Hi Roman, I finally got around to reading through that thread, very interesting!

I think I will go with the linistepper kit as it seems to be the best compromise between an accurate and affordable solution. Am I correct in thinking that all I would need in addition would be a suitable stepper motor and a suitable battery?

If your offer of programming the PIC is still open (If I ever get that far ) that would be most helpful.

Thanks!

 

Hi Roman, I finally got around to reading through that thread, very interesting!

I think I will go with the linistepper kit as it seems to be the best compromise between an accurate and affordable solution. Am I correct in thinking that all I would need in addition would be a suitable stepper motor and a suitable battery?
...

Yes and no. I think the main expense in money and in work will be your telescope mount and the gearing setup!

It's really worth sorting that out first, before committing to buying a stepper driver kit etc because that will give you critical info on what speed your motor needs to do (depends on the gearing) and on what motor power is needed (depends on gearing, telescope weight and balance).

If you are only starting the design process for your telescope driver I think it would be a really good idea to spend some time googling and reading a lot of other people's builds. Also you could start a thread on one of the telescope/astronomy forums asking for build advice. I'm not a telescope guy I'm just an electronics guy.

Having a motor that turns at a very accurate speed is cool, but the bulk of the build will be the design of the telescope mount and all the gearing design. If the gears are sticky or sloppy etc you can get a really bad result even if the motor turns perfectly. The telescope experts are the guys to ask about all that.

...
If your offer of programming the PIC is still open (If I ever get that far ) that would be most helpful.
...

I don't handle kit sales or programming or shipping, James in California does that. As the kit designer I just get a couple dollars royalty from each kit sold.

However I'm available to discuss the software operation and help you work out the math for that number that makes the motor turn at the exact speed you want (once you know the gearing). I did a similar thing with the telescope build guy in that other thread, so you get the idea.

 

I already have a design for the mount and drive, it is a lot simpler than the one which was designed in the thread you pointed me to on CNCzone, mostly made from wood, so should be cheap enough. I have the measurements all set up for arm length, thread pitch of the threaded rod etc so that a 1rpm drive should give me very accurate tracking (in theory )
Here is an example of a completed design which is very similar to how I would like mine to turn out.

You do make a good point about the gearing having to be smooth, but I would hope a stepper motor would have more than enough torque to drive this set up without any problems.
I am inclined to get the motor and related components first so that I can adapt the design of my mount to incorporate the motor and then the gearing.

Would you be able to help advise on a suitable stepper motor for the Linistepper and my needs? I know there is a link to various motors on the website but I am unsure what would be best for me. I assume a motor with 1.8 degree increments would be better but I guess they would be quite expensive.

 

That mount looks pretty simple. It looks to use a threaded brass rod for inclination.

As for motors, you won't need much power at all so a size17 (42mm) motor will do fine. Any that have a voltage >5v should be ok, a 12v 300mA type would be ideal. I would probably just check ebay.

 

I just looked at this thread. Several years ago, I saw an alternative to stepper motor telescope drives, in Popular Electronics I think. They used a 120VAC/60HZ synchronous timing motor driven by a home brew 12VDC-120VAC inverter. The frequency of the inverter was tuned to sidereal time, 60.164279HZ, rather than 60.000000HZ clock time.

Ken

 

I'm going to be a heretic here and say that maybe there isn't a good case for building much electronics: the original d.c. motor design is perfectly good enough. Looking at the star photographs, they are excellent. If a cheap motor and very simple electronics can do that, why look elsewhere? I understand that the motor isn't going to run at a precisely controlled speed, but maybe it doesn't need to.

One serious issue is that this rig has to run outdoors, and be portable. If it can run for a few hours on a small battery, I imagine that's a big advantage. With this stepper setup (a power-burning linear one, no less) what's the current consumption? Of course a stepper doesn't stop drawing power even when it's stationary! The original version talks about a 9V power supply. Little square transistor batteries are useless for most purposes, but if it works for this camera drive, it's an indication of how much (how little) power is actually needed. I can't see that happening with a stepper driver, even if it's fantastically good.

I see the words "large heatsink" being used. I'm sure it gets chilly out there under the clear night sky, and having something to warm one's fingers might be a good idea. But perhaps it's better to have the camera drive and the handwarmer be separate items.

