Ground Reaction Force (GRF)

The force the ground pushes back onto a foot; legged robots stand, walk, and jump entirely by means of it.

Ground reaction force is the force the ground exerts on whatever is touching it. By Newton's third law, whatever force a foot presses into the ground, the ground pushes back with an equal and opposite force. It splits into two parts: a vertical component, the supporting (normal) force, and a horizontal component, friction — pushing off to walk forward or pushing sideways to turn both draw on the horizontal part. Biomechanics researchers measure it with a force plate, combining it with motion-capture data to work out human joint torques. For a legged robot, aside from gravity, GRF is essentially the only external force that can accelerate the body's center of mass, and the robot can only adjust it indirectly, through joint torques. That's why many quadruped and humanoid controllers — MPC, whole-body control — treat each foot's GRF directly as an optimization variable, and require it to stay inside the friction cone: horizontal force can't exceed the friction coefficient times vertical force, or the foot slips.

ExampleA 60 kg person standing still on both feet has a combined vertical ground reaction force roughly equal to their weight: 60 × 9.8 ≈ 588 N. At the instant of push-off during running, this force exceeds body weight.

Also called

GRF

Related

Contact Force · Friction Cone · Zero Moment Point · Center of Pressure (CoP) · Foot Force Sensor · Centroidal Dynamics

Sources

Ground reaction force - Wikipedia
Inverse dynamics - Wikipedia