Harmonic Drive Hoopla?


Roland Christen
 

In a message dated 12/30/2009 8:37:16 AM Central Standard Time,
uthin8er@... writes:


Is there any other advantage to harmonic drives other then no periodic
error? I've read some posts on other forums where the new technology is being
hailed as the end to gear driven systems and no need to guide. I don't buy
it. Aren't there other considerations to an imaging mount besides PE. Like
atmospheric refraction, polar misalignment etc..?
And what happens to you gear when you have an unexpected power failure?
Ejamacate me. :)
Harmonic drives do have tracking error, on the order of 10 arc seconds P-V.
The tracking or "periodic" error is not periodic because there is no worm
involved. It can be compensated for with PEC training to a point, but not to
the same sub-arc sec value that you can train a worm drive to.

Harmonic drives are gear drives, so there is no problem holding the scope
in place when the power is removed. I think you are mixing up harmonic drives
with Direct Drive systems. Direct Drive mountings have no gears, each axis
is attached directly to the motor's rotor. When the power is lost, the scope
can swing free and can crash into the pier if it is not perfectly balanced
in both axes. There is not as much torque in a direct drive system as in a
geared system, so the scope has to be very accurately balanced. They also
require a lot of power and usually cannot run directly off a 12 volt power
source. Direct Drive mountings use large ring encoders to produce accurate
sidereal drive rates, and can have sub-arc second tracking with no periodic
error.

Both Harmonic drive systems and Direct Drive systems are much more
expensive to make than worm gear drives. We are always looking towards these and any
other new servo technology to adapt to our mount product line. Right now
the only source of harmonic gearing is a single company headquartered in
Japan, and the gearsets that they make have a number of drawbacks for our
application. The cost for a single gear is very high, there is no stock readily
available - everything is custom made, with their system there is no easy way
to do thru-the-mount wiring because the gears have little or no clearance in
the centers.

With direct drive the problem of cost is even higher - the motors and
encoders are very expensive. The fact that the mount loses holding torque when
power is removed means that some sort of braking system must be employed,
which raises the cost even more. Yes, a direct drive mount can position the
scope to any part of the sky in a mere second or two, but is this really
necessary? The fact that it uses a ring encoder to provide essentially perfect
tracking is nice, but this can also be done with a worm gear by adding a ring
encoder to the gear. The question is always, what are you willing to spend to
get that last ounce of performance enhancement?

Rolando


uthin8er
 

Is there any other advantage to harmonic drives other then no periodic error? I've read some posts on other forums where the new technology is being hailed as the end to gear driven systems and no need to guide. I don't buy it. Aren't there other considerations to an imaging mount besides PE. Like atmospheric refraction, polar misalignment etc..?
And what happens to you gear when you have an unexpected power failure?
Ejamacate me. :)


AstroDad <ccurran@...>
 

They stay in "tune" longer... :)

--- In ap-gto@..., "uthin8er" <uthin8er@...> wrote:

Is there any other advantage to harmonic drives other then no periodic error? I've read some posts on other forums where the new technology is being hailed as the end to gear driven systems and no need to guide. I don't buy it. Aren't there other considerations to an imaging mount besides PE. Like atmospheric refraction, polar misalignment etc..?
And what happens to you gear when you have an unexpected power failure?
Ejamacate me. :)


AstroDad <ccurran@...>
 

And what happens to you gear when you have an unexpected power
failure?

Regardless of mount tech, use a UPS - no unexpected power failures,
*and* clean power to boot.

I started doing this years ago while camping. I kept tripping the
breaker at a local county park with a questionable power grid..

--
cheers & beers,
Chris


uthin8er
 

Yes, but UPS's fail too. And if you have a remote set up, the backup power is only going to last so long. I've heard that these systems will cause you rig to "crash" during a power failure. I'm not familiar with them, so I don't know if there is a clutch system or how they operate.
This could be untrue and a moot point.


Regardless of mount tech, use a UPS - no unexpected power failures,
*and* clean power to boot.


AstroDad <ccurran@...>
 

??? Nevermind...

--
cheers & beers,
Chris
--- In ap-gto@..., "uthin8er" <uthin8er@...> wrote:

Yes, but UPS's fail too. And if you have a remote set up, the backup
power is only going to last so long. I've heard that these systems will
cause you rig to "crash" during a power failure. I'm not familiar with
them, so I don't know if there is a clutch system or how they operate.
This could be untrue and a moot point.

Regardless of mount tech, use a UPS - no unexpected power failures,
*and* clean power to boot.


