Type of automotive 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 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.
Principle
[
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]
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,
[1]
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). The dynamic elements comprise only the linear spring and the linear damper; therefore, no complicated calculations are necessary.
[2]
[3]
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
[
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]
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
[
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]
Hydraulically actuated suspensions are controlled with the use of
hydraulics
. The first example appeared in 1954, with the
hydropneumatic suspension
developed by
Paul Mages
at
Citroen
. 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.
[4]
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.
[5]
Millions of production vehicles have been built with variations on this system.
Electronic actuation of hydraulic suspension
[
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]
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.
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.
[6]
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
Federation Internationale de l'Automobile
decided to ban the technology to decrease the gap between Williams F1 team and its competitors.
[7]
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
.
The 1999
Mercedes-Benz CL-Class (C215)
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.
[8]
[9]
Active anti-roll bar
[
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]
Active anti-roll bar
stiffens under command of the driver or suspension
electronic control unit
(ECU) during hard cornering. First production car was
Mitsubishi Mirage Cyborg
in 1988.
Electromagnetic recuperative
[
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]
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.
[10]
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 (ECASS) technology was patented by the University of Texas Center for Electromechanics in the 1990s
[11]
and has been developed by L-3 Electronic Systems for use on military vehicles.
[12]
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
[
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]
- 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 1,100 Nm (811.3 lb-ft) 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.
[15]
[16]
[17]
[18]
[19]
[20]
[21]
Adaptive and semi-active
[
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]
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. While adaptive suspensions have generally a slow time response and a limited number of damping coefficient values, semi-active suspensions have time response close to a few milliseconds and can provide a wide range of damping values. Therefore, adaptive suspensions usually only propose different riding modes (comfort, normal, sport...) corresponding to different damping coefficients, while semi-active suspensions modify the damping in real time, depending on the road conditions and the dynamics of the car. 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.
Solenoid/valve actuated
[
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]
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).
[
citation needed
]
This type of system is used in
Cadillac's
Computer Command Ride
(CCR) suspension system. The first production car
[22]
was the
Toyota Soarer
with semi-active
Toyota Electronic Modulated Suspension
, from 1983.
In 1985, Nissan introduced a shock absorber using a similar version, called "Super Sonic Suspension," adding an ultrasonic sensor that would provide information that a microcomputer would then interpret, combined with information from the steering, brakes, throttle, and vehicle speed sensor. The adjustment information signals would then modify the shock absorbers when a driver-controlled switch was placed in "Auto". The automatic adjustment could be limited if the switch was placed in "Soft," "Medium," or "Hard" settings. A modified version that didn't use the sonar module was also used, allowing the settings to be manually selected.
[23]
[24]
This implementation is currently used industry-wide by a number of manufacturers, provided by
Monroe Shock Absorbers
called CVSAe, or Continuously Variable Semi-Active electronic.
In 2008, with the introduction of the
Nissan GT-R
, "DampTronic" was jointly developed by Nissan and Bilstein. DampTronic provides three selectable driver settings that can also interact with the
Vehicle Dynamics Control
technology to modify the transmission's shift points. The settings are labeled as Normal, Comfort, or R, and can be either set in Normal for automatic adjustment or the "R" setting for high-speed driving, while "Comfort" is for touring and a more compliant ride. The "R" mode enables the vehicle to utilize the
yaw angle
rate with a reduced steering angle for a crisper, more communicative steering, while the "Comfort" setting produces less vertical G-loading in comparison to the "Normal" or computer determined suspension setting.
[25]
Magnetorheological damper
[
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]
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 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.
[
citation needed
]
This system was 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 optimal 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.
[
citation needed
]
Production vehicles
[
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]
By calendar year:
- 1954:
Citroen Traction Avant
15-6H:,
self-leveling
Citroen
hydropneumatic suspension
on rear wheels.
[
citation needed
]
- 1955:
Citroen DS
, self-leveling Citroen hydropneumatic suspension on all four wheels.
- 1957:
Cadillac Eldorado Brougham
: premiere of self-leveling GM
air suspension
- 1967:
Rolls-Royce Silver Shadow
Partial load bearing hydropneumatic suspension on all four wheels. Front system deleted in 1969
[
citation needed
]
- 1970:
Citroen SM
, self-leveling Citroen hydropneumatic suspension on all four wheels.
