Page 305 - Understanding Automotive Electronics

2735 | CH 8 Page 292 Tuesday, March 10, 1998 1:19 PM

8 VEHICLE MOTION CONTROL

Figure 8.23
4WS Configuration

FPO

input from the driver, hydraulic pressure from an engine-driven pump is
applied to a hydraulic cylinder that boosts the steering effort of the driver.
Typically, the effort available from the pump increases with engine speed
(i.e., with vehicle speed), whereas the required effort decreases. It would be
desirable to reduce steering boost as vehicle speed increases. Such a feature can
potentially be incorporated into a power steering system featuring electronic
controls. An electronically controlled power steering system adjusts steering
boost adaptively to driving conditions. Using electronic control of power
steering, the available boost is reduced by controlling a pressure relief valve on
the power steering pump.
An alternative power steering scheme utilizes a special electric motor to
provide the boost required instead of the hydraulic boost. Electric boost power
steering has several advantages over traditional hydraulic power steering.
Electronic control of electric boost systems is straightforward and can be
accomplished without any energy conversion from electrical power to
mechanical actuation. Moreover, electronic control offers very sophisticated
adaptive control in which the system can adapt to the driving environment.
An example of an electronically controlled steering system that has had
commercial production is for four-wheel steering systems (4WS). In the 4WS-
equipped vehicles, the front wheels are directly linked mechanically to the
steering wheel, as in traditional vehicles. There is a power steering boost for the
front wheels as in a standard two-wheel steering system. The rear wheels are
steered under the control of a microcontroller via an actuator. Figure 8.23 is an
illustration of the 4WS configuration.

292 UNDERSTANDING AUTOMOTIVE ELECTRONICS