help with drive mechanism for barn door tracker

[danstar10]

Hello!
I am about to embark on a small project to build a star tracking platform for my camera, and I would greatly appreciate a little help regarding what design to use, as I am not very well versed in electronics. I have done a fair bit of research and it seems that a stepper motor may be my best bet. As I am sure you all know, I need it to turn the threaded rod in my barn door tracker at precisely 1rpm. This could come directly from the motor or through the resultant gearing.

I think what I want is a pre assembled stepper motor driver controller kit? (some come with, some without the motor) but there are various differing voltages etc, like the following:
http://www.jaycar.co.nz/productView....KV&Keyword2=KD
http://www.ebay.co.uk/itm/1906858573...=p5197.c0.m619
http://www.brocott.co.uk/electronic-...-kit-b211.html

Am I on the rite path? I am open to whatever would work best, be it a stepper motor or otherwise, so I would love to hear what other people have used and get some advice on what would work best for me. I am based in London UK.

Cheers!
Dan

[bojan]

Maybe you should try this.
It is for use with EQ2 mount (via additional gear stage, I believe 1:5 or close) and rotational speed is such that when driving 144 tooth worm gear, you end up with sideral rate.
Now, obviously the rotational speed is not 1r/min, it is 2 rot/min (if that additional gear reduction is indeed 1:5, - I am not sure, but someone may be able to confirm), so this only means your barndoor arm must be shorter (or you should use adequate, finer threaded rod).

[Scorpius51 (John)]

Originally Posted by bojan

Maybe you should try this.
It is for use with EQ2 mount (via additional gear stage, I believe 1:5 or close) and rotational speed is such that when driving 144 tooth worm gear, you end up with sideral rate.
Now, obviously the rotational speed is not 1r/min, it is 2 rot/min (if that additional gear reduction is indeed 1:5, - I am not sure, but someone may be able to confirm), so this only means your barndoor arm must be shorter (or you should use adequate, finer threaded rod).

Good idea Bojan!

This is available from Bintel http://www.bintel.com.au/Mounts---Tr...oductview.aspx

Cheers
John

[danstar10]

thanks for the link. That is a little above my budget, but I did some searching and found these:

http://www.green-witch.com/acatalog/...or_Drives.html

The first and third one down look promising. Do you think they could work for me?

[OzEclipse (Joe Cali)]

Originally Posted by danstar10

thanks for the link. That is a little above my budget, but I did some searching and found these:

http://www.green-witch.com/acatalog/...or_Drives.html

The first and third one down look promising. Do you think they could work for me?

I've never pulled apart an EQ1 or 2, so I do not know from personal experience what the gear ratio of the EQ2 is but I think you are likely to find it much more difficult than has been suggested.

The calculations posted further down in this thread don't add up. Even if the EQ2 has, as stated, a 144 tooth worm, the worm drive shaft has to rotate once every 10 minutes or 1/10th RPM. If the final reduction gear is 5:1 then the drive must rotate at 1/2 rpm not 2rpm.

However I found this URL : http://tinyurl.com/cn7lbzn states that

The EQ1 has 96 teeth.
The EQ2 has 100 teeth.
The EQ3-2 & NEQ3 have 130 teeth.
The HEQ5 & HEQ5 Pro have 135.
The CG-2 or CG-3 mount (used by Celestron AstroMaster / Powerseeker range) have 136 teeth..
The old EQ3 & EQ4, EQ5 & EQ5 PRO & Orion Skyview have 144 teeth.
The Meade LX and the Celestron CGE range have 180 teeth.

If this web site is correct, then the output shaft of an EQ2 will turn at 1 rev per 861.6 seconds x final reduction gear ratio. ie if it is a 5:1 reduction gear then you are looking at a rotation of 1 rev per 172.32 seconds. A little faster than 1/3 rpm. The EQ1 is a little easier at 900s per rev divided by the reduction ratio. These drives use quartz crystals so it is not that easy to change frequency to change speeds and you won't find gears of the appropriate ratio to solve the problem.

A barndoor drive is easy to make, but not easy to make to high precision. It's much easier to have a variable speed drive so that you can make a final speed adjustment to after it is made. These are effectively fixed speed drives unless you swap crystals.

