Page - 84 - in Maximum Tire-Road Friction Coefficient Estimation

4 Sensitivity Analysis the tendency is similar to the results for a lateral acceleration of 1 m/s2. The same factor accounts for the results presented in the upper row of diagrams in Figure 4.14, whichcomprises lateralaccelerationsbetween5and6m/s2. Hereagain, the longitudinal acceleration increases fromthe left totherightside inFigure4.14. Ithastobementioned that the driving states of bax= 0.6 and bay = 6 (upper left) and bax= 3 and bay = 5 (upper right) were only reached in a stable way atµmax = 1.1 andµmax = 1.2, so there is only one data point, rather than a line, as in the other diagrams. With increasing bay at low bax, the sensitivity of wheel speed ωfr (outer wheel in turn) is bigger than that of the inner wheel speedωfl. The higher the longitudinal acceleration, the higher the influence of the inner wheel speed ωfl. This is due to the decreasing tire load on this wheel during cornering, which leads to an increasing wheel speed in comparison to a higher-loaded wheel when a driving torque is applied. Region 5 (combined cornering and braking) Figure 4.15 shows the standarized sensitivities for Region 5, which comprises negative longitudinal and lateral excitation. Similar to Figure 4.14, bax decreases from the left to the right side, whereas bay increases from the bottom row up. Unlike in Figure 4.14, the order of magnitude of the sensitivities does not depend so much on bay, but rather on bax. One possibility is that the bax achieved during braking are two times higher than during accelerating, see Table 4.1. 4.5.2. Sensitivities for all-wheel-driven vehicle In addition to the front-wheel drive (FWD), an all-wheel drive (AWD) with a wheel torquedistributionof 50%onthe frontandthe rearaxlewas simulated. The simulation used the same manoeuvre and the same initial vx,0 as the simulation of the front-wheel drive in Region 1. As expected, the absolute sensitivities for the AWD configuration shown in Figure 4.16 are smaller than those of the FWD configuration, cf. Figure 4.11. This can be explained by the fact that the wheel slips on the front axle are lower for the AWD configuration. Thus, it is expected that it is more difficult to estimate µmax for an AWD. This is consistent with results from previous investigations on AWD from Holzinger, [Hol92, p.86]. However, other than for the FWD configuration, the sensitivities of the rear wheels are higher and have a different sign. This is due to the positive driving torque applied in the AWD configuration, whereas for FWD the only torque present is the comparably small and negative rolling resistance torque. 84