series-elastic actuator (SEA)

A series-elastic actuator is a motor with a deliberate spring placed between it and the thing it moves. Normally a motor connects to its load through stiff gears, so it can shove hard but feels harsh and rigid. The SEA breaks that rigid chain on purpose by inserting a spring in line — the motor pushes the spring, and the spring pushes the load.

That small change buys two big gifts. First, force becomes easy to measure and control: how far the spring is stretched tells you exactly how much force is passing through, so the robot can push with a chosen gentleness instead of guessing. Second, the spring cushions shocks — if the limb suddenly hits something, or something hits it, the spring soaks up the jolt that would otherwise hammer the gears, much like a car's suspension absorbs a pothole.

The cost is that the spring makes the joint a bit softer and slower to respond, so it is a trade between brute precision and graceful, safe force. SEAs are popular in legged robots, walking aids, and machines that work close to people, where feeling and forgiving contact matters more than being stiff as a rock.