I have used the Jaycar kits you identified but they are being phased out of production. I've swapped the timing capacitor for one that has good temperature stability and replace the timing potentiometer with two precision trimpots in series, one for course and one for fine adjustment and some push buttons to switch in different resistors for fast and slow.

The trimpots must be set with a screwdriver and once set can't be bumped out of calibration. However it is not a simple matter to attach a precision gearbox to the supplied motor. They come with a spur gear pressed onto the end of the shaft but I haven't found a suitable supply of gears. In my application, I took the spur gear off and purchased a separate high precision gearbox that was supplied with it's own spur gear. But I needed to use a lathe to make the parts to adapt it.

Joe

[OzEclipse (Joe Cali)]

Originally Posted by danstar10

thanks for the link. That is a little above my budget, but I did some searching and found these:

http://www.green-witch.com/acatalog/...or_Drives.html

The first and third one down look promising. Do you think they could work for me?

I've never pulled apart an EQ1 or 2, so I do not know from personal experience what the gear ratio of the EQ2 is but I think you are likely to find it much more difficult than has been suggested.

The calculations posted further down in this thread don't add up. Even if the EQ2 has, as stated, a 144 tooth worm, the worm drive shaft has to rotate once every 10 minutes or 1/10th RPM. If the final reduction gear is 5:1 then the drive must rotate at 1/2 rpm not 2rpm.

However I found this URL : http://tinyurl.com/cn7lbzn states that

The EQ1 has 96 teeth.
The EQ2 has 100 teeth.
The EQ3-2 & NEQ3 have 130 teeth.
The HEQ5 & HEQ5 Pro have 135.
The CG-2 or CG-3 mount (used by Celestron AstroMaster / Powerseeker range) have 136 teeth..
The old EQ3 & EQ4, EQ5 & EQ5 PRO & Orion Skyview have 144 teeth.
The Meade LX and the Celestron CGE range have 180 teeth.

If this web site is correct, then the output shaft of an EQ2 will turn at 1 rev per 861.6 seconds x final reduction gear ratio. ie if it is a 5:1 reduction gear then you are looking at a rotation of 1 rev per 172.32 seconds. A little faster than 1/3 rpm. The EQ1 is a little easier at 900s per rev divided by the reduction ratio. These drives use quartz crystals so it is not that easy to change frequency to change speeds and you won't find gears of the appropriate ratio to solve the problem.

A barndoor drive is easy to make, but not easy to make to high precision. It's much easier to have a variable speed drive so that you can make a final speed adjustment to after it is made. These are effectively fixed speed drives unless you swap crystals.

I have used the Jaycar kits you identified but they are being phased out of production. I've swapped the timing capacitor for one that has good temperature stability and replace the timing potentiometer with two precision trimpots in series, one for course and one for fine adjustment and some push buttons to switch in different resistors for fast and slow.

The trimpots must be set with a screwdriver and once set can't be bumped out of calibration. However it is not a simple matter to attach a precision gearbox to the supplied motor. They come with a spur gear pressed onto the end of the shaft but I haven't found a suitable supply of gears. In my application, I took the spur gear off and purchased a separate high precision gearbox that was supplied with it's own spur gear. But I needed to use a lathe to make the parts to adapt it.

Joe

[danstar10]

cheers for the low down OzEclipse.
The one on that page I am interested in is the EQ2 R.A Economy Motor Drive - I think there may be a misunderstanding as to how I plan to use it. I would attach a gear to shaft of the motor, which would then drive another gear which would be attached to a nut which in turn drives the threaded rod (equivalent of a worm gear). This means I could have 2 gears which I could change the ratio of to gear down the speed to 1rpm.

here is an equivalent finished barn door tracker
http://www.ellenrooneydesign.com/gsp...racker%201.jpg

and a diagram trying to explain it
http://img22.imageshack.us/img22/7586/barndoor.jpg

do you not think with the combination of that and the variable speed on the drive unit that I could achieve 1 rpm? I know the thread pitch of the rod and the distance from the hinge to the rod, so if I can turn it at 1rpm it should work.

