ABS control function

When the driver applies the brakes during critical driving situations (wet or slippery road surfaces, driver's frightened reaction to a situation, etc.), the wheels of the vehicle may begin to slip so that without ABS intervention it may not be possible to steer the vehicle and the vehicle may therefore swerve off the road and begin skidding.

In such situations the antilock brake system (ABS) intervenes. Using the rpm regulators (B79, B80, B81 and B82) the ABS quickly detects when one (or more) of the wheels is about to lock up and instantly ensures that the brake pressure is kept constant or reduced (by controlling the solenoid valves in the hydraulic unit and the ABS return flow pump). The vehicle follows the steering, remains stable and is optimally braked.

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Control circuit

The ABS control circuit is based on a simplified control system:

- One non-driven wheel

- The vehicle mass

- The wheel brake

- An idealized friction factor slip curve (friction factor: friction coefficient between weight pressure at the wheel and the road surface)

The control variable is the rotational speed of the wheel, from which the circumferential deceleration and the brake slip are derived. The actuating variable is the brake pressure at the wheel.

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Reference speed

The brake slip is not calculated by the ABS control module (A2) directly, but via a reference speed which corresponds to the wheel speed under ideal braking conditions.

The reference speed is constantly being recalculated by the electronic control module itself from the generated wheel speed signals.

Control cycle

The following describes the control cycle on one wheel. Approx. 4 to 10 control cycles are performed per second, depending on the road surface conditions (the ABS control module also detects the prevailing road conditions and adapts the control to suit them). The control sequence on the other wheels is identical.

If the ABS control module detects that the wheel speed has reached a defined threshold limit value, ABS brake control is initiated.

The pressure in the brake circuit is constant at the start of the ABS control process. If the wheel tends to lock up, the pressure is reduced. At the same time the brake fluid in the reservoir is pumped back into the brake circuit upstream of the brake master cylinder by the return flow pump.

When the pressure is too low for the wheel to start to accelerate again, there is no further pressure reduction and the pressure is again kept constant. If the wheel's acceleration exceeds the limit, the pressure is increased again. The pressure does not exceed the pressure in the brake master cylinder generated by the pedal force.

This control process is repeated until the wheel turns without risk of locking up.

The hydraulic unit is actuated in three control phases (pressure build-up, pressure hold and pressure reduction) by appropriate signals from the ABS control module (A2).