Compliant Locomotion

This article describes benefits of model-based approach in developing humanoids and presents experimental results as well. Regardless of the chosen actuation strategy, the design of humanoid locomotion controllers is greatly complicated by the underactuated and nonlinear nature of the associated multibody dynamics. Drawing inspiration from biology, researchers have begun to incorporate passive mechanical compliance into the design of legged robots, often by adding spring elements in series with the robot’s actuators. First introduced by the MIT Leg Laboratory, series elastic actuators (SEAs) have been shown to improve the fidelity and stability of closed-loop force controllers while simultaneously increasing shock tolerance. The chapter shows an example SEA utilized in the design of THOR, a compliant humanoid robot developed at Virginia Tech. Despite new advancements, several challenges remain before humanoids can be fielded in real-world applications that require a high degree of mobility. Model-based control approaches could greatly benefit from techniques found in the robust and adaptive control literature. The field is also interested in moving towards more efficient, human-like locomotion using biologically-inspired control strategies.