Zongquan Deng

Planetary rovers' wheel---soil interaction mechanics: new challenges and applications for wheeled mobile robots

With the increasing challenges facing planetary exploration missions and the resultant increase in the performance requirements for planetary rovers, terramechanics (wheel–soil interaction mechanics) is playing an important role in the development of these rovers. As an extension of the conventional terramechanics theory for terrestrial vehicles, the terramechanics theory for planetary rovers, which is becoming a new research hotspot, is unique and puts forward many new challenging problems. This paper first discusses the significance of the study of wheel–soil interaction mechanics of planetary rovers and summarizes the differences between planetary rovers and terrestrial vehicles and the problems arising thereof. The application of terramechanics to the development of planetary rovers can be divided into two phases (the R&D phase and exploration phase for rovers) corresponding to the high-fidelity and simplified terramechanics models. This paper also describes the current research status by providing an introduction to classical terramechanics and the experimental, theoretical, and numerical researches on terramechanics for planetary rovers. The application status of the terramechanics for planetary rovers is analyzed from the aspects of rover design, performance evaluation, planetary soil parameter identification, dynamics simulation, mobility control, and path planning. Finally, the key issues for future research are discussed. The current planetary rovers are actually advanced wheeled mobile robots (WMRs), developed employing cutting-edge technologies from different fields. The terramechanics for planetary rovers is expected to present new challenges and applications for WMRs, making it possible to develop WMRs using the concepts of mechanics and dynamics.

Terramechanics-based high-fidelity dynamics simulation for wheeled mobile robot on deformable rough terrain

Numerical simulation analysis of the motion of wheeled mobile robots is significant for both their R&D and control phases, especially due to the recent increase in the number of planetary exploration missions. Using the position/orientation of the rover body and all the joint angles as generalized coordinates, the Jacobian matrices and recursive dynamic models are derived. Terramechanics models for calculating the forces and moments that act on the wheel—as a result of the deformable soil—are introduced in consideration of the effect of normal force. A rough terrain modeling method is developed for estimating the wheel-soil interaction area, wheel sinkage, and the terminal coordinate. A simulation program that includes the above techniques is developed using Matlab and SpaceDyn Toolbox. Experimental results from a 4-wheeled mobile robot moving on Toyoura soft sand are used to verify the fidelity of the simulation. A simulation example of a robot moving on a random rough terrain is also presented.

Study of Locomotion Control Characteristics for Six Wheels Driven In-Pipe Robot

An articulated multi-unit in-pipe wireless robot inspection system was developed for inspecting the inner surface of pipelines using MFL (magnetic flux leakage), which consists of six modules, moves in a pipe 195 mm in diameter, and can be controlled by commands from an intelligent control center through a CAN bus. The proposed robot has six wheeled driving arms fixed circumferentially 60deg apart on the robot body frame, and driving wheels, which can stretch against the pipe wall, are mounted directly on the ends of driving arms and can be driven by a single DC servo motor, respectively, to form the six active wheel driven mode. Due to the power consumption of the robot and the capacity of batteries which contradicts the creation of great traction forces with low power consumption, we have developed a new type of compact robot control mode, namely, main-subordinate control mode. The locomotion control characteristics were tested in the laboratory, and the results proved that the modular structures of the robot driving arms and its multi-motor main-subordinate control mode have advantages of great traction force and low power consumption

Parameter identification for planetary soil based on a decoupled analytical wheel-soil interaction terramechanics model

Identifying planetary soil parameters is not only an important scientific goal, but also necessary for exploration rover to optimize its control strategy and realize high-fidelity simulation. An improved wheel-soil interaction mechanics model is introduced, and it is then simplified by linearizing the normal stress and shearing stress to derive closed-form analytical equations. Eight unknown soil parameters are divided into three groups. The highly complicated coupled equations, each of which includes all the unknown soil parameters, are then decoupled. Each decoupled equation contains one or two groups of soil parameters, making it feasible to make a step-by-step identification of all the unknown parameters that characterize the soil. Wheel-soil interaction experiments were performed for six kinds of wheels with different dimensions and wheel lugs on simulated planetary soil. Soil parameters are identified with the measured data to validate the method, which are then used to predict wheel-soil interaction forces and torque, with a less than 10% margin of error. The improved model, decoupled analytical model, and soil-characterizing method can play important roles in the development of both the planetary exploration rovers and the terrestrial vehicles.

Foot-terrain interaction mechanics for legged robots: Modeling and experimental validation

Contact mechanics plays an important role in the design, performance analysis, simulation, and control of legged robots. The Hunt–Crossley model and the Coulomb friction model are often used as black-box models with limited consideration of the properties of the terrain and the feet. This paper analyzes the foot–terrain interaction based on the knowledge of terramechanics and reveals the relationship between the parameters of the conventional models and the terramechanics models. The proposed models are derived in three categories: deformable foot on hard terrain, hard foot on deformable terrain, and deformable foot on deformable terrain. A novel model of tangential forces as the function of displacement is proposed on the basis of an in-depth understanding of the terrain properties. Methods for identifying the model parameters are also developed. Extensive foot–soil interaction experiments have been carried out, and the experimental results validate the high fidelity of the derived models.

