What is the use of a suspension system. Its main task is to provide a safe and stable ride for the vehicle. The components are usually passive force elements as they provide a nice trade-off between wear, stability and comfort. Suspension system dampens the shocks and does not allow the jerks to be transmitted to the human body. By selecting the right spring and damper properties, the suspension functions as a barrier to the jerks and only passes those frequencies which come in comfortable range for humans. But at the same time the wheel load variation must be minimum as we need contact of tire with road at all times.
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A system of links connects the un-sprung mass (wheel, brake, steering hub) to the sprung mass (car body). However there is a trade off at the cornering, as the spring should be stiff enough to avoid over roll of the body. Sometimes an anti-roll bar is used to overcome the exaggerated roll due to damping in corners. However the stiffness of the roll bar is not independent as we cannot transmit vibrations of one wheel to the other.
The traditional suspension will tilt the tire while cornering. This is because the linkage rotates and the tire connected to it also rotates. This causes the area of contact of the tire to reduce while cornering or turning. This results in the loss of grip and massive body roll of the vehicle. In case of cornering the requirement usually is to get a so-called counter camber. The negative camber angle will cause a favorable deformation of the contact patch, which in combination with the unfavorable deformation due to the cornering forces will lead to a desirable contact patch between the tire and the road. Examples of today’s suspension systems which provide a negative camber are the double wish-bone and the McPherson suspension system. Non-zero static camber can also improve the cornering and bump control of the vehicle.
Active Suspension system
An active suspension system can prevent suspension travel under a varying load, theoretically without consuming energy. That is why it is very suitable for leveling car during accelerating, braking and cornering, or for taking care of static load variations. And as these systems are computer-aided, there mathematical models can be fed to a controller to level the car or to improve the comfort level.
Design and Working
While cornering the average force per suspension strut varies from the static value and its length remains equal as to eliminate body roll. By using the principle of a lever, the varying load can be counter-balanced by a constant force by varying distance from the fulcrum. This system perfectly eliminates body roll and the system would not rotate. Because all relevant forces are perpendicular to the direction of adjustment of the fulcrum and constant force, the adjustment will not cost any energy.
A possible application of this principle is shown below:
The pre-tensioned secondary spring (inner one) is maximally assisting the primary spring (outer one). Initially, the adjustable arms are positioned at an angle of 90° with respect to the position in figure above in order to not produce any torque around the fulcrum. Fulcrum here is the hollow hole on the right side which will get attached to the chassis of the vehicle. This type of system is called a trailing arm suspension system.
As we can see, we will need two actuators to properly control this system. Another point to be noted is that the synchronization of both the arms is very important and difficult.
Primary Spring and Damper
Fixed to chassis
The above shown model is much more practical to be used in a car. The working mechanism of the above shown system is already been used in Delft Active Suspension (DAS). But in this version the active part has been integrated and is more customizable. Initially, the goal is to suppress the roll and pitch motion of the vehicle in an energy efficient manner. This system is basically used to maximize the comfort index while the lateral acceleration is not the main objective here.
The modelling is quit extensive and this being a review paper, the pre-calculated model is show. However its significance is important to understand. The model in fact is a transfer-function between the actuator and the adjustable arm. The function relates the stiffness of the secondary spring with the angle actuator has to turn. The equation developed is:
This equation is used by the controller to tell the actuator to find out the angel required for which the active suspension produces the correct height adjustment to get the right tire contact and maximum comfort.
Active body roll controll
Active suspension also prevents the body roll of a car. Instrumentations such as EPS and ABS are used for this purpose, however a modern active suspension designed to minimize body roll can replace these devices. Due to this body roll control, the velocity during cornering can be increased by a great extent. This also increases the stability of the car, hence increasing the safety of the car. As the car does not lean or hunch back, there is a relative improvement in the acceleration of the car.
Objectives of Body Roll Control:
It is generally designed keeping in mind the comfort level.
If a conventional suspension system is designed for excessive comfort, it will require the suspensions to be too soft, which in turn will cause massive body roll. To incorporate that manufacturers now a days have designed systems which have literally eliminated body roll. As a result the pitch is also nearly eliminated, resulting in better acceleration and braking.
