IV. OPTIMAL BRAKING PERFORMANCE AND RBS EFFICIENCY
In recent years, more and more advanced braking systems are in development, which allow us to control the braking force on each wheel independently. The fully controllable hybrid brake
system can be controlled to apply braking forces on the front and rear wheels by following the ideal braking force distribution curve (Fig. 11). This control strategy can obtain optimal brake performance. Fig. 9 illustrates the principle of this control strategy for the vehicle on which electric regenerative braking is available only on front wheels. When the required total braking force on the front wheels is smaller than that produced by the electric motor, the electric motor produces the total braking force, and no mechanical braking force is applied. Nevertheless, the mechanical braking produces the total braking force for the rear wheels to follow the I-curve, as shown by point a in Fig. 9.When the required total braking force on the front wheels is greater than that produced by the electric motor, both electric and mechanical brakes have to be applied. For more braking energy recapture, the electric motor should be controlled to produce its maximum braking force that is limited by the electric motor or energy storage. As shown by point b in Fig. 9, the remaining is applied by the mechanical brake. It should be noted that, in low front-wheel speed caused by the actual low vehicle speed or closely locked wheels, it is hard for the electric motor to produce the braking torque due to the low electric motive force (voltage) generated in the stator windings of the electric motor. Therefore, in this case, the mechanical brake has to produce the total braking force as required. As seen in Fig. 9, a significant amount of braking energy is consumed by the rear brake, especially for weak braking (small deceleration). For example, at z = 0.3, around 33% of the total braking energy is consumed by the rear brake; at z = 0.1, this percentage reaches 37.8%. The battery should take into account the relationship between the SOC and its charging characteristics. In this paper, the input/output power and SOC of the battery are calculated using the internal resistance model of the battery. The internal resistance is obtained through experiments on the SOC of the battery. The following equations describe the battery’s SOC at
discharge and charge. At discharge