Terradynamics For Legged Vehicle: Modeling And Experimental Validation

The neglect of interaction mechanism between the moving mechanism and the ground may cause serious consequences.In 2009,the NASA Mars Rover Courage in the Trojan Sands of Mars caused a serious slip failure due to wheel scraping,which eventually caused the Curiosity to fall into the soft sand dunes and unable to move.The wheel of Opportunity had also sunk on the surface of Mars Meridiani Plain to reach 30 cm,and later it struggled to escape.Due to the advantages of discrete foot-ground contact points,flexible adjustment of foot-rest points,and the ability to cross complex obstacles,legged vehicles have broad application prospects in military,planetary detection,and nuclear power station rescue.The high-performance legged vehicle can make full use of the advantages of legged equipment,and can better complete the tasks in soft and rugged areas.Terradynamics is the core and theoretical source of the movement mechanism of legged equipment,which plays an important role in the research of foot end trajectory planning,foot end attitude control,path planning,on-line identification of ground mechanics parameters and so on.Based on the theory of terramechanics,this paper studies the foot-ground dynamic modeling and experiment of walking feet on hard ground and soft ground.Modeling and analysis of terradynamics on hard surfaces.First,taking the hexapod platform as an example,the foot coordinate system and the COM coordinate system are defined.On this basis,the attitude angle a nd velocity angle of the flat foot,semi-cylindrical foot and spherical foot are defined.On the hard surface,the elastic foot with a spring at the end is taken as the research object.Based on the Hunt-Crossley continuous collision basic model and the Coulomb friction model,a mathematical model of the elastic foot on a hard surface is established.This model includes foot end force,foot end deformation,foot end equivalent stiffness,and equivalent damping.Modeling and analysis of foot-ground dynamics on soft ground.Based on an in-depth analysis of the bearing and shear properties of the soft ground,it is co ncluded that the main cause of foot slip is the lack of shear resistance of the soil.Based on the Bekker and Janosi semi-empirical formulas in ground mechanics,the stress distribution functions of the foot-soil contact area of flat feet,semi-cylindrical and spherical feet are established.Based on the linearization theory in ground mechanics,a foot-ground dynamic model is established.The established model contains parameters such as soil mechanical parameters,foot-to-ground attitude,foot-to-ground speed direction,and foot geometry.Development of single-leg terradynamics testing system.A data acquisition software and a single-leg terradynamics test system design were designed and developed.The software can complete the collection of foot propulsion,bearing capacity,lateral force,foot depression,foot posture angle and ground inclination during the foot-ground interaction.The single-leg towing,loading and unloading can be carried out through the servo control system.The test system can set up soft ground with different cohesion and internal friction angle,and can adjust the tilt angle of the ground.The test system provides support for the effective development of the experiment.Research on the identification of terradynamics parameters.The terradynamics models on the ground with different degrees of softness and hardness are quite di fferent,and different identification methods need to be designed according to the specific conditions of the parameters to be identified by the model.A parameter identification method was designed for the terradynamics model of elastic feet on hard surfaces and hard feet on soft ground.The stiffness and damping terms are identified separately on the hard surface.Using the quasi-static loading method,the stiffness term coefficient is obtained through identification.The bearing capacity model of the flat foot on the soft ground is established,and the be aring parameters of the soil identified by the pressure plate experiment are used to predict the bearing capacity of the foot.The prediction error is 18.24%.The stress distribution function of the flat foot is established,and the experimental data of th e foot end angle β of 5°,15°,25° and 35° are identified by nonlinear fitting,and the stress coefficient of the propulsive force and the bearing capacity is obtained.The maximum fitting error is18.57%,and the minimum error is 9.89%,which verifies the validity of the establishment of the stress distribution function of the flat foot.