Robot control
Capture point
Definition
The capture point is a model-dependent location where support can be placed to bring a moving robot toward rest without further steps. In the constant-height linear inverted pendulum model, the instantaneous capture point combines center-of-mass position and velocity.
Also known as: Instantaneous capture point, ICP
Updated
Velocity changes where a robot must step
Two robots can have the same center-of-mass position but need different recovery steps because one is moving faster. For the constant-height linear inverted pendulum model, the horizontal instantaneous capture point is xi = x + v / omega, where omega = sqrt(g / h). Here x is position, v is velocity, and h is height above the support plane.
Griffin and colleagues use this state to control divergent motion. Placing the appropriate support or effective moment pivot at that location holds the capture point stationary while the center of mass converges toward it.
A target is not an executable footstep
A recovery step must fit within the leg's reachable region and available swing time. The same study incorporates those constraints into capture regions and examines changes to step position and timing. A point outside the current foot does not alone specify a successful recovery action.
Assumptions behind the formula
The simple expression assumes constant height and the associated pendulum dynamics. Changing height, substantial angular momentum, finite feet, and multiple future steps alter the analysis. A capture point is therefore a balance-planning quantity tied to a model, not a universal boundary between falling and recovery.
Sources
Related terms
Linear inverted pendulum model
The linear inverted pendulum model approximates a walking robot by a mass moving at constant height above its support, with simplified angular-momentum dynamics. These assumptions make horizontal center-of-mass acceleration linear in the displacement from the support point.
Center of mass
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.
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.
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.