Series Elastic Actuators: Why Some Robots Build In Springiness

Putting a spring deliberately between the motor and the load sounds backwards, until you see what it buys you.

A series elastic actuator (SEA) puts a compliant element — usually a mechanical spring — between the gear train and the output, instead of connecting them rigidly. It sounds like it would just add imprecision, but it's a deliberate design choice.

What it buys you

The spring's deflection is easy to measure, which turns the actuator into a built-in force sensor: you can estimate output torque directly from how much the spring compresses, without a separate force/torque sensor. It also absorbs shock loads and impacts before they reach the gearbox and motor, and gives the joint some natural, passive compliance that helps with things like walking over uneven ground.

The cost

That same springiness lowers the actuator's control bandwidth and can make precise position control harder, since the output no longer moves in lockstep with the motor. SEAs tend to show up more in legs, where compliance and force sensing matter, than in wrists or fingers, where precision usually matters more.