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#1 edwardw

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Posted 05 August 2014 - 01:16 AM

There is little information about ATM direct drive equatorial mount. The only thing I can find is http://www.astrosurf...ives/index.html from a French gentleman. Two key pieces of the project seem to be the encoder and the motor. The encoder turns out to be Renishaw and the motor is made by an Italian company called Phase Motion Control. Apparently, the motor is rather special. It is a torque motor. All motors generate torque but torque motor is the one specifically designed to be operated in constant stalled or nearly stalled state, just like a tracking equatorial mount would do.

 

Sadly I don't have enough mechanical skill to take advantage of this information. Hopefully someone in the forum can and build a DIY direct drive mount!



#2 obin robinson

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Posted 05 August 2014 - 05:30 AM

I've been looking at that page for a while now. I am really interested in that motor and have been looking for a U.S. based supplier. It looks to be well worth the effort necessary to build it!

 

obin :waytogo:



#3 edwardw

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Posted 06 August 2014 - 12:42 AM

Let the quest of holy grail in DIY telescope mount begin!


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#4 brave_ulysses

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Posted 06 August 2014 - 06:37 AM

some good diy info here

 

http://www.altazinit...Tech & Demo.htm

 

and here

 

https://groups.yahoo...-torquemax/info



#5 edwardw

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Posted 06 August 2014 - 05:33 PM

some good diy info here

 

http://www.altazinit...Tech & Demo.htm

 

Aha, this is really useful for alt-az mount DIYers. Its motor, however, is an axial flux one. An equatorial mount should not be able to use it.

 

After reading that direct drive equatorial mount blog post, I did do some research about homemade dc motor. Most of them just revolve too fast. At several thousand RPM, they are meant to use in tandem with gearboxes. The closest one I found is from an MIT student: http://www.instructa...Motor/?ALLSTEPS. He built a direct drive brushless dc motor to drive scooters. It still not slow enough for a tracking equatorial mount though.

 

Maybe the real difference is not a mechanical one but merely in motor's controller. But I couldn't find any literature directly addressing torque motor to confirm that.


Edited by edwardw, 06 August 2014 - 05:36 PM.


#6 obin robinson

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Posted 07 August 2014 - 07:14 AM

What about the Kollmorgen motors?

 

http://www.kollmorge...-torque-motors/

 

The literature says they go down to .017rpm just for example. Shouldn't those work? Contact them and see.

 

obin :)



#7 edwardw

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Posted 07 August 2014 - 07:48 AM

What about the Kollmorgen motors?

 

http://www.kollmorge...-torque-motors/

 

The literature says they go down to .017rpm just for example. Shouldn't those work? Contact them and see.

 

obin :)

Yeah, Kollmorgen shows up a lot in my search too. Hope it works out for you, both mechanically and price-wise! Then it'll be a bless for ATMers :D

 

Also, the link you give is Kollmorgen's frameless torque motor, which provides maximum freedom for mechanical design. If, say, you intend to mount motor at the end of RA or DEC shaft, I reckon a normal pre-assembled torque motor will do. Kollmorgen has those motors, too. Just my 2 cents.


Edited by edwardw, 07 August 2014 - 07:49 AM.


#8 Spectral Joe

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Posted 07 August 2014 - 08:42 AM

Something like this?

 

http://www.cloudynig...uatorial-mount/



#9 edwardw

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Posted 07 August 2014 - 08:52 AM

Something like this?

 

http://www.cloudynig...uatorial-mount/

No, it is harmonic drive so still gear reducer. Although periodic error wise, it is better than, say, worm gear. It is also said to be very expensive. Direct drive, on the other hand, the RA or DEC shaft is directly bolted to the rotor of the motor.


Edited by edwardw, 07 August 2014 - 08:54 AM.


#10 Linn

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Posted 07 August 2014 - 12:56 PM

As I am looking at needing a stiff mount in the not to far future (I hope), this thread caught my eye. One of my Google searches, "direct drive motor "ultra low speed"" came up with some interesting hits. There are two SPIE digital articles on using the motors for large telescope drives, an IEEE article on motors and telescope tracking, and a PDF on designing motor and control systems for machine tools. There was also a lot on vinyl record turntables as well. I did not have the time to dig through my library permissions to retrieve the technical articles, but the PDF seemed to be a primer on system design. The link:

 

http://electronix.ru...e=post&id=66372

 

Hope this helps. Linn



#11 edwardw

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Posted 07 August 2014 - 01:35 PM

Very useful. It answers some of my questions regarding torque motor. So it is mechanically different than usual DC motor on that it has more substantial permanent magnets. And a precise encoder and feedback control are also crucial. Being frameless is not necessary something unique or mandatory to torque motor, though. If it is, however, how to assemble it on site is apparently worth a video of its own :grin: : http://www.youtube.c...h?v=59GXzwHm0FI

 

It also helps that the PDF is written by ETEL, another name often shows up in my search about torque motor.


Edited by edwardw, 07 August 2014 - 02:34 PM.


#12 James74

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Posted 10 August 2014 - 02:42 AM

I've been wanting to build one for a while but I don't have enough electronics experience to make it happen.
Sidereal Tech makes a bldc controller for this purpose, but it costs $3k. Their controller can also handle very large currents, allowing it to be applied to 1m class scopes.

I think the major issue is not making the motor or getting the Renishaw encoder working, but controlling the motor. It seems like you would need a very high voltage resolution to control each motor phasewell enough to achieve the pointing accuracy you want. If this is true, then the mosfet gate driver will probably be expensive.

