Page - 29 - in Maximum Tire-Road Friction Coefficient Estimation

2 Estimation of the friction potential accuracy of the friction potential significantly, [LKE13a]. When the tire characteristics arenotknown a priori (e.g. frommeasurementsona tire testbench), the tire character- istics have to be identified using defined driving manoeuvres, as for example shown by Kollreider, [Kol09, p.83-105], and Kerschbaumer et al., [KKP+10] or by e.g. using wheel torque measurements as shown by Albinsson et al., [AFBJ14]. One disadvantage of tire characteristics measured on different test benches is that the results vary significantly, as shown by Zamow, [Zam95], and have to be adapted, e.g. [HPRS09]. Nevertheless, a priori knowledge of the tire properties is assumed in the present work. 2.2. State of the art In the last 30 years, extensive research has been conducted on the topic of determin- ing friction potential. In addition to the efforts of individual institutions, Europe-wide projects suchasPROMETHEUS1 andFRICTI@Nhavemadecontributions to the iden- tification of the friction potential to increase traffic safety. PROMETHEUS’ long-term aim was to enable autonomous driving, wherein sensor-based adaptation to the current road conditions was one sub-project. Environmental sensors and vehicle-reaction-based approaches were developed within this project, see Sections 2.2.1 and 2.2.2. In the FRICTI@N project, information about different existing sensors was used in a novel sensorfusion system, see Section 2.2.4. Within the Intelligent Vehicle and Safety Sys- tems programme, different algorithms to estimate the friction potential were developed using optical sensors, force measurements and measurements of the vehicle’s dynamic state, [ABC+07]. The existing approaches can be divided into methods that focus on causes that influ- ence the value of the friction potential (i.e. cause-based approaches) and methods that observevariables thatareaffectedbythe frictionpotential (i.e. effect-basedapproaches). Both kinds of methods can utilize either signals from sensors that are mounted on the vehicle or sensors that are part of road-side infrastructure, see Figure 2.9. Car-to-x com- munication systems (C2x) are infrastructural systems, see Section 2.2.3. The current road conditions are measured in-situ and sent wirelessly to the vehicle. Since infor- mation about the road conditions is available before the vehicle travels on the road, identification of the friction potential ahead of the vehicle is possible. However, since thecurrent tireconditionsanddrivingstatesarenot taken intoaccount, it isnotpossible to calculate the safety margin. The performance of vehicle-fixed environmental sensors 1PROgraMme for aEuropeanTraffic ofHighestEfficiency andUnprecedentedSafety 29