Push Recovery Control of Bipedal Robots

Al-Tameemi, Ibrahim Salman Hussein

Etd

Push Recovery Control of Bipedal Robots

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Bipedal robots are expected to play a crucial role in daily life, supporting humans with tasks, functioning in industrial facilities, providing healthcare, and executing search and rescue missions in hazardous situations. Bipedal robots are complex mechanical machines requiring extensive research. Bipedal robots struggle to maintain balance in the presence of external disturbances, which limits their interaction with their surroundings. Therefore, developing a robust push recovery control is essential for these robots to maintain balance while executing the planned tasks. The objective of this work was to develop a push recovery control system that maintains the stability of the momentum’s rate of change (Disturbance Avoidance Phase) and subsequently restores our robot (HURON) to an upright position once the disturbance subsides (Posture Recovery Phase). We have developed and validated a reaching law-based Sliding Mode Control (SMC) to stabilize the rate of change of linear momentum. The proposed SMC incorporates a Variable Power Rate Reaching Law (s˙), dynamically adjusting to changes in the system. The study examined the regulation of angular momentum through two approaches, considering the CoP, CoM, the desired rate of change of linear momentum and Ground Reaction Forces. This study also proposed using the null-space method to restore the robot to the upright posture without interfering with its main momentum controllers. The Higher Priority task is for the momenta controllers (Disturbance Avoidance Phase), while the Lower Priority task is (Posture Recovery Phase). The simulation results demonstrated that the proposed push recovery control effectively preserved standing stability in the presence of external disturbances. Moreover, the proposed control method effectively reduced chattering in joint torques and eliminated oscillations. Additionally, the results demonstrated that the push recovery control, when implemented using the Second Approach of angular momentum controller, yields smaller CoP position values than the First Approach. This study also involved conducting experiments where two distinct pushing forces were applied to our robot (HURON) within a short time. The robot demonstrated remarkable stability, even under the influence of these forces. This stability can be attributed to the effective implementation of the null-space method, allowing the performing of both the Disturbance Avoidance Phase and Posture Recovery Phase simultaneously. A comparison with established controllers showed the superiority of the proposed approach, particularly in reducing the CoP peak. This thesis additionally presented our ongoing research focusing on the stepping strategy. If the robot cannot maintain its standing stability when subjected to a large pushing force, it must take a step. Our methodology involved selecting the step position based on variations in momenta. Subsequently, the desired CoM and swing leg trajectories were generated using a quadratic Bezier curve and a Gaussian function. Subsequently, a multi-objective optimization problem was employed to minimize the momenta and swing leg trajectory error while considering stability constraints. The experimental stage is currently in progress. Initial findings indicate promising results; nevertheless, a thorough examination and explanation are still needed. The ongoing data collection and analysis processes will yield conclusive results in the coming days, further validating the efficacy of the employed methodology. Our future plans include implementing our proposed push recovery control to our manufactured bipedal, HURON, once it is ready for experimental tests. Additionally, we plan to complete the current work related to push recovery via stepping.

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