Charlie Miller <cmiller@...>
 

Harmonic drives have periodic error and backlash -- just not very much.

Harmonic drives use very complex gear trains to put a high level of
reduction into a small space and provide very accurate rotational
control. They are highly used in aviation and aerospace applications.
They are very expensive and only recently available in the small sizes
needed in the typical amateur sized telescope mount.

When the power fails they just stop. Unless your scope is massively out
of balance, it will not move.

Theory:

http://en.wikipedia.org/wiki/Harmonic_drive


Real world:

http://www.harmonicdrive.net/



Charlie Miller


uthin8er wrote:


Is there any other advantage to harmonic drives other then no periodic
error? I've read some posts on other forums where the new technology
is being hailed as the end to gear driven systems and no need to
guide. I don't buy it. Aren't there other considerations to an imaging
mount besides PE. Like atmospheric refraction, polar misalignment etc..?
And what happens to you gear when you have an unexpected power failure?
Ejamacate me. :)


Harold
 

The first 90% of performance cost the first 90% of the budget. The last 10% of performance cost the second 90% of the budget. Based on forty years of engineering development experience.

--- In ap-gto@..., chris1011@... wrote:

In a message dated 12/30/2009 8:37:16 AM Central Standard Time,
uthin8er@... writes:


Is there any other advantage to harmonic drives other then no periodic
error? I've read some posts on other forums where the new technology is being
hailed as the end to gear driven systems and no need to guide. I don't buy
it. Aren't there other considerations to an imaging mount besides PE. Like
atmospheric refraction, polar misalignment etc..?
And what happens to you gear when you have an unexpected power failure?
Ejamacate me. :)
Harmonic drives do have tracking error, on the order of 10 arc seconds P-V.
The tracking or "periodic" error is not periodic because there is no worm
involved. It can be compensated for with PEC training to a point, but not to
the same sub-arc sec value that you can train a worm drive to.

Harmonic drives are gear drives, so there is no problem holding the scope
in place when the power is removed. I think you are mixing up harmonic drives
with Direct Drive systems. Direct Drive mountings have no gears, each axis
is attached directly to the motor's rotor. When the power is lost, the scope
can swing free and can crash into the pier if it is not perfectly balanced
in both axes. There is not as much torque in a direct drive system as in a
geared system, so the scope has to be very accurately balanced. They also
require a lot of power and usually cannot run directly off a 12 volt power
source. Direct Drive mountings use large ring encoders to produce accurate
sidereal drive rates, and can have sub-arc second tracking with no periodic
error.

Both Harmonic drive systems and Direct Drive systems are much more
expensive to make than worm gear drives. We are always looking towards these and any
other new servo technology to adapt to our mount product line. Right now
the only source of harmonic gearing is a single company headquartered in
Japan, and the gearsets that they make have a number of drawbacks for our
application. The cost for a single gear is very high, there is no stock readily
available - everything is custom made, with their system there is no easy way
to do thru-the-mount wiring because the gears have little or no clearance in
the centers.

With direct drive the problem of cost is even higher - the motors and
encoders are very expensive. The fact that the mount loses holding torque when
power is removed means that some sort of braking system must be employed,
which raises the cost even more. Yes, a direct drive mount can position the
scope to any part of the sky in a mere second or two, but is this really
necessary? The fact that it uses a ring encoder to provide essentially perfect
tracking is nice, but this can also be done with a worm gear by adding a ring
encoder to the gear. The question is always, what are you willing to spend to
get that last ounce of performance enhancement?

Rolando


[Non-text portions of this message have been removed]


dannysperry
 

Hi Rolando,

Pardon me for butting in on this thread but it seemed to be taking a direction of "what can be improved or changed in AP mounts to make them better" so I thought I'd oblige myself and present a few suggestions. :-)

That said, I don't think AP mounts need to take any dramatic technological turns because they're beautiful mounts that perform wonderfully. Good old worms and drive gears are working fantastically for AP. For anyone who thinks worms and drive gears are old technology, they must be using poorly machined mounts with subpar tolerances.

Anyway, here are a few things that came to mind after using my AP900 for a few months now.

(1) Motor locations: The Mach 1 places the RA motor at the top of the RA assembly instead of off to the side and the DEC gear assembly is more snug against the DEC housing. These would be excellent design features to integrate into future 900s and 1200s since it makes the mount more compact and transportable. For those of us who are mobile it reduces the odds of the motors getting knocked around during transport. It also makes everything slightly more compact to get into those tight spots in the car or truck.

