Hardware & control
Degree of freedom
A degree of freedom (DoF) is one independent parameter needed to specify a robot’s configuration; equivalently, a robot’s DoF is the dimension of its configuration space. It describes possible motion, not the number of motors. Joint constraints, closed kinematic chains and environmental contacts can make joint count, actuator count and controllable motion differ.
Also known as: DOF, Degrees of freedom
Updated
A dimension, not a motor count
Modern Robotics defines a robot’s degrees of freedom as the freedoms of its rigid bodies minus independent constraints on their motion. A free rigid body in three-dimensional space has six: three translational and three rotational. A revolute or prismatic joint normally contributes one freedom to an open chain.
This is a property of configuration, not necessarily the length of a stored coordinate vector. A representation may use redundant values subject to a constraint, while a closed-chain mechanism may have many named joints but fewer independent coordinates.
Why humanoid totals differ
A quoted humanoid DoF count often means its articulated joint freedoms with the base treated as fixed. A floating-base dynamics model adds six freedoms for the body’s pose in the world. Those base freedoms have no direct motors; foot or hand contacts instead constrain the motions that are currently possible.
The MIT Underactuated Robotics notes make this distinction explicit for legged systems: internal joint positions and the floating base belong in the configuration, while actuation and contact determine which generalised forces are available. Coupled joints and passive mechanisms create further differences between model coordinates and commands.
Why the convention belongs in training data
A joint-state or action array is ambiguous without its coordinate convention. A useful dataset states the joint order, fixed- or floating-base assumption, pose representation, units, limits, and which coordinates are actuated, passive or mechanically coupled.
This matters when retargeting motion or comparing policies. Two humanoids can advertise the same DoF count yet assign those freedoms to different joints, ranges and axes. Matching vector width does not establish a valid physical correspondence.
Sources
Related terms
Hardware & control
Actuator
An actuator is a robot’s power mechanism that converts electrical, hydraulic, pneumatic or other supplied energy into controlled mechanical force, torque or motion. In a humanoid, actuators drive joints, often through transmissions; they are distinct from the joints and sensors, and their arrangement need not correspond one-to-one with the robot’s degrees of freedom.
Hardware & control
Whole-body control
Whole-body control coordinates a humanoid’s full multibody state, actuated joints and environmental contacts to pursue several motion or force objectives while respecting constraints such as balance, joint limits and friction. It is a family of hierarchical or optimisation-based methods, not one algorithm; implementations may output joint positions, accelerations or torques.
Hardware & control
End effector
An end effector is a task-specific device attached to a robot manipulator’s mechanical interface so the robot can act on its environment, such as a gripper, hand or welding tool. It is distinct from the wrist or mounting flange, and from the tool centre point, which is only a coordinate frame used to plan the device’s motion.
Hardware & control
Proprioception
Proprioception is sensing of a robot’s own internal configuration and motion rather than the external scene. For a humanoid it commonly includes joint positions and velocities, actuator effort or torque, and inertial measurements of body rotation and acceleration. These signals support state estimation and feedback control but do not, by themselves, directly describe nearby objects or terrain.