Robotics
Center of mass
Definition
The center of mass is the mass-weighted average position of a body or a collection of bodies. For an articulated robot, its position changes as the links move.
Also known as: Centre of mass, CoM, COM
Updated
Combining the masses of the links
A humanoid's torso, arms, legs, and payload each contribute to the whole-system center of mass. MIT's derivation computes its position by multiplying each link's center position by that link's mass, adding those products, and dividing by total mass. Every position must be expressed in the same coordinate frame.
Extending a heavy arm therefore moves the robot's center of mass even if its feet stay still. Carrying a box changes the combined robot-and-payload center of mass; the model must include the box if that combined system is being controlled.
Position and motion both matter
In slow standing on level ground, projecting the center of mass onto the support polygon helps assess whether gravity can be balanced. During walking, acceleration and angular momentum also matter. A moving robot can require a recovery step even when its center of mass currently projects between its feet.
A useful summary with missing detail
Centroidal dynamics connect center-of-mass motion to external forces and momentum. This reduces planning complexity, but a feasible center-of-mass trajectory does not by itself establish that every joint can execute it. The MIT notes identify joint-position and effort limits as information that simplified centroidal planning can miss.
Sources
Related terms
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.
Support polygon
The support polygon is the convex hull of a robot’s active contact points or contact patches projected onto a common support plane. It describes the available support region in planar contact models.
Zero-moment point
The zero-moment point is a point on a chosen support plane where the net moment associated with the ground reaction wrench has zero components parallel to that plane. In flat-ground walking with the usual contact assumptions, it coincides with the center of pressure.
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.