Electronic Brake Control Module: Description and Operation

PURPOSE
The Antilock Brake System (ABS) control unit receives electrical signals from the wheel speed sensors which are proportional to the speed at which the wheels are rotating. From them the computer derives the acceleration, deceleration and slip values. Logic circuits then integrate these values and generate control instructions for the electromagnetic valves in the hydraulic pump assembly.

CONSTRUCTION
The ABS control unit comprises two printed-circuit boards carrying the necessary resistors, diodes, transistors and integrated circuits (IC). Only a few of these ICs are the equivalent of several thousand individual transistors, and are accommodated on a single tiny flake of silicon.

OPERATION
The electronic control unit can be divided into two functional zones:

- Signal processing and logic
- Safety circuit

Signal Processing And Logic
Signal processing has the task of accepting the wheel speeds as they arrive from the signal transmitters and deriving from them the regulating values such as peripheral deceleration or acceleration at the wheel and the slip information in a form suitable for the logic circuits.

The logic circuits, which receive the signals as stated above, generate the actual adjustment instructions such as pressure increase, pressure retention or pressure drop, which are passed to the solenoid valves on the hydraulic unit.

Faults (e.g. deriving from the speed sensors) which can arise as a result of normal manufacturing tolerances are filtered out before processing, that is to say before they pass to the amplifier stage.

Safety Circuit And Monitoring Devices
In order to prevent the regulator from responding incorrectly, with potentially dangerous results, the ABS incorporates a safety circuit.

The task of the safety circuit is to monitor the system before each journey and while in motion, to make sure that it remains operational.

If a fault is detected, or one occurs when the system is in operation, the ABS monitor circuit shuts down the anti-locking installation and thus ensures that conventional braking is not affected. An ABS warning light on the facia indicates to the driver that a fault has occurred or that the system has "failed safe".

BITE (built-in Test Equipment) Test Cycle
This test cycle forms an active part of the safety circuit. On the one hand it checks operation of the two regulating modules by means of a predetermined test program, and on the other hand it transmits a test signal to ascertain that the built-in safety circuit is itself in working order.

The safety circuit and its monitoring devices check the following items:

- The presence of the general switching signal (cycle rhythm monitoring)
- The signal which switches the return-flow pump "On" and "Off", or the presence of this signal (return pump monitoring)
- Control of the valves
- The valve operating periods (logic monitoring). The maximum valve operating times are here monitored by a combination logic circuit; these maximum times are governed by physical considerations.
- In a 20-second rhythm, various items including the pump motor, control sensitivity signals, the test cycle, acceleration etc.
- Battery voltage (over-or under voltages at the electronic control unit).

If an error is detected, the error memory is activated, the ABS warning light comes on, the ABS is rendered non-operative and any braking which is necessary takes place by conventional means.