Glossary · Hardware
Series elastic actuator
Also known as: SEA
In brief
A series elastic actuator places a calibrated spring between the motor output and the joint. The spring deflection is measured to estimate output torque, and the spring decouples motor inertia from output, providing inherent shock absorption and improved force control.
SEAs were pioneered at the MIT Leg Lab and used in Boston Dynamics PETMAN, the original Atlas, and many research humanoids. The spring acts as both a torque sensor (Hooke's law) and a passive shock absorber, smoothing impacts at heel-strike and protecting the gearbox.
The trade-off is added bandwidth limitations and mechanical complexity. Newer designs increasingly use QDD actuators where the motor itself provides enough compliance, eliminating the explicit spring.
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Glossary entries are upstream. The catalog is where the implementations live.