to swap two wires at the time until the motor starts to move in one direction. To change the direction, you just reverse the wires sequence (leaving the common where it was).

[dtrewren]

Hi Troy et al,

Here is the spec for the motor you have bought. The gearbox has a reduction ratio of 1:42.5 which gives a step angle of 3.75/42.5 ~ 0.088° or 4080 steps per revolution. Not sure how the MHP drives the motor ? My SharpSky design uses a stepping sequence that energizes two coils at once to increase the torque, the result is half the number of steps or 2040 per revolution. To be honest I find in practice, even at f/4, this is too fine and usually move the motor with a minimum of 5 steps giving me effectively 408 steps per revolution. So I usually start with movements of 100 steps for course, then 20 steps and finally maybe a very fine adjustment at 5 steps. Because of the gearbox I don't drive the motor when not moving and the gearbox holds the scope focuser totally solid. Because you can't move the motor spindle against the gearbox I designed in the electronic manual control with user selectable variable rate. The user rate selected is stored by the microprocessor and comes up at the pre-set value from power up.

Cheers & clear skies,

Dave

Rated voltage ： 12VDC
Number of Phase ： 4
Speed Variation Ratio ： 1/42.5
Stride Angle ： 3.75° /42.5
Operation Frequency(PPS) : 200 (PPS)
Resistance Ω/Phase(at25°C) ： 30Ω±7%(25℃)
Max.Freeload Pull-in Frequency : >450 (PPS)
Max.Freeload Pull-out Frequency : >550 (PPS)
Pull-in Torque(mN.m) : ≥ 147 (mN.m)
Detent Torque(mN.m) : ≥68.6(mN.m)(120Hz)
Insulated resistance : >50MΩ(500V)
Insulated electricity power ：600VAC/1mA/1s
Insulation grade ：A
Rise in Temperature <60K(120Hz)
Noise <40dB(120Hz,No load,10cm)
Model ： 35BYJ-412B

[bojan]

Ah, I didn't interpret the data sheet on ebay website correctly.
This is what I meant by "there is a room for improvement " as far as data sheet is concerned (now I remember I had similar dilemmas when trying to determine what the motor I was using really is...)
So Troy will be quite fine with this motor.

[dtrewren]

Hi Guys,

I just did a short motor demo using the motor Troy has just bought :D Thought it might be nice to see it in action :

http://www.youtube.com/watch?v=ofX_Hp_cEi0

Cheers & clear skies,

Dave

[troypiggo]

Excellent news, thanks for easing my mind guys. And thanks for the video, David. Looks like more than enough for me. Big grins all around. :D

[troypiggo]

Thought I'd followup this. I received the 35BYJ-412B motor ($7) earlier this week. Went to Jaycar yesterday to buy a serial cable and connectors and box to put motor in (about $20). I've ordered a beam coupler from eBay for about $5 I think. Just need a strip of aluminium as a mounting bracket and think I'm done.

I should say that I'm a complete gumby when it comes to soldering and all things electronic. I took a punt that the colour/wiring points in the SharpSky PDF that David provided were the relevant ones for the motor I bought. Searched all over the internet for this motor, but couldn't find anything that said what the coloured leads meant in terms of +2, +1, 12V etc. Did the soldering of the leads to a DB9 connector today and plugged it into the MountHub Pro. Very simple and just worked. Just a bench test, but it works forward and backward and seemed repeatable.

So for all up around $35 plus the MHP that I already had, I've got a stepper motor ASCOM focuser that works in Maxim. How cool is that? :)

[dtrewren]

Hi Troy,

This is indeed excellent news, always nice when a plan comes together.

I had a question for you. When you tested your motor did you plug it directly into the MHP or did you use your serial lead ? The reason I ask is because a standard serial lead is not straight through but has several crossed over pins. My SharpSky design uses a 9-pin monitor extension cable because it is pin to pin straight through therefore the motor can be plugged directly into the controller or via the lead. Maybe the MHP is wired such that using a serial cable results in the correct pinout for a MoonLite/Robo D-type.

Just curious :)

Clear skies,

Dave

[troypiggo]

I specifically bought a straight-through cable, not a crossover one. It's male at one end and female at other. So, yes, I used that bought cable between the MHP and the motor.

[dtrewren]

Hi Troy,

Ahh, ok that makes sense - I found it quite tricky to get hold of a straight through cable here in the UK so to find one locally is good going :)

Clear skies,

Dave

[dtrewren]

Just been looking at the Jarcar site :) looks a bit like the Maplin stores we have here in Blighty but Jaycar seems to have a more varied range of gear. I usually end up having to buy online to get the bits I need and keep the cost down.

[MrB]

Hi Dave,
In my experience Maplin has a much better range of components.
Last time I was in the UK I picked up a Maplin catalogue to bring home with me, it's about 3 times the size of the Jaycar one.
I'm very familiar with the Jaycar stock(used to manage a store) and it was very frustrating when I couldn't even buy some of the most basic components.