By 'tire-road friction estimation' it is meant to predict how easily a vehicle will slip on a road without actually making the vehicle slip. One way to quantify how easily a vehicle will slip is with the maximum coefficient of friction We develop a slip-based method that estimates tire-road friction during braking using data from standard ABS wheel speed sensors and a brake pressure sensor. During a braking maneuver, normalized traction force, My, vs. slip data is collected and the slope of this My vs. slip line is used to estimate the road friction. Results from field tests of the algorithm are presented. We can draw several conclusions from this work: 1. The problem of traction force estimation during braking can be reduced to the problem of measuring brake pressure and then estimating, KB, the brake pressure to brake torque gain. Numerous techniques can be used to estimate KB. Here, we have shown three: Recursive Least-Squares, an Equivalent Input Method, and the Acceleration Method. For our experiments, the Recursive Least-Squares technique gave the best results. 2. There is potential in using slip-based techniques during moderate excitation braking maneuvers to estimate My-max. Using wheel speed and brake pressure data from single braking maneuvers with My smaller/equal 0.5, we were able to distinguish in most cases between surfaces with mirco-max about 1,1 and My-max about 0,9. The braking approach to slip-based My-max estimation is complementary to approaches using traction data. A vehicle spends more time driving than braking, making the traction approach valuable. However, vehicles tend to reach higher My values during braking, making the braking approach valuable. 3. Data from our experiments shows a correlation between slip slope values obtained using data for My smaller/equl 0,5 and My-max. This adds to a small body of literature that indicates heuristically that such a correlation may exist, hut does not provide conclusive evidence. 4. One possible explanation for the slope of the slip curve-My-max correlation that contradicts neither established contact theory nor experimental evidence is an effect that we call the 'secant effect.' Even if the initial slopes of slip curves on a variety of surfaces are the same, a least-squares fit to experimental data taken using finite values of My can have different slopes for different surfaces.
Slip-based tire-road friction estimation during braking
Rutschbasierte Abschätzung der Straße-Reifen-Reibung beim Bremsen
2002
8 Seiten, 9 Bilder, 14 Quellen
Conference paper
English
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