Service brake, function

Service brake actuation

Pressing the brake pedal triggers transmission of pneumatic and electropneumatic braking commands from the brake value sensor.

Pneumatic control

A protected circuit supplies direct braking pressure from the pedal travel sensor's pneumatic circuit to the front and rear pressure-control units. Under normal braking conditions the pressure transferred to the rear circuit is approximately 40 % less than that relayed to the front circuit. During emergency braking the same brake pressure is transferred to both circuits (via direct mechanical circuit control).

During each braking operation conducted with electropneumatic control, the brake pressure is blocked by valves in the pressure regulator modules. In the event of a failure in the electropneumatic brake control system, the valves remain inoperative and the brake pressure flows directly to the brake cylinders.

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Electropneumatic control

The brake pedal actuation travel is registered by the sensor unit that joins the pneumatic circuit in the brake value sensor; the sensor unit then converts this data into electrical signals suitable for processing in the brake control (BS) control unit.

With various data concerning factors relevant to braking as reference parameters, the control unit uses the signals from the sensor unit to calculate praking pressures for the front and rear wheel brakes; this information is converted to electrical control signals for the front and rear circuits. The pressure regulator modules then relay the appropriate brake pressures to the wheel brakes.

The control unit monitors brake pressures throughout the duration of the braking operation, and responds to variations in the brake-relevant information or changes in sensor signals by adapting the pressure.

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Braking force distribution (ALB function)

In calculating the braking force distribution between the front and rear axles, the brake control (BS) control unit relies on signals from the wheel speed sensors.

Calculations, which also reflect dynamic axle load shift, are based on the difference in slip rates between the front and rear wheels.

The brake control (BS) control unit continuously monitors the differences in slip rates and responds to variations between front and rear axle by activating the differential slip control function to correct the distribution of braking force.

This differential slip control function makes it possible for the wheels at both axles to make maximum use of surface traction and arrive at the wheel-lock threshold almost simultaneously.

The slip stability control replaces the conventional ALB function. The ALB controller and the sensors for monitoring axle load are no longer required.

Braking control/deceleration control

Braking response should remain essentially constant, regardless of the vehicle's load factor. The brake control (BS) control unit achieves this by assigning a specific braking value to every position of the brake pedal. During braking the control unit then controls the amount of brake pressure needed to achieve the braking level that correlates with the pedal's position.

On level surfaces the programmed specified braking level corresponds to the vehicle's actual deceleration. The actual rate of deceleration will necessarily vary from the specified level on uphill grades and descents. The brake control (BS) control unit counters this tendency by (for example) interpreting extended pressure on the brake pedal as a deceleration command. This system then automatically switches from brake control to deceleration control. An immediate adjustment in brake pressure then brings the vehicle's actual deceleration rate back into line with the brake pedal's travel.

On long descents the deceleration control protects the brakes from overheating by avoiding continuous application.