In this study, the various characteristics of different suspensions namely Passive, Semi-Active and Fully Active suspensions have been studied briefly and compared to one another. Furthermore, the different dampers used for each type of suspension and their working have been explained. The different parameters used to rate each type of suspension were handling, performance, weight and cost. Applications of each type of suspension have also been discussed. Ultimately, it was concluded that Semi-Active Suspension Systems are more applicable for passenger cars because of its feasibility. However, it is observed that Active Suspensions have the best performance among the various suspension systems.
Introduction
The suspension system is one of the most important systems in an automobile. It's designed to support the mass of the vehicle body and also to prevent transfer of oscillatory forces from the ground to the passenger seats. Apart from the aforementioned factors, the other important functions of the suspension system in a vehicle are to maintain a constant contact area between the road and the wheels and also to be able to adjust to several disturbances inside the vehicle. Furthermore, the suspension systems should be in a position to respond to changes in load inside the vehicle [1]. The mass of the vehicle supported by the suspension system is defined as its sprung mass. This includes the vehicle body, passengers, the power train and the frame. The remaining components of the vehicle such as the rotor bearings, wheel and wheel bearings are not supported by the suspension system. Hence the collective mass of these components constitute the unsprung mass [2].
Therefore, the construction and effective functioning of a suspension system are the most important factors to be taken into consideration in matters of passenger comfort.
Each suspension system has two basic components. The first component is the spring. Its function is to absorb any energy attained when the vehicle jumps over an uneven surface and to release the energy back to the ground when the wheel is travelling on a flat surface. The second component is the damper. It behaves as a shock absorber. It means that it prevents transfer of vibrations from the wheels to the vehicle body [3].
The various types of suspensions discussed in this study are the Passive, Semi-Active and Fully Active suspension systems.
Discussion
1. Passive Suspension Systems
A passive suspension system consists of a spring and a damper. The stiffness of the spring cannot be varied. The spring absorbs the vibrational energy from the wheels while the damper dissipates the energy in the form of heat. The arrangement of the various components in a passive suspension system is given below for a quarter car model.
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Figure 1 - Passive Suspension System [10]
Where, ms - vehicle sprung mass
mus - vehicle unsprung mass
Fk - spring stiffness
Fc - damping coefficient
kt - tyre stiffness
zs , zus and zr represent the vertical displacements of the sprung mass, unsprung mass and the tyre respectively
The damper in a passive suspension system consists of a piston attached to the wheels. The region above the piston is either filled with a hydraulic fluid or a gas[1]. While travelling on an uneven surface, the wheel is exposed to sinusoidal forces which in turn pushes the piston inside the damper repeatedly thus compressing the fluid. This generates the necessary damping action as the energy from the wheel is not transferred to the passenger seats but rather dissipated into heat.
Passive suspension systems have the disadvantage of operating effectively only at smaller frequencies due to the absence of any control mechanism inside the suspension system[1].
In spite of this disadvantage, they are used in most of today's passenger vehicles [2]. This is because they are cheap and easy to manufacture. Most of the economical passenger cars employ the passive suspension system.
2. Semi-Active/Adaptive Suspension Systems
The construction of Semi-active suspensions is similar to that of passive suspensions as there is an absence of active control with the exception to the fact that the damping coefficient of the damper in a semi-active suspension can be altered electronically depending upon the situation [1].
The various components of a semi-active suspension in a quarter car model are shown in figure 2.
http://www.mathworks.co.uk/matlabcentral/fx_files/11119/2/SGA__suspension_quarter_semiactive.jpg
Figure 2 - Semi-Active Suspension [11]
Where , m2 - vehicle sprung mass
m1 - vehicle unsprung mass
k2 - spring stiffness
k1 - tyre stiffness
co - damping coefficient
fd - controller to adjust damping coefficient
Also, z2,z1 and q represent the vertical displacements of the sprung mass, unsprung mass and tyre respectively.
There are two kinds of dampers used in commercial vehicles. They are Solenoid/Valve Actuated dampers and Magneto Rheological dampers[4].
