Active suspension
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 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 chassis parallel to the road when turning corners, preventing unwanted contacts between the vehicle frame and the ground, 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.
Principle
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 is calculated based on the value provided by an acceleration sensor installed on the body of the vehicle. The dynamic elements comprise only the linear spring and the linear damper; therefore, no complicated calculations are necessary.
A vehicle contacts the ground through the spring and damper in a normal spring damper suspension, as in Figure 1. To achieve the same level of stability as the Skyhook theory, the vehicle must contact the ground through the spring, and the imaginary line with the damper, as in Figure 2.
Theoretically, in a case where the damping coefficient 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
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.Hydraulic actuation
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 milliseconds, 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.
Millions of production vehicles have been built with variations on this system.
Electronic actuation of hydraulic suspension
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.
In 1990, Nissan installed a hydraulic supported MacPherson strut based setup, called Full-Active Suspension that was used in the Nissan Q45 and President. The system used a hydraulic oil pump, a hydraulic cylinder, an accumulator and damping valve, which connected two independent circuits for the front and rear strut assemblies. The system would then recover motion energy to balance the car continuously. The system was revised and is now called Hydraulic Body Motion Control System, installed on the Nissan Patrol and Infiniti QX80.
Williams Grand Prix Engineering prepared an active suspension, devised by designer-aerodynamicist Frank Dernie, for the team's Formula 1 cars in 1992, creating such successful cars that the Fédération Internationale de l'Automobile decided to ban the technology to decrease the gap between Williams F1 team and its competitors.
Computer Active Technology Suspension 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.
The 1999 Mercedes-Benz CL-Class introduced Active Body Control, where high pressure hydraulic servos are controlled by electronic computing, and this feature is still available. Vehicles can be designed to actively lean into curves to improve occupant comfort.
Active anti-roll bar
stiffens under command of the driver or suspension electronic control unit during hard cornering. The first production car with this technology was the Mitsubishi Mirage Cyborg in 1988.Electromagnetic recuperative
In fully active electronically controlled production cars, the application of electric servos and motors married to electronic computing allows for flat cornering and instant reactions to road conditions.The Bose Corporation has a proof of concept model. The founder of Bose, Amar Bose, had been working on exotic suspensions for many years while he was an MIT professor.
Electromagnetic 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 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 Humvee exceeded the performance specifications for all performance evaluations in terms of absorbed power to the vehicle operator, stability and handling.
Active Wheel
- 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.
- Audi active electromechanical suspension system introduced in 2017. It drives each wheel individually and adapts to the prevailing road conditions. Each wheel has an electric motor which is powered by the 48-volt main electrical system. Additional components include gears, a rotary tube together with internal titanium torsion bar and a lever which exerts up to on the suspension via a coupling rod. Thanks to the front camera, the sedan detects bumps in the road early on and predictively adjusts the active suspension. Even before the car reaches a bump in the road, the preview function developed by Audi transmits the right amount of travel to the actuators and actively controls the suspension. The computer-controlled motors can sense imperfection on the road, and can raise the suspension up from the wheel which would go over the undulation, thus aiding the ride quality. The system will direct the motors on the outside to push up or pull down the suspension while cornering. This will result in a flatter drive and reduced body-roll around corners which in turn means more confident handling dynamics.
Adaptive and semi-active