DIY actuator – Brandon Lai’s second attempt at designing an actuator for a humanoid robot aimed for 40–60 rpm and 20 Nm of torque for up to an hour, but testing on a limited benchtop power supply produced only 7 Nm. He also found excessive backlash in a cycloidal gearbox bui
Brandon Lai thought he was getting close. In his second pass at building the actuator he wants for a humanoid robot, he set ambitious targets: 40 to 60 rpm while delivering 20 Nm of torque for up to an hour continuously.
The goal was practical as much as it was technical. The design was inspired by an MIT research paper, but Lai made changes to match his own needs. Where the MIT approach uses an inbuilt planetary gearbox. this build substitutes a cycloidal gearbox. with the expectation that it could offer better torque capacity with less backlash.
At the center of the actuator is a hand-wound stator made with an off-the-shelf core. The rest of the build leans on custom machining and prototyping: custom CNC parts and 3D printed components make up the motor housing.
Testing was where the optimism met reality. Running the motor from a benchtop power supply with limited current, the actuator reached only 7 Nm of torque. Lai’s conclusion was direct—an improved power supply would likely lift performance.
Even more frustrating was what happened inside the gearbox. Lai noted excessive backlash in the cycloidal gearbox, blaming poor tolerances. And the budget, too, didn’t cooperate. The construction cost landed at $400, but came in well over what he’d planned.
For all of that, he isn’t walking away. The next revision is where he plans to tackle the problems he ran into—power delivery, gearbox tolerances, and the overall cost.
CAD files are available online for anyone who wants to dig deeper into the design. If you’re hoping to replicate or build on the idea, the video that follows shows the actuator in motion.
DIY robot actuator humanoid robot cycloidal gearbox planetary gearbox torque RPM CAD files CNC parts 3D printed motor housing robotics testing