What do the ZWO AM5, the iOptron HEM27 telescope mounts and the wheels of the Apollo Lunar Rover have in common?
They’re all driven by a Harmonic Drive.
There’s been a lot of excitement about the recent announcement of affordable telescope mounts using Harmonic Drives.
These mounts offer some features that make them of great interest to observers and astrophotographers. First of all, they’re extremely lightweight compared to the load they can carry. The Harmonic Drive mechanism is inherently free from backlash for better drive accuracy.
The two Harmonic Drive mounts BINTEL are initially supplying are the ZWO AM5 and the iOptron HEM27.
A “Harmonic Drive” sounds like something Elon himself might build into his latest amazing new gizmo. But….
Harmonic Drives or Strain Wave Gears are a well proven mechanical design offering some advantages to normal gear drives that make them especially handy for astronomers. First patented in 1955, they’ve been used in a wide range of complex engineering applications in aerospace, robotics, Apollo Lunar Rovers and other precision equipment.
Telescope mounts normally have some form of gears to reduce the speed from the faster rotating electric motors to the more slowly revolving telescope axes. The motors in the mount will speed up to slew the telescope across the sky to another location or slow down dramatically to track an astro object across the sky as the Earth turns. Doing this, a mount will complete a revolution in 24 hours, or about half the speed of the hour hand on your watch.
As you probably know, traditional mechanical gear drives use a disk with grooves or teeth cut into them. As the gears turn the teeth will engage with other gears (or a gear with a long continuous groove called a worm gear.) These can change the speed, torque or direction of the output from the electric motor. One problem with gears is the gaps between the teeth the produces “backlash” resulting in a back and forward movement referred to as “play”.
Gears cannot have teeth that are exactly the same size as the gaps in the gear they’re driving otherwise they’d jam up and not be able to move. There needs to be a small gap between the either sides of the teeth to allow for the transfer of movement between them. Even small variation in the gaps of a gear’s teeth can affect the smoothness and accuracy of the movement of a device. The tiny differences in the spacing between gears might even show up in the tracking of a telescope mount during a long photographic exposure and could be one source of “periodic error” which needs to be taken into account and corrected.
There’s a few different ways to address the issue of backlash or play in a gear driven system. One is to make gears with higher manufacturing tolerances. In other words, carefully make sure the gaps between all the gear’s teeth are as close to the same as possible, while keeping the gaps in the gears it drives against small. Machining high precision gears is time consuming and expensive. Adjusting the spacing between the gears will reduce backlash but too bringing the gears too close together will increase the friction between the point of them binding together. In other words, the system will have zero backlash, zero play because it can’t move.
Another way is to replace part of the drivetrain with flexible belts, often made of rubber with rubber teeth. Belt drive telescope mounts can have reduced vibration and backlash. Some users even retro-fit belt drive kits to their mounts to help with drive accuracy. One downside to belt drives is the meshing of rubber belts with metal gears creates more friction than metal to metal. This results in a loss of carrying capacity of the mount or an increase in the size of the mechanism. It may also require more maintenance over time.
A Harmonic Drive uses a unique approach to solve some of the issues with conventional gear drives. In most gear drive systems, the shape of each component remains the same. (Belts or chain drives might change as they flex, but the shape they trace out stays the same.)
There’s three main components to a Harmonic Drive. A toothed outer gear, an elliptical inner drive and a toothed, flexible though rigid spline gear. Rather than these two gears constantly meshing directly at the same point, the spline gear is pushed into an elliptical shape by the inner drive shaft and this pushes the two toothed gears together and drives the outer gear in the direction of the “waveform” produced by this action.
This clever drive mechanism has some remarkable advantages compared to gears directly driving each other.
First of all, as one side of the flexible gear spline is engaged in one direction, the other side is engaged against the outer gear’s teeth bracing against it. There’s no movement back and forward and this results in zero backlash. There means there is no play or excess movement in a Harmonic Drive and this will dramatically reduce the period error found in many telescope mounts.
The accuracy of Harmonic Drive over long periods of time makes them a great match for the precision requirements of telescope mounts.
The two toothed sections of a Harmonic Drive can have a remarkable reduction in the ratio of the inner and outer gears in an extremely compact space. This leaves more room for a powerful motor and one of the reasons Harmonic Drive telescope mounts are extremely compact and lightweight for the carrying capacity.
ZWO AM5 Harmonic Drive
Harmonic Drive mounts have been used in high-end observatory telescope mounts for some time because of these advantages. It’s only now they’re starting to seen prices comparable to traditional amateur astronomer telescope mounts.
Here’s the main reasons why telescope mounts like the ZWO AM5 and iOptron HEM27 are ideal choice for serious visual observers and astrophotographers –
First of all, harmonic drives are extremely compact. Harmonic Drive telescope mounts are smaller and lighter for their telescope, camera and accessory carrying capacity compared to traditional telescope mounts. Due to the extremely high torque their design inherently produces, even the need for a counterweight to balance the mount is reduced, although not totally eliminated.
Harmonic Drives have zero backlash and no play. Their tracking and accuracy will be smoother and more accurate then other current telescopes mounts designs.
Long Term Reliability. Harmonic Drives are known for their extremely reliability over long periods of use.
Downside? Yes – they a still slightly more expensive than traditional telescope mounts with the same load carrying capacity.