Hardware & control
Floating base
A floating-base model represents a robot’s root body with six unconstrained spatial degrees of freedom—three for translation and three for rotation—instead of fixing it to the world. Those base coordinates are not directly actuated; a legged robot changes them through joint motion and external contact forces.
Also known as: floating-base model, free-floating base, free-floating-base model
Updated
The root pose becomes part of the state
A fixed-base manipulator attaches its base frame to the world. A floating-base model instead adds the root body’s position and orientation to the robot’s generalized coordinates. The ETH Zurich notes describe these base coordinates separately from the actuated joint coordinates.
The word “floating” describes the model, not necessarily a robot suspended in the air. A standing humanoid is commonly represented with a floating base while foot contacts are imposed as external constraints. When a foot lifts, slips or lands, those constraints change without redefining the robot’s coordinate structure.
The base cannot command itself
There is no motor that directly applies an arbitrary six-dimensional command to a humanoid’s pelvis in free space. Joint actuators alter the internal configuration, while gravity and contact forces change whole-body momentum. This makes the floating-base system underactuated and couples limb motion to balance.
Free-floating-base dynamics therefore include both the unactuated root and actuated joints. Whole-body controllers solve for joint commands and feasible contact forces together rather than treating the base as an independently driven Cartesian stage.
Floating-base data needs an estimator and a frame
Joint encoders measure relative joint positions but do not directly measure the root pose in the world. A state estimator may combine an IMU, kinematics, contact assumptions, vision or external tracking to estimate base orientation, position and velocity.
A dataset should declare the chosen root link, pose representation, reference frame, estimator, covariance where available and whether the base state is measured, estimated or simulator ground truth. It should also distinguish a model’s floating-base coordinates from ROS names such as base_link or base_footprint, which define frames rather than actuation.
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Related terms
Hardware & control
Biped robot
A biped robot is a legged mobile robot that travels using two legs. Biped describes the locomotion structure, not the robot’s overall resemblance to a person: a biped can lack arms or a head, while a robot described as humanoid may use wheels instead of two-legged locomotion.
Hardware & control
Underactuation
Underactuation is the condition in which a system’s available control inputs cannot independently produce every possible acceleration of its modelled degrees of freedom. It often arises because there are fewer independent actuators than degrees of freedom, but contact, actuator limits and the chosen model also determine whether a system is underactuated.
Hardware & control
Robot dynamics
Robot dynamics describes the relationship between forces and torques acting on a robot and the motion they produce, accounting for mass, inertia, gravity, velocity-dependent effects and external contact. Forward dynamics predicts acceleration from applied forces; inverse dynamics computes forces or torques for a specified motion.
Hardware & control
State estimation
State estimation is the process of inferring variables that are not known exactly—such as a robot’s base pose, velocity, contact state or sensor bias—from noisy measurements, control inputs and a model of how the system evolves. An estimator should provide both an estimate and, where possible, a representation of its uncertainty.
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.