Dynamic Modeling and Hybrid Compliant Control for a 2-DOF Compliant Robotic Leg With a New Biarticular Actuation

Abstract

It is well recognized that biarticular muscles cross over two joints instead of just one, granting them the ability to generate substantial forces across both joints. This type of muscle plays a crucial role in human movement, since it produces large forces. In literature, there have been numerous designs of mono- and biarticular actuations applied for a two- or three-link robotic leg, aimed at studying human locomotion. A novel biarticular actuation configuration for a two-link compliant robotic leg is proposed in this research. It features mono-articular rotational actuation for the proximal link and biarticular linear actuation for the distal link. The distal link is actuated by a linear Series Elastic Actuator (SEA) that is suitable for compliant ground interaction, while the proximal link is driven by a stiff rotary actuator. This biarticular actuation is bio-inspired by the biarticular muscles in humans, especially the hamstring muscle, which span two joints instead of one to provide high torque at both joints. This new configuration for the two-link robotic leg realizes the Spring-loaded Inverted Pendulum (SLIP) dynamics, making human locomotion during the compliant ground interaction simpler. In the biarticular coordinates, complete kinematic and dynamic analyses are developed for this new configuration. The approach also involves using a rotating task space to represent the swing and stance phases of human gait. The dynamic model is transformed into this rotating task space and a hybrid impedance and position controller is developed with optimized gains to minimize the tracking errors and prevent any disturbance. A disturbance observer controller is developed and integrated with the hybrid impedance control for the proposed linear SEA in order to improve the force control precision and the resilience against external disturbances. The simulation results show the feasibility of this new approach.