 

I am pretty set with this approach now. A simple DC motor would work but if the timing is out even by a little it will show up with long exposures, and i would like to use fairly long lenses (300mm) which will amplify any imperfections so I think a stepper is the my best bet.

Roman, I am having trouble finding a size17 12V 0.3A stepper motor that I can get hold of in the UK. What are the tolerances with those values? I.e. how low / high in voltage / current rating can I get away with? And why do you specifically suggest size 17?

Cheers

 

Try this for a motor.

Also for parts in small quantity and cheap delivery, here

 

I'm going to be a heretic here and say that maybe there isn't a good case for building much electronics: the original d.c. motor design is perfectly good enough. ...

... I see the words "large heatsink" being used. I'm sure it gets chilly out there under the clear night sky, and having something to warm one's fingers might be a good idea. But perhaps it's better to have the camera drive and the handwarmer be separate items.

A DC motor won't be that good for long exposures, 30 minutes or more can be common. Unless the DC motor has closed-loop encoder feedback to compentate for temperature and voltage changes, load changes due to the telescope weight moving etc.

Re the Linistepper and "large heatsink" that is mainly for machine use, ie CNC machines. For a very low-powered slow moving device like a telescope driver the PSU voltage can be low and the current also low, so heatsink size is greatly reduced.

...
Roman, I am having trouble finding a size17 12V 0.3A stepper motor that I can get hold of in the UK. What are the tolerances with those values? I.e. how low / high in voltage / current rating can I get away with? And why do you specifically suggest size 17?

Generally size17 are cheaper and smaller and lighter, and will do the job for your mount. The more common size23 are large heavy high current motors generally for fast CNC machine use etc.

This motor is really cheap, and smaller than size17 but it does have the advantage of a 64:1 internal gearbox, so it would still power your mount, and you could even attach the motor shaft direct to the screw drive so it saves you using those plastic gears in your photo;

http://www.ebay.co.uk/itm/5V-4-phas...ial_Automation_Control_ET&hash=item35be45fc2e

The motor is quite tiny but will be good for power consumption. Assuming you use that mount in your photo and are driving a light enough (or reasonably balanced) camera weight with a fine screw thread I think it would have enough torque.

 

How about this one, seems very similar except it is 12V and still has the internal reduction ratio

http://www.ebay.co.uk/itm/370601688902?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

and it is cheaper!


There are also these ones:

http://www.ebay.co.uk/itm/280992253698?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=400370561460&ssPageName=ADME:X:RTQ:GB:1123

I asked details about the bottom link and he posted this:

These are 64 steps per 360deg.

Electronic Parameters :

Rated voltage ? 12VDC

Number of Phase ? 4

Speed Variation Ratio ? 1/64

Stride Angle ? 5.625° /64

Frequency : 100Hz

DC resistance ? 200?±7%(25?)

Idle In-traction Frequency : >600Hz

Idle Out-traction Frequency : >1000Hz

In-traction Torque : >34.3mN.m(120Hz)

Self-positioning Torque : >34.3mN.m

Friction torque : 800-1800 gf.cm

Pull in torque : 450 gf.cm

Insulated resistance : >10M?(500V)

Insulated electricity power ?600VAC/1mA/1s

Insulation grade ?A



I assume 12V is better than 5V, so which of the above do you think is most suitable?

Cheers

 

The original design does run the motor via a voltage regulator, so "voltage changes" must be pretty minimal. I wonder how much the weight would shift, and how much it would affect the motor after lots of gearing. It's evidently workable.

What provides the timebase for the proposed stepper drive? It's only going to be as accurate as the clock that drives it.

 

...
I assume 12V is better than 5V, so which of the above do you think is most suitable?
...

I would definitely go for the 5v one. The Linistepper will drop your 12v battery voltage to run the 5v one, and it allows the option of overdriving it a bit if you need a bit more torque, and still gives perfect operation even once your battery might get a bit flat <12v.

Looking at that motor's output shaft torque after its gearing (800-1800g/cm) it will have plenty of power to turn a fine pitch inclination screw.

 

ok, sounds good. I think i will go for this one as it is a lot cheaper and is the same motor as far as i can tell just without the driver board. (I assume i will not need the driver board?)
http://www.ebay.co.uk/itm/New-28BYJ...al_Components_Supplies_ET&hash=item43b2aaa447

What kind of battery unit would I need to run this? would this 12V supply work?
http://www.ebay.co.uk/itm/370732522540?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

 

That 12v battery and charger looks like great value! I would expect the whole motor and driver to take well under an amp, probably closer to half and amp. So that 6.8Ah battery should run from about 7 to 12 hours constant.

