Tire and force distribution modeling and validation for wheel loader applications.
Abstract: This thesis describes the development of a machine force distribution estimator and the calibration of tire models for wheel loader applications. Forces generated in the contact patch between the tires and the ground are crucial for understanding and controlling machine dynamics. When it's not possible to directly measure these contact patch forces they are estimated from other sensor data. Validated models of the contact patch forces are also used in machine dynamics simulations and are very relevant to model based development. Vehicle dynamics is of crucial importance to the automotive industry. In contrast the modeling of these forces has not been very important to the construction equipment industry and as such wheel loaders haven't been studied as much as conventional cars. In order to model the forces in the contact patch, suitable tire models have been studied and calibrated. The tire models have been calibrated by using two different sources, field data and test rig data. Two steady state tire models were chosen for the field data. These were the brush model and the Magic Formula. The resulting fit from the data was not ideal, but this was due to that the given data were of low accuracy. The Magic Formula was used for the test rig data, which gave a good overall fit. The results from the test rig measurements were then used in a transient model, the single contact point model, and the MF-Tyre software. The models were implemented in Simulink and were validated against experimental data. They showed good correspondence, but deviated for some levels of slip. Another important aspect of the wheel loader is the force distribution over the entire machine. Two estimators have been developed, one to estimate the vertical forces on each tire, the normal force estimator and one to calculate the turning behavior due to different force outputs on the tires, the turning torque estimator. The normal force on each tire is information that is important for the tire model, but it can also be used to estimate when the wheel loader risk tipping on its side. The turning torque estimation is useful for control systems to optimize the driving behavior of the machine. Compared against measured data from an actual wheel loader the normal force estimator showed a high accuracy in estimating the individual wheel vertical forces. The turning torque estimator could estimate the behavior of the torque but had problems when estimating the magnitude.
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