Robotics
Floating-base robot
Definition
A floating-base robot is modeled with an unconstrained base pose relative to the world in addition to its internal joint coordinates. In a legged or humanoid robot, joint actuators change base motion indirectly through the coupled dynamics and external contact forces.
Also known as: Floating-base system, Floating base robot
Updated
Add the base to the configuration
A fixed industrial arm treats its pedestal as part of the world. A floating-base model instead gives the base a three-dimensional position and orientation, normally adding six degrees of freedom to the joint coordinates. The base can be the pelvis or torso of a humanoid, or a reference link on another free-moving multibody system.
Floating describes the model boundary, not a claim that the machine is airborne. A standing humanoid still has a floating base in its equations even though its feet constrain motion through the floor. The MIT humanoid notes formulate the robot with unactuated floating coordinates and external contact forces.
Contacts move what motors cannot command directly
Joint motors apply internal torques between links. They do not directly push the base through space. Gravity and contact wrenches supply external effects, which is why a free-standing humanoid is underactuated. Foot, hand, or knee contact can constrain some base motion and allow joint torques to change whole-body momentum.
Del Prete and colleagues formulate motion and partial contact-force control for constrained floating-base robots and evaluate the method on a simulated 23-degree-of-freedom humanoid. That simulation supports the reported controller, not every contact model or physical robot.
Coordinates and contacts need careful handling
Base orientation cannot be represented globally without a coordinate choice and its associated singularities or constraints. A quaternion adds a normalization constraint, while local rotation coordinates require updates between tangent spaces. The mass matrix also includes coupling between base and joint motion.
Contact assumptions are equally important. A foot treated as rigidly fixed can actually slip, roll, or deform. Making or breaking contact changes the constraints, and impacts can create discontinuous velocity. A floating-base model is therefore necessary for many legged-robot calculations, but it is not sufficient for accurate whole-body control.
Sources
Related terms
Underactuation
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.
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.
Centroidal dynamics
Centroidal dynamics describe the motion of a multibody system’s center of mass and the evolution of its total linear and angular momentum. External forces and moments determine the rates of change of those momenta.
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.