Improving Traction Efficiency in Off-Road Vehicles - A Sliding Mode Approach
Abstract: This report evaluates the option of using an equal slip controller, an effective rolling radius and rolling resistance force observer in a 4WD wheel loader. The vehicle studied is an under development- vehicle designed by Volvo CE. A wheel loader is an over actuated, articulated vehicle that is mainly used with low velocities in construction operations. The efficiency subject has been studied earlier by many manufacturers in order to analyze the environmental and economical losses and profits. This has provided research opportunities of optimizing efficiency of different kinds. Since the tire is the only part of a vehicle that is in contact with the ground, the characteristic of the tire effects the dynamics. The analysis shows that the efficiency of a tire is directly connected to the slip ratio which in turn is a component of the overall efficiency ratio. Studies show that the slip ratio should be controlled in such a way that the highest value of efficiency rate is obtained. This optimal value is dependent on all four wheel’s slip conditions. Therefore a strategy should be formulated in order to apply changes to all wheels and not only one. Further analysis shows that the maximum efficiency in a 4WD wheel loader is obtained when the vehicle runs in such a way that the slip ratio is equal for all wheels. I order to maintain same amount of slip for all wheels a control strategy is required. In the control strategy the current amount of slip of each wheel is determined. Further, the average value of slip ratio is calculated. Finally the equal amount of slip is achieved using corresponding optimal torque inputs for each individual wheel. Thus a stable, robust controller is required. The controller used to achieve this goal is a sliding mode controller that is popular among control engineers for its stability, robustness against uncertainties, speed and easy implementation. For an accurate control, states of the rolling resistance and the effective rolling radius need to be determined. The pressure acting on the tire will cause deformation on the tire itself. This leads to a dynamic radius of the tire. This deformation is highly dependent on vertical stress and the structure of tire. Further more velocity, inflation pressure, vertical load etc. also have effect on rolling resistance. Rolling resistance has a great impact on the fuel consumption of the vehicle and the driving characteristics. These estimated variations of effective radius and rolling resistance build a feedback system to the controller which in turn derives the system to the desired slip ratio. As it turns out, the slip efficiency is increased using an equal slip controller. However, it is highly dependent on the ratio of thrust between front and rear wheels.
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