Adaptive suspension (other name semi-active suspension) is a type of active suspension in which the degree of damping of the shock absorbers varies depending on the condition of the road surface, driving parameters and driver requests. The degree of damping refers to the rate at which vibrations attenuate, which depends on the resistance of the shock absorbers and the magnitude of the sprung masses. In modern adaptive suspension designs, two methods are used to regulate the degree of damping of shock absorbers:
- using solenoid valves;
- using magnetic rheological fluid.
When regulated using an electromagnetic control valve, its flow area changes depending on the magnitude of the acting current. The greater the current, the smaller the valve flow area and, accordingly, the higher the degree of damping of the shock absorber (rigid suspension).
On the other hand, the lower the current, the larger the flow area of the valve, the lower the degree of damping (soft suspension). The control valve is installed on each shock absorber and can be located inside or outside the shock absorber.
Shock absorbers with electromagnetic control valves are used in the design of the following adaptive suspensions:
Magnetic rheological fluid includes metal particles that, when exposed to a magnetic field, line up along its lines. The shock absorber, filled with magnetic rheological fluid, does not have traditional valves. Instead, the piston has channels through which fluid passes freely. Electromagnetic coils are also built into the piston. When voltage is applied to the coils, the particles of the magnetic rheological fluid line up along the magnetic field lines and create resistance to the movement of the fluid through the channels, thereby increasing the degree of damping (suspension rigidity).
Magnetic rheological fluid is used much less frequently in the design of adaptive suspension:
- MagneRide from General Motors (Cadillac, Chevrolet cars);
- Magnetic Ride from Audi.
Regulation of the degree of damping of shock absorbers is provided by an electronic control system, which includes input devices, a control unit and actuators.
The adaptive suspension control system uses the following input devices: ride height and body acceleration sensors, operating mode switch.
Using the operating mode switch, you can adjust the degree of damping of the adaptive suspension. The ride height sensor records the amount of suspension travel in compression and rebound. The body acceleration sensor detects the acceleration of the vehicle body in the vertical plane. The number and range of sensors varies depending on the design of the adaptive suspension. For example, Volkswagen's DCC suspension has two ride height sensors and two body acceleration sensors at the front of the car and one at the rear.
Signals from the sensors enter the electronic control unit, where, in accordance with the programmed program, they are processed and control signals are generated to actuators - control solenoid valves or solenoid coils.
In operation, the adaptive suspension control unit interacts with various vehicle systems: power steering, engine management system, automatic transmission and others.
The adaptive suspension design usually provides three operating modes: normal, sport and comfort.
Modes are selected by the driver depending on the need. In each mode, the degree of damping of the shock absorbers is automatically adjusted within the limits of the set parametric characteristic.
The readings from body acceleration sensors characterize the quality of the road surface. The more unevenness there is on the road, the more actively the car body sways. In accordance with this, the control system adjusts the degree of damping of the shock absorbers.
Thus, based on sensor signals, the control unit generates control signals for each shock absorber individually, which allows for maximum comfort and safety for each of the selected modes.
Cadillac Magnetic Ride Control struts and shock absorbers are designed to improve handling and comfort when driving on a variety of road surfaces. The system appeared quite a long time ago and turned out to be so effective that it was later repeated by many other European and German automakers, but initially it appeared on the Escalade, SRX, and STS models.
Operating principle
In general, the system works quite simply. Unlike traditional shock absorbers, shock absorbers of this type do not use oil or gas, but magnetic-rheological fluid, which reacts to the magnetic field created by a special electric coil located in the body of each shock absorber. As a result of the impact, the density of the liquid changes, and, accordingly, the stiffness of the suspension.
The Magnetic Ride Control system works very quickly; data from various sensors arrives at speeds of up to a thousand times per second, instantly responding to changes in the road surface. Sensors measure body sway, vehicle acceleration, load and other data, on the basis of which the current flowing separately into each of the shock absorbers at that particular moment is calculated.
In reality, everything happens exactly as the manufacturer describes; good handling is combined with a high level of comfort. But there is also a significant drawback when operating in our country.