In the Solenoid/Valve Actuated dampers, a solenoid valve is utilised. This is connected to the Electronic Control Unit (ECU). The ECU transmits a signal to the solenoid which changes the flow of the fluid inside the damper [5]. The nature of the fluid flow inside the damper is dependent on the situation. The ECU continuously monitors the road behaviour and transmits signals to the solenoid accordingly.
The Magneto Rheological dampers, on the other hand, work on a completely different principle. In this case , the damper consists of a Magneto rheological(MR) fluid. This fluid is under the influence of a magnetic field. This is attained by using an electromagnet. When the vehicle is riding on an irregular surface, the electromagnet increases the strength of the magnetic field. As a result, the viscosity of the Mr fluid increases which creates the damping effect in the system [6][7].
Examples of cars which use the MR damper in its suspension system are the Acura MDX, Audi R8, Ferrari 458 Italia etc. [6].
There are two methods of control in a semi-active suspension system namely the PID control and the Adaptive Fuzzy PID control. Among the two types of control, the Adaptive Fuzzy PID control has a better effect in controlling vehicle vibration [8].
3. Fully Active Suspension Systems
Contrary to the Semi-Active suspension system, the Fully Active/Active Suspension systems consists of an additional actuator which provides the active effect. This actuator exerts a force on the suspension thus improving ride quality. The basic diagrams for some of the Active Suspension systems are given below:
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Figure 3 - Low Bandwidth Active Suspension Systems[12]
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Figure 4 - High Bandwidth Active Suspension Systems[12]
Active suspensions can be broadly classified into Hydraulic Actuated and Electromagnetic Recuperative suspension systems.
In Hydraulic Actuated systems, sensors are deployed on the body frame as well as at the suspension. These sensors observe the vehicle behaviour and transmit the data to the ECU. Accordingly, the ECU transmits data to a hydraulic pump which, as a result, sends the proper amount of fluid to the actuator. The pump is either motor-driven or engine-driven. This generates the necessary active force required to counter the wheel vibrations and provide adequate ride and handling [9].
Hydraulic actuated active suspensions can be further classified into low bandwidth and high bandwidth active suspension systems. In low bandwidth suspension systems, the actuator is placed in series with a spring as shown in figure 3. In this system, the vehicle sprung mass vibrates at a frequency of 1Hz. These systems are cheaper and easier to operate compared to the high bandwidth suspension systems [13].
The disadvantage of this system is that it has a larger power consumption rate. Thus only large actuators are effective under this condition [13].
High bandwidth suspension systems on the other hand employ the actuator in parallel with the spring as shown in figure 4. The advantage of this system is that the vehicle's weight can be absorbed by the spring. Thus, the actuator size is usually very small. However, It is difficult to activate and control due to its larger requirements[13].
Electromagnetic Recuperative systems can work on a larger bandwidth than the hydraulic actuated systems. This system consists of an electromagnetic damper in addition to the mechanical spring. The system has the advantage of being able to regenerate energy developed due to wheel vibrations [1]. This means that when the vehicle encounters a bump, the electric motor generates energy to lift the wheel upwards thereby avoiding any transfer of energy from the road to the passenger seat.
Due to its complex construction and high cost compared to other suspension types, Active Suspensions are presently used mainly in racing cars ( e.g. Formula 1 cars) and high end luxury cars (eg. Range Rover Evoque).
Conclusion
The study on the various types of suspensions was conducted. Comparisons of these suspensions based on ride quality, structure, cost, weight have been made. It is concluded that the passive suspension systems are still widely used because they are cheap, reliable, lighter, simple to manufacture. Semi-Active suspensions are mostly used for the mid-sized vehicles. But, they are currently preferred over Active Suspensions because they are cheaper and lighter. However, Electromagnetic Recuperative Active Suspensions are the future of suspension technology because its ability to deliver passenger comfort without sacrificing vehicle handling is exponentially superior compared to other suspension types.
