Page 409 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CHAP TER 1 3. 1 Vehicle motion control

body acceleration measurement can be used to evaluate rough road calls for damping that optimizes tire normal
ride quality. The controller does this by computing force, thereby maximizing cornering forces.
a weighted average of the spectrum of the acceleration.
The relative body/wheel motion can be used to estimate
tire normal force, and damping is then adjusted to try to 13.1.6 Electronic steering control
optimize this normal force.
The yaw rate sensor provides data which in relation- The steering effort required of the driver to overcome
ship to vehicle speed and steering input measurements restoring torque generally decreases with vehicle speed
can be used to evaluate cornering performance. In certain and increases with steering angle. Traditionally, the
vehicles, these measurements combine in an algorithm steering effort required by the driver has been reduced
that is used to activate the electrohydraulic brakes. by incorporating a hydraulic power steering system in the
Under program control in accordance with the control vehicle. Whenever there is a steering input from the
strategy, the electronic control system generates output driver, hydraulic pressure from an engine-driven pump is
electrical signals to the various actuators. The variable applied to a hydraulic cylinder that boosts the steering
damping actuators vary either the oil passage orifice or effort of the driver.
the RH fluid viscosity independently at each wheel to Typically, the effort available from the pump increases
obtain the desired damping for that wheel. with engine speed (i.e., with vehicle speed), whereas the
There are many possible control strategies and many required effort decreases. It would be desirable to
of these are actually used in production vehicles. For the reduce steering boost as vehicle speed increases. Such
purposes of this book, it is perhaps most beneficial to a feature is incorporated into a power steering system
present a representative control strategy that typifies featuring electronic controls. An electronically con-
features of a number of actual production systems. trolled power steering system adjusts steering boost
The important inputs to the vehicle suspension control adaptively to driving conditions. Using electronic control
system come from road-roughness-induced forces and of power steering, the available boost is reduced by
inertial forces (due, for example, to cornering or controlling a pressure relief valve on the power steering
maneuvering), steering inputs, and vehicle speed. In our pump.
hypothetical simplified control strategy these inputs are An alternative power steering scheme utilizes a special
considered separately. When driving along a nominally electric motor to provide the boost required instead of
straight road with small steering inputs, the road input is the hydraulic boost. Electric boost power steering has
dominant. In this case, the control is based on the spectral several advantages over traditional hydraulic power
content (frequency region) of the relative motion. The steering. Electronic control of electric boost systems is
controller (under program control) calculates the spec- straightforward and can be accomplished without any
trum of the relative velocity of the sprung and unsprung energy conversion from electrical power to mechanical
mass at each wheel (from the corresponding sensor’s actuation. Moreover, electronic control offers very so-
data). Whenever the weighted amplitude of the spectrum phisticated adaptive control in which the system can
near the peak frequencies exceeds a threshold, damping is adapt to the driving environment.
increased, yielding a firmer ride and improved handling. An example of an electronically controlled steering
Otherwise, damping is kept low (soft suspension). system that has had commercial production is for four-
If in addition the vehicle is equipped with an ac- wheel steering systems (4WS). In the 4WS-equipped
celerometer (usually located in the car body near the vehicles, the front wheels are directly linked mechan-
center of gravity) and with motor-driven variable- ically to the steering wheel, as in traditional vehicles.
aperture shock absorbers, then an additional control There is a power steering boost for the front wheels as in
strategy is possible. In this latter control strategy, the a standard two-wheel steering system. The rear wheels
shock absorber apertures are adjusted to minimize are steered under the control of a microcontroller via an
sprung mass acceleration in the 2–8-Hz frequency actuator. Fig. 13.1-23 is an illustration of the 4WS
region, thereby providing optimum ride control. How- configuration.
ever, at all times, the damping is adjusted to control In this illustration, the front wheels are steered
unsprung mass motion to maintain wheel normal force to a steering angle d f by the driver’s steering wheel input.
variation at acceptably low levels for safety reasons. A sensor (S) measures the steering angle and another
Whenever a relatively large steering input is sensed sensor (U) gives the vehicle speed. The microcontroller
(sometimes in conjunction with yaw rate measurement), (C) determines the desired rear steering angle d r under
such as during a cornering maneuver, then the control program control as a function of speed and front steering
strategy switches to the smaller aperture, yielding angle.
a ‘‘stiffer’’ suspension and improved handling. In par- The details of the control strategy are proprietary and
ticular, the combination of cornering on a relatively not available for this book. However, it is within the

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