Unified Wheel-soil Interaction Modeling In Longitudinal Skid And Slip For Planetary Rovers And Applications

The lunar and Martian surfaces are covered with loose soil and numerous steep slopes along their crater rims,which brings great challenge to planetary exploration rovers.It is easy for wheeled rovers to get stuck on such surfaces,and in the worst case this could lead to the failure of a mission.In order to avoid such problems in the future Mars exploration missions,it is necessary to conduct thorough investigations of the contact mechanics between the wheels and soft soil.Wheel-soil interaction mechanics has critical affects for the system's design,sensing system development,and control algorithms.Wheel-soil interaction mechanics in longitudinal slip directly affects the tractive performance of wheeled rovers,and thus get extensive attention.When a wheeled rover is climbing down or in braking,the wheels could be in longitudinal skid.However,according to the published articles,very little research has been conducted on the wheel-soil interaction mechanics in longitudinal skid.Hence,experimental investigation,terramechanics-based modeling for longitudinal skid,unified and analytical modeling for longitudinal skid and slip,and applications are conducted in this paper.Experimental investigation is the most direct and effective approach to investigate the wheel-soil interaction mechanics aiming at wheeled rovers.To develop an instrumental wheel that can measure the wheel-soil interactions,the wheel-soil interaction variables are divided into three groups first.Methods of wheel-soil interaction forces,torque,entrance angle,wheel sinkage measurement,and data collection system are then proposed.An instrumental wheel is developed,and the selected sensors are calibrated.Finally,the longitudinal skid and slip experiments are conducted on a planetary soil simulant.The influence of bulldozing effect is detected using the directly measured entrance angle.A predictive model and an empirical model of bulldozing effect are established.Two terramechanics models aiming at longitudinal skid are established.The equations that are used to calculate the maximum normal stress angle,shear stress transition angle,soil shearing displacement,and sinkage exponent are modified.The definition of equivalent wheel sinkage is introduced,based on which the normal stress can be computed directly.The accuracy of these two terramechanics models is verified by experimental results.After analyzing the reasons why the longitudinal skid and longitudinal slip terramechanics models cannot be unified,linear normal and shear stress equations about the equivalent sinkage are derived to simplify terramechanics theories aiming at wheeled rovers.The analytical wheel-soil interaction equations are then established by integrating the linear normal and shear stress,which are validated using four wheels and two planetary soil simulants.A switching function is proposed on the basis of the direction of the wheel driving torque,through which unified analytical wheel-soil interaction models in longitudinal skid and slip are then established.After being verified by experimental results,the unified analytical models are used to compute the terrain equivalent mechanical parameters and drawbar pull in real time.An estimation method for the entrance angle is established based on the unified analytical models for situations where wheel sinkage cannot be measured or estimated.Terrain equivalent mechanical parameters for a six-wheeled rover test-bed both climbing up and downhill slopes are estimated using the computed entrance angle,which are then used to investigate the influence of multi-passing effect on terrain mechanical properties,and the maximum relative error of the computed drawbar pull is less than 11% when the slope angle ranges from 0 to 18°.Moreover,dynamic models for a six-wheeled rover test-bed with skid-steering mechanism moving on a plane are developed.Terramechanics models for binding force are established based on the equivalent wheel sinkage.The influence of binding force on the dynamics models can be eliminated.The dynamic models are then used to estimate wheel driving torque and motor power consumption using the estimated entrance angle.Skid-steering power consumption experiments are conducted using the six-wheeled rover test-bed developed at University of Michigan,and the experimental results are used to verify the accuracy of the simulation results.The unified analytical terramechanics models for planetary rovers in longitudinal skid and slip are successfully established,and are used to estimate terrain equivalent mechanical parameters,drawbar pull,and wheel sinkage in real time.Furthermore,they are also utilized to evaluate the climbing up and downhill capability of a six-wheeled rover test-bed and the skid-steering power consumption of a six-wheeled rover test-bed in a plane,and lay a foundation for the real-time control of planetary rovers.