Control Strategies For Stable And Flexible Running On Underactuated Biped Robots

Under-actuated bipedal robots have great advantage in fast and efficient locomotion, but are poor in stability and agility by the existing control theory methods. There are few control strategies of gaits regulation, disturbance rejection and varied unknown terrain adaption yet for a running biped with underactuation, which result the biped to fall easily. To overcome these difficulties, this dissertation proposes a new criterion and designs a series of control strategies to ensure both stability and agility on fast-running biped with underactuation. The contributions of the research are listed in the following aspects.Firstly, aiming at the poor agility in biped with multi degrees of underactuation, this paper presents control strategies for speed regulation on a running kneed biped with underactuation de-gree two. In order to achieve variable speed running, a set of desired gaits (each at a different speed) is designed with the controlled Lagrangian method for the under-actuated robot. Then an event-based control law is applied to stabilize the running gait according to the running speed specification, and an energy shaping control is added for attraction enlargement as well as converge speed improvement. At last a transitional control is developed for smooth transitions between dif-ferent running gaits.Secondly, to solve the contradiction requirements between stability and agility, a novel notion of sustainable running is proposed for the first time. The stability notion in systems and control theory has been defined in the sense that the system trajectories converge to an equilibrium point or a limit cycle, whereas in agile biped locomotion, with the goal of preventing fall, the system trajectories may not need to be convergent to the reference limit cycle, but switch to any period or non-periodic trajectories. Thus, it is essential to propose the sustainable running criterion to ensure that a biped can sustain a running motion without falling. A running-feasible state set is also estimated by using the orbit energy and the symmetric property of the inverted pendulum model, which allows a biped to follow any running gait without falling as long as the system states within a running-feasible set all the time. Finally, we tackle the problem of biped running under a sudden push and varied unknown terrain. The main idea is to maintain the state of every step at touchdown within a running-feasible set instead of a specific point, from which the biped is capable of running without falling down. To address these issues, the sustainable running criterion is used to generate non-periodic reference trajectories within a running-feasible set for disturbance rejection in real time, and control strate-gies are designed to track the reference trajectories in every running step. Thus an underactuated running biped resembles the reaction pattern of human being to against external disturbances.This dissertation gives rigorous theoretical proof for the proposed criterion analysis and con-trol design methods. Several simulations are given in each chapter to illustrate the results.