I’m developing a new actuator for a legged robot I’m building, and I thought you guys would find it interesting. It uses the quasi-direct-drive actuation scheme seen in the MIT Cheetah. The Cheetah’s actuators have large pancake motors and single-stage, low-ratio planetary gearboxes. This combines the best aspects of traditional geared servos (high torque density and efficiency) and direct-drive motors (good force control and compliance), allowing the Cheetah to run at 14 miles per hour.
Compared to harmonic or cycloidal drives, quasi-direct-drive lets you do accurate force control since the friction and inertia in the gearbox is low. Compared to pure direct-drive, it gives far better torque for the same weight of actuator.
As an alternative to the expensive custom motors in the Cheetah, I’m using large off-the-shelf drone motors. The 8318 motor size produces enough torque and is relatively cheap. Hobbyking sells them rebranded as the Multistar 9235-100KV, which is what I’m using.
The current design: (The black parts will be printed in NylonX and the white ones in Alloy 910)
An earlier prototype with PLA gears (very noisy)
Here are the specs. I’ll take some real measurements once I get it put together.
- 28 Nm of torque (possibly higher for short bursts)
- 0.95 kg
- 6:1 gear ratio
- Under $150 in materials
I’m going to open-source the design when I’m finished, since it seems like it would be useful to people. As far as I know there aren’t any off-the-shelf or DIY actuators that are lightweight, high torque, and compliant.
Here’s a paper explaining the concept of quasi-direct-drive in more detail: