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;
• joint forces can be estimated from motor current alone (proprioceptive force control), avoiding expensive torque sensors;
• force-control bandwidth stays high — essential for jumping and running.

QDD vs. High-Ratio Drives

High-ratio drives (harmonic, cycloidal) win on torque density and holding efficiency but sacrifice backdrivability and impact tolerance; QDD is the opposite trade. Legged robots overwhelmingly choose QDD. The BXI 85/70/50-series joint motors use a 19.5-ratio planetary design — a classic QDD architecture with MIT-protocol force control (see MIT protocol).