(2) Polar housing threads: The front polar scope cap and the rear threads for the polar scope are rather fine and can be easy to accidentally cross-thread in the cold and dark. Coarser threads would be handy.

(3) Polar scope position: When inserted, it sticks out quite a bit from the mount. Allowing it to be inserted further into the mount would be a bonus for us mobile users (we could more easily and safely keep it in place during transport).

(4) Through-the-mount cabling: Another great design feature of the Mach 1 that would be much appreciated in the 900s and 1200s.

(5) Hand controller cord: I'm sure this one is on the "not even remotely likely" list but it would be great to have a removeable/replaceable cord for the hand controller... for transport or to replace with a straight cord, etc.

I realize these aren't simple changes and that there may be technological and engineering implications I'm not aware of (not to mention the issues it presents to owners of older mounts and the availability of replacement parts down the line). These are just "gee, wouldn't that be nice" kind of things.

And to the group... please, don't let this be the start of a debate. These are just little things that popped into my little head. I'm not trying to start a campaign for change or get into any arguments over why my suggestion is lame or anything like that. :-)

Happy New Year to you and yours.

Best,
Danny
http://www.californiastars.net/

--- In ap-gto@..., chris1011@... wrote:


Harmonic drives do have tracking error, on the order of 10 arc seconds P-V.
The tracking or "periodic" error is not periodic because there is no worm
involved. It can be compensated for with PEC training to a point, but not to
the same sub-arc sec value that you can train a worm drive to.

Harmonic drives are gear drives, so there is no problem holding the scope
in place when the power is removed. I think you are mixing up harmonic drives
with Direct Drive systems. Direct Drive mountings have no gears, each axis
is attached directly to the motor's rotor. When the power is lost, the scope
can swing free and can crash into the pier if it is not perfectly balanced
in both axes. There is not as much torque in a direct drive system as in a
geared system, so the scope has to be very accurately balanced. They also
require a lot of power and usually cannot run directly off a 12 volt power
source. Direct Drive mountings use large ring encoders to produce accurate
sidereal drive rates, and can have sub-arc second tracking with no periodic
error.

Both Harmonic drive systems and Direct Drive systems are much more
expensive to make than worm gear drives. We are always looking towards these and any
other new servo technology to adapt to our mount product line. Right now
the only source of harmonic gearing is a single company headquartered in
Japan, and the gearsets that they make have a number of drawbacks for our
application. The cost for a single gear is very high, there is no stock readily
available - everything is custom made, with their system there is no easy way
to do thru-the-mount wiring because the gears have little or no clearance in
the centers.

With direct drive the problem of cost is even higher - the motors and
encoders are very expensive. The fact that the mount loses holding torque when
power is removed means that some sort of braking system must be employed,
which raises the cost even more. Yes, a direct drive mount can position the
scope to any part of the sky in a mere second or two, but is this really
necessary? The fact that it uses a ring encoder to provide essentially perfect
tracking is nice, but this can also be done with a worm gear by adding a ring
encoder to the gear. The question is always, what are you willing to spend to
get that last ounce of performance enhancement?

Rolando


[Non-text portions of this message have been removed]


Roland Christen
 

In a message dated 12/31/2009 12:09:45 AM Central Standard Time,
danny@... writes:


(1) Motor locations: The Mach 1 places the RA motor at the top of the RA
assembly instead of off to the side and the DEC gear assembly is more snug
against the DEC housing. These would be excellent design features to
integrate into future 900s and 1200s since it makes the mount more compact and
transportable. For those of us who are mobile it reduces the odds of the
motors getting knocked around during transport. It also makes everything
slightly more compact to get into those tight spots in the car or truck.

(2) Polar housing threads: The front polar scope cap and the rear threads
for the polar scope are rather fine and can be easy to accidentally
cross-thread in the cold and dark. Coarser threads would be handy.

(3) Polar scope position: When inserted, it sticks out quite a bit from
the mount. Allowing it to be inserted further into the mount would be a bonus
for us mobile users (we could more easily and safely keep it in place
during transport).

(4) Through-the-mount cabling: Another great design feature of the Mach 1
that would be much appreciated in the 900s and 1200s.

(5) Hand controller cord: I'm sure this one is on the "not even remotely
likely" list but it would be great to have a removeable/replaceable cord for
the hand controller... for transport or to replace with a straight cord,
etc.

All 5 of your suggestions are great. We will consider them for sure. I
don't know about #5 since we don't make the hand controller, but it' doesn't
hurt to ask.

Rolando