The original design does run the motor via a voltage regulator, so "voltage changes" must be pretty minimal. I wonder how much the weight would shift, and how much it would affect the motor after lots of gearing. It's evidently workable.
...

Workable for sure, but not great. Some of the cheap telescope drivers worked like that. They used an opamp and precision voltage reference for stability, much better than just a 3 pin voltage regulator.

Besides the stability, the other big issue is ease of adjustment. You still have to try to tune it by turning the pot to match the exact speed to match the planets rotation.

What provides the timebase for the proposed stepper drive? It's only going to be as accurate as the clock that drives it.

The Linistepper code uses a xtal to drive a PIC, at a typical <30PPM error. Then the PIC uses a math algorithm to generate any exact stepper speed by just changing one number. Adjustability is better than 1 PPM.

Setting the speed is a big bonus, as the user can do a calc allowing for the known total gearing, and the known planets rotational speed, and just set that number in the software. Then right off the bat the stepper will turn at the exact speed +- the xtal error of a few PPM.

The same basic xtal->stepper process is used in the high end telescope drivers.

 

Great so i think i am have all the parts i need to make a start! Just want to double check this is everything i will need and it is all correct:
1. linistepper: http://www.piclist.com/techref/io/stepper/linistep/index.htm
2. 5v motor: http://www.ebay.co.uk/itm/New-28BYJ...al_Components_Supplies_ET&hash=item43b2aaa447
3. 12v rechargable battery: http://www.ebay.co.uk/itm/370732522540?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649
4: some kind of small heatsink. I guess this does not need to be specific, just something to dissipate the heat? The thread you pointed me to has a link to his similar finished project using the linistepper kit, and it looks like he has used a very small heatsink, which just looks like a flat bit of metal. http://www.youtube.com/watch?v=X4niXQXGAUg&t=5m0s

I hope the assembly of the linistepper pcb is not beyond me, I think I will do a little research into good soldering practices so I dont just stumble into it. The instructions online seem comprehensive enough. I also need a fine nib soldering iron, I think I will go for this one http://www.ebay.co.uk/itm/230827670862?_trksid=p5197.c0.m619

cheers

 

Yep those parts should work.

Be careful assembling the Lini to check all resistor values and to make sure diodes and resistor networks are the correct way around BEFORE soldering them in.

You won't need a big heatsink for that tiny motor, if you use a metal box that could be enough heatsinking. Otherwise a piece of aluminium or a cheap heatsink long and flat enough to mount the 4 transistors to.

 

Great, i have purchased all those parts now, I look forward to the challenge of assembling it
Thanks very much for all your advice and patience, it is very much appreciated.
I'm sure I will be updating this thread with various questions once I have received the parts and made a start assembling it!

Cheers
Dan

 

I'll be watching!

Next time I order from ebay I will get one of those little 5v geared stepper motors, and do some testing to find the required current and torque etc. They look to be good value for money for a very slow speed stepper application.

I think commercially they are used in cheap pan/tilt remote controlled camera heads for security cameras, and motors made in large quantities for that application also find their way onto ebay sellers.

 

Hello again!
I have received most of the components I need to make a start on this now and have been reading up on various things to help ensure I can do a decent job of it

Roman, did you by any chance ever manage to get hold of one of those motors I will be using to test the current / torque? Just to understand what you mean by that, would you be testing with a view to then being able to recommend what resistors I can swap out to fine tune the current to my motor and reduce the heat / increase the battery life of the set-up?

when it comes to programming the PIC, would this do the job?
http://www.ebay.co.uk/itm/221164316598?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649
It lists the number printed on the PIC in the kit as compatible (PIC16F628A)

Hope to hear from you soon!

Cheers
Dan

 

That programmer looks capable of programming the PIC 16F628A, so that's fine.

The MikroC compiler from MikroElektronika has a "free demo" version that will do up to 2k code size, which is all that is needed for that project in that PIC.

Sorry I've been busy with work stuff and have not ordered any ebay goodies, I do order some from time to time though.

Do you have the motor yet? If so I can suggest a couple of tests.