Harmonic Drives and Star Tracker V1
Recently, the YouTube Algorithm recommended me this video on Harmonic Drives or Strain Wave gearing. Without going into too much detail, they’re a neat gearing system that uses a wave generator to force a flexible spline (flexspline) to engage the internal teeth of two circular splines that differ by two teeth. Through rotating the wave generator, the flexspline changes where it engages with the circular splines causing one of them to move slowly in the opposite direction to the wave generator. This means that between the wave generator and the moving circular spline, we have a large gear reduction in a very small space. This type of gearing has all sorts of benefits such as no backlash and high torque. For more detail, watch the video below.
At the end of this video, they explain that making harmonic drives through 3D printing would be a difficult challenge as a flexspline would require flexible 3D prints. I took this as a personal challenge.
My first attempts were to use flexible filament to create the flexspline. However, these attempts (red prints) went poorly as the filament was difficult to print with. The filament was not able to form the shape and often broke apart post print.
I soon gave up using flexible filament and went with regular PLA. These attempts went much better. The filament formed the correct shape and was flexible enough to be used as a flexspline. However, these too, were prone to breaking.
I noticed that the breaking was due to the way the print was being sliced. The layer seams were aligned and created a weak point in the flexspline. By randomising the seam position, it was possible to remove this weak point.
The video shows the first working harmonic drive I constructed. It used a stepper motor to drive the wave generator and had bearings to ensure smooth contact. It achieved a 1:25 gear reduction but was prone to skipping due to having shallow teeth.
In later versions of the design, I increased the number of teeth to create a 1:50 gear reduction and had deeper teeth to prevent gear skipping.
Star Tracker Version 1
When I realised how much of a gear reduction was possible from such a small design, I was reminded of a star tracker I had previously tried to build. A star tracker is a device that moves a camera such that its motion matches the motion of the stars. This allows for longer exposures of the stars and lets you capture more detailed images of the night sky. Since the stars move very slowly, you need a very slow-moving motor to mount the camera on.
I previously built a star tracker by 3D printing a series of gears that had a 1:3 ratio. Combined the gears had a 1:27 gear reduction. This mechanism was very bulky and had numerous issues with it. It had a lot of backlash and was not very sturdy. I realised that I could redesign the star tracker using a harmonic drive at its core and achieve a smaller and more robust solution.
The result was a harmonic drive with a 1:50 gear reduction driven by a stepper that was micro stepping every 540ms. This meant that in a day it would make one full revolution. At the end of the drive was a ball mount to mount the camera. The whole mechanism was mounted on a tripod which was aligned to the celestial pole using a phone app. Below shows the images that I captured using the star tracker.
This tracker was not perfect and though some shots were amazing, if the alignment was off by a bit, the stars would form streaks. Using the tripod to align to the celestial pole wasn’t the best method as the precision wasn’t enough to reliably align to the pole each time. I plan to create another tracker that has improved alignment abilities.
