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Classic worm-wheel gear or Direct Drive MOUNT?

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#1 Leonardo Priami

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Posted 12 October 2019 - 12:50 PM

Hi to all,
I would like to have feedback from you about direct drive mounts experiences in terms of  advantages and disadvantages with respect classical worm wheel geared, and in terms of what characteristics should be implemented or improved in direct drive mounts. What are the critical issues on direct drive mount that you would like a producer should resolve and what are the reasons you would choice one type respect the other one.

Very thanks for your comments!!

 

Regards,

Leonardo


Edited by Leonardo Priami, 12 October 2019 - 12:50 PM.


#2 Whichwayisnorth

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Posted 12 October 2019 - 01:22 PM

A good direct drive mount is the way to go.  Essentially zero periodic error, no belts to deal with and zero back lash of any kind.

 

So if you can afford a good one, I'd go that route.

 

http://planewave.com...t/#.XaIZrWZlAuU



#3 OldManSky

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Posted 12 October 2019 - 11:25 PM

Leonardo,

Thanks for asking.

I've owned close to a dozen equatorial mounts.  From cheap entry-level ones to AstroPhysics top-level ones.  All have been worm drive.  Some worked wonderfully, some were horrible -- most were adequate, and with tuning and adjustments could be made better.

 

I really like the *idea* of direct-drive mounts.  It's the price that is the problem.  My main mount now is a tiny little iOptron CEM25P.  It's a stepper-belt drive-worm mount.  It's got less than 5" PE that's nice and smooth, and is easily guided out.  And it was under $900.  

Get a direct-drive mount -- even with a limited payload of 25-35 pounds -- anywhere near that price point (even double it!), and you'll have a real winner on your hands.  



#4 Arie

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Posted 13 October 2019 - 11:06 AM

Talk to a ASA mount user and you'll pull your chequebook.


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#5 einarin

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Posted 13 October 2019 - 11:22 AM

Too bad DDM100 is now the 'entry level' mount from ASA. DDM60 or DDM85 no longer available.


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#6 Leonardo Priami

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Posted 13 October 2019 - 11:47 AM

Mmm...thanks for your experiences. May be my question need to be more articulated than proposed by me.

Let say we can consider at least two kind of mounts:

 

a) transportable mounts - for itinerant astroimagers

b) observatory mounts - for fixed backyard or remote observatory use

 

For transportable mount (weight <20/30 kg), probably we can asses that motors/electronic/development costs may  lead we ever can have a "small" Direct Drive at 1500/2000 USD. Moreover due to torque motor is strictly proportional to the rated current, also power supply maybe it will always be a problem. For these reason, in my opinion, for small transportable mount it will be hard for DDM to beat more economical worm-wheel mounts. Even if high quality traditional mount still show moderate high costs as Paramount MyT or Mach2 that should be near to a Direct Drive mount.

 

For observatory mounts, the considerations are different. In this field the battle is more interesting.

Tell me somethings about...brake problems, balancing problems, other issues that you have and that have conditioned your experience.

Thanks



#7 Leonardo Priami

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Posted 13 October 2019 - 12:40 PM

Talk to a ASA mount user and you'll pull your chequebook.

Because do you think that most of them are happy?



#8 Waldemar

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Posted 13 October 2019 - 01:18 PM

Because do you think that most of them are happy?

I am an ASA addict for about 9 years now. I sinserely believe it is the best mount around, having used a number of worm driven mounts. But... the price is outrageous, information is not the strongest point, software is different from any other and the ASA company is not really ready for the amateur market I think. Personally I never had any problems with the mounts (I had a DDM60Pro for 7 years and now own a DDM85 Styandard since 2017) nor the company.

Like is said before, it is a pitty they stopped production of the 'smaller' mounts, the DDM60 (pro) and the DDM85, and now the lightest mount available is the DDM100: a 100kg (220 Lbs) photographic payload beast. 

The advantages of direct drive: NO PE, NO backlash, NO wear and tear (because of no drivetrain) all come together in these incredible pieces of technique. 
As you can read, I am more than happy...
 


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#9 WadeH237

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Posted 13 October 2019 - 07:42 PM

One physical characteristic of direct drive mounts that's not mentioned here is that when they are unpowered, they can move pretty freely.  Normally, they are well balanced, so it's not an issue.  But if they are out of balance, they will move (potentially quickly) when the power is turned off.

 

And regarding price, I think that the reason that they tend to be so expensive is that they require high precision encoders to function.  As such, I doubt that they would get lots cheaper as they get smaller.  The encoders are expensive, large or small.


