Wheel-soil Interaction Terramechanics For Lunar/planetary Exploration Rovers: Modeling And Application

NASA's Mars exploration rovers Sojourner, Sprit and Opportunity have achieved fruitful results and greatly widened the knowledge horizon of humankind, as a result of which an upsurge of exploring planets with wheeled mobile robots (rovers) was set up in the world. The future planet exploration missions, such as the MSL, ExoMars, Chang'e SELENE, require the rovers to traverse over more challenging deformable rough terrain than had ever encountered with limited supervision from the operator.Wheel-soil interaction terramechanics, which can be widely applied to planetary rover's mechanical design, performance evaluation, soil parameter identification, dynamics simulation, mobility/navigation control, etc, is a bottle-neck problem for improving the performance of planetary rovers and becomes a hot research topic at the budding stage. During the research and development process of a planetary rover, the terramechanics knowledge for conventional terrestrial vehicles is usually used directly. However, there are many differences between the planetary rover and terrestrial vehicles from the aspects of running environment, control mode, running state, payload, chassis configuration/dimension and wheel type/dimension, etc. Moreover, the conventional models are oriented to vehicle design with relatively low precision. It is quite necessary to research on the terramechanics aiming at planetary rovers, including experiments, theory and application methods.Terramechanics is a subject that combines theoretical and experimental study closely. The factors that influence wheel-soil interaction terramechanics are analyzed in the beginning, according to which the experiments are designed. Then a wheel-soil interaction testbed and the El-Dorado II four-wheeled rover testbed are used for experimental study with lunar soil simulant. The influence on terramechanics caused by wheel dimensions (radius and width), lug parameters (height, number and inclination angle), terrain information (slope climbing angle and cross angle), normal load, running state information (slip ratio, steering angle, velocity, repetitive passing times) are tested, in order to provide basic data for further theoretical analysis and modeling.After analyzing the wheel lug effect and slip-sinkage principle, a high-fidelity driving model for a wheel moves forward with slip is derived, and a method for amending load effect is brought forward. The model is verified with experimental data. Based on it, the skid model for wheels moving forward, side slip model, steering model, and coupled model of moving forward and steering are deduced. The motion of a wheel moving on rough terrain is decomposed into two basic motions: climbing up/down and traversing across slopes and the mechanics is analyzed.The driving model of wheel-soil interaction terramechanics for a planetary rover's wheel is simplified. Two kinds of closed-form analytical decoupled models are derived and three kinds of parameter identification methods are brought forward. Eight parameters that can reflect the bearing performance, shearing performance and contact angles could be identified to characterize the planetary soil comprehensively. They are verified with experimental data. The method that identifies soil parameters based on the integrated model has high fidelity. Methods that are developed based on closed-form analytical decoupled models, are suitable for real-time parameter identification. Simplified model considering load effect is deduced, based on which the parameters of Toyoura soil are identified with the experimental data obtained by El-Dorado II rover.Both the absolute and relative indices on evaluating a wheel's driving performance are summarized, including sinkage indices, drawbar pull performance indices and motor performance indices, and equations are deduced to investigate the relationships among them. The influences of wheel radius/width and lug height/number/inclination angle on wheel performance are analyzed according to experimental data from micro and macro aspects, based on which the principles and methods for designing the dimensions and lugs of a wheel are brought forward. According to the mission requirements of China's Chang'e lunar exploration project, lunar rover's wheels are designed and the performances of them are analyzed.Kinematics equations are developed with recursive method, by using the position and orientation of rover's body and the joint angles as generalized coordinates, and Jacobian matrices for calculating the velocities of wheel-soil interaction point and mass centers of all the components are deduced. Lagrange dynamics equation and Newton-Euler equation are then used to deduce generalized dynamics model combined with wheel-soil interaction mechanics. A key issue of calculating wheel-soil interaction area and coordinate is solved. The simulation is implemented with Matlab and SpaceDyn Toolbox, and verified with experimental data of El-Dorado II rover.Path following strategy is researched to control a rover moving in deformable rough terrain. Non-holonomic kinematics model is established and steering algorithm considering slip-compensation is designed. Two on-line slip ratio estimation methods are developed, based on lug traces and terramechanics, respectively. The relationship between energy consumption and slip ratio is analyzed. It is proved that"equal slip ratio"is a sub-optimal solution for energy optimal control. The velocity loss caused by wheel slip is compensated with feed-forward and feedback of rover body's velocity. The control algorithms are combined and an energy-time optimal path following strategy for planetary rovers is brought forward. The control algorithms are verified by dynamics simulation using parameters of El-Dorado II rover and Toyoura sand.Wheel-soil interaction terramechanics models and closed-form analytical decoupled models with high-fidelity for planetary rover's wheels are deduced, which are then successfully used for soil parameter identification, wheel design, high-fidelity dynamics simulation and high-performance locomotion control. The results of this study could provide solutions for the mechanics-based research of mobile robot, especially planetary rovers moving in deformable rough terrain.