[
citation needed
]
- 1970:
Citroen GS
, self-leveling Citroen hydropneumatic suspension on all four wheels.
- 1974:
Citroen CX
, self-leveling Citroen hydropneumatic suspension on all four wheels.
- 1975:
Mercedes Benz 450 SEL 6.9
Hydropneumatic suspension on all four wheels.
- 1982:
Citroen BX
, self-leveling Citroen hydropneumatic suspension on all four wheels.
[
citation needed
]
- 1979:
Mercedes Benz W126
Hydropneumatic suspension on all four wheels as an option on the LWB v8 models
- 1983:
Toyota Soarer
: world first Electronically controlled (TEMS) that used a shock absorber control actuator (spring constant, variable attenuation force) installed
[26]
- 1985
Mercedes Benz 190E 2.3-16
Partial load bearing hydropneumatic suspension on all four wheels as an option on the 16v model. Standard on the Evo 1 and Evo 2 models
- 1985: Nissan introduced ultrasound
semi-active suspension
sensing "Super Sonic Suspension" optionally on the
Cedric
, Gloria and
Nissan Laurel
that integrated actuators inside the MacPherson struts on the front and rear suspension.
[27]
- 1986:
Jaguar XJ40
, self-leveling suspension.
[28]
- 1986:
Mercedes Benz W126
Hydropneumatic suspension on all four wheels with electronically controlled adaptive damping as an option on the LWB v8 models
- 1987:
Mitsubishi Galant
(sixth generation) - features Active Controlled Suspension (Dynamic ECS). The system enables a comfortable ride and handling stability by automatically adjusting the vehicle height and damping force.
- 1989:
Citroen XM
- self-levelling, semi-active
Hydractive
on all four wheels with automatically adjusted spring rates and dampeners.
- 1989:
Mercedes Benz R129
Partial load bearing hydropneumatic suspension with automatically adjusted spring rates and dampers as an option (ADS)
- 1990:
Infiniti Q45
and
Nissan President
"Full-Active Suspension (FAS)", active suspension system
- 1992:
Toyota Celica
(Toyota Electronically Modulated Suspension)
- 1992:
Citroen Xantia
VSX - self-levelling, semi-active
Hydractive 2
on all four wheels, with automatically adjusted spring rates and dampeners.
[
citation needed
]
- 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.
[
citation needed
]
- 1994:
Toyota Celsior
introduced first
Skyhook
air suspension
[29]
- 1994:
Citroen Xantia
Activa - self-levelling, fully active Hydractive on all four wheels with hydraulic anti-roll bars and automatically adjusted spring rates and dampeners.
- 1998:
Land Rover Discovery series 2
- Active Cornering Enhancement; an electronically controlled hydraulic anti-roll bar system was fitted to some versions, which reduced cornering roll.
- 1999:
Mercedes Benz C215
Self leveling fully active hydraulic
Active body control
. Available on the S, CL and SL models
- 2000 Citroen C5 Hydractive 3 or Hydractive 3+
- 2002:
Cadillac Seville
STS, first
MagneRide
[30]
- 2004:
Volvo
S60 R
and
V70 R
(Four-C, a short name for "Continuously Controlled Chassis Concept", semi-active)
- 2006 Citroen C6 - Hydractive 3+
- 2010:
Alfa Romeo MiTo
Cloverleaf (DNA System based on
Maserati
's Skyhook technology)
- 2012:
Jaguar XF Sportbrake
, self-leveling air suspension.
[31]
- 2013:
Mercedes Benz W222
: Optional
Magic body control
. Self leveling fully active hydraulic system with road surface scanning electronics
- 2013:
Volkswagen
Mk7 Golf R
User-Selectable Electronically Controlled Shock Dampening (Dynamic Chassis Control (DCC))
- 2019:
Toyota
Avalon
Touring model (Adaptive Variable Suspension (AVS))
- 2025:
Nio
ET9
(SkyRide fully active suspension)
[32]
See also
[
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]
References
[
edit
]
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