IMPORTANCIA DEL SOFTWARE ESPECIALIZADO DE DISEÑO

Braking on low-µ surface

Figures 3.7a)-d) show the test results of braking with ABS using SMC on a slippery road surface (similar to packed snow in the real life situation). The slip ratio for individual wheels is well controlled around the target value in order to avoid wheels locking. The stopping distance is improved by 18% compared to the non-ABS braking event. The braking torque demand by the driver is clearly greater than the available friction between the road surface and the tyres, hence the brake torques are limited and reduced to satisfy the target slip ratio value as illustrated in figure 3.7 c) and d).

Figure 3.7: SMC straight line braking with roadµ=0.3,Vinitial=50km/h,sre f=-0.1.

Braking on split-µ surface

Slip control is very important when there is uneven friction of road surfaces. In this example, a vehicle braking on a road with dry asphalt on the left wheels (µ=1) and on snow on the right wheels (µ=0.3). Yaw moment will be created towards the high friction side of the road if the same brake torque is applied between left and right wheels. ABS

will detect the high wheel slip ratio and quickly retard the brake torque to avoid wheel locking and vehicle from spinning. Lower brake torque is applied to the wheels on low-µ to maintain the wheel slip within acceptable value as illustrated in Figure 3.8e) and g). To prove this observation, we can see that the maximum driver steering angle required to maintain the intended path is 68deg and the maximum yaw rate achieved by the vehicle is 8.6deg/s which are shown in figure 3.8b)-c) respectively. This observation indicates that with sufficient countersteering by the driver model in CarMaker, the vehicle can be safely stopped and to demonstrate that the vehicle stability and steering response can be maintained.

Braking in turn

Emergency braking while cornering is a good example to demonstrate the benefits of having ABS in a vehicle. Without ABS, the vehicle will understeer once the friction between road surface and tyres is saturated and sliding instead of turning in following the cornering radius. On the other hand, slip control will try to maintain the wheel slip ratio following the target value. This will avoid saturation of the lateral grip and allow the driver to steer and maintain the vehicle turning radius. There is also a risk of oversteering the vehicle if only the rear wheels are locked. This observation is clearly shown in figure 3.9b) where maximum driver steering angle required to maintain the cornering path is 44degwhereas the driver will have to provide steering angle input of 191degin the ABS- off scenario. Based on Figure 3.9a), it is clearly shown that the individual wheel slip controller maintains the slip ratio of each wheel close to the reference value by varying the friction brake torques. Figures 3.9d)-g) show the capability of individual wheel slip control using EHB which indicates different braking torque commanded by the SMC for each wheel in order to satisfy the slip target.

Lane change braking

Figure 3.8: SMC split-µ braking,Vinitial=50km/h,sre f=-0.1.

formance of the ABS. Figure 3.10a) indicates the capability of the ABS to individually control the wheel slip by controlling the EHB braking torque for the individual wheel as shown in 3.10d)-g) to allow the vehicle to maintain its stability and steering response. Maximum yaw rate achieved on the vehicle is 9.8deg/s and the driver steering wheel angle input to control the vehicle throughout the manoeuvre is 58deg. In comparison,

Figure 3.9: SMC brake in turn onµ=0.5,Vinitial=50km/h,sre f=-0.1.

the non-ABS case causes the vehicle to understeer and failed to follow the intended path. Again, the lateral grip between tyre and road surfaces is saturated if the wheels are locked during braking. Additionally, the vehicle stability and steering controllability are main- tained during ABS braking scenario even without the vehicle stability control assistance.

Figure 3.10: SMC lane change braking onµ=0.3,Vinitial=50km/h,sre f=-0.1.

Braking onµ-jump

A transition of road surface’s friction coefficient is another critical situation which in- volves emergency braking. ABS has to be able to adapt to the change of the road surface condition and avoid wheels locking. Based on figures 3.11b), there is a spike of the slip ratio (sx=0.55) overshooting when the vehicle is just entering the slippery road condition

(µ =0.3) from a high adhesion road surface (µ =1). Braking torques illustrated in figure 3.11c)-d) for both front and rear wheels are quickly reduced to allow the wheel slip ratio to satisfy the reference value ofsx=0.1.

In other situation which is the transition from low-µ to high-µ (positive µ-jump), there is a clear observation that the controller is trying to quickly minimise the slip error by increasing the commanded braking torque as shown in figure 3.12. In other words, the slip controller is robust enough to identify any change on the road surface and adapt accordingly.