Series elastic actuator

A series-elastic actuator (SEA) is a robot joint architecture with an elastic element (a spring) placed in series between the motor's output and the joint output. The spring deflection is measured (via encoder or linear sensor) to compute joint torque indirectly, and the spring absorbs impact loads that would otherwise damage the gearing. Trades some control bandwidth for [backdrivability](/glossary/backdrivability), impact tolerance, and direct torque sensing.

The distinction matters because SEAs are why most modern humanoids can operate near humans without injury. Apptronik's Apollo, Agility's Digit, and most cobot designs use SEA or its derivatives (variable-stiffness actuators). Boston Dynamics' Atlas does NOT. It uses hydraulic and rigid electric actuators with impedance control instead, which is why Atlas is faster and stronger but harder to certify for human-collaborative environments. The actuator architecture cascades into the entire control stack.