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New ZWO AM5 Harmonic Drive Mount

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590 replies to this topic

#576 OzAndrewJ

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Posted 24 June 2022 - 01:41 AM

Gday Clouzot

I'm pretty sure this has been discussed at length in the RST-135 thread

Part of it has.

The thought of finding a sweet spot and rotating the spline drive to match it is new and interesting.

 

Anyway, there was even a link to an academic paper analyzing, in the context of strain wave reducers, the PE dependency to actual load and angle.

Oooh do please find the paper

I have seen several papers on the topic, but in all cases, the test rigs had rigid axles on each side

then slotted in a harmonic drive.

This removes the longitudinal bending moment effects that come about when driving

a cantilevered load.

As i mentioned in the other thread, i can visualise the different loadings in the flex spline and cross bearing

but how it results in an error in PE and swash ( at the arcsec level ) is beyond me.

 

Andrew Johansen Melbourne Australia



#577 Clouzot

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Posted 24 June 2022 - 12:42 PM


Oooh do please find the paper

I have seen several papers on the topic, but in all cases, the test rigs had rigid axles on each side

then slotted in a harmonic drive.

This removes the longitudinal bending moment effects that come about when driving

a cantilevered load.

As i mentioned in the other thread, i can visualise the different loadings in the flex spline and cross bearing

but how it results in an error in PE and swash ( at the arcsec level ) is beyond me.

 

I like the idea of finding the sweet spot too.

Regarding the limited test apparatus: I think that's the case for most (if not all) test rigs, and probably because that's easier to test this way. I found a recent thesis that does a thorough FEM/BEM-based analysis at the mesh level, with no specific assumptions about load, but the author's goal is mainly about fatigue, not precision. Yague-Spaude, E. (2021). Computational Design, Simulation of Meshing, and Stress Analysis of Strain Wave Gear Drives. Rochester Institute of Technology. https://scholarworks...du/theses/10697

 

It seems that the reference paper about SWG errors is this one: Ghorbel, F. H., Gandhi, P. S., & Alpeter, F. (2001). On the kinematic error in harmonic drive gears. J. Mech. Des., 123(1), 90-97., 

https://doi.org/10.1115/1.1334379

At least it's one of the most cited papers in the field. Sadly I don't have access to this one journal (mechanical engineering was my major long ago, but I'm rather into signal processing these days).

 

 

Sorry ZWO's AM5 lovers for the slightly off topic post: at least you'll be able to know how your next precious mount performs, and why it does (or not). I'm glad to see this technology getting "momentum" in our field, it means that prices will drop, performance will improve and we'll perhaps see an entry-level harmonic mount sooner than later

 

 

EDIT: that was indeed the correct paper. In the RST-135 thread, see:

- lukasik original pointer to the paper: https://www.cloudyni...ount/?p=9490793

- astrojedi's comments about the paper and its implication regarding balancing: https://www.cloudyni...ount/?p=9495511

- the paper itself, available as a PDF from there: https://api.semantic...rpusID:15119268


Edited by Clouzot, 24 June 2022 - 03:49 PM.

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#578 OzAndrewJ

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Posted 24 June 2022 - 05:01 PM

Gday Clouzot

mechanical engineering was my major long ago,

Mine too, hence why these new drives are so interesting,

but the way they are designed, mounted and driven

makes them a far more complicated beast to understand.

Unlike a simple worm/wormwheel, which only has a few variables,

these units will probably have lots of side effects based on loading etc,

so understanding what to watch for is a bit harder.

Again time will tell, and each model will probably be different.

 

that was indeed the correct paper.

Yep, but it doesnt address how a highly unbalanced longitudinal load works,

which is what we have in the current mounts.

I see the recent thread from an RST135 user once more indicates

the DEC is precessing with the cam and at a measurable size.

As the drives have virtually no backlash, it can be guided out smoothly

but its a new thing to watch for with these types of drive.

Be interesting to see what the reconstituted guide logs for the AM5s look like

to see if it shows in them???

