Drrect drives and harmonic drive mount retrofits

[skybadger]

Hi all 

has anyone got any experience of fitting direct drive motors to a telescope mount, possibly replacing the worm wheel with a different drive train ?

I'm looking at the catalogues and trying to work out what the likely cost and engineering effort mighth be and talking to someone who has already pursued this would be helpful. 

I am looking into converting my AE type B mount into a gearless mount driven from a direct drive motor on the botto of the RA shaft or belt driven at the top.

The payload of the mount is typically around 30Kg and so I reckon a output of about 15Nm will do the trick, although higher for direct drive. 

I also read that direct drive with inbuilt encoder can do 13M ticks per rotation but the spec says the positioning precision is 90" - what does this mean ?

 

Any thoughts ?

Mike

 

[hughgilhespie]

44 minutes ago, skybadger said:

I also read that direct drive with inbuilt encoder can do 13M ticks per rotation but the spec says the positioning precision is 90" - what does this mean ?

Hi Mike,

A link would help.

Regards, Hugh

[Gina]

I can only hazard a guess as I have no experience of direct drives.  If this is the spec for the mount as would seem logical for a direct drive mount then this is nowhere near accurate enough if this is the tracking precision.  90" means 90 seconds of arc or 1.5 minutes of arc.  Seeing in the UK is around 2 seconds of arc and this would be roughly the maximum resolution for imaging.  ie. 2" would be the effective pixel size.  Star alignment can cope with say 10 pixel error but I wouldn't like it to be much worse or you'll loose too much of the image round the edge.

You need tracking/guiding to be accurate to a few seconds of arc or you will get star trailing in the images.  It seems to me that the accuracy specified is nowhere near good enough for imaging and is far worse than conventional mounts with worm or belt drive.

Edited February 21, 2017 by Gina

[skybadger]

Hi Gina. Fair enough but I assumed most of that, which is where the question is coming from. 13M ticks is a small fraction of an arc second per tick, so maybe it's a case of precision not resolution. 

Hi Hugh. I was looking at the www.harmonicdrive.net product selector pages and also some motors on the bay from FHA. Assuming you behave as if the harmonic gear head is an exceedingly stiff planetary gear, typical ratios are still only 100:1, which means yet more gearing is required.

So still looking for that understanding.

Mike

[Vroobel]

Hi,

I just began building a heavier EQ fork mount for my 10" Bresser Messier. I own used 100:1 harmonic drive... 

...which will be driven by Nema 23 stepper motor equipped with 25:1 worm gear (picture attached). Even without using micro steps mode I can obtain 500,000 steps per revolution. Simple use of 16 micro steps mode gives 8m steps in total, that is much more than needed for an astro-photo. 

 

Here is its live story:

 

Tomasz.

I'd like to add, that the harmonic drive should easily hold a static load 50+ kg as it worked in 5-axes milling machine similar to a robot's arm. So the advantages are: zero backlash, high precision and high load. 

 

Edited April 11, 2021 by Vroobel

[Michele Scotti]

On 21/02/2017 at 07:53, skybadger said:

I also read that direct drive with inbuilt encoder can do 13M ticks per rotation

That's Planewave direct drive specification - 10 ticks/arcsec. Maybe it works with less but they kind of set the standard in case the DD is coupled directly to the axis.

[DroneSaif]

Following 

[whitfieldp]

Greetings all.

Encoders are a powerful tool for precision science/engineering - just a quick cautionary note though. The terms precision and accuracy are not interchangable - there are explanations using the analogy of darts/archery that are easy to find online. The number of ticks per revolution will give you the maximum resolution but real-life accuracy is never that good.

I know someone who built a bit of analytical equipment for certifying standards using Heidenhain RON905 encoders (well known for many years as excellent bits of kit). If you read the brochure linked on the page the positional error and system accuracy of different encoder models are frequently quite different. Much of the expense in a system goes into reducing the gap between theory and practice, the gap for the RON905 is very small.

https://www.heidenhain.co.uk/en_UK/products/angle-encoders/sealed-angle-encoders/ron-905/product/detail/229352-01/

Edited May 16, 2021 by whitfieldp
fix typo