If engineers are building or deploying robots that operate around people, actuator selection becomes a critical design decision. While rigid actuators can deliver high precision, they also introduce the risk of dangerous force spikes when unexpected contact occurs. Series Elastic Actuators (SEAs) address this challenge through a simple but effective concept: placing a spring between the motor and the load.<\/p>\n
A Series Elastic Actuators (SEAs) are a type of actuator that places a compliant spring element between the gearbox and the robot\u2019s output link, so force passes through the spring rather than a rigid drive train. The concept was first pioneered in 1995 by Pratt and Williamson from MIT and was developed specifically for robots that interact with people, where rigid force spikes can cause injury or damage.<\/span>[1]<\/sup><\/span><\/p>\n The spring is placed in series with the actuator, meaning the drive chain goes from the motor to the gearbox, then through the spring before reaching the output, rather than using a parallel elastic element that assists the motor from the side. In a nutshell, the spring acts not just as a cushion, but as a force sensor, where deflection corresponds to force according to Hooke\u2019s Law, as well as a shock absorber and an energy store, all in a single mechanical element.\u00a0<\/span><\/p>\n The motor drives through the gearbox as normal, but before that torque reaches the output, it passes through the spring. The controller measures how much the spring deflects, and that deflection indicates how much force is being applied, without requiring an external force sensor.<\/span><\/p>\n Most traditional actuators are position-controlled, whereas series elastic actuators are designed for force control. The spring deflection provides a direct and low-noise torque reading, making these systems well suited for tasks where contact force matters, such as pushing, lifting, or interacting with a human arm.<\/span><\/p>\n The spring also stores kinetic energy during braking and releases it when needed. This behavior is particularly useful in legged robots, where it functions similarly to a human Achilles tendon by storing energy on impact and returning it during push-off.<\/span><\/p>\nHow Does it Work?<\/b><\/span><\/h2>\n