There are 2 basic types of Active suspension systems:
Hydraulic and pneumatic Systems
Hydraulic and Pneumatic System:
Mostly pneumatic or hydraulic elements are introduced to the traditional suspension to eliminate body roll. The most famous of these systems is Citroën’s Hydractive suspension system. To develop an understanding, this system would be primarily discussed. A large Sphere is positioned on the top of each suspension strut. The sphere contains two compartments separated by an elastic membrane. It is shown below:
The compartments are filled with Fluid and a gas as shown. The gas compartment acts as a spring. When the strut moves, it passes its change in pressure to the fluid via piston and compresses the fluid against the membrane. Now the compressed air smoothly releases the energy back. Fluid restrictions smoothens the variation of pressure and hence making the fluid compartment act as a damper. Now there is another sphere mounted in the mid of the front and rear axle. This mid sphere connects the 2 corner spheres with each other. The front and the rear central spheres are also inter-connected to provide a high-pressure fluid connection between all the spheres. Due to this inter linkage between spheres, the total volume of gas and fluid experienced by one wheel increases which results in smoother spring and softer damper and therefore a more comfortable ride.
Now if during drive, the central spheres are closed, they will cause the suspension to be stiffened. Fast reacting solenoids are used in the central spheres which ensure quick response of the system. So, the system makes the system softer while moving straight for smooth ride and during cornering or braking, the system is stiffened so that the body does not roll much and a better grip is obtained.
This type of system is a Semi-Active system as it only varies the spring and damping of the system and cant in fact apply a force if necessary to stabilize the car. To improve this system and to nullify the body roll, an extra sphere filled with gas is connected to the anti-roll bar. The sphere with gas acts as a cushion and because of this cushioning effect the anti-roll bar acts as a very soft linkage and does not allow the propagation of vibrations from one end of the anti-roll bar to the other. Now when the car is cornering, the tilt due to body roll is detected and the valves in the gas sphere are closed to eliminate the cushioning effect and stiffening the anti-roll bar. To lemmatize the body roll, sometimes an additional hydraulic ram is used which adjusts the anti-roll bar as required by the scenario.
Comparison of traditional and Active body roll suspension systems.
To reduce the body roll, Mercedes use another approach (Active Body Control (ABC)). A conventional spring mounted damper is connected in series with a fluid chamber. When the spring is contracted (cornering), the fluid chamber is filled so that strut length remains same. Now when the spring is stretched, the fluid chamber will be emptied, resulting in no change in length of the strut. However, this mechanism is a little “delay-ish” so it does not provide high level of comfort.
Active Body Control
While BMW’s “Dynamic Drive” system is by far the most integrated system for hydraulic and pneumatic active suspensions. It uses a hydraulic coupling element which can cause torque on any end of the anti-roll bar. While moving straight, the toe ends of the bar are disconnected for zero transaction of shocks between the ends. While when cornering, the ends are connected and an anti-torque is applied to reduce body roll.
Zero Body Roll (left)
The major advantage of Hydraulic and pneumatic Active suspension is that ride height can be adjusted by simply filling or removing fluid. Right height is a very important criterion for stability. It can be reduced while driving at high speeds and the height will remain invariant during static loading, improving comfort.
The major disadvantage however is the requirement of a ridiculously high power energy consuming hydraulic pump to maintain the pressures required for stabilized running of the vehicle.
Electromagnetic Suspension System:
Boseâ„¢ recently developed an Active Suspension System. It uses a set of electric motors which produce forces required for nullifying body roll and pitching effects. The conventional spring damper systems are replaced by these simple motor circuits as shown below:
The improvement in ride comfort is unimaginable. Deflections and body roll effects are detected via sensors which continuously map the quality and roughness of road. Now the actuators direct the motors to directly control the height of individual tire strut. This keeps the car literally horizontal and at a same height at all times. All the motors and actuators are interconnected. Due to this inter connection even when the car encounters a bump, the height remains totally horizontal and unchanged. Yes it requires energy to be continuously in action, but this draw-back can be neglected when we can regain some energy from it unlike hydraulic systems. When the force is to be made against the direction of velocity, the energy can be regenerated.