[bojan]

Everything that Joe mentioned is OK - I made an error in calculation, my apologies ....
However, I do not think that you really need exactly 1 rpm - because barn door arm length can be calculated to suit the speed of the motor shaft (after reduction, as per images you supplied).
This can be done by moving the hinge into appropriate position - much better than fiddling with potentiometers, especially if one is not handy with electronics. Using the coarser threaded rod is another way to adapt slower motors to desired output)

The good supply of various gears and timing belt reductor stages are old printers (they are very easily found here in Oz, people are often throwing them on the grass in front of their houses).
I would simply go for EQ1 Single Axis D.C. Motor Drive (hopefully motor is also included in the kit?) and then see what I can do with available gears (from old printers) in terms of arm length.

Of course, check the website Joe suggested, for possible motor speeds. You can also up-gear your reduction (speed up instead of slowing down. This will increase the PE, but for wide angle photography (short focal lengths of lenses) it doesn't matter much)

EDIT: I will be back to you with detailed design parameters around EQ1 Single Axis D.C. Motor Drive shortly.

[danstar10]

Why would you go for the EQ1 drive over the EQ2 drive, when the EQ2 has variable speed and the EQ1 does not?
Yep I actually went to a local store today and the had the EQ2 R.A. Economy drive; I looked at and it does indeed include the motor.

What does PE stand for when you say "This will increase the PE"?

Great, look forward to seeing your designs.

[bojan]

PE is periodic error

EQ1/EQ2 multi speed has 2x and 4x speed, so essentially it's not variable... and this is not really needed for barn door.

I suggested EQ1 economy (or EQ2 economy) motor kit because it is basic (but has all you need with no fuss, apart from additional gears), and cheaper.

Bear with me for a while, I will post spreadsheet with calculations soon - so you will be able to see from it what you need to do to build your barn door mount

[danstar10]

I think we are getting names confused!
from this link http://www.green-witch.com/acatalog/...or_Drives.html

The top one 'EQ1 Single Axis D.C. Motor Drive Ref: OV-449'
is £29.99 and 'Provides multi-speed quartz-controlled electronic motor tracking'

the third one down 'EQ2 R.A Economy Motor Drive Ref: OV-998'
is also £29.99 and 'provides single speed (1x sidereal) motor tracking'

I think the 2x and 4x speed ones are the other design which are around £60 and up, I am not looking at them.
So I actually had it back to front, I think the EQ1 Single Axis D.C. Motor Drive will have a variable speed by turning a knob to fine tune it which would help me...

May be it is just me that is getting confused, but what do you think of the EQ1 economy drive over the EQ2 economy drive as it offers variable speed? By the looks of it it is a twist know meaning it would could be fine tuned.

[bojan]

They look exactly the same to me (except being for EQ1 and EQ2, so they will have different output motor speeds, because EQ1 has 96 and EQ2 has 100 teeth on worm gear).

Provides multi-speed quartz-controlled electronic motor tracking of the R.A. axis. Battery-powered.

This doesn't make sense to me - quartz frequency can not be adjusted enough to be significant for this purpose...

But it all doesn't matter much really - your barn door reduction rate should be equivalent to EQ1 or EQ2 anyway.

[bojan]

Hmm.. now I see something else:
Top one has DC motor!!! So it is probably widely adjustable.. unless it is designed with PLL, in which case this motor drive is suspiciously cheap.
If it's not PLL, then I would not suggest it because the motor speed will be dependent on load (and temperature and grease viscosity and what's not..).

Let's concentrate on EQ2 single speed driver (the third one), for a start. This model will have stepper motor, speed of which is controlled by quartz crystal (so it is accurate more than enough for our purpose)

[bojan]

OK..

Attached below is a spreadsheet which will calculate the required (simple) barn door arm length.

It is assumed we are dealing with EQ2 motor, and the associated gear reduction between motor box and worm on EQ2 is 1:5 - however those parameters can be adjusted in the spreadsheet.

If the above assumptions are correct, with the screw pitch of 1.25mm (which is a standard M8 screw coarse pitch), and up-gearing 2:1 (additional gears you have to find somewhere, from old printer for example), the required arm length is 199mm.

Up-gearing would be possible to avoid if the crystal is replaced with 2x higher frequency, for example - but this is a job for someone who has some experience with soldering.

