Active suspension is a type of automotive suspension that controls the vertical movement of the wheels relative to the chassis or vehicle body with an onboard system, rather than in passive suspensions where the movement is being determined entirely by the road surface; see Skyhook theory. Active suspensions can be generally divided into two classes: pure active suspensions, and adaptive/semi-active suspensions. While 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 perpendicular to the road in corners, allowing better traction and control. An onboard computer detects body movement from sensors throughout the vehicle and, using data calculated by opportune control techniques, 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.
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Skyhook theory is the idea these systems are tageting - the vehicle maintains a stable posture as if suspended by an imaginary hook in the sky, unaffected by road conditions.
Since there is no imaginary line or hook in the sky, so instead, an actuator will be operated. The imaginary line (acceleration = 0) is calculated based on the value provided by an acceleration sensor installed on the top of the vehicle (Fig. 3). Since the dynamical elements are only made up of the linear spring and the linear damper, no complicated calculations are necessary.
A vehicle contacts the ground through the spring and damper in a normal spring damper suspension, as in Fig. 1. To achieve the same sustainability in the Skyhook theory, the vehicle must contact the ground through the spring, and the imaginary line with the damper, as in Fig. 2. Theoretically, in a case where the coefficient of the damper reaches an infinite value, the vehicle will be in a state where it is completely fixed to the imaginary line, thus the vehicle will not shake.
Active suspensions, the first to be introduced, use separate actuators which can exert an independent force on the suspension to improve the riding characteristics. The drawbacks of this design are high cost, added complication and mass of the apparatus, and the need for frequent maintenance on some implementations. Maintenance can require specialised tools, and some problems can be difficult to diagnose.
Hydraulically actuated suspensions are controlled with the use of hydraulics. The first example appeared in 1954, with the Hydropneumatic suspension developed by Paul Magès at Citroën. The hydraulic pressure is supplied by a high pressure radial piston hydraulic pump. Sensors continually monitor body movement and vehicle ride level, constantly supplying the hydraulic height correctors with new data. In a matter of a few seconds, the suspension generates counter forces to raise or lower the body. During driving maneuvers, the encased nitrogen compresses instantly, offering six times the compressibility of the steel springs used by vehicles up to this time. 
In practice, the system has always incorporated the desirable self-levelling suspension and height adjustable suspension features, with the latter now tied to vehicle speed for improved aerodynamic performance, as the vehicle lowers itself at high speed.
This system performed remarkably well in straight ahead driving, including over uneven surfaces, but had little control over roll stiffness.
Electronic actuation of hydraulic suspension
Colin Chapman developed the original concept of computer management of hydraulic suspension in the 1980s to improve cornering in racing cars. Lotus fitted and developed a prototype system to a 1985 Excel with electro-hydraulic active suspension, but never offered it for sale to the public, although many demonstration cars were built for other manufacturers.
Sensors continually monitor body movement and vehicle ride level, constantly supplying the computer with new data. As the computer receives and processes data, it operates the hydraulic servos, mounted beside each wheel. Almost instantly, the servo-regulated suspension generates counter forces to body lean, dive, and squat during driving maneuvers.
Computer Active Technology Suspension (CATS) co-ordinates the best possible balance between ride quality and handling by analysing road conditions and making up to 3,000 adjustments every second to the suspension settings via electronically controlled dampers.
This type of active suspension uses linear electromagnetic motors attached to each wheel. It provides extremely fast response, and allows regeneration of power consumed, by using the motors as generators. This nearly surmounts the issues of slow response times and high power consumption of hydraulic systems. Electronically controlled active suspension system (ECASS) technology was patented by the University of Texas Center for Electromechanics in the 1990s and has been developed by L-3 Electronic Systems for use on military vehicles. The ECASS-equipped HMMWV exceeded the performance specifications for all performance evaluations in terms of absorbed power to the vehicle operator, stability and handling.
