Page 290 - Understanding Automotive Electronics

2735 | CH 8 Page 277 Tuesday, March 10, 1998 1:19 PM

VEHICLE MOTION CONTROL 8

Figure 8.11
Typical Sample-and-
Hold Circuit

FPO

Switch S is used to disable the speed controller by interrupting the
2
control signal to the throttle actuator. Switch S disables the system whenever
2
the ignition is turned off, the controller is turned off, or the brake pedal is
pressed. The controller is switched on when the driver presses the speed set
switch S .
1
For safety reasons, the brake turnoff is often performed two ways. As just
mentioned, pressing the brake pedal turns off or disables the electronic control.
In certain cruise control configurations that use a vacuum-operated throttle
actuator, the brake pedal also mechanically opens a separate valve that is located
in a hose connected to the throttle actuator cylinder. When the valve is opened
by depression of the brake pedal, it allows outside air to flow into the throttle
actuator cylinder so that the throttle plate instantly snaps closed. The valve is
shut off whenever the brake pedal is in its inactive position. This ensures a fast
and complete shutdown of the speed control system whenever the driver presses
the brake pedal.
ANTILOCK BRAKING SYSTEM
One of the most readily accepted applications of electronics in automobiles
has been the antilock brake system (ABS). ABS is a safety-related feature that
assists the driver in deceleration of the vehicle in poor or marginal braking
conditions (e.g., wet or icy roads). In such conditions, panic braking by the driver
(in non-ABS-equipped cars) results in reduced braking effectiveness and,
typically, loss of directional control due to the tendency of the wheels to lock.
In ABS-equipped cars, the wheel is prevented from locking by a
mechanism that automatically regulates braking force to an optimum for any

UNDERSTANDING AUTOMOTIVE ELECTRONICS 277