Active DoF / Passive DoF

A degree of freedom is active when a motor drives it independently, and passive when it just follows a linkage or an external force.

An active degree of freedom is a joint driven directly and independently by an actuator — a motor, a hydraulic cylinder, and so on. A passive degree of freedom has no independent actuator; its motion is instead determined by mechanical constraints, linkages, or external forces. Passive joints show up in two common places: ball joints and universal joints in parallel mechanisms — a Stewart platform, for instance, is driven by six linear actuators while its ball joints are entirely passive, their positions set by the whole closed kinematic chain — and finger joints in dexterous hands or underactuated grippers, which are coupled to an active joint through linkages or tendons. This is why reading a dexterous hand's spec sheet means distinguishing 'degrees of freedom' from 'number of joints': the former usually counts only the active DoF, which sets how many dimensions the policy can independently control — the size of its action space — while the latter also counts the passive joints that just follow along. Passive joints also need special handling in simulation (URDF mimic joints, closed-chain constraints) and when retargeting motion for teleoperation.

ExampleInspire Robots' RH56DFX dexterous hand is specified as having 6 active degrees of freedom and 12 joints: 6 motors each control one dimension, while the remaining joints follow along through linkages. The Shadow Hand, by contrast, has 20 active degrees of freedom plus 4 underactuated coupled joints, for 24 joints in total.

Also called

Active Joint, Passive Joint, Actuated / Passive Joint

Related

Degrees of Freedom (DoF) · Underactuation · Parallel Mechanism · Dexterous Hand · Adaptive (Underactuated) Gripper · Linkage Transmission

Sources

Parallel manipulator - Wikipedia
Inspire RH56DFX Dexterous Hand (product page)
Shadow Dexterous Hand Series (Shadow Robot)

As of

2026-09