Abstract on Finite Element Analysis of AUTOMOBILE SUSPENSION SYSTEM

Finite Element Analysis of AUTOMOBILE SUSPENSION SYSTEM

 ABSTRACT

The equation of motion for suspension system consists two degrees of freedom, as two independent special coordinates are required to define the complete configuration namely rectilinear motion and rotary motion. The equations of motions are assumed to be simple harmonic with the presence of static coupling and with the absence of dynamic coupling.

Finite element technique is applied to solve the equations of motions by choosing BEAM3, MASS21 and COMBINATION14 with their properties. The reduced option for analysis is used and a comparative study is performed on the automobile suspension system

FUNCTIONS OF SUSPENSION SYSTEM:

The primary function of suspension system is to isolate the vehicle structure from shock loading and vibration due to irregularities of road surface. Secondly it must do it without impairing the stability, steering or general handling qualities of vehicle. The primary requirement is met by the use of flexible elements and dampers, while the second is achieved by controlling, by the use of mechanical linkages. These linkages may be either as simple as a semi-elliptic spring and shackle or as complex as a double transverse link and anti-rollbar or some other such combination of mechanisms.

FACTORS AFFECTING SUSPENSION SYSTEM:

The diameter of the tyre, size of contact patch between tyre and road, the rate of tyre acting as a spring, and weight of the wheel and axle assembly affect the magnitude of shock transmitted to the axle, while the amplitude of wheel motion is affected by all these factors and the rate of suspension springs, damping effect of the shock absorbers and the weights of sprung and unsprung masses. The unsprung mass can be defined as that between the road and the main suspension springs .The sprung mass is that supported on suspension springs, though both may also include the weights of the parts of the springs and linkages.

 Two types of shocks are applied to the wheels.

1)     Shock due to the wheel’s striking on the bump. This is influenced by the geometry of the bump and the speed of the vehicle.

2)     Shock caused by the wheels falling into a pothole. This is influenced by the geometry of the hole, the unsprung masses and spring rates, speed being an incidental influencing factor.

     SUSPENSION SYSTEM

A suspension system consists of a spring shock absorber. The hydraulic damping force of the shock absorber can be taken as proportional to the square of the vertical velocity of the sprung mass relative to that of the unsprung mass, the dynamic friction damping force is, in effect, constant regardless of velocity. It follows that while small amplitude, small velocity movements of the suspension are virtually unaffected by the hydraulic damping, the force applied by the friction damping is same for these small movements as it is for large once.

Dampers have two functions. First they reduce the tendency for the carriage unit to continue to bounce up and down on its springs after the disturbance that caused the initial motion has ceased. Secondly, they prevent excessive buildup of amplitude of bounce as a result of period excitation at a frequency identical to the natural frequency of vibration of the sprung - mass system. This natural frequency is a function of the rate of the sprung mass and spring rate.

Dampers are required to cause a rapid die-away of any vibrations forced either randomly or periodically at the natural frequency of the suspension system and thus introducing a state of resonance.

The damper piston, or pistons, forcing the hydraulic fluid at high velocities through small holes, affects damping. Thus energy is absorbed by the fluid, converted into heat, and dissipated partly by conduction into the surrounding structure of the vehicle, but ultimately all passing into the air stream flowing past these components. The amount of energy thus absorbed and dissipated, for any given rate of energy input, is a function of the volume and viscosity of fluid and the numbers, sizes and geometry of the holes through which it is forced. A major advantage of hydraulic damping is the resistance to deflection of the damper is a function of the square of its velocity. 

The aim in damper design is to obtain maximum possible potential for energy absorption for any given size, and this would imply equal damping on the pump and rebound strokes.

Types of dampers:

         1) Telescopic damper

2) Lever – arm- type damper   

Springs used in suspension are,

1)     Leaf springs.

2)     Coil springs.

3)     Torsion springs.

4)     Rubber springs.

5)     Air springs.

                  Modern Tech Today, the science of springs and shock absorbers is very sophisticated, with strategies like high-pressure nitrogen gas inside the damper for more constant performance and complex fluid controls to get just the right amount of   resistance in every conceivable situation. Even some charmingly simple items like anti-roll bars (sway bars) used in modern cars. These pivoting, flexible bars link the suspension on one side of the car to the suspension on the other side of the car to reduce body roll in corners. There are even active systems that replace the spring and damper with computer-controlled arms that lift the wheels precisely over bumps and set them back down seamlessly on the other side. All of these systems strive to accomplish the same basic thing, which is to let each of the vehicle's wheels absorb bump impact comfortably without causing any added handling upset to the vehicle

 RECENT TRENDS IN AUTOMOBILE SUSPENSION SYSTEM

Variable Damping System

To obtain optimum vehicle vibration characteristics, you need to have high-quality suspension and damping systems. Most suspension systems are based on the design principle of progressive action, enabling them to adapt to changes in vehicle loads. At the expense of comfort, most shock absorbers have defined damping characteristics and are only able to adapt to changes in load and road surface across a narrow range. Comfort and safety problems have been resolved using multi-stage damping action - at best a compromise because these systems need several damping valves with different characteristics, requiring a lot of space and higher levels of cost.

