Why direct drive is the Future

[avandonk]

Without any comment check out this drive.

http://www.acrodyne.com.au/wp-content/uploads/2009/09/04_ddm_kdueng.pdf


Bert

[Bassnut]

Yeah, whopp de do, show me the money!. Direct drive is not "WOW the future", its old hat and its future is "depending" on quite a few things. For us astro plebs, its an end performance/cost ratio that has a long way to go before it becomes universal and default.

[ballaratdragons]

That is a ridiculously arrogant and rude response!

It is actually quite a stunning piece of equipment.

8.38 MILLION steps per revolution is not old hat.
And I'm sure there are a few people in here that would be happy to part with the dollars for accurate tolerances like this.

So Whoop de do that!

No wonder some people refuse to post interesting items when they get repsonses like Freds!

[MrB]

Agreed.

I was not aware such drives existed so thanks Bert.
Will be doing a bit more reading.

[Zaps]

Is that a direct drive system, such as the ASA DDM and Astelco NTM-500 type mounts, or a harmonic drive (with gears) like the Chronos mount? True direct drive systems do not use gears, and are therefore PE-less, so to speak.

The resistance to these new direct drive systems mostly comes from those who have invested heavily in the traditional (geared) type mounts.

[Shawn]

that would be 7 steps per arc sec yeah?, math never my strong point...

[DavidTrap]

Amazing that such technology still uses the RS-232 interface back to a computer...

DT

[AndrewJ]

Gotta semi agree with Fred here.
While it looks good, its a long way from useable ( for a mount )
Reading the tolerances etc
"after correction" its 10arcsec position accuracy, so the quoted encoder resolution doeasnt appear to directly be useable.
I cant tell from the specs but i'm pretty sure the encoder is just a std unit, ie its absolute position is "calculated" by keeping track of a quadrature pulsetrain, vs having a true absolute encoder.
Must be used indoors, no moisture, thermally stable environment.
Holding torque is pretty low and it would appear to lose alignment if bumped.
Dont get me wrong, I like the idea of direct drives or harmonic drives,
but a good worm drive with a precision encoder ( preferably absolute )
on the output shaft will still achieve the same end, for a lot lower cost.
IMHO, the biggest gains for astro work will come when precision encoders become cheaper.
And even then, this still doesnt address refraction, flexure, other misalignments downstream of the axle etc etc,
Andrew

[icytailmark]

whats the difference between this direct drive and the maxxor motors you can get with a losmandy mount?

[DavidU]

Very nice bit of engineering.I wonder if the company is still operating after the earthquake.

[Barrykgerdes]

Please don't take Fred too lightly. I know he likes to stir. but he does know what he is talking about.

Barry

[renormalised]

CDK700 (http://www.planewave.com/index.php?page=1&id0=6&id=0)

[ballaratdragons]

I'm not debating his knowledge on the subject, just the rudeness of his post.
It is not worded as a 'stir'. It is total lack of respect to a poster.

[Bassnut]

Sorry about that Ken, no offence intended.

[TrevorW]

Direct drive mounts are available as you Know but are still quite expensive and generally beyond the pockets of us poor amatuers

[renormalised]

That's the problem...but it's a matter of supply and demand, apart from anything technical.

[TrevorW]

True but each of the directs I've seen advertised are over $10k

ASA and Chronos

demand won't be an issue because of the price

look how well the EQ6 (in all it's guises) has done because it is a cheap mount that does what it does well for what you pay for

Personally I believe the Chinese are missing a complete market segment by not producing a good quality heavy duty mount around the $5k mark

If the EQ8 comes in at that price or thereabouts to be competitive then they'll be on a winner IMO

Sorry off track a bit

[OzRob]

You want to lend me the $185 000!!??....lol

Nice looking setup though!

[renormalised]

Put a lotto in for tonight....it's $20M :):)

[wasyoungonce]

About $25000 (http://www.optcorp.com/product.aspx?pid=10306).:D

Well to answer this a little better...these direct drive units use harmonic drive mechanism (http://www.optcorp.com/product.aspx?pid=1411-1429-1424-12275) that virtually eliminate backlash (they drive the axis directly ...no gear train) and also have very accurate encoders that can drive a scope (http://www.griffel.com/) (edit: keep the drive position accurate) so accurately that you probably don't need guiding (edit: auto guiding etc).

Well that's the principle. I guess they should work but they haven't yet reached mainstream ..yet. Probably due to high costs. More here (http://www.astrosysteme.at/eng/monts.html).

[renormalised]

That's the main problem...the cost.

[AndrewJ]

My original comment was referencing the original article,
not direct drives in general.
As others have posted, direct drives suitable for mounts are still
a tad expensive at present.
Again, the biggest step forwards here is in having encoders on
the output shaft, not the motor.
Having a good feedback loop tied to this is where the gains come from,
and even a std worm drive could probably achieve the specified positioning/driving tolerances with this sort of feedback.

Andrew

[ballaratdragons]

What it needs is some young smarty-brains to design and develop a home-made direct drive mount from bits in his shed, then post pics and info in Forums like this.

