Robotics
Quasi-direct drive
Definition
Quasi-direct drive is an actuation approach that combines a torque-capable motor with a relatively low transmission reduction to preserve useful backdrivability and force-control behavior. It differs from direct drive because it still uses a transmission.
Also known as: QDD, Quasi direct drive, Quasi-direct-drive actuation
Updated
Keeping transmission effects manageable
A transmission increases output torque while reducing speed, but it also changes friction and the motor inertia felt at the joint. Modern Robotics shows that ideal reflected rotor inertia scales with the square of the reduction ratio.
Quasi-direct-drive designs use modest reduction together with a motor suited to producing substantial torque. The aim is to keep backdrivability and controllable interaction forces while obtaining more joint torque than the motor supplies directly.
Examples in manipulation and locomotion
Gealy and colleagues describe this approach in the Blue manipulation arm, including belt transmissions and motor choices. Katz's MIT thesis develops related low-ratio, backdrivable actuators for dynamic robots. These examples establish particular design choices, not a universal performance level for anything described as QDD.
What the label does not specify
There is no single reduction ratio that alone establishes good force control. Motor inertia, transmission friction, thermal limits, and controller bandwidth all matter. A low-ratio actuator may need a larger motor or more current to deliver the required torque. Compare continuous and peak torque under stated conditions, rather than treating quasi-direct drive as a guarantee of high payload or safe contact.
Sources
Related terms
Backdrivability
Backdrivability is the ability of an external load applied at a mechanism’s output to drive motion back through its transmission. In a robot joint, it describes how readily an outside force can move the joint and its actuator.
Torque control
Torque control regulates the turning effort delivered by an actuator or robot joint. It provides an actuation interface from which motion, force, and impedance controllers can produce the joint torques their tasks require.
Series elastic actuator
A series elastic actuator places an elastic element in the force-transmission path between the drive and its load. Measuring the element’s deflection can support force or torque feedback while the elasticity changes the actuator’s response to impacts.
Impedance control
Impedance control shapes the dynamic relationship between a robot’s motion and the forces it exchanges with its environment. A common goal is for the robot to respond like a chosen mass, spring, and damper at a joint or end effector.