Michelin's Active Wheel from 2004 incorporates an in-wheel electrical suspension motor that controls torque distribution, traction, turning maneuvers, pitch, roll and suspension damping for that wheel, in addition to an in-wheel electric traction motor.
Adaptive or semi-active systems can only change the viscous damping coefficient of the shock absorber, and do not add energy to the suspension system. Though limited in their intervention (for example, the control force can never have different direction than the current vector of velocity of the suspension), semi-active suspensions are less expensive to design and consume far less energy. In recent times, research in semi-active suspensions has continued to advance with respect to their capabilities, narrowing the gap between semi-active and fully active suspension systems.
This type is the most economic and basic type of semi-active suspensions. They consist of a solenoid valve which alters the flow of the hydraulic medium inside the shock absorber, therefore changing the damping characteristics of the suspension setup. The solenoids are wired to the controlling computer, which sends them commands depending on the control algorithm (usually the so-called "Sky-Hook" technique). This type of system used in Cadillac's Computer Command Ride (CCR) suspension system.
Another fairly recent method incorporates magnetorheological dampers with a brand name MagneRide. It was initially developed by Delphi Corporation for GM and was standard, as many other new technologies, for Cadillac Seville STS (from model 2002), and on some other GM models from 2003. This was an upgrade for semi-active systems ("automatic road-sensing suspensions") used in upscale GM vehicles for decades. It allows, together with faster modern computers, changing the stiffness of all wheel suspensions independently. These dampers are finding increased usage in the US and already leases to some foreign brands, mostly in more expensive vehicles.
In this system, being in development for 25 years, the damper fluid contains metallic particles. Through the onboard computer, the dampers' compliance characteristics are controlled by an electromagnet. Essentially, increasing the current flow into the damper magnetic circuit increases the circuit magnetic flux. This in turn causes the metal particles to change their alignment, which increases fluid viscosity thereby raising the compression/rebound rates, while a decrease softens the effect of the dampers by aligning the particles in the opposite direction. If we imagine the metal particles as dinner plates then whilst aligned so they are on edge - viscosity is minimised. At the other end of the spectrum they will be aligned at 90 degrees so flat. Thus making the fluid much more viscous. It is the electric field produced by the electromagnet that changes the alignment of the metal particles. Information from wheel sensors (about suspension extension), steering, acceleration sensors - and other data, is used to calculate the optimizal stiffness at that point in time. The fast reaction of the system (milliseconds) allows, for instance, making a softer passing by a single wheel over a bump in the road at a particular instant in time.
Some production vehicles with active and semiactive suspension
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- 1955: Citroën DS: premiere of fully active self-leveling Citroen hydropneumatic suspension
- 1957: Cadillac Eldorado Brougham: premiere of active self-leveling GM air suspension
- 1983: Toyota Soarer: premiere of semi-active Toyota Electronic Modulated Suspension (TEMS)
- 1986: Toyota Soarer: world first Electronically controlled (TEMS) full air suspension (spring constant, variable attenuation force) installed
- 1989 Citroën XM (Hydractive, semi-active)
- 1987 BMW M3 "BOGE adjustable damping" system
- 1990 *First semi-active suspension scanning the road ahead (sonar) - 1990 Nissan Leopard/Nissan Cedric/Nissan Maxima/Nissan J30 DUET-SS Super Sonic Suspension
- 1990 Infiniti Q45 "Full-Active Suspension (FAS)", active suspension system, although it did still have conventional coil springs
- 1992 Toyota Celica (Toyota Electronically Modulated Suspension)
- 1992 Citroën Xantia VSX (Hydractive 2, semi-active)
- 1993 Cadillac, several models with RSS road sensing suspension. RSS was available in both standard and CVRSS (continuously variable road sensing suspension) systems. It monitored damping rates of the shock absorbers every 15 milliseconds, selecting between two settings.
- 1994 October: Toyota Celsior introduced first Skyhook air suspension
- 2002 Cadillac Seville STS, first MagneRide
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