Brake by Wire Mode
After the voltage supply is switched on, the hybrid brake system carries out a self-test of all system components required for proper function of the by-wire mode. If this self-test is completed successfully, by-wire mode is activated. Otherwise, the hybrid brake system remains in conventional mode.
In by-wire mode, the brake pedal is mechanically decoupled from the brake servo. The driver's brake request is evaluated by the SBA control unit using the brake pedal angle sensor. Depending on the driving situation and the state of the hybrid components, this braking request is split into a regenerative portion and a hydraulic portion. To do so, the SBA control unit sends a nominal value to the hybrid master control unit to implement the regenerative portion. The hybrid master control unit then implements this nominal value via the hybrid electric machine control unit A and B control units. The electrical energy generated by the electric machines in this way is stored in the high voltage battery. Here, too, the control units of the power electronic box are involved (converting voltage and current level).
To transmit the hydraulic portion, the SBA control unit supplies current to the solenoid valve in the active brake servo. This allows air to flow into the work chamber, and due to the prevailing vacuum, a force is generated on the pistons in the tandem brake master cylinder. Thus pulls the push rod into the brake servo. As a result, the bolt of the brake pedal, which is plugged through the fork-shaped end of the push rod, cannot reach the mechanical limit position. Thus there is no counterforce when the brake pedal is actuated.
Instead, the counterforce is generated by the pedal force simulator. A force curve has been implemented that is practically identical to that of a conventional brake system. The shut-off unit acts as a rigid element in by-wire mode. The enclosed brake fluid cannot be compressed. Here, it cannot escape into the expansion chamber with the spring, as the chamber is blocked by a solenoid valve.