Series elastic actuator development for a biomimetic walking robot

TLDR

Series elastic actuators use linear springs in series with the motor to enable force estimation and control, and a second‑order linear model distinguishes fixed‑load and free‑load dynamics. The study aims to guide the selection of the spring constant in series elastic actuators. The authors use a dimensional‑analysis based second‑order model that incorporates friction and motor saturation, measuring spring strain to estimate force and analyze nonlinear effects such as stiction. The model clarifies the beneficial and detrimental effects of the springs and informs a design procedure for series elastic actuators.

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. 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.