Tire Models for Use in Braking Applications

University dissertation from Department of Automatic Control, Lund Institute of Technology (LTH)

Abstract: The tire is a significant part for control of a vehicle. For a well-working brake system the contact properties between the tire and the ground is the limiting factor for a safe braking. To get optimal performance it is important that the system can utilize all friction resources. The brush tire model was a popular method in the 1960's and 1970's before the empirical approaches became dominating. The brush model gives an educational interpretation of the physics behind the tire behavior and explains that a part of the tire surface in the contact patch to the ground slides on the road surface. Information about the friction coefficient is revealed in the tire behavior even when low tire forces are transmitted. If the the brush model is sufficiently good it is possible to estimate the friction coefficient. In the thesis the influence of velocity-dependent friction and asymmetric pressure-distribution on the brush model are examined. The latter is used to introduce a calibration factor to improve the agreement of the model to real data. Performed vehicle tests show that sufficient accuracy might be obtained. The coupling between the longitudinal and lateral tire forces is discussed in detail and a new proposal to derive the combined slip forces from pure slip models is presented. This method relies on the physics from the brush model and includes a velocity dependency which is derived from the pure slip models. All information is extracted automaticly from the models, which allows continuous changes of the tire characteristics. The method shows good agreement to real data.