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#10 EFT

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Posted 14 October 2019 - 12:08 AM

People think that worm drives are not good because they are used in every poorly designed or poorly manufactured mount out there.  When made with high precision and placed in mounts made with high precision, they are an excellent way to drive a mount while being simplistic in their design.  Look at the top three premium manufacturers out their and you will see that they all use worm drives in their average-sized mounts (some use direct drives when they get up into the 1 meter or so range.  When coupled with high-precision axis encoders, you aren't going to really get anymore accurate and precise than these mounts in regards to exceeding the useful precision of a telescope mount in the amateur market.  The drawbacks are primary related to potential maintenance and slew speed which is probably limited to somewhere around 20 degree/sec. as opposed to the ridiculously high slew speeds of direct drives. 

 

Direct drives are kind of a dream drive system but they have inherent drawbacks the most significant is the price.  The torque motor drives are very expensive, the high precision encoders are very expensive, and most of the mounts available are made in even smaller quantities than other premium mounts which also makes them expensive.  There are other issues like the freewheeling when not under power.  That's not a particularly difficult issue to address and is addressed in most machines that use torque motors, but I know of only one company that has bothered to address the issue in a telescope mount.  Nonetheless, the chances of loosing power when slewing at high speed are reasonably low and a lot of effort can be put in to try to balance the mount as well as possible so that it does not quickly rotate to a different position if power is lost.  You do have to remember that if one of these unlike events does occur, the likely weight of the system and the associated momentum when it looses power will hurt you.  In other words, don't sit in its way. 

 

When it comes to the question of which type of drive system is better, there is really no significant mechanical advantages either way as long as the systems are well made.  The advantages are primary the cost and the different level of user friendliness which are variable throughout all of the premium mounts regardless of the drive system.


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#11 Waldemar

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Posted 14 October 2019 - 01:45 AM

One physical characteristic of direct drive mounts that's not mentioned here is that when they are unpowered, they can move pretty freely.  Normally, they are well balanced, so it's not an issue.  But if they are out of balance, they will move (potentially quickly) when the power is turned off.

 

And regarding price, I think that the reason that they tend to be so expensive is that they require high precision encoders to function.  As such, I doubt that they would get lots cheaper as they get smaller.  The encoders are expensive, large or small.

Both remarks are valid, but the balance needs to be near perfect with ASA mounts, otherwise they start using more power to keep the scope in place, which in excessive cases will cause a stop. There is a kind of 'magnetic residu', that helps to keep the mount in place as well as a very light bearing friction. I never had any issues with this. I think, they could have solved the potential problem, by making a sort of electro-magnetic anti-brake system so that if a power failure happens, it would stop the mount from moving. 

 

And yes indeed, the price is very high, but I think they are worth every penny.

The software needs a bit more user attention than most, but does not present a huge problem.

 

I notice a tendency at ASA to make bigger mounts, not smaller... 



#12 Leonardo Priami

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Posted 14 October 2019 - 04:23 AM

Summarizing at this moment we can say that for Direct Drive mounts the major issues are:
a) Critical balancing procedure
b) Unpowered mount behavior - related to point a)
c) Companies tends cease production of small mounts
d) High costs

 

My opinions and my questions:
a) I do not have experience on that, someone has said the ASA implemented an effective guided software balancing procedure. Does it work well? How do the other brands?
b) We know if all is perfect, no slip will occur during unpowered time. But how much unbalance the frictions can handle to hold the system motionless? And what happen when power-off heppen during a slew?It make sense for you implement a real magnetic brake?
c)We know that to produce a small mount it takes more or less the same money than to produce a big one. So it is clear that small mount generate much lower gains. It is also true that more small ones can be sold than large ones. But this probably does not compensate.
d) Yes DDM have high cost. It is also true that high-end  worm-wheel mounts equipped with encoder, costs close to a direct drive mounts with same load capacity.



#13 Waldemar

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Posted 14 October 2019 - 06:38 AM

Summarizing at this moment we can say that for Direct Drive mounts the major issues are:
a) Critical balancing procedure
b) Unpowered mount behavior - related to point a)
c) Companies tends cease production of small mounts
d) High costs

 

My opinions and my questions:
a) I do not have experience on that, someone has said the ASA implemented an effective guided software balancing procedure. Does it work well? How do the other brands?
b) We know if all is perfect, no slip will occur during unpowered time. But how much unbalance the frictions can handle to hold the system motionless? And what happen when power-off heppen during a slew?It make sense for you implement a real magnetic brake?
c)We know that to produce a small mount it takes more or less the same money than to produce a big one. So it is clear that small mount generate much lower gains. It is also true that more small ones can be sold than large ones. But this probably does not compensate.
d) Yes DDM have high cost. It is also true that high-end  worm-wheel mounts equipped with encoder, costs close to a direct drive mounts with same load capacity.

