Distributed Functions

The SBA control unit is the primary control unit of the hybrid brake system. It controls all processes, from detecting the braking request to activating the actuators of the brake system.

The actuator for regenerative braking is the drive train: using the power electronic box, the electric machines are activated such that they work as generators. So that they can generate electrical energy, they have to be driven mechanically. Thus they absorb torque, which acts as braking torque on the drive train. Therefore, with the possible decelerations of up to 3 m/s² , unstable driving situations could result if the braking torque acted on the rear axle only. Therefore, in regenerative braking, the multidisc clutch in the transfer box is also closed. Then, the speeds at the front axle and rear axle are the same, which is the prerequisite for splitting the braking torque almost equally between the two axles.

In this "by-wire" mode, as much brake energy as possible is recovered, i.e. fed to this first, electrical path. The conventional service brake is used for the remaining amount of energy only for decelerations greater than 3 m/s² or if the hybrid drive cannot convert the full brake energy. For this purpose, the SBA control unit activates the active brake servo. The servo generates the brake pressure for both brake circuits, which is distributed to the four wheel brakes by the dynamic stability control.

Only in fault situations or special situations are there emergency functions in which the SBA control unit does not assume the master role. For example, in situations with unstable dynamic handling characteristics, the dynamic stability control assumes the master role, taking higher priority in order to stabilize the vehicle. In that case, regenerative braking is no longer possible.

If one of the components required for regenerative braking fails or the power supply is interrupted, the hybrid brake system switches from "by-wire" mode to a conventional mode. In conventional mode, the mechanical connection between the brake pedal and service brake is once again made. This allows the vehicle to be decelerated safely using the conventional hydraulic brake system.

Fig 1: Implementation Of Brake Request By Hybrid Brake System

Courtesy of BMW OF NORTH AMERICA, INC.