What Is a Quasi-Direct Drive Actuator?

A quasi-direct drive (QDD) actuator pairs a large-diameter, high-torque motor with a low reduction ratio gearbox (typically 5–20), an approach popularized by legged-robot projects such as MIT Cheetah. "Quasi-direct" means close to direct drive (no reduction) while keeping a small amount of gearing.

The Key Property: Backdrivability

The higher the reduction ratio, the harder it is to drive the motor backwards from the output (reflected friction and inertia scale with the ratio squared). A low ratio keeps the joint backdrivable:

• external impacts are absorbed by the motor yielding rather than the gears fighting them, protecting the drivetrain during landings and collisions;
• output torque can be estimated from motor current after calibrating torque constant, efficiency, and friction, reducing reliance on dedicated joint torque sensors in some applications;
• force-control bandwidth stays high — essential for jumping and running.

QDD vs. High-Ratio Drives

High-ratio harmonic and cycloidal drives typically provide high output torque and holding capability but have more backdrive resistance. QDD favors dynamic response and compliant control, so it is common in dynamic legged robots. BXI 85/70/50-series joint motors use a 19.5-ratio planetary design with MIT-protocol control (see MIT protocol).