suspension system

1. INTRODUCTION.

A suspension system supports an automobile and keeps its wheel in contact with uneven road surfaces. Modern suspensions produce a smooth, stable ride as well as predictable cornering and stopping characteristics. The suspension system determines how an automobile “handles” and how much weight can be carried safely.

SUSPENSION FUNCTIONS :

• The suspension systems support the weight of the vehicle chassis and provide comfortable and safe rides over many types of road surfaces.
• In addition, the suspension minimizes irregular and excessive tyre wear.
• It isolates the vehicle from road surface irregularities.
• Controls body motion caused by road inputs, inertia forces and aerodynamic forces.
• Maintain average vehicle attitude and ground clearance.
• Maintain vehicle directional stability during maneuvers.

A vehicle is most stable and controllable when its body is level and the forces acting on the tyre contact patches are the areas of tyre on which the vehicle rests on the road. Forces created by acceleration, braking, and cornering cause a vehicle’s body to tilt front to back and lean from side to side. As the body tilts or leans, weight shifts from one end or side of the vehicle to another. As a result, the traction of the tyre becomes uneven and the vehicle is less controllable.

Figure illustrates the range of conditions that occur during driving.

To deal with changes in vehicle attitude that occur during driving, three types of suspension systems are studied,

1. Passive suspension system.
2. Semi active suspension system.
3. Active suspension system.

2. PASSIVE SUSPENSION.

Passive suspension system is found on most vehicles. Vehicle height and damping depend on fixed, non adjustable spring and shock absorber or struts. When weight is added, the vehicle lowers as the spring are compressed. Vehicle body motion and tyre traction varies with the road surface and driving conditions.

The characteristics of the spring and shock absorber or struts used to determine how stiffly or softly the vehicle rides. The amount of traction that keeps the vehicle under control changes during cornering and breaking.

For examples, the suspension design and parts of some luxury cars produces a high, “soft” ride on smooth roads for maximum passenger comfort. Soft springs and soft shock absorber damping allow the wheels to move upward over small bumps without transferring this motion to the body.

However, this choice of parts allows the body and the wheels to move excessively during sudden maneuvering or hard braking. The vehicles may experience excessive body roll under these conditions.

At the other extreme, sports cars are built low to the ground and have stiff springs and shock absorbers. This design and choice of parts produces a “hard”, stiff ride on most surfaces. Stiff springs and shock absorbers control body lean and wheel motion rapidly and efficiently. Thus sports cars corner well at high speeds and are more controllable during hard braking and quick emergency maneuvers. However sports cars are not well known for soft, comfortable rides.

Passive suspension systems have been developed over a number of years to a high level of sophistication. Control over rigid body and wheel dynamics is attained by selecting suitable spring rates and damper characteristics. Body motions caused by inertial forces are reduced by addition of roll springs and suspension geometry; “refinement” is added by inserting compliant bushes and damper isolators. Vehicle lateral stability is tuned by changing the ratio of front / rear roll spring rates.

When fully developed, a passive suspension system is cheap, reliable and works very well over small range of vehicle weight. However, a particular vehicle will have been tuned to exhibit good ride qualities or good handling qualities, but rarely both simultaneously. Furthermore, the trend in automobile is towards reduction in vehicle weight (in order to improve fuel economy) and hence towards an increase in the ratio of payload / vehicle weight. This trend makes the design of a satisfactory passive suspension system increasingly difficult.

3. SEMI ACTIVE  SUSPENSION.

To provide some of the benefits of both “soft” and “hard” suspension, semi active suspensions were first introduced during the mid 1980s. These systems use computer controlled adjustable springing devices and adjustable shock absorbers.

3.1 Computer control. Like other computer controlled systems semi active suspension receive input data in the form of electrical signals from sensors. Sensors provide input data for front and rear suspension height, G forces (acceleration), steering wheel movement, vehicle speed and other conditions. The computer unit processes the data and controls output units – air suspension units and adjustable shock absorbers.

Semi active suspension controls some vehicle body motions. The system controls overall height, shock absorbers or strut stiffness or softness, and spring stiffness.

3.2 Springing control. Spring stiffness is resistance of a springing device to compression or extension. To adjust spring stiffness rapidly in a semi-active suspension system, flexible air chambers can be used.

