Research On Hydrostatic Driving System Of Single Degree Of Freedom Articulated Wheeled Off-road Vehicle

Multi-functional wheeled robot for complex unstructured terrain has been one of the hotest issues all the time. This paper comes from the school-enterprise cooperation project: Development of Single Degree of Freedom Articulated Wheeled Off-road Vehicle. Aiming at the complex environment of wheeled off-road vehicle driving, a flexible hydrostatic scheme with multi drive mode is proposed, and the three driving modes are “engine-variable displacement hydraulic pump-four parallel hydraulic motors”, “engine-variable displacement hydraulic pump-double circuit parallel hydraulic motors” and “engine-variable displacement hydraulic pump-four series hydraulic motors”. With the three driving modes corresponding to the low-speed, medium-speed and high speed driving requirements respectively, the off-road vehicle can adapt to the complex unstructured terrain. Aiming at the proposed hydrostatic driving scheme, this paper uses the methods of mathematical modeling, test, Co-simulation by 1D +3D and experimental verification and carries out following works:(1) Based on the analysis of characteristics of different conjunction form of hydraulic transmission, the advantages and disadvantages of the variable displacement hydraulic pump-multi mode hydraulic motors driving scheme is analyzed in detail. For the “wheel slip” caused by different road adhension coefficient and “drag skid” phenomenon caused by inconsistent traveling path of different wheels, this paper presents a balanced tractive force control scheme based on fluid resistance technology.(2) By establishing the mathematical model of hydrostatic transmission system, this paper analyzes the high efficient working area of engine and the hydrostatic transimission(hydraulic pump and motor) efficiency, and determines the static matching principle of system parameters to ensure that the engine works in governor area and the operating point close to the power point. A pump control strategy associated with engine speed at a given throttle opening is made, and the travel motors combined mode is automatic controlled according to system pressure to adjust the engine operating point.(3)The steering process is a typical working condition that lead to difference of the wheel running path. Based on the traveling system kinematics and dynamics analysis, this paper research the power characteristics of each drive motor during steering process. The tractive force balance control system based on fluid resistance technology is drawn out by the difference of the driving power of each wheel in the process of the front wheel steering and the four wheel steering. Analyze the position of tractive force balancing orifice R1 and R2 in low-speed and medium-speed conditions, as well as the flow distribution characteristic of transmission apparatus during steering process. In order to combine the slip problem of vehicle wheels and walking path inconsistent problem, this paper puts forward a tractive balance control strategy by adjusting diameter of the orifices based on a fixed differential pressure.(4) Variable displacement pump’s efficiency is studied, which provide the basis for the simulation analysis. The simulation model of engine, hydraulic transimisson system and each control units are built through AMESim software. For the complex tire-road model, use LMS Virtual.Lab Motion software for modeling, and run multi-physics co-simulation. Thus a higher accuracy quantitative analysis for articulated wheeled off-road vehicles can be executed.(5) Based on the theoretical analysis and simulation research, a matching method of the engine-switch mode hydrostatic transmission is put forward, that is, the variable displacement control strategy is related to the throttle opening and the engine speed. Simulation results show that: the variable displacement pump’s efficiency is higher than 77.5% during starting, accelerating and climbing process, NFPE control has the function of preventing flameout of engine, and static pressure braking function can be realized by controlling the throttle opening.(6) Establish Motion model of the vehicle based on LMS Virtual.Lab motion, and combined with AMESim model to analyze the applicability of tractive force balance control in steering travel, and verify the effectiveness of tractive force balance control in the single side wheel slip, and study the diameter of orifice’s influence on tractive force balance control system. Summarized the balance tractive force control scheme studied in this paper can effectively eliminate the parasitic power during low-speed and medium-speed steering process, and in order to ensure a balanced tractive force in a straight line driving, avoid inconsistencies adhesion coefficient leading to a deflection body, the orifice diameter should be set as 1mm. At last, the simulation model is verified by analyzing the tractive force balance control characteristics during the process of steering through the experimental method.