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.
Also known as: Robot actuator, Actuators
Updated
From control signal to joint motion
ISO 8373:2021 defines an actuator by its energy-conversion role. On an electric humanoid, the practical joint unit may package a motor, drive electronics, transmission, brake and sensors, but those parts should not automatically be treated as one signal or component.
Software abstractions are narrower still. The ROS 2 actuator interface represents a one-degree-of-freedom actuator. Physical mechanisms can instead couple one actuator to several joints, use several actuators around one joint, or leave a freedom unactuated. Actuator count, joint count and degree-of-freedom count are therefore not interchangeable.
Why actuator design matters for humanoids
An actuator must produce motion within torque, speed, travel and thermal limits while carrying its own mass. Transmission choices also affect efficiency, reflected inertia, mechanical compliance and how readily an external push can move the joint. The MIT Underactuated Robotics notes show why available actuation and force limits determine which accelerations a robot can command, even when its kinematic model has many movable joints.
What training data should preserve
Actuation records should separate command values from measured output. Useful fields include the control mode, motor-side and joint-side position, velocity, current, estimated or sensed torque, gear ratio, limits, temperature, fault state and timestamps.
That distinction is hardware-specific: Franka’s ROS 2 interface exposes desired joint torque and measured link-side torque as separate interfaces. Without equivalent metadata, identical-looking action vectors can describe different inner control loops and cannot be compared safely across humanoids.
Sources
Related terms
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.
Hardware & control
Torque control
Torque control is a control mode in which desired joint torques, or equivalent linear forces, are the command variables sent to a robot’s low-level actuators. Unlike position control, it does not directly command where each joint should be. Motion and contact instead emerge from the applied efforts, robot dynamics, feedback, constraints and any higher-level controller.
Hardware & control
Impedance control
Impedance control regulates the dynamic relationship between a robot’s motion error and the force or torque it exerts, usually by giving the robot virtual stiffness, damping and sometimes inertia. It lets an end effector or limb yield predictably during contact instead of trying to hold an exact position regardless of interaction forces.
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.