Robotics
Series elastic actuator
Definition
A series elastic actuator places an elastic element in the force-transmission path between the drive and its load. Measuring the element’s deflection can support force or torque feedback while the elasticity changes the actuator’s response to impacts.
Also known as: SEA, Series-elastic actuator
Updated
A spring in the load path
The defining feature is intentional compliance in series with the output, rather than only a flexible cover or a spring acting alongside the drive. For a calibrated linear spring, force is related to extension by F = k * delta; a torsional spring has an analogous torque-angle relationship.
Williamson's MIT thesis studies this arrangement for force-controlled actuation. Katz's actuator thesis explains how measuring spring deflection turns output torque control into a problem of regulating that deflection.
Why use it in a robot
Elasticity can separate the motor and gearbox from abrupt output disturbances and provide a useful force measurement. In some designs, the spring also stores and returns energy. A leg actuator can therefore respond differently to touchdown than an otherwise rigid transmission would.
Compliance brings tradeoffs
Spring stiffness, travel, sensing, and controller design affect the achievable force response. Williamson explicitly describes a tradeoff between bandwidth and the benefits of compliant force control. Adding elasticity also introduces motion between motor and output, which must be represented in control and estimation. An SEA is not automatically accurate, safe, or energy-efficient under every load; those properties require evidence for the particular design and operating conditions.
Sources
Related terms
Torque control
Torque control regulates the turning effort delivered by an actuator or robot joint. It provides an actuation interface from which motion, force, and impedance controllers can produce the joint torques their tasks require.
Impedance control
Impedance control shapes the dynamic relationship between a robot’s motion and the forces it exchanges with its environment. A common goal is for the robot to respond like a chosen mass, spring, and damper at a joint or end effector.
Quasi-direct drive
Quasi-direct drive is an actuation approach that combines a torque-capable motor with a relatively low transmission reduction to preserve useful backdrivability and force-control behavior. It differs from direct drive because it still uses a transmission.
Backdrivability
Backdrivability is the ability of an external load applied at a mechanism’s output to drive motion back through its transmission. In a robot joint, it describes how readily an outside force can move the joint and its actuator.