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                                           INTRODUCTION
In the present work, the shortcomings of conventional passive vehicle suspensions will be discussed. Traditionally, in automotive suspension designs have been a compromise between the two conflicting criteria of road holding and passenger comfort. The suspension system must support the weight of the vehicle, provide directional control during handling, and provide effective isolation of passengers and payload from road disturbances. A passive suspension has the ability to store energy via a spring and to dissipate it via a damper. The parameters are generally fixed, being chosen to achieve a certain level of compromise between road holding and ride comfort. Once the spring has been selected based on the load-carrying capability of the suspension, the damper is the only variable remaining to specify. Low damping yields poor resonance control at the natural frequencies of the body (sprung mass) and axle (unsprung mass), but provides the necessary high frequency isolation required for a comfortable ride. Conversely, large damping results in good resonance control at the expense of high frequency isolation. Due to these conflicting demands, suspension design has had to be something of a compromise, largely determined by the type of use for which the vehicle is designed.

1.1.1.Pitching:
1.1.2.Rolling:
1.2.Various kinds of suspension systems includes:
 1)Passive suspension system.
2)Semi-active suspension system.
3)Active suspension system.

                                FIG::SUSPENSION SYSTEM IN CAR

1.2.1) Passive Vertical Quarter-Car Model:
When, suspension modelling and control are considered, the well-known vertical quarter-car model is often used. This model allows us to study the vertical behaviour of a vehicle according to the suspension type (passive or controlled). In this book, this model is largely used for control design and for performance analysis.  The passive (both general and simplified forms)
Quarter-car model is shown.
                                           
                                       FIG::PASSIVE SUSPENSION SYSTEM

2) SEMI-ACTIVE SUSPENSION SYSTEM:
The suspension system has been considered in many case studies of control design. Most of the papers have been concerned with active suspensions, since they allow us to obtain greater performances while the control synthesis does not require some dissipativity properties to be handled. The semi-active suspension control literature is also quite large, and an important number of control strategies exist for such a system. In this chapter, the most important or at least the most developed ones, are recalled together with some “ad hoc” references. This chapter aims at presenting and evaluating some of the usual existing control strategies. The emphasis is mainly put on performance analysis, using the tools described, rather than on an exhaustive and complete description. The chapter is organized as follows: some usual and simple semi-active suspension strategies, focusing on comfort and road-holding respectively, are presented and evaluated using the frequency response diagrams. Then, these strategies are compared using the performance criteria presented. In, some modern semi-active methods are also recalled but, due to their “complexity” (especially for performance tuning), only briefly evaluated.

6.1 Comfort Oriented Semi-Active Control Approaches
In this section, the most common comfort oriented semi-active suspension control strategies
Skyhook Control
The principle of this approach is to design an active suspension control so that the chassis is
“linked” to the sky in order to reduce the vertical oscillations of the chassis and the axle

 

                            FIG::    Semi-Active Suspension Control Design for Vehicles

Independently of each other. Thus a fictitious damper is considered between the sprung mass and the sky frame, as shown in
Through the isolation of the sprung mass from the road profile, it allows a reduction of
Vibration. This desired behaviour is then modelled as:
M¨z = −k(z−zt )−csky ˙z
m¨zt = k(z−zt )−kt (zt −zr )

where csky is the damping coefficient of the Skyhook behavior.
Since this is not theoretically possible, this “ideal” system is realized, starting from the model
(3.46), using a damper force csky ˙z, that allows us to reproduce the Skyhook behavior for the
sprung mass (but not for the unsprung mass).

                      FIG::SEMI-ACTIVE SUSPENSION SYSTEM
3)ACTIVE SUSPENSION SYSTEM:
                             
                      FIG: :ACTIVE SUSPENSION SYSTEM
Quarter-Car Performance Specifications and Signals of Interest
Considering the previous remarks, and considering the quarter-car model given in
the following signals are considered for performance analysis and
Characterization of a suspension system:
• To evaluate the comfort, the vertical displacement z (or acceleration ¨z) of the chassis is
 Analysed.
• To evaluate the road-holding, the tire deflection (zdeft ) is analysed.
• Eventually, the deflection limits (z min def and zmax def ) could be analysed.

                                  
                                            FIG::A Quarter Car-model Suspension System.

                              Fig::Damping ratio trade-off
 Two main conclusions can be drawn:

• The fixed-damping configurations have an intrinsic trade-off: a low-damping provides Superior high-frequency filtering performance, but it is affected by a badly un-damped body resonance; on the other hand, a high-damping setting removes the resonances, but strongly deteriorates the filtering capabilities. Intermediate damping settings simply deliver different combinations of this trade-off.
• A wise semi-active algorithm can (almost) completely remove the classical trade-off good damping of the body resonance can be guaranteed, together with good filtering performance.

Applications and Technologies of Semi-active Suspension System
Today semi-active suspensions are used over a vast domain of applications. In vehicle applications, semi-active suspensions are used at different layers:

• At the (classical) wheel-to-chassis layer, in primary suspension systems.

• At the chassis-to-cabin layer, in large vehicles where the driver cabin is separated from the main chassis (e.g. large agricultural tractors, trucks, earth-moving machines, etc.).

• At the cabin-to-seat layer: in large off-road vehicles the driver seat is also frequently equipped with a fully-fledged suspension system, in order to reduce the vibration suffered by the driver during the typically long hours spent in the vehicle.

Many types of vehicle are equipped (or are being equipped) with semi-active suspension; the
list is long, multi-faceted, and continuously increasing. Such vehicles range from small vehicles like motorcycles, ATVs, snowmobiles, etc. to large off-road vehicles (agricultural
Tractors, earth-moving machines, etc.), passing through classical cars, and duty-vehicles such
as trucks, ambulances, fire-trucks, etc.

If we look inside a semi-active damper, today there are three main available technologies,
which allow a fast-reacting electronically controlled modification of the damping ratio of a
shock absorber.

FIG:: Examples of electronically controlled semi-active shock absorbers, using three
different technologies. From left to right: solenoid-valve Electrohydraulic damper (Sachs),
Magnetorheological damper (Delphi), and Electrorheological damper (Fludicon).

 

In a semi-active suspension system, the variation of damping may be achieved by introducing mouldable mechanisms in the shock absorber such as solenoid valves (electrohydraulic dampers), or by the use of fluids which may vary their viscosity if subject to an electric or magnetic field (electrorheological and magnetorheological dampers). A simple but effective description of this kind of actuator is extremely important for control design purposes.

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