As shown in figure, the top of a coil spring rests against flexible air chambers. In effect, the flexible air chambers from an adjustable “cushion” between the spring and the vehicle body. The computer unit controls an air valve system to increase or decrease air pressure in the chambers.

When air pressure is decreased, the flexible chambers become softer and more flexible, just as removing air from the tire makes it softer. When the spring is compressed as the wheels move upward over a bump, it presses against the air chambers. The soft flexible chamber expands as it absorbs some of the force of spring compression. This action provides a smooth ride.

Pumping air into the flexible chambers makes them stiffer. Then, when the spring compresses, the force is transmitted more directly to the body and the springing becomes “stiffer” for greater vehicle control.

       3.3 Height Control. The flexible air chambers also can raise or lower the vehicle’s height. The chambers expand as air is pumped into them. Because the body is resting on top of the chambers, it rises as air pressure increases. To lower the vehicle, air pressure is reduced in the chambers.

For example, if vehicle load is increased by adding passengers and cargo, semi active suspension reacts to height sensor information. The system increases air pressure in the chambers to raise the vehicle and compensate for the increased load.

Another use of height control is to improved aerodynamics at highway speed. As vehicle as speed increased, the vehicle lowers with the front and angled downward. This action reduces wind resistance for better gas mileage and greater stability. When the driver switches the headlights on, the system lowers the rear of the vehicle to level the vehicle and maintain the correct headlight aiming. As the vehicle slows, the semi active suspension raises the body to normal height and level conditions.

3.4 Damping Control. Damping refers to the actions of shock absorbers or struts in controlling the bouncing motion or oscillation of spring devices. Special adjustable shock absorbers or struts units control spring damping in semi active suspension.

Shock absorbers dampen spring oscillation by providing a resistance to the bouncing movements of the spring. Within the shock absorbers, a piston with orifices or holes is forced to move hydraulic fluid when shock absorber is compressed or expanded. This restricted movement through fluid provides the damping resistance.

For semi active suspension system, a computer controls the motion of an actuator ( an air motor or an electrical stepper ). The actuator turns a control rod that extends through the shock absorber.

On the control rod is a rotary valve through which the shock absorber fluid flows during compression or expansion. A rotary valve includes orifices that can be aligned with holes in a surrounding hollow enclosure.

When the holes are fully aligned fluid can flow through the passages easily. If the fluid passages are not fully aligned, fluid flow is partially blocked. The rotary valve varies the size of the shock absorber orifices through which hydraulic fluid flows. The larger the passageway, the more easily the fluid flows and the less damping resistance the shock absorber provides.

The rotary valve controls the amount of damping by providing a large passageway (soft damping) or a small passageway (hard damping). Intermediate positions produce medium damping.

                        

4.  ACTIVE SUSPENSION.

In addition to all of the functions provided by semi active suspension, fully active suspension reacts to many other types of body motions. Fully active suspensions also controls body tilt from front to back and body lean from side to side during cornering, braking and acceleration. Additional solenoid air valve are used to transfer air pressure and control body motion.

4.1 THE SYSTEM:

Major parts of pneumatically operated fully active suspension includes,

·         Adjustable strut / shock absorber units with strut chambers

·         Solenoid air valves

·         Air compressor

·         Front and rear height sensors

·         Vehicle speed sensor

·         G sensor for side to side acceleration

·         Throttle position sensor

·         Steering wheel angular velocity (turning speed sensor)

·         Information / control panel

·         Electronic control unit

In this fully active suspension design, an air control system similar to that of semi active suspension system is used. However, additional air control valves are added so that air pressure is transferred from side to side and front to back. This transfer of air pressure changes suspension height and counteracts body motions caused by turning, braking and acceleration.

Roll control.  During cornering, the side of the vehicle nearest the inside of a turn tends to lift upwards, as the vehicle leans towards the outside of the turn. This motion is known as roll. Roll reduces traction on the tyres at the inside of a turn.

To counteract these forces, the active suspension system increases air pressure in the air chambers at the outside wheels. At the same time, valves open to reduce air pressure in the inside wheel air chambers. These actions counteract the motions caused by turning the vehicle body remains relatively level for better traction and driver control.

After the turning motion is completed, valves are opened to equalize the pressure on both sides of the vehicle and return it to a level position.

Dive control.  During hard braking, weight transfer tends to push the front of the vehicle downward and lift the rear upward. This motion is known as dive. Dive reduces traction on the rear end of vehicle and may cause it to slide or spin during emergency braking.