Since ASA does not produce the 'littler' mounts like DDM60 and DDM85 anymore, and I have no experience with the new series,
I think it is not in my nor anybody elses interest to go into this deeper.



#14 Wildetelescope

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Posted 14 October 2019 - 08:29 AM

People think that worm drives are not good because they are used in every poorly designed or poorly manufactured mount out there. When made with high precision and placed in mounts made with high precision, they are an excellent way to drive a mount while being simplistic in their design. Look at the top three premium manufacturers out their and you will see that they all use worm drives in their average-sized mounts (some use direct drives when they get up into the 1 meter or so range. When coupled with high-precision axis encoders, you aren't going to really get anymore accurate and precise than these mounts in regards to exceeding the useful precision of a telescope mount in the amateur market. The drawbacks are primary related to potential maintenance and slew speed which is probably limited to somewhere around 20 degree/sec. as opposed to the ridiculously high slew speeds of direct drives.

Direct drives are kind of a dream drive system but they have inherent drawbacks the most significant is the price. The torque motor drives are very expensive, the high precision encoders are very expensive, and most of the mounts available are made in even smaller quantities than other premium mounts which also makes them expensive. There are other issues like the freewheeling when not under power. That's not a particularly difficult issue to address and is addressed in most machines that use torque motors, but I know of only one company that has bothered to address the issue in a telescope mount. Nonetheless, the chances of loosing power when slewing at high speed are reasonably low and a lot of effort can be put in to try to balance the mount as well as possible so that it does not quickly rotate to a different position if power is lost. You do have to remember that if one of these unlike events does occur, the likely weight of the system and the associated momentum when it looses power will hurt you. In other words, don't sit in its way.

When it comes to the question of which type of drive system is better, there is really no significant mechanical advantages either way as long as the systems are well made. The advantages are primary the cost and the different level of user friendliness which are variable throughout all of the premium mounts regardless of the drive system.


YES!!! In industry, direct drive robots gave a mandatory perimeter that people are required to stay out of. Thank you for pointing out this safety issue.

Jmd

#15 Leonardo Priami

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Posted 14 October 2019 - 08:53 AM

Sorry, in my posts DDM means "Direct Drive Mount", not only ASA series mount, but in general speaking.



#16 psandelle

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Posted 14 October 2019 - 09:32 AM

Leonardo - are your questions academic, or are you thinking of building a new mount that's direct drive? grin.gif

 

Paul


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#17 Jared

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Posted 14 October 2019 - 10:02 AM

Both direct drive mounts and worm/wheel mounts can give excellent results. Direct drive has the advantage of no periodic error. However, a good worm drive will have periodic error that is low enough and smooth enough to be guided out effectively. I believe all currently produced direct drive mounts include high precision encoders. If you compare like for like quality levels on mounts with absolute encoders, both approaches work extremely well. Both will have essentially zero periodic error. Both will be without meaningful backlash. Both will allow unguided exposures with a good pointing model. In short, for permanent installations the mount is going to just fade into the background and work. The direct drive mount will be quieter and will slew faster, but that won’t affect the quality of the views or images.

If you are talking about something more portable that you will setup for the night, unguided imaging is not likely to be a major goal. You simply lose too much time building up a good pointing model. So you are likely to be guiding with any portable setup. In that case, again, you just need good mechanical quality and either drive approach can be very effective.

Personally, I wouldn’t choose a mount based on direct drive vs worm and wheel. I would go on quality of the manufacturer, weight/capacity ratio, cost, and availability of support. For example, I would want the mount manufacturer to be in my country/economic zone so I could ship back and forth as required without worrying about customs or long delays. For a transportable mount I would want a mount whose real carrying capacity was greater than the weight of the mount. I would want a manufacturer with a reputation for well engineered products. And I would want the least expensive solution that meets all the above requirements and had the capacity I needed. I wouldn’t care between worm, belt, and direct drives. Other aspects are much more important.
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#18 psandelle

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Posted 14 October 2019 - 10:45 AM

 

If you are talking about something more portable that you will setup for the night, unguided imaging is not likely to be a major goal. You simply lose too much time building up a good pointing model. 

Like the post, but wanted to point out that this one point isn't quite correct (and I hear people say it a lot). I go strictly mobile and unguided when I have 10Micron mounts, and it only takes 20 or so minutes to do a 45 point model, and that's done waaaaaay before the end of astronomical twilight (I do three 5-point models for PA, focus, do flats, as well and still have time to spare). People I've talked to with ASA mounts, the same.