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Let's first understand the concepts, since now various terms are in use - active suspension, adaptive... So, we will assume that active suspension is a more general definition. After all, changing the characteristics of suspensions in order to increase stability, controllability, get rid of rolls, etc. it can be done either preventively (by pressing a button in the cabin or by manual adjustment) or fully automatically.
It is in the latter case that it is appropriate to talk about an adaptive chassis. Such a suspension, using various sensors and electronic devices, collects data on the position of the car body, the quality of the road surface, and driving parameters, in order to independently adjust its operation to specific conditions, the driver’s driving style, or the mode chosen by him. The main and most important task of an adaptive suspension is to determine as quickly as possible what is under the wheels of the car and how it is driving, and then instantly rebuild the characteristics: change the ground clearance, the degree of damping, the suspension geometry, and sometimes even... adjust the steering angles of the rear wheels.
HISTORY OF ACTIVE SUSPENSION
The beginning of the history of active suspension can be considered the 50s of the last century, when outlandish hydropneumatic struts first appeared on cars as elastic elements. The role of traditional shock absorbers and springs in this design is performed by special hydraulic cylinders and hydraulic accumulator spheres with gas pressure. The principle is simple: change the fluid pressure - change the chassis parameters. In those days, such a design was very bulky and heavy, but it fully justified itself with its smooth ride and the ability to adjust ground clearance.
The metal spheres in the diagram are additional (for example, they do not work in hard suspension mode) hydropneumatic elastic elements, which are internally separated by elastic membranes. In the lower part of the sphere there is a working fluid, and in the upper part there is nitrogen gas
Citroen was the first to use hydropneumatic struts on its cars. This happened in 1954. The French continued to develop this theme further (for example, on the legendary DS model), and in the 90s the more advanced Hydractive hydropneumatic suspension debuted, which engineers continue to modernize to this day. It was already considered adaptive, since with the help of electronics it could independently adapt to driving conditions: it was better to smooth out shocks coming to the body, reduce dive when braking, combat roll in corners, and also adjust the vehicle’s ground clearance to the speed of the car and road conditions. covering under the wheels. The automatic change in the stiffness of each elastic element in the adaptive hydropneumatic suspension is based on the control of liquid and gas pressure in the system (to thoroughly understand the principle of operation of such a suspension scheme, watch the video below).
VARIABLE STIFFNESS SHOCK ABSORBERS
And yet, over the years, hydropneumatics has not become simpler. Quite the contrary. Therefore, it is more logical to start the story with the most common method of adapting suspension characteristics to the road surface - individual control of the stiffness of each shock absorber. Let us remind you that they are necessary for any car to dampen body vibrations. A typical damper is a cylinder divided into separate chambers by an elastic piston (sometimes there are several of them). When the suspension is activated, liquid flows from one cavity to another. But not freely, but through special throttle valves. Accordingly, hydraulic resistance arises inside the shock absorber, due to which the swing damps.
It turns out that by controlling the speed of fluid flow, you can change the stiffness of the shock absorber. This means seriously improving the car’s performance using fairly budgetary methods. After all, today adjustable dampers are produced by many companies for a variety of car models. The technology has been proven.
Depending on the design of the shock absorber, its adjustment can be done manually (using a special screw on the damper or by pressing a button in the cabin), or fully automatically. But since we are talking about adaptive suspensions, we will consider only the last option, which usually also allows you to adjust the suspension proactively - by choosing a specific driving mode (for example, a standard set of three modes: Comfort, Normal and Sport).
In modern designs of adaptive shock absorbers, two main tools are used to regulate the degree of elasticity: 1. a circuit based on solenoid valves; 2. using the so-called magnetorheological fluid.
Both versions allow you to individually and automatically change the degree of damping of each shock absorber depending on the condition of the road surface, vehicle driving parameters, driving style and/or proactively at the request of the driver. A chassis with adaptive shock absorbers significantly changes the car’s behavior on the road, but in the range of regulation it is noticeably inferior, for example, to hydropneumatics.
- How does an adaptive shock absorber based on electromagnetic valves work?
