Active Brake Booster
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 in severe stability assist events and makes sure that the hydraulic control unit 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 release 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 release switch indicates when the brake pedal has been depressed. 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 switch itself provides indication from two positions. In its rest position, it supplies two signals, normally open (NO) and normally closed (NC). Upon application of sufficient force to initiate movement of the key away from the valve body, the brake booster 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 release switch is held in position by the input rod spring and remains against the valve body. Although the BPP switch changes state when the brake pedal drops, the release switch remains inactive until such time as it encounters an external force from the driver, whether in a stability assist event controlled by the stability assist module, or in a standard mechanical braking event.
The BPP switch sends brake application inputs to the stability assist module. This signal is called driver brake application. 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 application when either the BPP switch or the release 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 will attempt to keep the driver brake application signal accurate. If the stability assist system detects faults on the driver brake apply 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 a stability assist event when the driver is not applying the brake pedal. The switch can also change states during the system check of the brake booster. Only the stability assist system can interpret these changed states and determine which are actually due to driver brake application. Similarly, the stability assist system can take advantage of its other sensors to detect BPP switch failures.