Reduction of Body Roll
Adaptive or Alive Damping
Adaptive dampers are dampers whose characteristic can be changed as per requirement to overcome pitch and roll. These dampers can vary the rate of roll and pitch. We can use magneto rheological fluid; the viscosity of this type of fluid can be changed by changing magnetic field. It is only effective for quick change of direction. In long corners the system ultimately allows roll but at a slow rate. As only a change of conventional dampers is required for this system to be installed, many sports cars are using this method to stabilize their ride.
It is a Semi-Active system. It consumes less energy than a typical Active System.
advantages of Active suspension over conventional systems
Major advantages are listed below:
It is independent of overall suspension condition and stabilizes the vehicle independently.
The trade off between ride height, comfort, tire wear.
Eliminates body roll during high speed cornering.
The tires can be aligned to axis which give the optimum performance when encountering a bump or in case of cornering.
Anti-roll bar is omitted while using a fully active system.
A simple Trailing arm suspension replaces the traditional complex suspension linkages.
The simple trailing arm reduces the space requirement for suspension and the car can be designed low. Resulting in low air drag.
Ride height is not affected by varying static loads.
Natural Frequency of the vehicle does not change and can be selected to be lower than the human body comfort threshold.
Stiffness of the system is lower as compared to traditional systems. This increases the comfort index.
Adjustable characteristics of the system even during driving.
As it increases the contact of tire with road at all times, the overall grip of the vehicle is increased. Hence creating a safer ride.
Lateral Acceleration Improved!
The Active suspension system eliminates pitch. When the car accelerates, its centre of gravity shifts backwards. This causes the acceleration to be reduced. Now when the stiffness of the suspension system can be changed, the centre of gravity does not change any more and the acceleration does not decrease. This results an improved lateral acceleration. Also, when we corner, the car has a better grip and is in equilibrium hence the acceleration afterward also increases and the vehicle does not slip.
The reduced pitch also decreases the stoppage length of the vehicle when braking. As the car does no lean forward, the fulcrum is reduced and the care stops early due to less momentum.
Road Grip Improvements:
Rotating tire can be considered as spring with a low damping effect. It takes some time to actually let the shock pass through it. Active suspension quickens this process. Forces produced are almost likely to a damper positioned between the road and the un-sprung axel. The force production does not satisfy the requirements for damping the vibrations and thus improving road holding. The wheel load variation improves and the variation in jerk absorption rate reduces, creating better grip.
Suspensions are designed to get a good trade-off between traction, comfort, load variations and axel travel. Advanced suspension systems provide a better trade-off between but do not remove the conflicts. They also consume alot of space due to their increased linkages.
However, Adaptive or Active Suspensions have active elements which can apply torques of forces and provide a better trade-off between the above listed items. It almost zeros pitch and body roll providing stability and comfort. It is very small in size and does not involve complicated linkages. This reduces the area required and lets the designer design the vehicle more openly. It takes care of the static load variations by automatically adjusting the stiffness of the system.
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It is an adaptive utility like a computer. We can add new components to it to improve the system. We can introduce performance evaluation controller which determines the actual force for the actuator required to stabilize the car and provide comfort. The actuators produce the exact force at the exact time required using the mathematical model of active suspension system.
As the system eliminates body roll, it improves the car cornering. It also allows a greater cornering velocity. This system is also responsible for providing a smooth and a comfortable ride. Independent suspension travel is used to isolate passengers from shocks and jerks. The system also is responsible for safety. The tires may not leave the ground aver, even while experiencing a bump.
Active Suspension System is a ground breaking technology. It takes the driving experience to a new level. It literally increases the comfort index to 8.9 standard. This technology is all aces in my perspective and research. However, the drawback is that it makes the driver lose feel of the actual speed while cornering. This is the reason why it was banned in F1 motor-sport. The other drawback is that it requires energy to run continuously.
 Mohammad Rizal Bin Ahmed, University of Technology Malaysia, Simulation and Experimental Analysis of Active Suspension System 760217-02-5103.
 M.S.P. Leegwater, Eindhoven University of Technology The Netherlands, S010527 (TU/e).
The refrences are available as a .rar file attached with the paper.
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