Page 404 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Vehicle motion control C HAPTER 13.1

brake pressure. The cycle continues until the vehicle is motor-driven pump. For ACC application of the brakes,
stopped. the apply and isolation valves independently regulate the
It should be noted that by maintaining slip near S o , the braking to each of the four wheels.
maximum deceleration is achieved for a given set of For ABS applications, the braking pressure is regulated
conditions. Some reduction in lateral force occurs from by alternately opening and closing the apply and release
its maximum value by maintaining slip near S o . However, valves. These valves are operated by output signals from
in most cases the lateral force is large enough to maintain the ABS controller in accordance with an algorithm
directional control. applied to wheel speed measurements as described
In some ABSs, the slip oscillations are shifted below above, which attempts to maintain slip near a value
S o , sacrificing some braking effectiveness to enhance di- corresponding to the maximum friction coefficient.
rectional control. This can be accomplished by adjusting
the upper and lower slip limits.
13.1.5 Electronic suspension
13.1.4.1 Tire-slip controller system

Another benefit of the ABS is that the brake pressure Automotive suspension systems consist of springs, shock
modulator can be used for tire-slip control. Tire slip is absorbers, and various linkages to connect the wheel as-
effective in moving the car forward just as it is in braking. sembly to the car frame. The purpose of the suspension
Under normal driving circumstances, the slip that was system is to isolate the car body motion as much as
defined previously for braking is negative. That is, the tire possible from wheel motion due to rough road input; and
is actually moving at a speed that is greater than for the performance of the suspension system is strongly
a purely rolling tire. In fact, the traction force is pro- influenced by the damping parameter of the shock
portional to slip. absorber.
For wet or icy roads, the friction coefficient can become The two primary subjective performance measures are
very low and excessive slip can develop. In extreme cases, ride and handling. Ride refers to the motion of the
one of the driving wheels may be on ice or in snow while car body in response to road bumps or irregularities.
the other is on a dry (or drier) surface. Because of the Handling refers to how well the car body responds to
action of the differential, the low-friction tire will spin and dynamic vehicle motion such as cornering or hard
relatively little torque will be applied to the dry-wheel braking.
side. In such circumstances, it may be difficult for the Generally speaking, ride is improved by lowering the
driver to move the car even though one wheel is on a rel- shock absorber damping, whereas handling is improved
atively good friction surface. by increasing this damping. In traditional suspension
The difficulty can be overcome by applying a braking design, the damping parameter is fixed and is chosen to
force to the free spinning wheel. In this case, the dif- achieve a compromise between ride and handling (i.e., an
ferential action is such that torque is applied to the rel- intermediate value for shock absorber damping is
atively dry wheel surface and the car can be moved. In chosen).
the example ABS, such braking force can be applied to In electronically controlled suspension systems, this
the free spinning wheel by the hydraulic brake pressure damping can be varied depending on driving conditions
modulator (assuming a separate modulator for each drive and road roughness characteristics. That is, the suspen-
wheel). Control of this modulator is based on measure- sion system adapts to inputs to maintain the best possible
ments of the speed of the two drive wheels. Of course, ride subject to handling constraints that are associated
the ABS already incorporates wheel speed measure- with safety.
ments, as discussed previously. There are two major classes of electronic suspension
The ABS electronics have the capability to perform control systems: active and semiactive. The semiactive
comparisons of these two wheel speeds and to determine suspension system is purely dissipative (i.e., power is
that braking is required of one drive wheel to prevent absorbed by the shock absorber under control of
wheel spin. a microcontroller). In this system, the shock absorber
Antilock braking can also be achieved with electro- damping is regulated to absorb the power of the wheel
hydraulic brakes. An electrohydraulic brake system was motion in accordance with the driving conditions.
described in the section of this chapter devoted to ACC. In an active suspension system, power is added to the
Recall that for ACC a motor-driven pump supplied suspension system via a hydraulic or pneumatic power
brake fluid through a solenoid-operated apply valve to source. At the time of the writing of this book, com-
the wheel cylinder. In the case of ABS, the driver mercial suspension systems are primarily semiactive. The
supplies the pressurized brake fluid instead of the active suspension system is just beginning to appear in

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