Quasi-Direct Drive (QDD) Actuators: The Backdrivability Advantage

Lower gear ratios trade some peak force for a joint that can absorb an impact instead of fighting it.

Quasi-direct drive actuators pair a high-torque-density motor with a much lower gear ratio than a harmonic drive — typically 6:1 to 10:1 — so the motor itself is doing more of the torque production rather than relying on gearing to multiply it.

What you get in exchange

Low reflected inertia and easy backdrivability: an external force pushing on the joint can move the motor shaft directly, rather than being blocked by high-ratio gearing. That's exactly the property you want when a robot falls, gets bumped, or needs to absorb an impact without the gearbox taking the damage.

Where it shows up

Boston Dynamics' move from a hydraulic Atlas to a fully electric one leaned heavily on QDD-style actuation, and it's become a common choice for legs and feet across the industry more broadly, even on robots that use harmonic or planetary gearing elsewhere for joints that need to hold sustained load.