Quasi-Direct Drive

A joint design pairing a high-torque motor with a low gear ratio, balancing strength and compliance.

Quasi-direct drive is a joint design approach: it pairs a large-diameter brushless motor with high torque density with just a single low-ratio gearbox stage (usually under 10:1), sitting between fully direct drive (no gearbox at all) and a high-reduction design like a harmonic drive. The low gear ratio gives good backdrivability and low reflected inertia (the motor rotor's inertia as seen at the joint, which grows with the square of the gear ratio), so an outside force can push the joint back, and impacts on landing are less likely to damage the gears; because motor current is roughly proportional to output torque, the joint's force can be estimated and controlled without a dedicated torque sensor. MIT's Cheetah series popularized this “proprioceptive actuator” approach, and today it's used in the leg joints of most quadruped and humanoid robots, including Unitree's — it also suits locomotion policies trained with reinforcement learning particularly well.

ExampleAll 12 joints on the MIT Mini Cheetah use the same module: a brushless motor plus a 6:1 single-stage planetary gearbox. Unitree's Go1 uses a similar design in its GO-M8010-6 joint motor, with a 6.33:1 gear ratio.

Also called

QDD Motor, Low-Reduction-Ratio Actuator

Related

Direct Drive · Proprioceptive Actuator · Backdrivability · Planetary Gearbox · Joint Actuator Module · MIT Mini Cheetah Actuator

Sources

Proprioceptive Actuator Design in the MIT Cheetah (IEEE T-RO 2017)
Mini Cheetah: A Platform for Pushing the Limits of Dynamic Quadruped Control (ICRA 2019)
Unitree GO-M8010-6 关节电机参数（宇树官网） (Chinese)