Biped Locomotion: Stability analysis, gait generation and control

Abstract

Locomotion is an important domain of research in Bipedal Robots. Dynamics of the foot-link plays a key role in the stability of biped locomotion. Biped locomotion can be either with flat-foot (foot-link does not loose contact with ground surface) or with foot-rotation (foot-link rotates about toe). The initial part of this dissertation presents a flat-foot optimal walking gait generation method. The optimality in gait is achieved by utilizing Genetic Algorithm considering a tradeoff between walking speed and stability. The optimal flat-foot walking gaits are implemented on a biped robot - BRAIL 1.0. The robustness of such gaits in presence of disturbances is enhanced by applying zero-moment-point (ZMP) compensation into the robot's ankle-joint. Effectiveness of the ZMP compensation technique is validated by utilizing the technique to maintain postural stability when a humanoid robot, MaNUS-I, is subjected to disturbances (in the form of push from front or back, carrying weight in the back and climbing up/down slopes). Such flat-foot gaits are suitable when the biped is moving slowly. However, the foot-link can rotate during relatively faster bipedal activities.The bipeds, with foot-rotation, have an additional passive degree-of-freedom at the joint between toe and ground. Such bipeds are underactuated as they have one degree-of-freedom greater than the number of available actuators during the single-support phase. Underactuated biped dynamics (with foot-rotation) has two-dimensional zero-dynamics submanifold of the full-order bipedal model. Stability of the associated zero-dynamics is essential for the stability of the biped locomotion with foot-rotation. The nature of zero-dynamics is governed by the structure of the biped, foot/ground contact surface and certain control parameters.Landing stability of bipedal jumping gaits is studied considering the stability of the associated zero-dynamics. In the landing phase of jumping gaits, switching occurs between configurations with flat-foot and with foot-rotation. The associated bipedal zero-dynamics in jumping gait is modeled as a switching system. Stability of the switching zero-dynamics is investigated by two novel concepts - critical potential index and critical kinetic index. Utilizing the stability concepts, stable landing is achieved while implementing the jumping gait on a biped robot - BRAIL 2.0.A novel concept of rotational stability is introduced for the stability analysis of biped locomotion with foot-rotation. The rotational stability of underactuated biped is measured by introducing a ground-reference-point Rotational Stability Index (RSI) point. The concepts of rotational stability and Rotational Stability Index point investigates the stability of associated zero-dynamics. A stability criterion, based on Rotational Stability Index point, is established for the stability in biped locomotion with foot-rotation.