 

Paul


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#19 Raginar

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Posted 14 October 2019 - 07:49 PM

Checks with my experiences with 10u. Super simple to get everything running before dark.
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#20 555aaa

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Posted Yesterday, 01:26 AM

Worm gear mounts, even premium mounts, have backlash, because you always have some tiny gap between the trailing edge profile of the worm wheel tooth relative to the trailing edge of the worm thread. If these were always in contact then the worm could not turn if there was any dimensional error at all.  You can't get out of that without using a split wheel or a deformable worm or a torque preload motor which none of the "premium" mounts have. Larger observatory mounts often use a large DC torque motor to create preload against the worm for exactly this reason. Direct drive and a couple other drive designs eliminate backlash. In a worm gear, the stiffness of the drive is ultimately limited by the gear tooth profile and width, but a direct drive mount has "active" stiffness. Encoders on a worm gear driven mount only partially improve the situation because while they eventually provide backlash compensation, there is a time lag between when a disturbance force occurs and when it is possible for the mount to react, if the disturbance is reversing load. The drive system has to see the upset, then reverse the motor, and in doing so unwind all of the backlash, and then start the compensating motion. In direct drive the compensating motion is happening all the time at the same rate as the servo loop is closed.

 

The plot below shows what a direct drive can achieve in terms of dynamic performance (during slewing no less) with a reversing load.

 

In the plot, the blue line is the commanded profile which is a rate-limited trapezoid which is why it has rounded corners. The vertical axis on the left side is encoder tics for a 26 bit encoder and the right side, which corresponds to the green line, is the position tracking error in tics. There's about 52 tics per arc second. The profile produces about 13 degrees of movement in 1.7 seconds, and reverses, and repeats continuously. The mount shows that it only has 15 arc seconds of following error when the direction reverses, and that is only for a small fraction of a second; the rest of the time the tracking error (this is while slewing mind you) in the arc second range.  You can't achieve that kind of performance except with a direct drive mount.

 

 

 

following_error_performance2.png


Edited by 555aaa, Yesterday, 01:31 AM.


#21 Leonardo Priami

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Posted Yesterday, 07:52 AM

 

In the plot, the blue line is the commanded profile which is a rate-limited trapezoid which is why it has rounded corners. The vertical axis on the left side is encoder tics for a 26 bit encoder and the right side, which corresponds to the green line, is the position tracking error in tics. There's about 52 tics per arc second. The profile produces about 13 degrees of movement in 1.7 seconds, and reverses, and repeats continuously. 

 

 

attachicon.gif following_error_performance2.png

Very interest aspect. Please send a better resolution plot. Thanks!



#22 EFT

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Posted Yesterday, 11:47 AM

There comes a point at which the increasing level of performance becomes irrelevant.  A worm-drive mount with axis mounted encoders can exceed that point.  While there are technically precision benefits to a torque motor drive and there are certainly some applications where the difference might be significant, the only really significant advantages of torque motor drives are torque and speed.  The high speed of a torque motor drive with are large (e.g., 24-inch or larger) scope mount can be good for survey work, but it makes little difference when it comes to smaller amateur mounts.  The settling time for the system will exceed the slewing time by a significant margin.  The torque advantage is essentially meaningless when it comes to a reasonably well-balanced telescope mount in the amateur size range.  It is one thing to consider the application of these drives in high-precision devices located in more controlled conditions with positioning that is not based on something that is significantly influenced by seeing, refraction, etc. or in applications involving the movement are much larger machines.  It is another thing to consider their use in amateur telescope mounts.  On paper, a torque motor drive has benefits over a worm drive but in application to amateur telescope mounts, the benefits are of far less, if any, importance.

 

There is a nice little comparison of mechanical drives (e.g., worm drives) and torque motor drives here: https://www.machined...motors-do-trick


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#23 Leonardo Priami

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Posted Today, 04:10 AM

Hi,

I agree, a high-end worm-wheel mount with high resolution encoders can give similar performances to a Direct Drive mount, not in terms of slew speed, but similar in terms of guiding precision and in the most cases in amateur use.
But, I want underline another aspect that make very different the two approach. A worm-wheel mount is more mechanically complex, reducers, gearboxs, springs, clutches for balancing, bearings, etc. All these mechanisms need to be perfectly sized, perfectly realized, perfectly assembled. For larger mounts, all these mechanisms need to be correctly sized with respect to the payload rating.
In a direct drive mount the system is extremely simplified mechanically speaking and there are only three most aspects: direct drive motors and its performances in terms of continuous torque (and relative dimensions, weight, currents, etc), bearing system, electronics (including the closed control loop, encoders, and so on).
In my opinion this simplification make, direct drive mounts, intrinsically more reliable and with more constant and repeatable performance, less dependent on other external factors - i.e. great temperature changes -.




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