U-041

Series Elastic Actuator Development for a Biomimetic Walking Robot

Authors: David W. Robinson, Jerry E. Pratt, Daniel J. Paluska, and Gill A. Pratt
Affiliation: MIT Leg Laboratory

Abstract
Series Elastic Actuators have linear springs intentionally placed in series between the motor and actuator output. The spring strain is measured to get an accurate estimate of force. Despite using a transmission to achieve high force/mass and high power/mass, the spring allows for good force control, high force fidelity, minimum impedance, and large dynamic range.

A second order linear actuator model is broken into two fundamental cases: fixed load -- high force (forward transfer function), and free load -- zero force (impedance). This model is presented with dimensional analysis and extends previous linear models to include friction. Using the model and dimensionless groups, we examine nonlinear effects of motor saturation as it relates to large force bandwidth and nonlinear friction effects such as stiction. The model also helps to clarify how the springs help and hinder the operation of the actuator.

The information gained from the model helps to create a design procedure for Series Elastic Actuators. Particular emphasis is placed on choosing the spring constant for the elastic element. Large force bandwidth requires a high spring constant. Minimizing nonlinear friction and impedance requires a low spring constant. The design procedure tries to balance these competing requirements and is used to construct a physical actuator.

David W. Robinson
dwrobin@ai.mit.edu