ZWO harmonic drive


Chad Gray
 

I had to look up what the heck Harmonic Drive was... interesting mechanics!

https://www.youtube.com/watch?v=7QidXf9pFYo&t=392s

Zero backlash is it's main feature it appears.

Reminds me of the wankle rotary engine.


kevin
 

I fancied one fore my lathe but the are expensive, even second hand, Several people on youtube has been 3d printing them, e.g https://www.youtube.com/watch?v=Emvo3bLT-Z4
and also  cycloidal drives, no quite zero backlash, which i didn't even now exiisted  https://www.youtube.com/watch?v=OsS9-FzKN6s


Derek Regan
 

Pegasus and sharp Star are joining the party too


Robert Benward
 

I was looking at (really just dreaming) harmonic drives in the mechanical industry magazines many years ago, when even a small one was in the multi thousands $$$.  Now ZWO is talking about $1999 for the whole thing!

Bob


Cloud You
 

The harmonic equatorial instrument was first made in Japan, but due to the high cost, there was no result in the end. Then South Korea. The first harmonic equatorial instrument in China is Ruixing optics and Mark III medium harmonic equatorial instrument, with a load of 18-28kg. The Am5 light harmonic equator instrument made by zwo is loaded within 20kg. The characteristic of harmonic reducer is that there is no return difference, which does not mean that there is no return difference in the whole machine, because there can be no harmonic in primary deceleration, but there must be secondary deceleration.


 

Strain wave gearing (marketed as harmonic drive) has been around for a while. Frankly, I don't understand people's obsession with it. It's zero backlash feature is main selling point, but there are some downsides, such as being exclusively axial by design or excessive vibrations when running at high speed due to input shaft mass distribution on elliptical gear that noone speaking about. For astronomy application these can be easily be ignored, but I would question it's input/output curve.

What is the point of having ZWO AM5 besides just saying that you have harmonic drive mount? It's advertised to have 40 arcsec periodic error (absolute). Is it worth having zero backlash and so much of PE?


Wayne Hilliard
 

I would say on the RA drive no, no advantage to Harmonic Drive. Once your plate solved and guiding the RA axis should never encounter backlash. Now the Dec axis is different. You can reverse the Dec drive depending on several factors but having zero backlash on the Dec I would see as a huge advantage.

On Sat, Jan 15, 2022 at 8:15 PM Vladimir <cyberianice@...> wrote:
Strain wave gearing (marketed as harmonic drive) has been around for a while. Frankly, I don't understand people's obsession with it. It's zero backlash feature is main selling point, but there are some downsides, such as being exclusively axial by design or excessive vibrations when running at high speed due to input shaft mass distribution on elliptical gear that noone speaking about. For astronomy application these can be easily be ignored, but I would question it's input/output curve.

What is the point of having ZWO AM5 besides just saying that you have harmonic drive mount? It's advertised to have 40 arcsec periodic error (absolute). Is it worth having zero backlash and so much of PE?


George Cushing
 

I leaned Vlad's way until I saw how Kevin Ma used such a drive to convert a Meade LX fork. The fork originally was equipped with a tangent arm Dec adjuster. I had been advising folks to avoid these old forks due to the difficulty of retrofitting a worm gear assembly. Kevin's solution made these mounts a viable choice.


bryan_ramsey_pterodyne
 

Having made several harmonic drive based mounts I can tell you that the upsides are: Way more efficiency than worms, which is why you can do away with counterweights (in most cases).  Zero backlash. And compactness.  Also the large periodic error is very long so it's easy to guide out. A properly designed PEC curve should completely eliminate it due to it's long and very repeatable nature

It has allowed me to get a large capacity in a very small package.  My issues are more of flexure, but that is my own engineering and design fault, nothing to do with harmonic drives.  I suspect ZWOs mount also will suffer some flexure do to it's small size vs capacity.

The main downside is expense.  Also, how many precision industrial robots are using worm drives?

Ive also made mounts with byers gears, so I have no opposition to the tried and true, but for compactness and torque, you'd be hard pressed to find something better.


 

On Sun, Jan 16, 2022 at 11:23 PM, bryan_ramsey_pterodyne wrote:
The main downside is expense.  Also, how many precision industrial robots are using worm drives?
I think precision industrial robot requirements are somewhat different from AP appliances so the two categories are hardly comparable and see no point. But, you're right about expences. In AP field, this technology is bleeding edge, premature and expensive. Once the building process is perfected and manufacturing expences are cut, it will become interesting. Let's get back to the topic which is not about harmonic drive in general, but about ZWO harmonic drive mount. Personally, I don't have first hand experiance with them, I just speak from what I seen in specs. I can't afford them, but even if I could, I think the expences are not justified. I know no man who would rather spent 2000 on ZWO AM5 than 1800 EUR on iOptron CEM60 or even SkyWatcher EQ6-R Pro.


bryan_ramsey_pterodyne
 

To each their own I guess.  I dont understand a lot of things people buy, but there must be a market for them as new ones are coming out.  I still maintain mounts like this are hard to beat for carrying capacity vs size/compactness. Ive had great luck with them.  There are a lot of tried and trueism in this hobby, and a lot of people doing their own thing in the face of it. Both have success and failures.  As far as I can see the cost of the ZWO is all harmonic drive.  Used prices are doable for HD sets. But no, I wouldn't buy one either. I have made 2 though :)


smith_barry
 

Having been following the harmonic drive issue for almost twenty years partly for 4th/5th axis CNC applications and to a lesser extent AP applications I would like to make the following points:

You can find bargain priced harmonic drives second hand either as an actuator, actuator with motor (dc servo, ac servo, stepper) or just the cups to fit into your own housing.  But like most things the second hand market fluctuates and some items are priced not to sell but just because some one else is asking a high price.  Its annoying and impacts on other items of interest like encoders and motor drive electronics.  You can also pick up used robotic arms as a source, for example I picked up a SCARA for £150 which includes 3 harmonic actuators / motors.

A lot of the early drive/motor setups were axial and quite long (partly due to the harmonic cup and use of dc servo motors) but they ranged in size from 1cm by 10cm for a gearbox/dc motor and encoder to 20cm by 30cm.  Its now increasingly common to see much shorter versions with AC servos but of course the drive electronics get more expensive, and there are increasing numbers of bare actuators out there that can be adopted to AP use especially with steppers and hence my interest in onstep.   Common gear ratios on auction sites range from 30:1 to 160:1 but you can get much higher ratios if you buy through the manufacturers as the likes of ZWO do.  Whilst some tend to be long they are small compared to some worm drive setups, I have one from Beacon Hill which is 275mm dia!

Like I think Bryan used, I have some 17-80 actuators which measure 85x40mm (1:80) ratio and looked on one of the far east sites last night for suitable stepper based gearboxes to drive these, to be surprised by a few things.  Firstly a ZWO AM5 for £250.00 (obviously an error) but more importantly a range of harmonic drives ranging from short integrated NEMA 17-Harmonic drives 1:30 ratio for <<$100 new and a wide range of other HDs priced competitively .  This perhaps explains why ZWO and others are starting to use harmonic drives in consumer based mounts.  I have bought some of the NEMA 17 drives so will let you know what they are like when they turn up.  I also now have some boards for the Max onstep 3.6 so can move forward on the electronics.
Clear skies
Barry