Mechanical design and simulation studies of a quadruped robot motion control system




Sheng, Xiang

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This thesis focuses on mechanical design and simulation studies of a quadruped robot motion control system, targeting at designing an autonomous legged robot. The designed quadruped robot with ``X"-configuration is developed for traversing rocky and sloped terrain with a static walking gait. The mechanical design of the quadruped robot is illustrated in Chapter 2, including the main body design, leg design and component selection. In the design process, appropriate mechanical structures are utilized to minimize the energy consumption. To improve energy efficiency, a set of principles is proposed. Corresponding implementations are also concretely introduced in this chapter. In addition, to simplify the mechanical structure of the quadruped robot, the mass is symmetrically distributed about the frontal and lateral planes. To improve the leg agility, the leg mass is minimized. On the one hand, the lightweight design is implemented by optimizing the mass distribution of the leg mechanism. On the other hand, the key components are assembled in the body part instead of the legs as many as possible. A sufficient leg length is also selected not only to allow the robot to step on or over obstacles, but also to avoid the leg getting caught by objects. Particularly, the leg structure is demonstrated, including the hip joint, thigh part, knee joint and limb part with a telescoping joint. When the robot sustains extensive payload, the deformed shape in joints may lead to structural failures, thereby influencing the quadrupedal locomotion. Finite element analysis (FEA) is performed when designing the structural components in reasonable structures. The design processes of the shoulder part and brass rod are demonstrated as examples. Based on the setup of loads and fixtures, the maximum deformed shape of these structural components are analyzed. From FEA simulation results, the yield strength is two orders of magnitude larger than the maximum of von Mises stress. Hence, these components are suitable to be incorporated in the quadruped robot. Based on the designed mechanical structure, simulation studies of the quadruped robot motion control system are analyzed in Chapter 3, including the modeling for a robotic leg and animated simulation. Since the quadrupedal locomotion is executed by manipulating the postures of four legs, the leg model is significant to the motion control system, thereby being analyzed mathematically. Two links kinematic conversion is implemented between the foot-end trajectory and joint angles. The dynamic model of the leg is also computed to discovery the relationship between the actuating torques and joint angles. To animate the quadrupedal locomotion, a CAD robot model is converted into MATLAB. Following the predefined footfall pattern, four legs move in sequence to execute the creeping gait. The segment of the desired trajectory of the foot-end fits a fifth order polynomial and does not include the set of singular configurations. Then, the PD control is utilized to adjust the leg posture to track the desired path. Furthermore, the actual joint angles are calculated in the MATLAB/SimMechanics quadruped robot by using Euler-Lagrange equations. Lastly, simulation results are presented to analyze the tracking performance in the joint angles and foot-ends. Finally, conclusions of the thesis are summarized, and future work is presented in Chapter 4.



Quadruped Robots