During hard braking, the active suspension increases air pressure in the front air chambers and reduces air pressure in the rear chamber. These actions minimize dive to keep the vehicle level and make it easier for the driver to control.

After braking, valves operate to equalize air pressure in front and rear air chambers and level the vehicle again.

Squat control.  When the driver depresses the accelerator quickly during hard acceleration, the front end of the vehicle tends to lift up. The rear end lowers. This motion is known as squat.

The active suspension system controls squat by operating valves that increase air pressure in rear wheels air chambers and reduce air pressure in front wheel air chambers. When the vehicle is no longer accelerating quickly, the control system operates valves to equalize air pressure and level the vehicle.

Thus, an active suspension changes the height of the front, rear or ether side of the vehicle to counteract tilting and leaning. These active attitude control functions improves vehicle stability and increase tyre traction and driver control.

4.2 ACTUATORS USED IN ACTIVE SUSPENSION SYSTEM:

Recently with the advent of active control technology utilizing microprocessors, sensors and actuators, practical systems can be designed and applied to such concepts as automobiles and railway and magnetically levitated vehicles.

Hydraulic suspension.

In automobiles, the active system involves replacing the conventional suspension elements with hydraulic actuators, which is controlled by high-frequency response servo valves manipulated by way of feedback compensation.

In order to realize good vibration isolation property, the vehicle body vertical acceleration is measured & input to the microprocessor, which calculates the required force.

The active suspensions with hydraulic actuators can also control the rigid vehicle body attitude by measuring the lateral acceleration or yaw rate responses of the vehicle body. With this total vehicle body attitude control, unnecessary roll motion during cornering can be suppressed & the roll moment of the vehicle body can be designed in order obtain good handling & stability for driving at high speeds.

In the practical application, with consideration of the decrease of energy consumption, an auxiliary air chamber is attached to the hydraulic actuator, by which high frequency vibration is absorbed by the air cushion effect. Such practical active suspensions have been already applied to passenger cars.

Air suspension.

Active suspension with pneumatic actuators have been applied to automobiles & railway vehicles in order to the vehicle body attitude, decreasing unnecessary body roll motion in curves and leading to good running stability at high speeds.

Because of air compression properties, the high frequency vibration is expected to be absorbed by air cushion effects, leading to good ride quality. However, the pneumatic actuators act at rather low frequencies compared with hydraulic actuators. It is therefore necessary to compensate for the pneumatic force by considering the time delay of the actuator & utilizing feed forward compensation as well as feedback loop, measuring vertical acceleration, velocity displacement & other variables if necessary.

Magnetic suspension.

Magnetic levitation systems are divided into two types: electromagnetic suspension (EMS) which is the controlled attractive force type and electrodynamic suspension (EDS) which is the repulsive force type. These magnetic noncontact suspensions are used for the primary suspension replacing the wheel on rail mechanism in high-speed bullet trains, low noise urban trains and dust free carriers.

As is well known, electromagnetic suspension using electromagnets is inherently unstable. So feedback compensation is indispensable. Moreover, as the attractive force type EMS allows only about 10mm clearance between the magnets and the guide way surface, it is very important to control the air gap between the guide way surface and electromagnets in order to prevent contact between the magnets and the guide way.

The fundamental EMS control system can be illustrated as shown in figure. This electromagnetic suspension can be classified as active suspension.

                  

On other hand, electrodynamics suspension without control is classified as passive suspension although this type generates a strong repulsive force to obtain a large gap. It has some problems from the viewpoint of vibration isolation, such as an extremely low damping force and a small mass ratio of the vehicle body to the unsprung mass. Therefore, actively controlled primary or secondary suspensions are needed in order to gain high stability & good ride quality.

Magneto Rheological fluid suspension.

            Magneto Rheological fluids are materials that exhibit a change in rheological properties (elasticity, plasticity or viscosity) with the application of a magnetic field. The magneto rheological effects are often greatest when the applied magnetic field is normal to the flow of the magneto rheological fluid. A schematic of this damper is shown in the figure.

 The hydraulic cylinder houses the damper piston, in which is mounted a magnetic circuit. At the base and inside the hydraulic cylinder is a nitrogen accumulator that is used to pressurize the approximately 50 ml of magneto rheological fluid to above atmospheric pressure. This is a standard technique to prevent cavitation on the low pressure side of the piston while it is in motion.