If in a conventional shock absorber the channels in the moving piston have a constant flow area for uniform flow of working fluid, then in adaptive shock absorbers it can be changed using special solenoid valves. This happens as follows: the electronics collects a lot of different data (shock absorber reactions to compression/rebound, ground clearance, suspension travel, body acceleration in planes, mode switch signal, etc.), and then instantly issues individual commands to each shock absorber: to release or squeeze for a certain time and amount.
At this moment, inside one or another shock absorber, under the influence of current, the flow area of the channel changes in a matter of milliseconds, and at the same time the intensity of the flow of the working fluid. Moreover, the control valve with the control solenoid can be located in different places: for example, inside the damper directly on the piston, or outside on the side of the body.
The technology and settings of adjustable shock absorbers with solenoid valves are constantly being improved to achieve the smoothest possible transition from hard to soft damping. For example, Bilstein shock absorbers have a special central DampTronic valve in the piston, which allows the resistance of the working fluid to be continuously reduced.
- How does an adaptive shock absorber based on magnetorheological fluid work?
If in the first case solenoid valves were responsible for adjusting the stiffness, then in magnetorheological shock absorbers this is controlled, as you might guess, by a special magnetorheological (ferromagnetic) fluid with which the shock absorber is filled.
What super properties does it have? In fact, there is nothing abstruse about it: in the ferromagnetic fluid you can find many tiny metal particles that react to changes in the magnetic field around the shock absorber rod and piston. When the current strength on the solenoid (electromagnet) increases, the particles of the magnetic fluid line up like soldiers on a parade ground along the field lines, and the substance instantly changes its viscosity, creating additional resistance to the movement of the piston inside the shock absorber, that is, making it stiffer.
It was previously believed that the process of changing the damping rate in a magnetorheological shock absorber was faster, smoother and more precise than in a solenoid valve design. However, at the moment, both technologies are almost equal in efficiency. Therefore, in reality the driver hardly feels the difference. However, in the suspensions of modern supercars (Ferrari, Porsche, Lamborghini), where the reaction time to changing driving conditions plays a significant role, shock absorbers with magnetorheological fluid are installed.
Demonstration of the operation of Audi's Magnetic Ride adaptive magnetorheological shock absorbers.
ADAPTIVE AIR SUSPENSION
Of course, in the range of adaptive suspensions, air suspension occupies a special place, which to this day there is little that can compete in terms of smoothness. Structurally, this scheme differs from a conventional chassis in the absence of traditional springs, since their role is played by elastic rubber cylinders filled with air. Using an electronically controlled pneumatic drive (air supply system + receiver), you can delicately inflate or deflate each pneumatic strut, automatically (or preventively) adjusting the height of each part of the body within a wide range.
And to control the stiffness of the suspension, those same adaptive shock absorbers work in tandem with air springs (an example of such a scheme is Airmatic Dual Control from Mercedes-Benz). Depending on the design of the chassis, they can be installed either separately from the air cylinder or inside it (pneumatic strut).
By the way, in the hydropneumatic scheme (Hydractive from Citroen) there is no need for conventional shock absorbers, since the rigidity parameters are controlled by electromagnetic valves inside the strut, which change the intensity of the flow of working fluid.
ADAPTIVE HYDRO SPRING SUSPENSION
However, the complex design of the adaptive chassis does not necessarily have to be accompanied by the abandonment of such a traditional elastic element as a spring. Mercedes-Benz engineers, for example, in their Active Body Control chassis simply improved the spring strut with shock absorber by installing a special hydraulic cylinder on it. And in the end we got one of the most advanced adaptive suspensions currently existing.
Based on data from a lot of sensors that monitor the movement of the body in all directions, as well as on readings from special stereo cameras (they scan the quality of the road 15 meters ahead), the electronics are capable of finely adjusting (by opening/closing electronic hydraulic valves) the rigidity and elasticity of each hydraulic spring strut. As a result, such a system almost completely eliminates body roll under a wide variety of driving conditions: turning, accelerating, braking. The design reacts so quickly to circumstances that it even made it possible to abandon the anti-roll bar.
And of course, like pneumatic/hydropneumatic suspensions, a hydrospring circuit can adjust the height of the body, “play” with chassis rigidity, and also automatically reduce ground clearance at high speed, increasing vehicle stability.
