Subaru SVX 1992 Manual page 845

BRAKES
[M5B3] 4-4
Figure 36 shows a block diagram of the electronic
circuits . When the speed sensor sends an alternating
voltage corresponding with the wheel speed to the
input amplifier circuit, the signal voltage is converted
into a rectangular wave which is sent to the digital signal
generating circuit. (This circuit receives two channels of
signal at a time .)
The LSI circuit, which consists of approximately 16,000
transistors, computes wheel speed in relation to the
signal sent from the speed sensor . It then emits the
required control signal as a result of computation . This
circuit also contains a safety circuit for monitoring pur-
poses. The control signal emitted from the LSI circuit is
then sent to the current-control and -amplification cir-
cuits where a signal is produced to operate the magnet
valve of the hydraulic control unit .
The memory circuit, which serves to memorize system
failure, and monitor the regulated power circuit and
others, is housed in a separate IC. When the E.C .U.
power is applied with the ignition switch "ON", the
safety circuit begins to monitor electronic circuits, sen-
sors, the hydraulic control unit, etc . If any circuits or
units malfunction, a warning lamp (dual circuit design)
comes on to warn the driver of a problem . The LED in
the ECU illuminates to show a trouble code . The brake
system then functions as a conventional brake system
in place of the ABS .
I I
I
Wheel I Input
Digital signal
Current control
I
Power
I
speed amplifier
circuit circuit
amplifier
I
I
Sensor I circuit (ch 1)
(ch 1) circuit
I
I -
I
+ I
Monitor circuit I
I (ch 1)
1
(ch 1) I
I
I
I Digital signal
Current control
~
I
~ ( I
I (ch 4) circuit circuit
F-+i
(ch 4)
(ch 2)
Ich 2) `
I
Monitor circuit
I
l
1 I I I
1 I
I -J
Fail memory
I _ ( Driving relay in hydraulic l -
control unit (H/U)
Battery + I
I
I Stabilized power
I supply I
1 I
I Constant j
1 voltage +
I
I
I Power supply I
I
monitor circuit
Warning light
I
Magnet valve
in hydraulic
control unit
B4-037A
Fig . 25
3. ABS CONTROL CYCLE CURVES
As the brake pedal is depressed, brake fluid pressure
increases correspondingly, which in turn decreases
wheel speed . When brake fluid pressure reaches point
"A" (where wheel deceleration exceeds "- b,"), the
control unit transmits a "hold" signal to hold the brake
fluid pressure in wheel cylinder at that point . At the
same time, the control unit computes a "dummy" vehi-
cle speed . When the wheel speed drops below the
slip-ratio setting (= speed less than the dummy vehicle
speed based on the predetermined value) at point "B"
of the brake fluid pressure, the control unit then trans-
mits a "decrease" signal to prevent wheel lock-up . This
causes the brake fluid pressure to decrease .
After brake fluid pressure is decreased, wheel acceler-
ation increases . When it exceeds the wheel acceleration
setting "+ b,0" at point "C" (brake fluid pressure, the
control unit transmits a "hold" signal to hold the brake
fluid pressure at that point. When wheel acceleration
setting value "+ b,," is exceeded and when brake fluid
pressure is at point "D", the control unit judges that
wheel lockup will not occur and then transmits an
"increase" signal to increase brake fluid pressure.
When wheel acceleration drops below "+ b," at point
"E" (which occurred due to a brake fluid pressure
increase), the repetition of the "hold" and "increase"
signals takes place at a constant cycle.
When wheel deceleration exceeds "- b,", at point "F"
of the brake fluid pressure, the control unit immediately
15