3816 Mercedes Benz Brake Assist BA

The Mercedes-Benz BA system was introduced at the end of 1996, to cater for the tendency of the majority of drivers to under-react to emergencies. Even if their cars are equipped with ABS, more than 90% of drivers tend to be either fearful of stamping on the brake pedal too hard lest they lose control, or they fail immediately to realise the seriousness of the situation and do not apply maximum braking soon enough. The first mentioned type of inadequate reaction can increase, by up to 45% the stopping distances from a speed of 100 km/h.

In the event of excessively rapid depression of the brake pedal, indicating a panic stop, the full power of the booster is applied instantly by a solenoid-actuated valve housed within it, Fig. 38.30. As soon as the driver releases the brakes, the solenoid and, with it the booster, are deactivated. Since the system is used in conjunction with ABS, wheel lock is inhibited.

If, in an emergency, a driver without BA were to fail to apply instantly maximum force to his brake pedal, the stopping distance of a car travelling at 100 km/h could be 73 metres but, with BA, the stopping distance would be only 40 metres. Even in the event of a hesitant reaction by the driver, BA can reduce that stopping distance by about 6 metres. Incidentally, hesitant is defined as an initial braking reaction producing a deceleration of 7 to 10 m/s,

Control Valve Handbook

Fig. 38.30 If the electronic control of the Brake Assist system senses rapid depression of the brake pedal, indicating an emergency stop, it activates a solenoid valve in the brake servo unit to apply fully, instead of partially, atmospheric pressure to the right-hand side of the diaphragm. This provides maximum braking, although still modulated by the ABS system

Fig. 38.30 If the electronic control of the Brake Assist system senses rapid depression of the brake pedal, indicating an emergency stop, it activates a solenoid valve in the brake servo unit to apply fully, instead of partially, atmospheric pressure to the right-hand side of the diaphragm. This provides maximum braking, although still modulated by the ABS system

Fig. 38.31. Reactions producing deceleration values of less than 6 m/s or less are classified as inadequate.

From Fig. 38.30, it can be seen that the brake actuation unit comprises a fairly conventional brake servo with the addition of a pedal travel sensor, a solenoid valve which in fact is the air valve, an electronic control unit, and a brake release switch. So long as the brakes are inactive, induction manifold depression acts equally on each side of the diaphragm. When the driver moves the brake pedal, the push rod opens the air valve, applying atmospheric pressure to the chamber on the right in the illustration. This moves the diaphragm to the left, until the air valve is closed. Thus, without BAS, the pressure in the hydraulic brake system is at all times proportional to the pedal travel.

If the pedal travel sensor recognises a fear-induced excessively rapid movement of the pedal, the electronic control energises the solenoid in the centre of the brake servo unit, which opens the air valve fully, instead of partially: wheel lock is prevented by the ABS system. As soon as the driver releases the brake pedal, the release switch shown in the illustration breaks the circuit to the solenoid thus cutting out the boosting effect of the servo, Fig. 38.32.

The speed of operation of the brake pedal is not, however, the only signal upon which the electronic control bases its decision to activate BA. Other

Electronic Brake Assist

Pedal force

400

z

300

ce,

for

200

al d

100

e

P

Pedal force

With BAS

Without BAS

Time, sec

100 I 80

Pr 20 0

Brake pressure

100 I 80

Pr 20 0

With BAS

Time, sec

Fig. 38.32 As soon as the driver releases the brake pedal, a switch breaks the circuit to the solenoid to cut out the boosting effect of the servo factors include the speed of the vehicle, the state of wear of the brakes, signals from the electronic control systems for the engine and transmission management and from other systems such as ABS and, in some installations, those controlling wheel-spin and vehicle stability.

A major difficulty with such systems, however, is setting the threshold beyond which an emergency stop is automatically put into effect. This setting inevitably has to be a compromise that might not be appropriate for some drivers. For example, consider a nervous driver in an overtaking lane on a motorway, where traffic situations are liable to change with frightening rapidity. He might observe a change in the traffic movement ahead that does not call for emergency braking but, to be sure that he is ready to brake if the situation does become critical, he rapidly puts his foot on the brake, intending to apply relatively gentle braking yet, because of his nervousness, he moves exceptionally quickly. If the control interprets this as an emergency, he could find himself in a crash stop situation causing the driver of the car behind him to run into his back end. A similar situation could also arise at slower speeds in urban traffic, or if the driver suddenly realises that he is exceeding the speed limit in embarrassing circumstances!

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