Crashworthiness design optimisation of metal honeycomb energy absorber used in lunar lander

To provide a theoretical basis for metal honeycombs being used as buffering and crashworthy structures in a lunar lander system, this paper investigates the energy absorption properties of hexagonal metal honeycombs, and the size optimisation of the metal honeycomb energy absorber is performed by using response surface method (RSM). Specific energy absorption (SEA) is set as the design objective; the cell length and foil thickness of the metal honeycombs are optimised, while the applied mean crash load is set to not exceed allowable limits. The results demonstrate that this method is effective in solving crashworthiness design optimisation problems. Besides the design optimisation, parametric studies are carried out to investigate the influences of foil thickness and cell length on the metal honeycombs’ crash performances. The pre-processing software Patran is used to build up the finite element models and the explicit solver LS-DYNA is employed to perform the crashworthiness analyses.

Slip ratio for lugged wheel of planetary rover in deformable soil: definition and estimation

The wheel slip ratio is an important state variable in terramechanics research and the control of planetary rovers. Definitions of the slip ratio for a wheel with lugs and methods of estimating it for all wheels onboard have seldom been attempted. This paper presents several definitions for the slip ratio of a lugged wheel, which can be interconverted by altering the shearing radius. Equations for calculating the longitudinal velocity and slip ratio of a wheel moving on rough terrain are deduced from the horizontal speed of the wheels axle. Wheel-soil interaction experiments were performed for two types of wheels with different radii and lugs of different heights. The drawbar pull, torque, and wheel sinkage were measured using sensors. These data confirmed the effectiveness of the proposed slip ratio definition methods. Furthermore, two slip ratio estimation methods are proposed and verified: a visual information-based method by analyzing the lug traces marked on the terrain with high precision, and a terramechanics-based method in which the equations for the vertical load and torque are solved to estimate the slip ratios of all wheels.

Synthesis of Deployable/Foldable Single Loop Mechanisms With Revolute Joints

This paper presents a geometric approach for design and synthesis of deployable/foldable single loop mechanisms with pure revolute joints. The basic kinematic chains with symmetric mobility are first synthesized, and an intuitive geometric method is proposed for the mobility analysis of these kinematic chains. The deployable/foldable single loop mechanisms can be regarded as a combination of the basic kinematic chains with nontrivial mobility intersection, under this approach, the 5R to 8R single loop mechanisms with symmetric mobility are synthesized systematically. The method for determining the positions of the joint axes on polyhedral links is also proposed, so that the mechanism can be fully deployed or fully folded without suffering from physical interference. Under this framework, a class of novel deployable/foldable single loop mechanisms is developed. The computer-aided design models for typical examples are built to illustrate their feasibility. [DOI: 10.1115/1.4004029].

Experimental study and analysis of the wheels' steering mechanics for planetary exploration wheeled mobile robots moving on deformable terrain

Due to the requirements of challenging planetary exploration missions with wheeled mobile robots (WMRs), the driving mechanics of WMRs’ wheels moving on the deformable terrain has been researched intensively, but the mechanics of the wheels’ steering is lacking research. Systematic steering experiments were carried out using a single-wheel testbed for wheels moving on a lunar soil simulant with different radii, widths, lug heights, and lug numbers under different vertical loads. The influence of the eccentric distance and motion state, such as the steering motor’s angular velocity, steering angle, and initial wheel sinkage, were also studied. The experimental results are illustrated with plenty of figures and analyzed based on the preliminary steering mechanics model to draw conclusions. The steering resistance moment is caused by the lateral bulldoze stress and the shearing stress at the bottom of the wheel. The wheel sinkage and steering moment of resistance increase with an increase in steering angle, which could be fitted with exponential functions. The steering moment is the increasing function of the wheel sinkage, eccentric distance, vertical load, and wheel width. The conclusions, empirical models, and experimental data can be taken as references to the optimal design of a steering mechanism and the development/verification of a wheel’s steering mechanics model.

Slip-ratio-coordinated control of planetary exploration robots traversing over deformable rough terrain

Wheeled exploration robots are prone to slip during locomotion on deformable rough planetary terrain, which leads to loss of velocity and extra consumption of energy. Experimental results show that the power required for driving a wheel is an increasing function of its slip ratio; further, the tractive efficiency decreases rapidly after it reaches a peak value when the slip ratio is between 0.05 and 0.2. In this study, wheel-soil interaction terramechanics, which considers the slip ratio as an important state variable, is applied to analyze the quasi-static equations of a planar robot system. The slip ratios of wheels are controllable, but the degree of freedom is the number of wheels minus 1. A generalized algorithm for distributing the slip ratios of all the wheels of a robot to optimize the energy consumption is presented. Experimental and simulation results show that the “equal slip ratio” is at least a sub-optimal solution for optimizing energy consumption. Further, a more robust control method has been developed; this methods aims to equalize the slip ratios of all the wheels while maintaining a constant body velocity on rough terrains, without solving the values of the slip ratios. This method is verified by controlling a virtual four-wheeled robot using dynamics simulations.