Again, just need lots more data

 

Andrew Johansen Melbourne Australia



#579 arbit

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Posted 24 June 2022 - 08:54 PM



Yep, but it doesnt address how a highly unbalanced longitudinal load works,
which is what we have in the current mounts.


Hi, not a mechanical engineer, so asking purely from a user point of view.

Is the "highly balanced longitudinal load" inherent to the design, or is it affected by how well balanced the OTA is (even if it cant be as precisely balanced as in a traditional mount )?





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#580 OzAndrewJ

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Posted 24 June 2022 - 09:35 PM

Gday Arbit

Is the "highly unbalanced longitudinal load" inherent to the design,

You missed the "un"

and yes, the way these mounts are designed ( that i have seen so far ) means the load on

the bearings and gearing is unbalanced.

Have attached a quick piccy of the general loading used in mounts

1) Is the traditional shaft between two rigid bearings with an RA head rigidly fixed on the end.

2) Is the newer CGEM/english yoke type design, where an axle is supported rigidly in bearings

    and the RA head is fixed rigidly between the bearings

 

In the above two, the longer the distance "L", the more rigidly the axle is held.

A wormwheel ( or other drive ) is then fixed to the axle so once more it is rigidly supported

and only has to deal with simple tangential loading and normal PE

 

3) Is how the Flexdrives "appear" to be mounted so far

 

All the loads are taken by the one rigid bearing and ( to a much lesser extent ) the flex spline.

The full weight of the RA head plus DEC head + OTA creates a bending moment "M"

and the bulk of this normally has to be taken by the bearing,

but i suspect some is also taken by the flex drive spline

The load taken by the flex spline will change as the cam rotates

and my suspicion is this is what creates swash in the DEC axis

in some mounts, at some times.

Andrew Johansen Melbourne Australia

RA Head supports.jpg

 



#581 Mike Sandy

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Posted 24 June 2022 - 10:30 PM

I’m thinking this topic is important enough that I will start a new topic that focuses on strain wave gearing/mounts and lets this AM5 topic resume the reports of new mount deliveries, complaints about delivery delays, and descriptions of performance results for the ZWO’s offering.  

 

There is a lot of stuff to consider with SWGs that simply is not brand specific, and things about the unique different designs that should be compared, discussed and examined openly.  If that doesn’t make sense to others - the topic will die out quickly.  I would hope that together we could unpack some of the myths that surround these drives, and create a useable resource for folks considering one of these mounts.


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#582 arbit

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Posted 24 June 2022 - 10:53 PM

Gday Arbit

You missed the "un"

and yes, the way these mounts are designed ( that i have seen so far ) means the load on

the bearings and gearing is unbalanced.

Have attached a quick piccy of the general loading used in mounts

1) Is the traditional shaft between two rigid bearings with an RA head rigidly fixed on the end.

2) Is the newer CGEM/english yoke type design, where an axle is supported rigidly in bearings

    and the RA head is fixed rigidly between the bearings

 

In the above two, the longer the distance "L", the more rigidly the axle is held.

A wormwheel ( or other drive ) is then fixed to the axle so once more it is rigidly supported

and only has to deal with simple tangential loading and normal PE

 

3) Is how the Flexdrives "appear" to be mounted so far

 

All the loads are taken by the one rigid bearing and ( to a much lesser extent ) the flex spline.

The full weight of the RA head plus DEC head + OTA creates a bending moment "M"

and the bulk of this normally has to be taken by the bearing,

but i suspect some is also taken by the flex drive spline

The load taken by the flex spline will change as the cam rotates

and my suspicion is this is what creates swash in the DEC axis

in some mounts, at some times.

Andrew Johansen Melbourne Australia

attachicon.gifRA Head supports.jpg

Hi Andrew,

 

No I didn't miss the "un", but I did mis-interpret it :-)

 

Thanks for the drawings - pretty clear now.



