Impact

When two objects collide, a very brief but very large contact force that abruptly changes their velocities.

An impact is a collision between two objects in which the contact force spikes to a large value over an extremely short time, changing the objects' velocities almost instantaneously. Rigid-body simulation and control commonly treat it as an instantaneous event: the impulse (the time-integral of force) equals the change in momentum, and the coefficient of restitution e determines how much rebound results — e = 0 means no bounce at all, e = 1 means no energy loss. Impacts are common in robotics: a legged robot's foot touching down each step, a jump landing, or an arm bumping into a person or a table. Peak impact forces can damage gearboxes and sensors, and they can also make a foot slip or destabilize a controller. On the hardware side, engineers cushion impacts by reducing the motor inertia reflected to the output and improving backdrivability; Wensing and colleagues proposed an impact mitigation factor (IMF) in 2017 to quantify this ability for the MIT Cheetah, which had contact durations as short as 85 ms and peak forces over 450 N while running with the bound gait.

ExampleA 1 kg ball falls vertically at 2 m/s and lands with a coefficient of restitution of 0.5, so it bounces back at 1 m/s; momentum changes from −2 to +1 kg·m/s, an impulse of 3 N·s, and if contact lasts only 10 ms, the average contact force is about 300 N — roughly 30 times the ball's weight.

Also called

Collision Impact, Impulsive Contact

Related

Coefficient of Restitution · Backdrivability · Reflected Inertia · Quasi-Direct Drive · Contact Model · Ground Reaction Force (GRF)

Sources

Wikipedia: Impact (mechanics)
Wikipedia: Coefficient of restitution
Wensing et al. 2017, Proprioceptive Actuator Design in the MIT Cheetah: Impact Mitigation and High-Bandwidth Physical Interaction（IEEE T-RO）