Research On System Design And Control Method Of The Hydraulic Dual Leg-Wheel Robot

Legged animals in nature can adapt to various complex terrains such as mountains,grasslands,and ravines,and wheeled vehicles are the most widely used means of transport for humans due to their fast movement and high energy efficiency.The leg-wheel compound mobile robot can effectively combine the advantages of both to achieve the best of both worlds.Since the advent of Boston Dynamics’ Handle,many researchers at home and abroad have begun to study dual wheel-leg robots.According to the driving method,these robots are mainly divided into two types:motor-driven and hydraulic-driven.Compared with motor drive,hydraulic drive has the advantages of high power density and strong load capacity.Therefore,the research on the hydraulic dual wheel-leg robot has more practical significance.At present,the research on hydraulic dual leg-wheel robots is still in its infancy,and the relevant theoretical system is not yet mature.This paper focuses on the system integration and motion control of a high-performance hydraulic dual leg-wheel robot.According to the sequence from the hydraulically driven single leg to the onboard hydraulic power unit and then to the whole robot integration.The focus is on the design and optimization of high-bounce hydraulically driven single-leg mechanism,single-leg jump trajectory optimization and variable stiffness buffer control method,high power density onboard hydraulic power unit design and load adaptive control,stability control method for dual leg-wheel robots,and highperformance physics prototype integration and other aspects of research.The main contents are as follows:1.Aiming at the problems of small range of motion and unmatched force-position jump characteristics of traditional articulated joints,a hydraulic-driven single-leg optimization design method with large range of motion and high torque output was proposed.A hydraulic single leg with a large joint motion range of 140 degrees and high bouncing moment distribution is designed.Firstly,the biomechanical analysis of the skeletal structure of the lower limbs of the human body is carried out,and the equivalent simplified model of the hydraulically driven single leg is constructed in combination with the motion characteristics of the dual leg-wheel robot.Secondly,by introducing a four-bar linkage mechanism,the range of motion of the knee joint is expanded and the torque distribution suitable for jumping is obtained,which meets the motion requirements of the robot with high bouncing.Then,the knee joint parameter model is established by comprehensively using kinematics and dynamics analysis methods,and the link parameters are optimized by nonlinear optimization method,and the optimal knee joint configuration is obtained.Finally,according to the optimized knee joint,a hydraulically driven single-leg physical prototype is designed,and based on this,the integration of the mechanical system of the hydraulic dual leg-wheel robot is realized.2.Aiming at the problems of insufficient efficiency of the robot and large landing impact force during the jumping motion,inspired by the phase plane theory and the biological bounce mechanism,an optimal jump trajectory planning and variable stiffness landing buffer control method are proposed.The hydraulic-driven single leg achieved the goal of reaching a jumping height of lm under a large load of 50kg and landing softly.Firstly,the floating base kinematics and dynamics model of the hydraulically driven single leg is established to provide a mathematical basis for the optimization of the jump trajectory.Secondly,the load characteristics of the hydraulic servo cylinder are analyzed,and the mathematical relationship between the output force of the hydraulic cylinder and the moving speed of the piston is obtained.On this basis,the nonlinear dynamic optimization problem is decomposed into multiple simple linear programs by numerical iteration method,and in this process,so as to obtain Optimal single-leg takeoff trajectory.Then,a compliant landing strategy based on variable stiffness-variable damping virtual model control is proposed.The virtual stiffness and damping are set as functions of time,and the starting time is the moment of landing,and the stiffness and damping parameters are gradually increased from the initial minimum value to the set target value according to the method of linear interpolation.Finally,the effectiveness of the proposed method is verified by a hydraulically driven single-leg squat jump experiment.3.Aiming at the problems of changing postures,small internal space and limited battery capacity of the hydraulic dual leg-wheel robot,a design method and load-adaptive control strategy of a high-power-density onboard hydraulic power unit with a closed positive pressure loop are proposed,which realizes the goal of robot power autonomy and energy-efficient control.Firstly,the working principle of the hydraulic system of the dual leg-wheel robot is analyzed.The high-speed motor drives the micro piston pump as the power source,and the lowpressure accumulator is used as the booster oil tank to establish a closed positive pressure system.Secondly,considering the large difference in the demand for flow of the robot under different working conditions,a scheme of supplying energy to the robot by the pump source and the high-pressure accumulator is proposed,which effectively improves the power density of the onboard power unit.Then,qualitatively analyzes the energy consumption of hydraulic servo system,and proposes a control method based on load self-adaptation,so as to realize the optimal power matching between the pump source and the actuator,and improve the energy efficiency of the hydraulic servo system.Finally,the performance of the power unit and the energy-efficient control strategy are verified by electromechanical-hydraulic multiphysics simulation and physical prototype experiments.4.Aiming at the problems of complex full-order dynamic model of the dual leg-wheel robot,low solution efficiency,and difficulty in real-time control,a multi-mode motion control method based on a hierarchical decoupling strategy is proposed,which realizes the highly dynamic and stable motion of the dual leg-wheel robot.Firstly,the dual legg-wheel robot is decoupled into two-wheeled and leg-to-body subsystems,which simplifies the dynamic model of the robot.Secondly,for the above two subsystems,a time-varying linear quadratic regulator and a model predictive controller are designed respectively to realize the basic balance and multi-mode flexible motion of the robot.Then,Kalman filter is used to estimate the system state optimally,which improves the accuracy of feedback information.Finally,the effectiveness and robustness of the control method are verified by simulating various motion modes such as squatting,external disturbance suppression,speed tracking,omnidirectional motion,and jumping on the virtual prototype platform.5.Realize the integration of the physical prototype system of the hydraulic dual leg-wheel robot and carry out a series of verification experiments.Firstly,a dual-leg-wheel robot controller with a real-time embedded system as the core is built,which realizes the effective combination between the upper-layer motion control algorithm and the lower-layer actuator drive.Then,on the basis of the mechanical structure of the robot and the onboard hydraulic power unit,the mechanical system,power system and control system are integrated,and the physical prototype of the hydraulic two-legged-wheeled robot is realized.Finally,the layered decoupling control strategy is transplanted to the prototype,and experiments such as squat motion,anti-interference,speed tracking,in-situ steering and uneven terrain adaptation are carried out to verify the robustness of the control method and the reliability of the physical prototype.