#583 arbit

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Posted 24 June 2022 - 10:56 PM

I’m thinking this topic is important enough that I will start a new topic that focuses on strain wave gearing/mounts and lets this AM5 topic resume the reports of new mount deliveries, complaints about delivery delays, and descriptions of performance results for the ZWO’s offering.  

 

There is a lot of stuff to consider with SWGs that simply is not brand specific, and things about the unique different designs that should be compared, discussed and examined openly.  If that doesn’t make sense to others - the topic will die out quickly.  I would hope that together we could unpack some of the myths that surround these drives, and create a useable resource for folks considering one of these mounts.

From a user point of view, that would certainly be very useful to understand how and why the traditional SOP should change.



#584 alphatripleplus

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Posted 25 June 2022 - 06:37 AM

I’m thinking this topic is important enough that I will start a new topic that focuses on strain wave gearing/mounts and lets this AM5 topic resume the reports of new mount deliveries, complaints about delivery delays, and descriptions of performance results for the ZWO’s offering.  

 

 

Great idea!



#585 RobertM

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Posted Yesterday, 11:33 AM

This is the PE report for one that arrived today together with a dark grey sky (as I was unboxing) and cloudburst.

 

52176562966_49de0f67e0_w.jpg

 

Initially I'm quite impressed with the fit and finish.  There is a loudish beep on startup but then it's very quiet.  It's good to see that the power button latches in making it observatory friendly.

 

Will have to see how it performs in the field to see whether it was the right thing to get.


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#586 calypsob

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Posted Yesterday, 07:42 PM

This is the PE report for one that arrived today together with a dark grey sky (as I was unboxing) and cloudburst.

52176562966_49de0f67e0_w.jpg

Initially I'm quite impressed with the fit and finish. There is a loudish beep on startup but then it's very quiet. It's good to see that the power button latches in making it observatory friendly.

Will have to see how it performs in the field to see whether it was the right thing to get.


I could live with the pe

#587 arbit

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Posted Yesterday, 11:01 PM

I could live with the pe

Yes. CEM26 is 20as p2p and CEM40 is 14as p2p. Couldn't find EQ specs but I suppose is same or more.

So a max periodic of 16as p2p is pretty decent. And for a lot of the cycle it's 10as p2p or less, which is even better! Hope all pieces are in a similar range though.

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#588 Mike Sandy

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Posted Today, 01:48 AM

Yes. CEM26 is 20as p2p and CEM40 is 14as p2p. Couldn't find EQ specs but I suppose is same or more.

So a max periodic of 16as p2p is pretty decent. And for a lot of the cycle it's 10as p2p or less, which is even better! Hope all pieces are in a similar range though.

Sent from my SM-S908E using Tapatalk

I don’t believe you will only ever use about 180 degrees of the full periodic error chart.  The mount never goes below to the lower half of the RA rotation when used as a GEM.  So maybe you get lucky and the part of the drive rotation you do use has the lower net error.  Looks like you have a keeper in any event.  Congratulations!



#589 Ballyhoo

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Posted Today, 03:16 AM

wow this could upend the mount market.    I could never get my mounts to work the way I wanted them to reliably until I spent hard money.  I wonder whether this changes all of that.  Yet the mount appears limited to about a C8 payload.   Very exciting to follow this development for sure. 



#590 akdwivedi

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Posted Today, 05:45 AM

Please post a review on CN. So far all the reviews are on FB and YouTube and some of them really look like the influencer was reading a script (so animated) that it doesn't look real at all.

A real field test, and your opinion will be highly appreciated. I am probably in august batch for delivery and hop zwo continues the same consistent level of quality of their mounts as yours.

#591 arbit

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Posted Today, 06:23 AM

I don’t believe you will only ever use about 180 degrees of the full periodic error chart.  The mount never goes below to the lower half of the RA rotation when used as a GEM.  So maybe you get lucky and the part of the drive rotation you do use has the lower net error.  Looks like you have a keeper in any event.  Congratulations!

Well, was kind of hoping you, Andrew and all the other engineers could figure that out on the other thread you started smile.gif


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