ANTI-LOCK BRAKE SYSTEM
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PRINCIPLE OF ABS CONTROL
When the brake pedal is depressed during driving, the wheel speed decreases and the vehicle
speed does as well. The decrease in the vehicle speed, however, is not always proportional to the
decrease in the wheel speed. The non-correspondence between the wheel speed and vehicle
speed is called “slip” and the magnitude of the slip is expressed by the “slip ratio” which is defined
as follows:
Slip ratio = (Vehicle speed – Wheel speed)/Vehicle speed × 100%
When the slip ratio is 0%, the vehicle speed corresponds exactly to the wheel speed. When it is
100%, the wheels are completely locking (rotating at a zero speed) while the vehicle is moving.
The braking effectiveness is represented by the “coefficient of friction” between the tire and road
surface.The larger the coefficient, the higher the braking effectiveness. The diagram below shows
the relationship between the coefficient of friction and the slip ratio for two different road surface conditions
(asphalt-paved road and icy road), assuming that the same tires are used for both the conditions
and the vehicles are moving forward. Although the braking effectiveness (coefficient of
friction) depends on the road surface condition as shown and also on the type of the tire, its peak
range generally corresponds to the 8 — 30% range of the slip ratio.
The ABS controls the fluid pressure to each wheel so that a coefficient of friction corresponding to
this slip ratio range is maintained.
ABS WHEEL SPEED SENSOR
Each of the ABS wheel speed sensors detects the speed of the corresponding wheel. The sensor
consists of a permanent magnet, coil and tone wheel. The magnetic flux produced by the permanent
magnet changes as each tooth of the tone wheel (which rotates together with the wheel) passes
in front of the magnet’s pole piece. The changing magnetic flux induces voltages at a frequency
corresponding to the wheel speed.
EBD (ELECTRONIC BRAKE FORCE DISTRIBUTION)
The EBD system utilizes the function of the conventional ABS. This system prevents premature
locking of the rear wheels by using electronic control instead of a proportioning valve. The system
provides a feature to optimize the brake force distribution to the front and rear wheels according to
change in loading conditions or displacement of the center of gravity during deceleration.
The ABSCM calculates the optimum brake force distribution from the difference in speed between
the front and rear wheels. Based on the results, the ABSCM controls the rear wheel brake pressure
through the ABS hydraulic unit (H/U) to achieve the optimum brake force distribution suitable for the
driving condition.
ABS CONTROL MODULE AND HYDRAULIC CONTROL UNIT (ABSCM & H/U)
ABS control module section (ABSCM)
The ABSCM contains two micro control modules (MCMs) which communicate with each other. Both
the MCMs process the same program and each MCM monitors the other’s outputs. If a mismatch
occurs between their outputs, the ABSCM cuts off the system and activate the fail-safe function.
The ABSCM can store a maximum of 3 diagnostic trouble codes in a ROM. If more than 3 faults
have occurred, only the 3 most recent failures are stored and others are erased. Trouble diagnostic
trouble codes remain stored until they are internally or externally erased.
The ABSCM has a test routine (sequence control pattern) which facilitates checking of the hydraulic
control unit.
ABS control
Using primarily the wheel speed data from each ABS wheel speed sensor and secondarily the vehicle
deceleration rate data from the G sensor as parameters, the ABSCM generates a simulated
vehicle speed when there is a risk of wheel lock-up. Using the simulated vehicle speed (called “dummy”
vehicle speed) as a reference, the ABSCM determines the state of the wheel in terms of the
tendency toward lock-up.If the result shows that the wheels are about to lock, the ABSCM issues
commands to energize or de-energize the solenoid valves and activate the motor pump of the H/U
to modulate the brake fluid pressures that act on the wheel cylinders, thereby preventing the wheels
from locking.
The ABSCM controls the right and left front wheel fluid pressures independently and the rear wheel
fluid pressures based on the wheel which is the most likely to lock (select-low control). The STi models
have an additional function which independently controls the rear wheels according to the value
sent from the lateral G sensor.
Functions available using SUBARU select monitor
When the SUBARU select monitor is connected, the ABSCM allows it
To read out analog data
To read out ON/OFF data
To read out or erase diagnostic trouble codes
To read out data showing conditions under which a trouble code has been stored (freeze frame
data)
To initiate ABS sequence control pattern
Indication functions
Under the control of the ABSCM, the ABS warning light provides the following three indication functions:
ABS fault alerting
Diagnostic trouble code indication (by flashing in the diagnosis mode)
Valve ON/OFF indication (when sequence control pattern is initiated)