Then we all say "Build me one too please" :lol:

Actually the Belt-drive system available for the EQ6 mount is probably good enough and would cost a whole lot less!
It works well, with zero backlash.

But if they can get mount drives to 8.38 million steps within a reasonable price it would be lovely.

[ballaratdragons]

:thumbsup:

[AndrewJ]

Gday Ken


One ( at least ) already did, ( tho i dont know how good it really is )
http://www.mda-telescoop.com/index.php?option=com_frontpage&Itemid=1
IIRC there was a thread on this gizmo about a year back.
There was also a mob called NORTEK who did encoder mods
for Meades ( 14" and 16" ) to put absolute encoders on the output shafts.
Once you can get a true reading of the output shaft,
the drive mechanism,
ie direct drive, worm drive or water wheel powered clepsydra becomes irrelevant.
The clepsydra would probably have a prohibitively slow max slew speed tho:D

Andrew

[ballaratdragons]

Absolutely amazing!
I have never heard of this product before!
Thanks for the link.

EDIT: Hmmm . . . only available from Germany or Austria
http://www.meade.de/en/products/astro-specials/product.html?act=pdis&actid=0721061&cHash=6a9f0e3eca52d427ab49cfa6a871d 9e8
but for AU$355 it would be worth it.

[TrevorW]

You forgot the tracking unit required for an additional $1600

[ballaratdragons]

:eyepop: Yikes!

I didn't see any $1600 mentioned. I just saw the price of 249 Euros.

Here: http://www.meade.de/en/products/astro-specials/product.html?act=pdis&actid=0721061&cHash=6a9f0e3eca52d427ab49cfa6a871d 9e8

[avandonk]

With the new Gemini II coming out for Losmandy mounts I will keep an eye out for an older well loved Titan Mount.

I would then replace the servos and gearboxes with harmonic drives. These have encoded servos and with a 100:1 harmonic drive the number of encoder pulses or resolution is 800,000 per rev of the output shaft. This corresponds to 800,000 per single worm turn which is 1.333r degrees of mount movement. This is 21.6 million for a full 360 degrees of mount movement.

Of course a stepper motor could be used with a harmonic drive or 'gearbox' and with a ratio of 100:1 a 200step motor would have 20,000 steps per worm rev before microstepping.

Harmonic drives have no backlash. They do have a small PE but that would disappear when divided by the worm ratio.

There is no 'correct' solution for drives and mounts. It is only the best that is affordable to us amateurs. Advances in drive technology and computers puts previously undreamt of precision within reach.

Bert

[renormalised]

If it's as good as they say it is, I'd pay the price for the adapter and tracking module....$2000 would be worth it.

[terrynz]

AndrewJ, Very well written.

I looked into harmonic drive and direct drive when upgrading last year. In the end a Paramount ME was hard to beat. If I could have afford it, I would have brought this http://www.astelco.com/products/ntm/ntm.htm - NMT-500 direct drive. But it was a BIG unknown.

Fact is, for astroimaging and 95% of the other work I do, a rock solid GEM with "wheels and worms" was the way to go. Also, the majority of high end images out there use a bisque or AP mount and support from Daniel Bisque wonderful.

cheers
Terry

[renormalised]

That's just going with what you know is the tried and true method (despite the flaws). Fact is the professionals use this type of drive and it's proven to be more accurate than worm geared systems. The stock standard EQ system is so widespread because it is fairly accurate and it's easy to produce and (relatively) cheap. Harmonic and direct drive systems are dear at present, so aren't as widespread.

The big unknown comes from the fact of its price and not having any confidence in if it'll work (for you). The manufacturers have to bring the prices down to allow for these drives to become more popular.

[netwolf]

Ken,

The Sitech Goto system can do pretty much the same thing, all you need is a Hi-res encoder on the RA Shaft. This Hi-res encoder is the expensive part, the cheaper version is about 400USD. But the beauty is that the Goto system already has the brains to use this encoder to improve perforamce. You dont need to buy a secondary controller to achieve it.

But here is what you can achive with it on a typical G-11

https://sites.google.com/a/sitechservo.info/chuck-shaw-s-site/Home/g-11-gurley-mounting

1.41 arc seconds p-p (almost the Seeing conditions of the night).

I think for the near future the above type of systems using Hi-res encoder feedback can improve exisiting system performance. While we wait for Direct Drive motors to be realized in a more ecconomical package.

That being said, DIY Direct Drive is live and kicking. And it was done in shed by a braniac.

The Sitech is a also the system used by the Planewave mounts, and has been tested (with modifcation) to run Direct Drive motors. This particular motor in the bellow link is a "Home made" direct drive made by Davd Rowe (more well known for the design of the CDK optics).

http://www.siderealtechnology.com/DirectDrive/DirectDriveTest.html

More on the home made Direct drive motor by Dave Rowe.
http://starryridge.com/telescopes/index.php?title=Direct_Drive_Motors

Regards
Fahim

[ballaratdragons]

I'd be extremely happy to have a 10 Micron GM4000 QCI Mount :)

I never see anyone talk about 10 Micron Mounts.
http://www.10micron.com/