And this is a video demonstration of the operation of a hydraulic spring suspension with the Magic Body Control road scanning function
Let us briefly recall the principle of its operation: if the stereo camera and lateral acceleration sensor recognize a turn, then the body will automatically tilt at a small angle towards the center of the turn (one pair of hydraulic spring struts instantly relaxes a little, and the other slightly tightens). This was done to eliminate the effect of body roll when turning, increasing comfort for the driver and passengers. However, in reality, it’s more likely only... the passenger who perceives a positive result. Because for the driver, body roll is a kind of signal, information thanks to which he feels and predicts one or another reaction of the car to a maneuver. Therefore, when the anti-roll system works, the information comes with distortion, and the driver has to once again psychologically readjust, losing feedback with the car. But engineers are also struggling with this problem. For example, specialists from Porsche have tuned their suspension in such a way that the driver feels the very development of roll, and the electronics begin to remove undesirable consequences only when a certain degree of body tilt is reached.
ADAPTIVE ROLL STABILIZER
Indeed, you read the subtitle correctly, because not only elastic elements or shock absorbers can be adapted, but also secondary elements, such as the anti-roll bar, which is used in the suspension to reduce roll. Do not forget that when the car moves in a straight line over rough terrain, the stabilizer has a rather negative effect, transmitting vibrations from one wheel to another and reducing suspension travel... This was avoided by the adaptive anti-roll bar, which can perform a standard purpose, be completely turned off and even “play” with its rigidity depending on the magnitude of the forces acting on the car body.
The active anti-roll bar consists of two parts connected by a hydraulic actuator. When a special electric hydraulic pump pumps working fluid into its cavity, the parts of the stabilizer rotate relative to each other, as if lifting the side of the machine that is under the influence of centrifugal force
An active anti-roll bar is installed on one or both axles at once. Outwardly, it is practically no different from the usual one, but does not consist of a solid rod or pipe, but of two parts, joined by a special hydraulic “twisting” mechanism. For example, when moving in a straight line, it releases the stabilizer so that the latter does not interfere with the operation of the suspension. But in corners or when driving aggressively, it’s a completely different matter. In this case, the stiffness of the stabilizer instantly increases in proportion to the increase in lateral acceleration and forces acting on the car: the elastic element either works in normal mode or also constantly adapts to the conditions. In the latter case, the electronics itself determines in which direction the body roll is developing and automatically “twists” parts of the stabilizers on the side of the body that is under load. That is, under the influence of this system, the car tilts slightly when turning, as with the aforementioned Active Body Control suspension, producing the so-called “anti-roll” effect. In addition, active anti-roll bars installed on both axles can influence the car's tendency to drift or skid.
In general, the use of adaptive stabilizers significantly improves the vehicle’s handling and stability, so even the largest and heaviest models like the Range Rover Sport or Porsche Cayenne have the opportunity to “roll” like a sports car with a low center of gravity.
SUSPENSION BASED ON ADAPTIVE REAR ARMS
But the engineers from Hyundai did not go further in improving adaptive suspensions, but rather chose a different path, making... the rear suspension arms adaptive! This system is called Active Geometry Control Suspension, that is, active control of suspension geometry. In this design, each rear wheel has a pair of additional electric levers that vary toe-in depending on driving conditions.
Due to this, the vehicle's tendency to skid is reduced. In addition, because the inside wheel rotates during a turn, this clever technique simultaneously actively combats understeer, acting as a so-called full-steering chassis. In fact, the latter can be safely attributed to the car’s adaptive suspensions. After all, this system also adapts to different driving conditions, helping to improve the vehicle's handling and stability.
FULL CONTROL CHASSIS
For the first time, a fully controlled chassis was installed almost 30 years ago on the Honda Prelude, but that system could not be called adaptive, since it was completely mechanical and directly depended on the rotation of the front wheels. Nowadays, everything is controlled by electronics, so each rear wheel has special electric motors (actuators), which are driven by a separate control unit.