Computerized Electronic Suspension:

Computerized Electronic Suspension (CES) describes a damper than contains oil and an electro-magnetically controlled valve. There are no washer stacks, no check valves, and no blow-offs. A sensor tells a control computer the position and velocity of the damper rod. The damping curve, relating damping force to damper velocity and direction, exists only in software, implemented by the rapidly changing resistance of the damping valve in the shock. Uploading a new damping curve to it from a laptop can therefore instantly alter it. This is especially useful in a racing situation because it eliminates removing the shocks from the vehicle, and it eliminates disassembly-reassembly to change damping curves. When a wheel hits a bump, the bump accelerates it upward to some velocity. What the vehicle feels is a sudden increase in spring pressure, plus the compression damping force appropriate to the motion. The vehicle would be less upset if it felt spring force only, with the damping applied only after upward bump acceleration had ceased. This is possible with a fast-acting system such as C.E.S. Once you start thinking in terms of systems that can respond in special ways to special circumstances, the suspension horizon becomes suddenly much larger. CES delivers a comfortable ride without sacrificing the safety of sure handling.

.           At the heart of the CES system is an electronic control unit (ECU) that processes driver inputs and data from sensors placed at key locations on the vehicle. The sensors include three accelerometers mounted on the vehicle body and four suspension position sensors, which feed data to the ECU on steering wheel angle, vehicle speed, brake pressure and other chassis control information.

            The ECU utilizes control software that processes the sensor information in real time and sends signals that adjust independently the damping level of each shock absorber valve. CES dampers allow a large separation between maximum and minimum damping levels and adjust instantaneously to assure riding comfort and firm, safe vehicle control.

Continuous Damping Control System:

The CDC system (Continuous Damping Control), can be adapted to suit virtually any vehicle, including cars, trucks and buses. The modular design of this system makes it possible to enhance performance while at the same time reducing overall system costs. The self-checking CDC control system consists of a set of sensors, a computer unit with intelligent software and actuators. The new design of the actuator is a proportional damping valve, which can be used to alter the damping forces between a minimum and a maximum value, working across an infinitely variable range.
            Ever greater degrees of networking within vehicles yield new forms of vehicle control. Sensors can determine various conditions such as load, driver activity, and vehicle movement, and trigger active interventions. One example of this is CDC, the active suspension control system. The CDC control unit calculates the optimum damping forces for every situation and adjusts each damper within fractions of a second – for greater safety and comfort, as well as cost-effectiveness thanks to less strain on both the cargo and the vehicle itself.

                        The term “CDC with Skyhook control“ refers to a complete system consisting of two spring dampers on the front axle, two dampers on the rear axle, a set of sensors and an Electronic Control Unit (or ECU). While the vehicle is in motion, the sensors supply all the information including braking and road speed signals, which the ECU needs in order to produce optimum damping effort. The ECU then controls individual damping action to each wheel separately in accordance with parameters stored in the software. The internal or the external proportional valve responds within milliseconds to the ECU signals by performing continuous adjustment of damping action. The optimum damping force is calculated for each wheel and is then delivered: by detecting, and responding to, changes in traction and pressure in any given driving situation, the CDC system is able to generate precisely the right level of damping force for each situation. The aim of this system is to keep the vehicle passenger cell as peaceful unaffected as possible by changes in the road surface.

CONVENTIONAL ANALYSIS OF AUTOMOBILE SUSPENSION SYSTEM

                                                         (Neglecting damping)

The automobile suspension system can be represented as shown in figure above neglecting damping. An automobile suspension is simplified to consider only two major motions of the system

1) Up and down linear motion of the body.

2) Pitching angular motion of the body.

If the body is idealized as lumped mass with weight W and radius of gyration r the natural frequencies, f₁ and f₂ can be determined by taking the following numerical values.

W=3220lb   

l₁=4.5ft

l₂=5.5ft

k₁=2400lb/ft

k₂=2600lb/ft

r=4ft

l=10ft

m=100lb-sec²/ft

J=mr²

The equations of motion indicate static coupling.

mx”+k₁(x-l₁q  )+k₂(x+l₂q )=0

J q“-k₁(x-l₁q )l₁+k₂(x+l₂q)l₂=0

Assuming harmonic motion, we have

From the determinant of the matrix equation, the natural frequencies are

w₁=6.90rad/sec;     w₂=9.06rad/sec

f₁=1.10Hz;              f₂=1.44Hz

FINITE ELEMENT ANALYSIS OF AUTOMOBILE SUSPENSION SYSTEM

Analysis assumptions

The beam geometric properties are input (all as unity) but not used for this solution. The torsional moment of inertia I_t=W r²/g=1600 lb-sec²-ft. A lateral master degree of freedom (MDOF) and a rotational MDOF are selected at the mass. The spring length is used only to define the spring direction.

 Result

f₁=1.0981      Hz

f₂= 1.4406     Hz

COMPARISION OF RESULTS

	Conventional Analysis	Finite element Analysis	Ratio
f1, Hz	1.11	1.0981	0.989
f2, Hz	1.44	1.4406	0.999

CONCLUSION:

The suspension system in a automobile requires accurate analysis to find the natural frequencies of the system otherwise which will lead to unbalance of rotary or reciprocating type. The finite element analysis furnishes almost accurate results compared to exact method as tabulated above.

REFERENCES:

1) The motor vehicle ----T.K.Garret

2) Theory of vibration with applications---W.T.THOMSON  (pp-120—140)

3) Finite Element Method----Chandrapatla   (pp-238-256)

                        

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