Page - 17 - in Maximum Tire-Road Friction Coefficient Estimation

2. Estimation of the friction potential The first part of this chapter describes the physical effects in the contact patch between tire and road that mainly influence the friction-based force transmission and the friction potential. The friction potential is described on different levels of scale (e.g. in the contact patch or globally for the vehicle) and factors that influence the friction potential arediscussed. Thesecondpartof this chapterpresents the stateof theart for estimating the road condition. This includes a short overview of cause-based and effect-based methods, as well as infrastructural systems and sensorfusion. Finally, the requirements for an estimation algorithm are presented. 2.1. Definition of the friction potential The force transmission between tire and road depends on the frictional behaviour in the contact patch. The coefficient of friction between rubber and asphalt results from com- plexmechanisms,whichthemselvesdependonseveralparametersandvariables. Mostof these influences show non-linear behaviour on the coefficient of friction, [WHW02]. The relation with the two main influences, namely the normal force and the sliding velocity, are described within this section. The coefficient of friction µ can be used to charac- terise the grip when it is treated as a direction-dependent variable with different values forµ for the longitudinal and lateral direction. Generally, the coefficient of frictionµ is defined as µ= FF FN (2.1) with the friction forceFF acting in the contact plane and the normal forceFN acting on the contact plane, [Bac96, p.8]. The maximum achievable coefficients of friction µmax between tire and road limit the maximum transferable horizontal tire forces. In this work, µmax is referred to as friction potential. IntroducingFmaxF as the maximum transferable force in the contact plane, the friction potential reads µmax = FmaxF FN . (2.2)