PROSPECTS FOR THE DEVELOPMENT OF ADAPTIVE SUSPENSIONS
Today, engineers are trying to combine all the invented adaptive suspension systems, reducing their weight and size. Indeed, in any case, the main task driving automotive suspension engineers is this: the suspension of each wheel at each moment of time must have its own unique settings. And, as we can clearly see, many companies have been quite successful in this matter.
Alexey Dergachev
It begins in the mid-50s of the last century, when the French company Citroen installed hydropneumatics on the rear axle of the representative Traction Avant 15CV6, and a little later on all four wheels of the DS model. On each shock absorber there was a sphere divided by a membrane into two parts, which contained the working fluid and the pressurized gas supporting it.
In 1989, the XM model appeared, on which the Hydractiv active hydropneumatic suspension was installed. Under electronic control, it adjusted to the traffic situation. Today, Citroen is running the third generation Hydractiv, and along with the regular version, they also offer a more comfortable one with the Plus prefix.
In the last century, hydropneumatic suspension was installed not only on Citroens, but also on expensive executive cars: Mercedes-Benz, Bentley, Rolls-Royce. By the way, cars crowned with a three-pointed star still do not avoid this design.
Active Body and other systems
The Active Body Control system differs in design from Hydractiv, but the principle is similar: by changing the pressure, the suspension stiffness and ground clearance are set (hydraulic cylinders press the springs). However, Mercedes-Benz also has chassis options with air suspension (Airmatik Dual Control), which set ground clearance depending on speed and load. The stiffness of the shock absorbers is monitored by ADS (Adaptive Damping System). And as a more affordable option, Mercedes buyers are offered the Agility Control suspension with mechanical devices that regulate stiffness.
Volkswagen calls the system that controls the shock absorber settings DCC (aDaptive Chassis Control). The control unit receives data from sensors about the movement of the wheels and body and changes the chassis rigidity accordingly. The characteristics are set by solenoid valves installed on the shock absorbers.
Audi uses a similar adaptive suspension, but some models have the original Audi Magnetic Ride system. The damping elements are filled with a magnetoresistive fluid that changes viscosity under the influence of a magnetic field. By the way, Cadillac was the first to use a design that works on the same principle. And the “Americans” have a similar name - Magnetic Ride Control. Having fit into this family, Volkswagen is in no hurry to part with proper names. Porsche's intelligent chassis with electronically controlled shock absorbers and, on some models, also air suspension, is designated PASM (Porsche Active Suspension Management). Another signature weapon PDCC (Porsche Dynamic Chassis Control) helps to effectively combat rolls and dives. Anti-roll bars with hydraulic pumps practically prevent the body from swaying from side to side. Opel has been installing IDS (Interactive Driving System) on production models for almost a decade. Its main component is CDC (Continuous Damping Control), which adjusts the shock absorbers depending on road conditions. By the way, other manufacturers, such as Nissan, also use the CDC abbreviation. In new Opel models, clever electronic and mechanical devices are called “flexes”. The suspension was no exception - it was called FlexRide.
BMW has another cherished word - Drive. It is therefore logical that the adaptive suspension is called Adaptive Drive. This includes Dynamic Drive roll suppression systems and EDC (Electronic Damper Control) shock absorber stiffness control systems. The latter will probably soon also come up with a designation with the word Drive. Toyota and Lexus use common names. The stiffness of the shock absorbers is monitored by the AVS (Adaptive Variable Suspension) system, and the ground clearance is controlled by the AHC (Active Height Control) air suspension. The KDSS (Kinetic Dynamic Suspension System), which controls the hydraulic drives of the stabilizers, allows you to take turns with minimal roll. An analogue of the latter in Nissan and Infinity is the original HBMC system (Hydraulic Body Motion Control - hydraulic control of body movement), which changes the characteristics of the shock absorbers and thereby reduces the sway of the car from side to side.
Hyundai implemented an interesting idea by installing the AGCS (Active Geometry Control Suspension) rear suspension on the new Sonata. Electric motors drive the rods, changing the angles of the wheels. Thus, the electronics help the stern to steer in turns. By the way, in some cars, electric motors controlled by active steering change the steering angle together with the front ones. For example, RAS (Rear Active Steer) for Infinity or Integral Active Steering for BMW.
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