Traditionally automotive suspension designs have been a compromise between the three conflicting criteria of road holding, load carrying and passenger comfort.
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.
In a high bandwidth (or ``fully active'') suspension system we generally consider an actuator connected between the sprung and unsprung masses of the vehicle. A fully active system aims to control the suspension over the full bandwidth of the system. In particular this means that we aim to improve the suspension response around both the ``rattle-space'' frequency (10-12 Hz) and ``tyre-hop'' frequency (3-4Hz). The terms rattle-space and tyre-hop may be regarded as resonant frequencies of the system. A fully active system will consume a significant amount of power and will require actuators with a relatively wide bandwidth. These have been successfully implemented in Formula One cars and by, for example, Lotus [Wright 84]
Also known as slow-active or band-limited systems. In this class the actuator will be placed in series with a road spring and/or a damper. A low bandwidth system aims to control the suspension over the lower frequency range, and specifically around the rattle space frequency. At higher frequencies the actuator effectively locks-up and hence the wheel-hop motion is controlled passively. With these systems we can achieve a significant reduction in body roll and pitch during manoeuvres such as cornering and braking, with lower energy consumption than a high bandwidth system.
These aim to increase the bandwidth of a band-limited system by using feed-forward or knowledge of future road inputs. Some systems [Foag 89] aim to measure road disturbances ahead of the car (using perhaps a laser system [Prem 87]), and then use both standard feedback control and feed-forward from the sensor to achieve a superior response. Others eg. [Crolla 91] aim to use the information available from the front strut deflection to improve the performance of the rear suspension.
Active suspension systems that have been successfully implemented include the high profile examples found on Formula One racing cars. Most major motor manufacturers are researching there own systems and some are near to fruition. These include Jaguar [Williams 94], Mercedes Benz [Acker 91], and Toyota [Hayakawa 93] to name but three.
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.
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