In the preventive safety field, AEBS (Advanced Emergency Braking System) has been widely spread to the market. AEBS is composed of alert-braking phase and full-braking phase. When AEBS functions, ABS (Antilock Brake System) is often used simultaneously. ABS has the work braking to be a target tire slip ratio which is constant in general. In order to use braking force to its maximum range, it is desirable for ABS to use information of the road surface condition, the road friction coefficient. Because the relations between the tire slip ratio and the tire braking force are varied with the road friction coefficient. However, the road friction coefficient estimator is difficult to be realized. As the control of ABS depends on only tire slip ratio, it is impossible to generate enough tire braking force on the various road surface conditions. In this paper, we propose a method to estimate the road friction coefficient during alert-braking phase. In addition, we changed the target tire slip ratio required by ABS and hastened the start timing of full-braking phase based on the road friction coefficient. The possibility of reduction of the collision speed compared with the conventional system was shown by using the method mentioned above.

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