General system manner.The rate of decrease is deter-
8.3.3 General system manner.The rate of decrease is deter-
mined by the evaluation of all signals
description
received from the wheel sensors driven and non-driven wheels on the vehicle
As with other systems ABS can be considered
must be treated in different ways as
as a central control unit with a series of inputs
they behave differently when braking.
and outputs. An ABS system is represented by
A logical combination of wheel decelera-
the closed loop system block diagram shown in tion/acceleration and slip are used as the
controlled variable.The actual strategy
Figure 8.8. The most important of the inputs are
used for ABS control varies with the
the wheel speed sensors and the main output is
operating conditions
some form of brake system pressure control. The task of the control unit is to compare signals from each wheel sensor to measure the
8.3.4 ABS components
acceleration or deceleration of an individual wheel. There are a few variations between manufactur- From this data and pre-programmed look up tables,
ers involving a number of different components. brake pressure to one or more of the wheels can
For the majority of systems, however, there are
be regulated. Brake pressure can be reduced, held
three main components.
176 Advanced automotive fault diagnosis
Wheel speed sensors
The ECU performs a self-test after the ignition Most of these devices are simple inductance sen-
is switched on. A failure will result in disconnec- sors and work in conjunction with a toothed wheel.
tion of the system. The following list forms the They consist of a permanent magnet and a soft
self-test procedure:
iron rod around which is wound a coil of wire. As
● current supply;
the toothed wheel rotates the changes in induct- ● exterior and interior interfaces; ance of the magnetic circuit generates a signal;
● transmission of data;
the frequency and voltage of which are propor- ● communication between the two micro- tional to wheel speed. The frequency is the signal
processors;
used by the ECU. The coil resistance is in the ● operation of valves and relays; order of 800 to 1000 ⍀. Coaxial cable is used to
● operation of fault memory control; prevent interference affecting the signal. Some
● reading and writing functions of the internal systems now use ‘Hall effect’ sensors.
memory. All this takes about 300 mS!
Electronic control unit
The function of the ECU is to take in information
Hydraulic modulator
from the wheel sensors and calculate the best course of action for the hydraulic modulator. The
The hydraulic modulator as shown in Figure 8.9 heart of a modern ECU consists of two micro-
has three operating positions: processors such as the Motorola 68HC11, which
● pressure buildup brake line open to the run the same programme independently of each
pump;
other. This ensures greater security against any ● pressure holding brake line closed; fault which could adversely affect braking per-
● pressure release brake line open to the formance, because the operation of each proces-
reservoir.
sor should be identical. If a fault is detected, the ABS disconnects itself and operates a warning
The valves are controlled by electrical solen- light. Both processors have nonvolatile memory
oids, which have a low inductance so they react into which fault codes can be written for later
very quickly. The motor only runs when ABS is service and diagnostic access. The ECU also has
activated. Figure 8.10 shows an ABS hydraulic suitable input signal processing stages and output
modulator with integrated ECU. This is the latest or driver stages for actuator control.
Bosch System version 8.1.
Pressure from master cylinder
Return pump
Solenoid
Accumulation to store fluid
operated valve
Normal – build of pressure
One-way valves
Actuator winding
Electrically
Motor to return
controlled M
fluid to master
winding
cylinder Non-return valve Solenoid
To brake
shuttle valve
cylinder calliper
Hold pressure
Reduce pressure
Figure 8.9 ABS hydraulic
Chassis systems 177