Hardware & control
Support polygon
The support polygon is the convex hull of a robot’s contact areas on a declared support surface. For a biped on level ground it is formed by the loaded parts of one or both feet; it changes when contacts are added, removed or reduced to an edge or toe.
Also known as: base of support
Updated
The polygon comes from active contact
ISO 19649:2017 defines the support polygon as the convex hull of travel-surface contact areas. With one flat foot, that region follows the contacting sole. With two flat feet on one plane, the convex hull also spans the area between them.
The nominal outline of a foot is not always the active contact area. Heel lift, toe-off, an uneven sole or partial contact can shrink or reshape the region. Contact detection and the selected support plane therefore affect the calculated polygon.
Static and dynamic uses differ
For a stationary robot under suitable friction and planar-contact assumptions, constraining the vertical projection of the centre of mass to the support polygon is a common static-equilibrium test. Drake’s centre-of-mass constraint documentation gives this as a typical inverse-kinematics use.
Dynamic walking cannot be judged from centre-of-mass position alone. Controllers commonly compare a centre of pressure or zero moment point with the support polygon while also considering momentum and contact forces. Remaining inside the polygon does not prevent slipping, actuator saturation or failure caused by an inaccurate model.
Non-coplanar contacts need a broader model
Feet on different stair treads, a hand against a wall or a knee on the floor do not share one natural support plane. Projecting those contacts into a polygon can still be a chosen approximation, but the projection surface and assumptions must be stated. General multi-contact control instead works with contact wrenches, friction cones and whole-body dynamics.
Dataset fields should identify active contacts, contact geometry, reference frame, projection plane and whether the region is commanded, estimated or measured. A fixed polygon copied from the robot model should not be presented as observed contact.
Sources
Related terms
Hardware & control
Centre of mass
The centre of mass is the mass-weighted average position of a body or multibody system. For a humanoid, the whole-robot centre of mass changes as its joints move or its payload changes, and its position and acceleration are central variables in balance, locomotion and whole-body control.
Hardware & control
Centre of pressure
The centre of pressure is the point on a support surface at which the resultant of a distributed contact pressure or ground-reaction force can be represented as acting. In legged robotics it summarises how load is applied through one foot or across coplanar contacts and is commonly estimated from a measured contact wrench.
Hardware & control
Zero moment point
The zero moment point is a support-surface point used in legged-robot dynamics at which the tipping moment vanishes. ISO defines it from the resultant support forces, while classical biped literature uses the corresponding gravity-and-inertia formulation; for dynamically balanced planar contact, these formulations coincide with the centre of pressure.
Hardware & control
Gait
A gait is a recurring pattern of leg motion and contact phases used for legged locomotion. For a biped, it describes how left and right stance, swing, double-support and any flight phases repeat over a cycle; gait is narrower than locomotion, which also includes starts, stops, turns, transitions and recovery.
Hardware & control
Force–torque sensing
Force–torque sensing is the measurement of forces and rotational moments transmitted through a robot interface, commonly with a six-axis sensor reporting three force components and three torque components. The combined quantity is a wrench expressed at a specified origin and in a specified coordinate frame. It measures the resultant load at the sensor, not a spatial pressure map across every contact.