Any electrical engineers out there that have any insight?

Btw, I think we should put together an open source project to develop an inexpensive direct drive mount for smaller telescope.


Edited by James74, 10 August 2014 - 02:47 AM.


#13 soldevilla

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Posted 10 August 2014 - 04:17 AM

Are we talking about this?

 

http://www.astronomy...drivemotor.html



#14 edwardw

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Posted 10 August 2014 - 07:47 AM

Are we talking about this?

 

http://www.astronomy...drivemotor.html

Not really, and it is basically the same as descibed in post #4.

 

It is a direct drive motor, yes. But it is a axial flux one, probably not for equatorial mount.



#15 DRobb

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Posted 10 August 2014 - 09:01 AM

Hi, 

Really interested in this topic.

 

Excuse my ignorance but I was just wondering why the axial flux type motor is not suitable for an equatorial mount? 
Thanks, 

Dan  :)



#16 edwardw

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Posted 10 August 2014 - 10:08 AM

Excuse my ignorance but I was just wondering why the axial flux type motor is not suitable for an equatorial mount? 

Thanks, 

Dan  :)

Oh, the conclusion is solely mine and empirical ;) Both ASA and Skyvision seem to use radial flux (i.e. the usual one) motor in their direct drive mount. And I yet to find a commercially available axial flux torque motor. On the other hand, there're quite a few RF torque motor manufacturers.

 

Besides availability, another reason no one use AF torque motor in EQ mount, I suppose, is that it needs to be large enough in diameter in order to provide sufficient torque. So hard to make it compact. In one article describing that alt-az direct drive mount using DIY AF motor, they also mentioned that they had to use glass ball in ball bearing. It isn't very convenient, is it?


Edited by edwardw, 10 August 2014 - 11:23 AM.


#17 James74

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Posted 10 August 2014 - 11:55 AM

It may not be convenient, but it does seem cheaper to make your own axial flux motor. Another issue is that the axial flux motor will require more current to achieve the same force that a radial flux motor will produce.

With either motor you will have to build the mount axes and use some sort of bearings.
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#18 edwardw

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Posted 10 August 2014 - 12:29 PM

Another issue is that the axial flux motor will require more current to achieve the same force that a radial flux motor will produce.

Ah, fair point.

 

Regarding your call for a open source bldc controller, that's another very good point. I dig up a paper written by orlyandico in the "Are we in a mount revolution?" thread of mount forum in which he used an Arduino to read out data from a homemade encoder and correct PE of a conventional EQ mount: https://www.dropbox....landoAndico.pdf. Could be a good reference and starting point.



#19 gregj888

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Posted 11 August 2014 - 12:59 AM

In general the axial or radial flux doesn't matter much.  Axial flux motors look like a worm gear.  Radial flux fit better (IMHO) on an equitorial and have more tourque for the same power in, but you can always add more power.

 

Remember you are only turning about 1 revolution per day, so your encode needs to have many ticks/sec... hence the Renishaw engoders. also not cheap.

 

We hold out ATM workshops (RCA) in Dan's shop (Siderial Tech), so have watch this project from the begining.  It is flipping awsome.  It can move the scope so fast it will scare you... if not "turned down".

 

The BLDCs are basically the same as the 3 phase generators used on a lot of DIY wind turbines.  Dans first was made of plywood, the new versions are a bit more robust.

 

I hope to use a BLDC drive on my 20" if I can afford it...

 

Here's a few links-

http://wvi.com/~rber.../bulletin07.htm

http://www.youtube.com/watch?v=UfvRGd9OuWE

http://www.youtube.com/watch?v=cNRsbwZ83rY



#20 James74

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Posted 12 August 2014 - 12:42 AM

Hey y'all, here's what I gathered when I was doing some research a while back.

 This has some good info on the "pancake motor" (axial flux): http://www.altazinit... Demo.htm#Flux_

It also has a link to a word doc that is pretty good (search for "Reference: 18 Direct Drive Telescope - Cal Poly").

 

A little bit of the math behind axial flux motors: http://ro.uow.edu.au...&context=theses

 

Dan Gray's write-up on the si-tech prototype: https://www.google.c....72676100,d.cGE

 

Their small wiki page: http://starryridge.c...ct_Drive_Motors

It also has an excel sheet with some axial flux specs: http://starryridge.c...ancakeMotor.xls

 

Some light reading on encoders from Renishaw: https://www.google.c....72938740,d.cGU



#21 obin robinson

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Posted 12 August 2014 - 05:48 AM

Take a look at this auction. I wish I had the spare coin so I could use it in a mount project:

 

http://www.ebay.com/...=item4619700ce4

 

That would make a heck of a telescope mount!

 

obin :)



#22 edwardw

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Posted 12 August 2014 - 09:42 AM

Take a look at this auction. I wish I had the spare coin so I could use it in a mount project:

 

http://www.ebay.com/...=item4619700ce4

 

That would make a heck of a telescope mount!

 

obin :)

Oh, boy, that's only one axis. But yeah, what a heck of a combination that is!

 

Obin, in case you're still looking, I found this torque motor supplier list today: http://www.controlen...-rotary-servos/. Applimotion looks promising in particular. They are specialized in torque motor. In one occasion I read that they provided one of their surplus to a RC model plane player in very reasonable price.

 

Ed.







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