Series Elastic Actuator (SEA) · serves as: Actuator, Transmission, Force sensor.
Which group Agility Digit v4 SEA belongs to and how it is built
Actuators with a physical spring element between motor and output. Torque sensing via spring deflection.
Actuator with a physical series spring enabling torque sensing and shock absorption.
Successor of the fiberglass springs from ATRIAS
Encoders on both sides of the spring
Encoders on both sides of the spring
The Series Elastic Actuator (SEA) in the Digit v4 robot is the evolution of a unique actuator lineage developed at the Dynamic Robotics Lab at Oregon State University since 2009. It is the heart of Agility Robotics’ approach to dynamic bipedal locomotion — passive compliance integrated mechanically into the drivetrain, not simulated in software.
ATRIAS (2009–2015) was the first bipedal robot in history to reproduce the dynamics of the SLIP (Spring-Loaded Inverted Pendulum) model — a foundation of biological locomotion. Lightweight legs with fiberglass springs and motors close to the hips allowed it to walk, run and recover from pushes. Cassie (2016) preserved the SEA philosophy but replaced fiberglass springs with compact steel leaf springs, enabling a self-contained, battery-powered platform. Digit (since 2019, this entry covers the v4 generation released in 2023) carried the SEA architecture into a full humanoid with torso and arms, retaining passive shock compliance.
A Series Elastic Actuator places a purposefully designed compliant element (a spring) between the gearmotor and the joint output. Measuring the deflection of this spring with encoders on both sides allows precise torque estimation without expensive in-joint torque sensors. This delivers three key properties: (1) high-resolution torque sensing and control, (2) shock absorption from ground impacts during walking, and (3) elastic energy return on push-off, reducing energy consumption by roughly 20–30% compared with stiff actuators.
Each of the 16 powered joints in Digit v4 uses an SEA with a high-torque PMSM motor, a compact planetary gearhead and a steel leaf-spring element. Control runs at 1 kHz on distributed joint controllers, with a whole-body controller orchestrating motion on a central PC. The architecture enables 4-hour continuous operation with up to 16 kg payload, walking on uneven floors and disturbance recovery — all without needing to control the full dynamics model actively.
Agility’s SEA is one of three dominant actuator paradigms in humanoids today (alongside rigid quasi-direct drive — electric Boston Dynamics Atlas, Unitree H1; and hybrid harmonic drive — Figure 02, 1X NEO). The choice is deliberate: Agility trades peak positional precision and bandwidth for mechanical robustness, natural gait and human-safe collaboration — the qualities that matter most on a warehouse or factory floor.
