Robotics
Underactuation
Definition
Underactuation means that a system’s available control inputs cannot independently command acceleration in every degree of freedom of its model. It often occurs when a mechanism has fewer independent actuators than degrees of freedom.
Also known as: Underactuated system, Underactuated robot
Updated
Fewer independent inputs than motions
A two-joint pendulum with a motor at only one joint is a simple example. The unpowered joint still moves, but its acceleration follows from gravity and coupling with the actuated joint. MIT's Underactuated Robotics notes formalize the distinction using the rank of the mapping from control inputs to accelerations.
A humanoid also has an unactuated floating base: its motors apply internal joint torques rather than directly commanding the torso's position in space. Ground contacts provide the external forces through which it can move and support its body.
Underactuated does not mean uncontrollable
Coupled dynamics can allow a robot to reach useful states over time even when it cannot independently select every acceleration now. Swinging an unpowered joint by moving another joint illustrates the difference between instantaneous actuation and longer-term control.
The chosen model matters
MIT's definition emphasizes that actuation depends on the model and state. A rigid-link model may appear fully actuated while a more detailed model includes unactuated flexible modes. Contacts can also change the feasible motions. Counting motors is therefore a useful first check, but it does not replace examining input directions, constraints, and the modeled degrees of freedom.
Sources
Related terms
Degrees of freedom
Degrees of freedom are the number of independent coordinates needed locally to describe a system configuration. In robotics, this count depends on the bodies, joints, and independent constraints in the model.
Bipedal locomotion
Bipedal locomotion is movement using two legs, with body motion coordinated through changing contacts between the feet and the environment. It includes walking and running.
Whole-body control
Whole-body control coordinates a robot’s joints and contacts to satisfy several motion and force objectives together. In humanoids, it commonly combines balance, foot motion, hand tasks, and posture subject to physical constraints.
Forward dynamics
Forward dynamics predicts a robot's acceleration from its current configuration, velocity, applied joint forces or torques, and external forces. It uses the robot's mass, inertia, and other modeled dynamic properties.