Point Foot vs. Flat Foot

Whether a leg's foot meets the ground at a single point or across a flat sole determines whether ankle torque can hold balance.

Foot morphology refers to how a legged robot's foot contacts the ground. A point foot is a small ball or rubber tip making roughly point contact, so the ankle can't produce a moment against the ground and the center of pressure is pinned to that single contact point; as a result, a biped with point feet is underactuated (it has fewer controllable quantities than degrees of freedom to control), cannot stand still, and must balance by continuously stepping and adjusting where it plants its feet. Most quadruped robots also use point feet, but they can still stand firmly with multiple legs down at once. A flat foot has a sole, so the center of pressure can shift within the footprint, the ankle can apply a moment, the support polygon is larger, and the robot can stand still and can also plan its gait using zero-moment-point (ZMP) methods — at the cost of needing more motors at the ankle and a heavier leg tip. LimX Dynamics' TRON 1 can quick-swap its foot end among point-foot, flat-foot, and wheeled configurations; the company describes the point foot as the simplest and easiest to control leg form.

ExampleThe point-footed biped Hume, from a 2015 paper, has to continuously choose new footholds and keep stepping to stay balanced; a flat-footed humanoid such as Unitree's G1, by contrast, can stand still with its feet together.

Also called

Point-Foot, Sole Foot, Foot Morphology

Related

Zero Moment Point · Center of Pressure (CoP) · Support Polygon · Underactuation · LimX Dynamics TRON 1 · Parallel Ankle Mechanism

Sources

LimX Dynamics TRON 1 产品页（Point-Foot / Sole / Wheeled 足端） (Chinese)
Assessing Whole-Body Operational Space Control in a Point-Foot Series Elastic Biped (arXiv 1501.02855)
Wikipedia: Support polygon