Research On Modeling And Simulation Of Mars Rover With Active Suspension System For Varialbe Moving Conditions

China is going to carry out Mars exploration in 2020.Different from the surface characteristics of the moon,the surface environment of Mars is more complex,including not only soft soil/sand,but also hard rocks and gravals.The complex and variable terrain makes the rover prone to slip,skid or stuck into soil during operation,which brings unprecedented challenges to the Mars exploration missions.Therefore,China’s Mars rover adopts an active suspension structure,which significantly promoting its mobility.In this paper,the wheel-terrain interaction model,kinematic and dynamic model and simulation technology for the prototype of six-wheeled planetary rover are studied.The experiments on the wheel-terrain interactive mechanism are carried out.According to the angular speed and linear speed of the wheel,the moving conditions of the Mars rover are classified,and the basic wheel-terrain interaction model is analyzed.Considering the special moving conditions of the Mars rover with active suspension,such as high slip sinking,pure skidding and steering,the corresponding experiments are designed and carried out with the high-performance single wheel testbed developed by the Harbin Institute of Technology.In consideration of the difference between the experimental results of single wheel and those of rovers,mechanical experiments and suspension deformation experiments are carried out,and the law of the rover moving across the complex and variable terrain is obtained,which provides the experimental basis for the following wheel-soil interaction model and the fidelity verification of simulation system.Based on the in-depth analysis of the experimental results of the single wheel,the wheel-soil interaction models corresponding to the wheel’s steering in-situ,slip in-situ and pure skidding are established,respectively.In order to calculate the contact force between the wheel and composite terrain conposed of soft soil and hard rock,this paper further analyzes the conventional mechanical model for wheel-rock interaction.It is found that the wheel-soil interaction model and the wheel-rock interaction model have the same essential characteristics,thus a unified wheel-terrain interaction model is proposed,considering variable terrain surfaces from soft soil to hard rocks and covering rover’s motion states of skidding,slipping and steering.Through further analysis of the factors in the model,it is found that the dynamic sinkage can be described by the variable sinkage exponent equation,thus wheel-terrain interaction model under multiple moving conditions is obtained.Single-input-multi-output(SIMO)joints are adopted to reduce the size and mass of the rover.The dynamic model of plenatary rovers is usually assumed that the joint only connects two links,but SIMO joint connects three or more links,which brings new constraints to the dynamic model of planetary rovers.In order to solve this problem,a new dynamic model is established.In the model,the planetary rover is regarded as a floating base system,and the space vectors are used to describe the joint position of the rover.On this basis,the Jacobian matrix and the second-order influence coefficient matrix describing the relationship between wheel speed and joint speed are obtained.According to the principle of virtual work,the general dynamic model of the Mars rovers is devrived.Then a novel multi-body dynamic model and its solutions of special forms for Mars rovers with SIMO joints are derived by supplementing acceleration constrained equations.By analyzing the different uses of the model,the solutions of forward dynamics,control oriented inverse dynamics,simulation oriented acceleration of joints and perception oriented wheel’s motion state parameters are obtained respectivelyBased on the unified wheel-terrain interaction model,a map called digital elevation map with physical properties(DEMP2)is proposed.The DEMP2 combines the physical and geometric features of the terrain,so in the simulation,the corresponding physical parameters of the terrain can be obtained according to the position of the wheel.Then,a linear interpolation algorithm is used to solve the problem of mechanical calculation errors caused by the sudden change of physical parameters in the simulation program.Furthermore,the calculation method of the wheel-terrain contact frame is established,which can be used to solve the motion information such as the sinkage of the wheel.Finally,a set of numerical simulation system of plenetary rovers is developed by MATLAB,and the visual simulation based on Vortex is introduced.Based on the experimental data of planetary rovers,the model and simulation established in this paper are systematically and comprehensively verified,proving that the simulation system has high fidelity.The results of this paper can be applied to the design,manufacturing,control and teleoperation in orbit of Mars rovers with passive suspension,active suspension or even active suspension with SIMO joint,and have great significance to the dynamic research of wheeled mobile robot on both theoretical guidance as well as engineering.