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INTRODUCTION
1.1
Suspension System
Every vehicle moving on the randomly profiled road is exposed to vibrations
which are harmful both for the passengers in terms of comfort and for the durability of the
vehicle itself. Therefore the main task of a vehicle suspension is to ensure ride comfort and
road holding for a variety of road conditions and vehicle maneuvers [22]. This in turn would
directly contribute to the safety. Shock absorption in automobiles is performed by suspension
system that carries the weight of the vehicle while attempting to reduce or eliminate
vibrations which may be induced by a variety of sources, such as road surface irregularities,
aerodynamics forces, vibrations of the engine and driveline, and non-uniformity of the
tire/wheel assembly [33]. Usually, road surface irregularities, ranging from potholes to
random variations of the surface elevation profile, acts as a major source that excites the
vibration of the vehicle body through the tire/wheel assembly and the suspension system
(Wong, 1998).
In general, a good suspension should provide a comfortable ride and good
handling within a reasonable range of deflection. Moreover, these criteria subjectively
depend on the purpose of the vehicle. Sports cars usually have stiff, hard suspensions with
poor ride quality while luxury sedans have softer suspensions but with poor road handling
capabilities. A suspension system with proper cushioning needs to be “soft” against road
disturbances and “hard” against load disturbances. A heavily damped suspension will yield
good vehicle handling, but also transfers much of the road input to the vehicle body.
When the vehicle is traveling at low speed on a rough road or at high speed in a
straight line, this will be perceived as a harsh ride. The vehicle operators may find the harsh
ride objectionable, or it may physically damage vehicle. Where as a lightly damped
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suspension will yield a more comfortable ride, but would significantly reduce the stability of
the vehicle at turns, lane change maneuvers, or during negotiating an exit ramp. Therefore, a
suspension design is an art of compromise between these two goals. A good design of a
passive suspension can work up to some extent with respect to optimized riding comfort and
road holding ability, but cannot eliminate this compromise [22].
1.2 Classification of suspension systems
Suspension systems are classified in to three groups:
1.
Passive
2.
Semi Active
3.
Active suspension systems.
1.2.1 Passive suspension system
Passive suspension system consists of an energy dissipating element, which is
the damper, and an energy-storing element, which is the spring. Since these two elements
cannot add energy to the system this kind of suspension systems are called passive. Figure
1.1 shows Passive suspension system considered in this study.
Fig 1.1 Passive suspension system/Quarter car representation
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1.2.2
Active Suspension system
Figure 1.2 shows an active suspension system in which a force actuator is
placed in parallel to passive system. In active suspension systems, sensors are used to
measure the accelerations of sprung mass and unsprung mass and the analog signals from the
sensors are sent to a controller. The controller is designed to take necessary actions to
improve the performance abilities already set. The controller amplifies the signals which are
fed to the actuator to generate the required forces to form closed loop system (active
suspension system). The performance of this system is then compared with that of the open
loop system (passive suspension system) [22]. It should be noted, that an active suspension
system requires external power to function, and that there is also a considerable penalty in
complexity, reliability, cost and weight [8].
Fig 1.2 Active suspension system/Quarter car representation
1.2.3 Semiactive suspension system
To replace complexity and cost while improving ride and handling the concept
of semi active suspension has emerged. In this kind of suspension system, the passive
suspension spring is retained, while the damping force in the damper can be modulated
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(adjusted) in accordance with operating conditions. Figure 1.3 shows the schematic view of a
semi active suspension system. The regulating of the damping force can be achieved by
adjusting the orifice area in the damper, thus changing the resistance of fluid flow. Most
recently the possible application of electrorheological (ER) and magnetorheological (MR)
fluids to the development of controllable dampers has also attracted considerable interest.
Fig 1.3 Semiactive suspension system/Quarter car representation
Nowadays the passive suspension systems cannot fulfill the conflicting
requirements anymore, it is more urgent to introduce the active and semiactive suspensions in
the practical use [42].
Active vehicle suspension systems were introduced in the early 1970’s to
overcome the drawbacks of passive suspensions, namely the inherent tradeoff between ride
quality and handling performance [13]. Published research spanning 30 years has
demonstrated significant improvements in ride quality and handling performance using
prototype active suspensions.
Despite the published benefits and recent advances in active suspensions, these
systems remain complex, bulky, and expensive and are not common options on production
vehicles. Additionally, they typically require considerable power and impose heavy loads on
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the engine. A notable exception is electromechanical actuator technologies, which allow
regeneration of energy otherwise dissipated as heat. Additional design and control challenges
inherent in active suspension systems have not fully been resolved. These include actuator
nonlinearities, stiction, bandwidth limitations, durability and maintenance issues. The
application of active suspensions systems has thus been limited to prototype off-road,
military, and racing vehicles.
Semiactive suspensions overcome many of these limitations of active
suspensions, although generally with a reduction in achievable ride quality and handling
performance, though some researchers have concluded that this reduction is quite small [13].
Semiactive suspensions can be considerably more cost effective, compact, and functionally
simple as they require only a variable damper and a few sensors to achieve adequate
performance. Conventional semiactive suspensions rely on servo-controlled damper valves to
achieve “continuously variable damping”. The primary drawbacks of such systems are
associated with a large number of mechanical parts. These systems suffer many of the
bandwidth, durability, and maintenance limitations of active suspensions. Hence even
semiactive systems have not gained commercial popularity.
The recent advent of commercial
magnetorheological (MR) fluid dampers has made it possible to vary the damping force
almost instantaneously with very few mechanical parts. In comparison to typical passive
suspension systems, semiactive technology allows for softer damping when needed, in addition to
harder damping for situations that demand it [14].
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