Venus Pasandi (2014)
Year: 2014 - Present
Thesis Title: Sub-Optimal Stable Trajectory Planning for a Point foot Biped using Central Pattern Generator
Online reference trajectory generation for legged robots and especially bipeds is associated with many difficulties such as coupled high-order highly nonlinear dynamic and unilateral constraints. In point-foot bipeds, due to inherent instability and lack of actuator in the ground contact point of stance leg, this issue is more complicated. In this study, the design of a comprehensive control system including generation of a sub-optimal reference trajectory and stable control of speed and length step for a point-foot biped walking in sagittal plane is considered. The generation of reference trajectory is divided into two steps. In the first step, for each joint of the robot, an appropriate path that provides the desired motion cycle (i.e. required speed and length step) for the biped, guarantee motion stability and have the minimum energy consumption is designed. In the second step, a central control system based on Central Pattern Generator (CPG) is proposed to produce the reference trajectory for each joint. This trajectory for different initial conditions converges to appropriate path, designed in the first step. Finally, a supervisory control is designed that in the presence of disturbances, such as impact or uneven ground, adjust speed and step length to maintain stability of the robot. In various stages of this research, the stability of the proposed algorithm is proved analytically. Albeit in this study, it is considered that contact of stance leg with ground is limited to a point, thus stability criteria based on support polygon such as ZMP criterion are unusable and stability of proposed algorithm is ensured according to other criteria such as Poincare map and Lyapunov exponent. It should be noted that this study is concentrated on sagittal motion of a five link point-foot biped and hence, performance of the proposed comprehensive control system is examined and evaluated in this special case.
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