 The magneto rheological fluid flows through an annular orifice in the piston head, where it can be activated by a current applied to the magnetic circuit.

         Characteristics of the magneto rheological actuator are,

·         Electrically controllable and energy can be supplied externally without contact.

·         No mechanical sliders are used.

·         No risk of leak of fluid is imposed.

5. FUTURE DEVELOPMENT AND APPLICATIONS.

Future research and development is directed in the following areas:

1. Frequency response.   To improve the vehicle ride in terms of noise and harshness, the frequency response of the system must be improved to at least the performance of an optimized spring/damper at any given frequency.

2. Power consumption.     With the emphasis in motor vehicle development so much on reducing fuel consumption, any new system that requires power input has built in disadvantage. Reducing the authority of the system and the incorporating passive springs in parallel with the active component will result in a large reduction in mean power consumption. Variable displacement ‘smart’ hydraulic pumps are being developed for current vehicle hydraulic systems, promising further efficiencies.

3. Safety. Advances in digital electronics may make it cost effective to multiplex parts of the systems to ensure failure modes are safe. However, valve design and the authority of the active component must be such that in the event of failure the suspension will continue to offer a reasonable degree of stability and control that the driver has adequate warning. It must fail at least as safely as a tire.

4. Cost. Hydraulic systems area becoming fairly common place on motor vehicles and are already subjected to the economics of mass production. Electronics and microprocessor development are similarly affected and the technology is progressive too fast to make accurate predictions about cost in five year’s time. The keys to the costs of an active suspension system are the number and the type of transducers and the servo valves. New markets for these components, other than automotive, are developing all the time, being fundamental to computer control of ‘muscle’. An automotive application would provide the incentive for the investment necessary to achieve major cost reductions.

Potential applications exist on any vehicle which has conflicting suspension requirement that result in compromise solutions when passive spring and dampers are used. A fully active system provides an invaluable research tool to explore all the possibilities.

Potential applications include:

1.      Passenger cars.   More optimized ride and handling. Commonality of hardware between models with different software for each model. Suspension configurations and geometry optimized for weight, space utilization compliance.

2.      Commercial vehicle. Optimized suspension for wider range of payload. Improved stability of vehicles with a high center of gravity (coaches, ambulances, fire engines).

3.      Trains. Attitude control in bends.

4.      Off-road vehicles. Improved ride and traction. Lower foot print pressure by controlled load distribution among all wheels.

5.      Earthmovers and tractors. A prime requirement is attitude control when subjected to externally applied loads of the same order as the weight if the vehicle. Most vehicles have no suspensions other than that provided by the tires. Because the tires are only slightly damped, resonance problems occur at relatively low speeds. Active suspension would provide conventional suspension characteristics with attitude control.

6.      High speed off road military vehicles. Suspension could be optimized for road and off road use, and could provide a stable weapons platform and ballistic recoil absorption.

7.      Aircraft under carriages. Aircraft under carriages double as a suspension system and an energy absorption system. NASA research indicates that a 20-30% reduction in structure loads on landing possible.

8.      Spring / dampers. The control systems involved in active suspension are applicable to all forms of computer controlled springs and dampers, particularly velocity, acceleration or load controlled catapults or shock absorbers.

6. CONCLUSION.

            The purpose of an automobile suspension is to reduce the transmission of vibrations from the road and thus ensure ride comfort. The problem of isolating the vibration induced by the road surface has been tackled by design of passive, semi active and active means.

            The design of the passive suspension system involves the selection of spring stiffness and damping such that the effect of road surface induced vibration is minimized over a wide range of excitation frequencies. However, this system has fixed characteristics, once the spring has been selected based on load carrying capability of the suspension, the damper is the only variable remaining to specify. Therefore, the passive suspension system have limitations of vibration isolation and lack of altitude control of the vehicle body.

            In order to overcome the problem encountered in passive suspension system, various semi active and active suspension designs have been developed.

            In semi active suspension system, the damping force is controlled to dissipate energy from the system thereby reducing vibration transmission.

            However, with the coming of fast and inexpensive DSP (Digital Signal Processing), it has now become possible to create an effective active suspension system, which uses different types of actuators to both add and dissipate energy from the system based on signals obtained from various sensors which is processed through controller to provide effective isolation.