Study On Control Strategy And Application Of Underactuated Biped Robot

For the humanoid robot's potential application, related researches have aroused much attention since its appearance. In the past forty years, research on humanoid robot has acquired remarkable achievements and lots of research organizations built their prototypes to show and validate related results. But to the developmental level of humanoid technology, it achieves far from the expectation of human being. One of the critical reasons is that movement capability of current humanoid robots with huge mass is very poor, and the biped walking is slow and inefficient. So, underactuated biped robot is brought forward to research high speed and dynamic walking. The robot has no soles and touches ground with points, which brings great challenges to biped and humanoid stability control. For examples, it has no stable region, still standing is almost impossible and its supporting points may keep moving to achieve dynamic stability. But the greatest merit is that the underactuated biped robot can walk naturally with high speed and high efficiency. Therefore, the appearance of underactuated biped robot offers new methods for humanoid motion planning and control. The key of this dissertation covers the control strategy and application of underactuated biped robot, and its content as follows:As the basis of research on underactuated biped robot control, the hybrid control system model has been built. Several models are built respectively: kinematics model by D-H reference frame, control system model of single-supporting by Lagrange function, and states-switching model by angular momentum conservation. Then, the realizable conditions of the underactuated gait are discussed in great detail.The planning and control of the time-invariant gait of underactuated biped robot are researched. The gaits are planned and optimized by Genetic Algorithm, which takes the state variables as the optimized parameters, the minimum energy-expend as the optimized goal, the realizable conditions as the restriction, and then the problem of gait-planning is changed into a nonlinear-planning one. The gait is controlled by feedback linearization, so dynamic singularity states and finite-time stability controller are studied.Stability is the principal goal of biped motion control and it is also the most important in this dissertation. The current stability criterion can not be used directly to estimate the stability of underactuated biped robot for its special configuration. Based on Lyapunov stability theory, states of the robot can be classified into two parts: viable and inviable. Then the stability of biped robot is defined and the Lose Balance Degree (LBD) is taken as the stability criterion of underactuated biped robot. LBD is used to evaluate the degree of current state to balance one and synthesizes the pose and speed of the robot. Stable conditions of underactuated gait are given based on LBD, and the characters of underactuated gait are validated based on planar biped robot. Underactuated gait has the characters of gradually stability, so time-invariant gait has strong robustness and adaptability although it is planned offline.The stability control of 3D underactuated biped robot is the main goal of this dissertation. One of the critical conditions for stable biped walking is that the lateral motion should be in accordance with frontal one, where the frontal motion is the dominative part. Based on this fact, the stability control strategy of 3D underactuated gait is brought out which extends the planar one with step width adjustment to control the lateral walking cycle. By using this strategy, the lateral step cycle can be controlled to follow the frontal one and finally the stability of 3D underactuated gait is controlled. This strategy is robust to initial error and realizable for real robot. The stable still standing is very difficult for underactuated biped robot, but the posture is controllable. Then the stability controller based on T-S model and LQR controller has been designed to realize the asymptotic stability of double-support upright standing.Using above proposed strategies, application on humanoid robot has been studied. The humanoid robot can be look on as an underactuated system by active underactuated method and high-speed dynamic walking can be realized by underactuated gait. Emphases of this part include: how to realize ankle Zero-Moment control, how to improve the stability and convergence speed and how to control the walking speed by the moment adjustment of supporting ankle. Bipedal walking always starts and ends with still standing, so the control of starting and stopping processes are studied and fully underactuated walking process for humanoid robots can be realized.Finally, the main content of this dissertation is summarized and key points of future research are discussed.