Glossary · Hardware
Quasi-direct-drive
Also known as: QDD
In brief
A quasi-direct-drive (QDD) actuator pairs a high-torque motor with a single-stage low-ratio gearbox (typically 6:1 to 10:1). The result is a joint that's naturally back-drivable, torque-transparent, and compliant — at the cost of more motor mass and higher current draw.
Made famous by MIT's Cheetah quadrupeds and now widely adopted, QDD actuators sit at the opposite end of the design space from harmonic drives. By keeping the gear ratio low, you can estimate output torque directly from motor current, the motor itself becomes a useful spring, and external forces back-drive the joint without much friction. That makes torque control, impedance control, and whole-body force control practical.
The downside: you need a much beefier motor to get the same output torque, which adds mass to the robot. Most QDD humanoids accept that mass penalty in exchange for the control properties.
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