The suspension system must support the vehicle, provide directional control during handling manoeuvres and provide effective isolation of passengers/payload from road disturbances [Wright 84]. Good ride comfort requires a soft suspension, whereas insensitivity to applied loads requires stiff suspension. Good handling requires a suspension setting somewhere between the two.
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 was designed. Active suspensions are considered to be a way of increasing the freedom one has to specify independently the characteristics of load carrying, handling and ride quality.
A passive suspension system has the ability to store energy via a spring and to dissipate it via a damper. Its parameters are generally fixed, being chosen to achieve a certain level of compromise between road holding, load carrying and comfort.
An active suspension system has the ability to store, dissipate and to introduce energy to the system. It may vary its parameters depending upon operating conditions and can have knowledge other than the strut deflection the passive system is limited to.
Formula one cars represent the extreme of active suspension implementation, being fully active systems using high bandwidth aerospace specification components [Wright 84]. For wide spread commercial use much cheaper actuators and control valves must be used, and so semi-active or low bandwidth systems are the norm here. The oleo-pneumatic actuator is a popular choice [Williams 94], giving both a low frequency active element and a high frequency passive element in one unit.
(C) Gavin Walker 1997
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