Active Brake Booster (ABS With Stability Assist)
The active brake booster functions like a conventional brake booster with the added feature that it can be actuated electrically by the stability assist module. The electrical brake booster actuation is necessary during panic assist and severe stability assist events. During severe stability assist events, an electrical actuator of the booster ensures that the hydraulic control unit (HCU) can generate enough brake pressure to maintain vehicle stability. The brake assist from the brake booster is especially useful in cold weather when the hydraulic control unit cannot draw the brake fluid from the reservoir quickly at cold temperatures.
A solenoid within the active brake booster has a brake pedal force switch to indicate when the driver is stepping on the brake pedal. The solenoid provides electrical actuation of the brake booster. Without the force of the input rod, the air valve is directly opened due to the movement of the energized solenoid. With increasing current applied to the solenoid, the air valve opens and output force is created. With decreasing current applied to the solenoid, the air valve is closed and the vacuum valve opens, reducing output force. The brake pedal force switch indicates when the brake pedal has been applied. It is integrated into the booster key that normally sits against the rear shell of the boot in its rest position and it adjoins the valve body when in its balance position. The brake pedal force switch provides indication from 2 positions. In its rest position, it supplies 2 signals, normally open and normally closed. Upon application of sufficient force to initiate movement of the key away from the valve body, the brake pedal force switch changes state, signifying a driver application.
When the stability assist module is activating the brake booster, the brake pedal is pulled forward as the valve body moves. The brake pedal release switch is held in position by the input rod spring and remains against the valve body. Although the brake pedal position (BPP) switch changes state when the brake pedal drops, the brake pedal force switch remains inactive until it encounters an external force from the driver or a stability assist event controlled by the stability assist module.
Modules which are normally connected to the BPP switch are connected to a signal from the ABS module. This signal only changes state when the driver has applied the brake. If the ignition switch is in the RUN position, the brake booster is not electrically actuated, and no failure has been detected, the driver brake application signal will indicate a brake applied when the brake pedal force switch built into the brake booster indicates that the driver has applied the brake pedal. If the ignition switch is in the OFF position, the driver brake application signal will indicate a brake apply only when the BPP switch indicates that the driver is applying the brake pedal. When the ignition switch is in the OFF position, failures cannot be detected. If there are failures, several layers of redundancy try to keep the driver brake application signal accurate. If the ABS module system detects faults on the driver brake application signal line it will also attempt to maintain correct stoplamp operation by communicating on the standard corporate protocol (SCP) network. Control of the stoplamp operation is also required because the BPP switch can change states during an ABS module event when the driver is not applying the brake pedal. The BPP switch can also change states during the system check of the brake booster. Only the ABS module system can interpret these changes and determine which ones are actually due to the driver applying the brake. Similarly, the ABS module system can take advantage of its other sensors to detect failures in the BPP switch.