Page 296 - Understanding Automotive Electronics

2735 | CH 8 Page 283 Tuesday, March 10, 1998 1:19 PM

VEHICLE MOTION CONTROL 8

Figure 8.16
Brake Pressure
Modulating
Mechanism

FPO

Whenever the wheel slip limit is reached, the solenoid valve is closed and
the piston (11) retracts, closing the check valve. This action effectively isolates
the brake cylinders from the master cylinder, and brake line pressure is
controlled by the position of piston 11. This piston retracts, lowering the brake
pressure sufficiently so that slip falls below S . At this point, the control system
o
detects low T and the piston moves up, thereby increasing brake line pressure.
w
The ABS system will continue to cycle until the vehicle has stopped, the
braking conditions are normal, or the driver removes the brake pressure from
the master cylinder.
In the latter case, the operation of the brake pressure modulator restores
normal braking function. For example, should the driver release the brake
pedal, then the pressure at the inlet (1) is reduced. At this point, the check valve
(9) opens and brake line pressure is also removed. The solenoid valve opens and
the piston returns to its normal position (fully up) such that the check valve is
held open.
Figure 8.17 illustrates the braking during an ABS action. In this
illustration, the vehicle is initially traveling at 55 mph and the brakes are
applied as indicated by the rising brake pressure. The wheel speed begins to
drop until the slip limit is reached. At this point, the ABS reduces brake
pressure and the wheel speed increases. With the high applied brake pressure,

UNDERSTANDING AUTOMOTIVE ELECTRONICS 283