The Development Of Software For The Wheel Rolling On The Soft Ground

The vehicle-ground mechanics, is a discipline studying the interaction between vehicle and ground, has a broad development space and great value, and many scholars between home and abroad have done a mountain of research and scored plenty of achievements. The research subject, based on the basic idea of the vehicle-ground mechanics, combined with the Discrete Element Method (DEM) and the computer simulation technology, is to develop the software for emulating the wheel rolling on the soft ground by the force, which is the application of the vehicle-ground mechanics, at the same time, has certain characteristics.There are two research methods, including the theoretical research based on physical test and emulation research based on computers technology, which have been used for the development of the vehicle-ground mechanics so far, and each has sphere of application by itself. In broad outline, three characteristics on the vehicle-ground mechanics can be summed up from the research home and broad:the wheel is simplified rigid, the research is from a global point of view of the movement, and the method is physical experiment. The research subject has completed the following work:model for the wheel-ground interaction system by the use of DEM; the emulation type of the wheel can be chosen as "rigid" or "elastic"; computer emulation is the application methods; the driving force to the wheel is the fundamental starting point of the study.The core idea of the DEM is that:through the movement of discrete unit at each time step the micro-level on the micro level, the overall performance of the system within the specified time is revealed, where the macroscopic research object is composed by a large number of discrete unit such as "granule" and "shape", and the emulation time is divided into "time-step" equally.In order to complete the research target, this paper follows the main aspects of work done.1. The data origin has been designedThe data origin is used to store the CAD metafile, where all the shapes used in the emulation system are supplied.2. The wheel and the soft ground have been modeledThe elements needed by the emulation models are "granule" and "shape", the former of which is used to simulate elastic entities, and the latter of which is used to simulate rigid entities. The one innovation of this research object is that:the rigid wheel is constructed by shapes. which are read in from the data origin, and have the same movement properties with granules. It is necessary of modeling the soft ground that shapes are used for limited the ground's emulation space, where the granules are accumulated in a definite pattern.In this paper, the two-dimensional analytical expression for the granule comes from the "Super Circle" equation, and Modeling of the wheels and ground, is to determine all the geometric parameters of the granules and shapes in this system, where the granules are divided into three types:"rim", "tire" and "soil"The elastic wheel is constructed by one rim and a layer of tire granules, and multi-layer model of the elastic wheel is based on the one-layer model being completed. According to the discipline on the distribution of soil granule size, the modeling method of the ground falls into three types:invariableness, chance distribution, and normal distribution。The ground model based on circular shape is implemented in the motion-computing model of the system, which ensures there will be interaction between the wheels and the ground always.3. The interaction between the granules and the ground has been computedThe force of granules must be computed ahead of the movement of the wheel being computed, when action models are needed. According to the different types of the two granules in the model constitutive relation, in this paper, the action models have been divided into six types:M1 is used for computing the force between rim and tire granule; M2 is used for computing the force between two tire granules; M3 is used for computing the force between tire and soil granule; M4 is used for computing the force between rim and soil granule; M5 is used for computing the force between two soil granules; M6 is used for computing the force between granule and shape. In brief, different action model adopts different computing formula to get the specifically interaction."Overlap" is a key physical quantity of mechanics computation, by which the system can determine whether there is a force between the two discrete units and what the value and direction of the force are. In order to compute the motion of the unit, the emulation procedure follows the calculation process including force, acceleration, velocity and displacement to obtain the movement locus of the granule or shape in each step-time in accordance with those classical mechanics laws.The main computation module of the emulation system is computing the motion of all the discrete units within the specified time, and storing the emulation data into the outer files for users'use.Based on the outer driving force that the system has given the wheel, the wheel is divided into the two types:driving wheel and engaged wheel, and the former is given a torque, while the latter is given a force. No matter which type'the wheel belongs to, the computation procedure about the wheel rolling on the ground is accordant.4. The demand analysis and design of the emulation software has been doneIn the process of demand analysis, the most important is user's demand, since user's degree of satisfaction is the first measure of the success of this product and the function and performance demand of this system start from user's demand.The main functions of this system include:selecting emulation objects; setting the physical parameters of emulation objects; modeling the wheel and grounds; setting the mechanics parameters of emulation objects; setting the parameters of emulation situation; emulating the wheel rolling on the ground by force; demonstrating the emulation results; storing the emulation data. The most important performance of this system is enforcement efficiency, so the research subject has made an effort to low the Time Complexity of the system and achieved significant results.This emulation system still leaves much to be desired for author's limited knowledge, and it must be improved especially in the implementation of efficiency, interface design, model parameter selection, etc.Because this system is based on MFC platform, it is necessary that unit modules are compatible with the main modules and the security, robustness, scalability of the system can be guaranteed. The testing for this software is mainly unit testing, which ensures the function of every module accurate, and based on which integration testing ensuring the wheel-ground interaction system meet the needs of users perfectly.This emulation system still leaves much to be desired for author's limited knowledge, and it must be improved especially in the implementation of efficiency, interface design, model parameter selection, etc.