I hope this will be helpful for you

[danstar10]

Nice work on the spreadsheet, thanks.
What makes you think that the EQ2 driver will have a stepper motor as opposed to a DC motor? I think it will just have a DC motor set to a given speed.

All the barn door projects I have looked at have the same fixed distance for the arm and thread pitch which result in a 1rpm turning of the worm gear giving the sidereal rate. I would like to stick to this if possible as if I cannot get the drive working, I can operate it manually and in that case 1 rpm is easy to operate.
Ideally I could get from the motor rpm to 1rpm just using gears.

[bojan]

Originally Posted by danstar10

....
What makes you think that the EQ2 driver will have a stepper motor as opposed to a DC motor? ...

Because ad doesn't say it is DC motor.. and it is mentioning quartz control (so this implies stepper motor - in general, processor drives stepper with pulse rate derived from quartz reference. Then again, for the first item, it says DC motor with quartz.. which I believe is an error, deliberate or just not. Of course, it is possible to control DC motor rotational speed with quartz reference, but then it simply can't be priced at $50).

I will check with our local retailer (Bintel) if this is so or not, first thing Monday morning.

However, if you really want to stick to 1rpm, why not try this (or similar)?

http://www.ebay.com.au/itm/ROBUST-SM...item3a78351d2e

[bojan]

I checked with Bintel.. and learned something new today
Both Economy models have DC motors.

So I assume you will be able to adjust (with possibly internal pots) the required speed (1rpm.. I will try to ask someone who has that motor driver though).
However, I am not sure how stable this will be - without adequate feedback (PLL, which is not so cheap, and I doubt they are using this in this simple product), DC motor speed is susceptible to variations due to external factors (mainly temperature).
But, it may work for you.

[mswhin63 (Malcolm)]

My very first DC motor operation was 1RPM and I used a PWM controller to reduce speed (.1RPM) and was still able to push my 12" DOB. The gearing increases the torque capability till it reaches the stress point of the gear.

[danstar10]

yeah I suspected it would be the same build as the EQ1, shame. Thanks for checking it out.

So are you suggesting at the min it looks like my best bet is the EQ1 economy drive with variable speed? The possibility of varying speed with temperature does worry me a little, but may be it will be very slight? The torque should remain fairly constant.

Ideally I would want to use a stepper motor like you say. I have been trying to find a suitable set up which would work for me (pre built kits / kit which you assemble yourself) but have not had much luck so far. I don't suppose you know of where I could fine something like this?

These are a few things I have been looking at but have been advised they will not work, what do you make of them?

http://www.ebay.co.uk/itm/Stepper-Mo...item3cc7ff5613
http://www.brocott.co.uk/electronic-...-kit-b211.html
http://www.piclist.com/techref/io/st...step/index.htm

If I wanted to use the 1rpm motor you linked from ebay what would I need to attach to it to drive it? Just a battery or would it need circuitry?

Thanks

[bojan]

Originally Posted by danstar10

.. If I wanted to use the 1rpm motor you linked from ebay what would I need to attach to it to drive it? Just a battery or would it need circuitry?
Thanks

You will need only transformer, and mains power - those timing asynchronous motors (with internal gearbox) are designed to operate at AC voltage only.
Just make sure they are designed for 50Hz - those for US market will turn slower at 50Hz (they are deigned for 60Hz mains, so in UK they will be turning with 0.8333 rpm).


The circuits you mentioned require external PC to operate.

This is the reason I suggested to you to use "non-standard" arm length for your barn-door, and available electronics, with no fuss.
Economy driver may be OK for wide field photography, provided the frequency is properly adjusted. This can be achieved by fixing a small telescope (with maximum available power) on the mount, and adjusting the pot until the star field in the eyepiece is steady over longer period of time.
Then you can try it with camera and (short focal length) lens.

For the totally independent, battery operated, processor controlled barn-door (or eq) mount, have a look here.
I build this with very old processor (motorola 705, because I have a few leftovers from previous projects) and this works a treat - I still use it for wide fields.
But, it requires some experience with electronics and mechanics...

There were other discussions here on this topic.. for example this one:
http://www.iceinspace.com.au/forum/s...ht=PIC+stepper