Research On Torque Control Based On Motion Planning And Foot-ground Interaction For Hydraulic Quadruped Robot

The high autonomy,high compliance and high dynamic stability of the hydraulic quadruped robots in complex environments are the prerequisites and guarantees to entry into future military,production and life services.Based on above macro requirements,quadrupedal control system is generally divided into the planning layer,compliance layer and driving layer.The achievement of motion task goals often depends on the autonomy and anti-disturbance robustness of the motion planning layer;the robot's torso stability and foot-ground compliance depend on the foot-ground dynamic interaction in the compliance layer;and the implementation of motion trajectories and interactive force commands in planning and compliance layers must be realized through the driving layer torque control.To this end,this paper addresses the core issues of motion planning and force control of hydraulic quadruped robots in complex environments,focusing on breaking through the key technical difficulties within each layer of planning,compliance and driving layer,and taking into account the inter-layer linkages to ensure the torque tracks the task space trajectory and interaction forces simultaneously.This thesis addresses the planning and control of hydraulic quadruped locomotion in complex environments,starting from the analysis of quadrupedal locomotion characteristics,a point-mass model that can characterize the nature of the motion of the quadruped robot is established in the planning layer.The state variable dimensionality is then reduced.Based on the dynamics analysis of the point-mass model,a nonlinear optimization model with the Co M trajectory and footholds as decision variable is designed by combining the task commands,environment and gait characteristics.In order to achieve global optimality and online real-time responsiveness,this thesis discusses a stepwise optimization strategy based on offline preplanning and online replanning.The offline preplanning stage is aimed at the overall motion task optimality,the global Co M trajectory and footholds is generated based on the above optimization model.To achieve a good integration of global planning results and online replanning,the research of vector field tracking algorithm with global path space convergence is carried out by using the spatial convergence property of vector field path tracking.Combining the advantages of robust time-domain optimization of the model predictive control,an MPC online motion planning algorithm based on vector field path tracking is designed to solve the dimensional mismatch between the time function-based convergence speed and spacial global path.And then the purpose of motion planning with global and real-time is realized.To further explore the problem of quadrupedal trajectory reconstruction based on point-mass model,a gait pattern mode between typical walking gaits of quadrupeds and the virtual biped gait is studied.Based on the correspondence between the pointmass model and the virtual bipedal,a geometric mapping to the quadrupedal footholds is then established.Combined with the swing phase trajectory planning,the task space quadrupedal trajectory is finally planned.On the basis of the desired trajectory generated by the planning layer,this thesis draws on the compliance characteristics of the skeletal-muscular system of mammals and establishes a task space full-body compliance model in the compliance layer,including foot-end impedance model and virtual model of torso.The stiffness mapping between the two models is derived based on the equilibrium equation.Then geometric incremental connection between the torso and foot-end is established.In order to establish the force connection between the torso and foot-end,this thesis carries out the research on the foot force distribution algorithm based on analytical calculation and numerical optimization.Integrates the relationship of the force and position of the torso and foot-end,an equilibrium point control strategy based on position compensation algorithm and GRF tracking algorithm is constructed to establish the explicit synergistic relationship between torso and foot-end from the system perspective.The self-stabilizing compliance interaction performance of the system is then improved.To further explore the connection between the above task space motion planning layer,the foot-ground compliance layer and the joint space torque driving layer,centroidal dynamics of the quadruped robot is derived from the conservation of momentum of multiple rigid bodies,and then an explicit spatial mapping between Co M trajectory and the joint torques is established.Based on the centroidal dynamics,a model error compensation mechanism based on a PD feedback compensation is further adopted to integrate the multi-objective tracking evaluation indexes of the Co M,foot-end,torso trajectory and ground reaction forces.Then a multi-objective optimal torque control algorithm based on quadratic programming is established to solve the under-actuated ill-posed problem of the inverse dynamics and then achieve robust simultaneous tracking of motion trajectories and ground reaction forces.Based on the theoretical research and simulation analysis of the quadruped robot,a physical prototype of the hydraulic quadruped robot EHbot was built.The design process of the on-board hydraulic power source based on three motors driving gear pump and the hydraulic actuation system were elaborated.the research on force tracking compensation algorithm of hydraulic actuator unit based on force feedforward and disturbance compensation is carried out to improve the desired force tracking performance of hydraulic actuator unit under asymmetric chamber areas and foot-ground impact.The proposed locomotion planning and control algorithms were then carried out on EHbot,implementing the experiments of online motion planning,self-stabilizing compliance interaction and torque-controlled dynamic stability,as well as the comprehensive performance tests on speed,load and climbing capacity.The results show the feasibility of the physical prototype and the effectiveness of the motion planning and force control algorithms.In summary,this thesis provides a valuable theoretical basis and engineering experience for the future application of the hydraulic quadrupedal robots.