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Concept
Active suspension
Summary
An active suspension is a type of automotive suspension that uses an onboard control system to control the vertical movement of the vehicle's wheels and axles relative to the chassis or vehicle frame, rather than the conventional passive suspension that relies solely on large springs to maintain static support and dampen the vertical wheel movements caused by the road surface. Active suspensions are divided into two classes: true active suspensions, and adaptive or semi-active suspensions. While semi-adaptive suspensions only vary shock absorber firmness to match changing road or dynamic conditions, active suspensions use some type of actuator to raise and lower the chassis independently at each wheel.
These technologies allow car manufacturers to achieve a greater degree of ride quality and car handling by keeping the tires consistently perpendicular to the road when turning corners, preventing unwanted contacts between the vehicle frame and the ground (especially when going over a depression), and allowing overall better traction and steering control. An onboard computer detects body movement from sensors throughout the vehicle and, using that data, controls the action of the active and semi-active suspensions. The system virtually eliminates body roll and pitch variation in many driving situations including cornering, accelerating and braking. When used on commercial vehicles such as buses, active suspension can also be used to temporarily lower the vehicle's floor, thus making it easier for passengers to board and exit the vehicle.
Skyhook theory is that the ideal suspension would let the vehicle maintain a stable posture, unaffected by weight transfer or road surface irregularities, as if suspended from an imaginary hook in the sky continuing at a constant altitude above sea level, therefore remaining stable.
Since an actual skyhook is obviously impractical, real active suspension systems are based on actuator operations. The imaginary line (of zero vertical acceleration) is calculated based on the value provided by an acceleration sensor installed on the body of the vehicle (see Figure 3).
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Height adjustable suspension
Height adjustable suspension is a feature of certain automobile suspension systems that allow the motorist to vary the ride height or ground clearance. This can be done for various reasons including giving better ground clearance over rough terrain, a lower ground clearance to improve performance and fuel economy at high speed, or for stylistic reasons. Such a feature requires fairly sophisticated engineering. Height adjustment is most often achieved by air or oil compression used for the "springs" of the vehicle - when the pressure is varied - the vehicle body rises or lowers.
Air suspension
Air suspension is a type of vehicle suspension powered by an electric or engine-driven air pump or compressor. This compressor pumps the air into a flexible bellows, usually made from textile-reinforced rubber. Unlike hydropneumatic suspension, which offers many similar features, air suspension does not use pressurized liquid, but pressurized air. The air pressure inflates the bellows, and raises the chassis from the axle. Air suspension is used in place of conventional steel springs in heavy vehicle applications such as buses and trucks, and in some passenger cars.
Hydropneumatic suspension
Hydropneumatic suspension is a type of motor vehicle suspension system, designed by Paul Magès, invented by Citroën, and fitted to Citroën cars, as well as being used under licence by other car manufacturers, notably Rolls-Royce (Silver Shadow), Bmw 5-Series e34 Touring, Maserati (Quattroporte II) and Peugeot. It was also used on Berliet trucks and has been used on Mercedes-Benz cars, where it is known as Active Body Control.
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