Aktif süspansiyon sistemleri

Abstract

Bu tez çalışmasının ilk bölümündetaşıt titreşimlerine genel bir bakış yapılmıştır. Basitleştirilmiş taşıt modeli üzerinde titreşim denklemleri çıkarılmış, taşıta gelen uyarılar tanıtılmıştır. İkinci bölümdeyse titreşimlerin taşıt üzerindeki etkileri irdelenmiş, konfor ve dinamik tekerlek yükü gibi önemli kavramlar incelenmiştir. Üçüncü bölümde, taşıtlardaki süspansiyon sistemleri ele alınarak pasif, aktif, yarı aktif olarak üçe ayrılmış bunların ayrıntılı açıklamaları yapılmıştır. Dördüncü bölümde, basitleştirilmiş taşıt modeli üstünde pasif ve yarı aktif süspansiyonların bilgiayarla simülasyonu yapılmıştır. Beşinci bölümde de simülsayonun sonuçları verilerek yorumlanmış, süspansiyon sistemleri hakkında genel bir değerlendirme yapılmıştır.

The design of a vehicle suspension is a process in which a compromise has to be made between conflicting property requirements. In order to obtain a good ride comfort it is desired to isolate the vibrations from the car body as well as possible, especially the resonances have to be damped properly. On the other hand the wheels maintain contact with the road with as little load variation as possible, which is desirable in connection with the car's handling properties. In addition the wheel travel with respect to the car body is limited. In conventional suspension systems only passive components are used and it is well known from theoretical examination what the principal restrictions of such concepts are. The applications of active or semi-active control within the vehicle suspension introduces new possibilities and some extra degrees of freedom. In order to eliminate some of the principal barriers of a passive suspension system one would like to apply a fully active suspension system in the motorcar. However, such a system has some major disadvantages amongst which the most important are : A high energy consumption and high costs. By theoretical examination and computer similation it - xiii - is shown that some aspects of the improvement, available by a fully active suspension. Characteristics for any semi -act ive suspension is the variation of some parameter ( most obviously the damping coefficient ) over a large bandwidth within a closed-loop control system. Thus by only varying one or more parameters rather than using a linear actuator one is only capable of regulating the energy dissipation, which implies only a low level of energy cnsumption. A semi-actively damped suspension system is characterized by a rapidly adjustable damper parallel with a spring to support the stsatic load. The damping can be adjusted by controlling the oil flow path area in a conventional shock absorber. We can distinguish between two types of semi-active dampers : 1- Continuously variable dampers. These dampers are capable of tracking a force demand independently of the instantaneous velocity across it. 2- Switchable dampers. These dampers have two or more states. A semi-active damping system can be a lot chpear than a fully active suspension, because some very expensive hydraulic parts can be left out of it. The property on which semi-active damping can compete with fully active - xiv - suspension is the isolation of body vibrations in the frequency range from the body naturel frequency. This means with respect to a passively suspended car a good damping of the body naturel frequency is possible wihhout a deterioration of the isolation between body axle- naturel frequency. For the evaluation of a semi-active damping system two things are required, namely : a) Software."Control strategy, control law design and the computer program representation of the control 1 aw. b) Hardware : A vehicle equipped with the right modifications and outfit to perform the required functions. In the first part of this thesis, vehicle vibrations are in general glanced. The differential equations of the vibrations are on the quarter car model derived and the vibration impulses are described. Also the main subject is mechanical vibrations. For this reason essential technical instructions of the vibrations are remembered. In the second part, the effects of vibrations on the car are cleared and important concepts like dynamic tire load, ride comfort, suspension displacement stroke and the _ xv _ comfort number are explained to understand better what happens mechanicly on the car during it goes on a smooth road or waved road. In the third part, suspension system are introduced, which consist of passive, active and semi-active suspension systems. Spring and dampers are the basic elements of covnential (passive) suspensions. Therefore detailed informations about them are given, which have a constanat coefficient. They provide a global osilation because no automatic control is applied. Since the improvement studies of the ride comfort without decrease the ride security began, active suspension systems has been actual. Some kind of active systems and general principles of them are summarized. The main point here is the closed automatic control loop. Also the trend from mechanic to electronic control is used in active systems. Switchable actuators (changeable damping states accordingly to road distrubances) have been consequent elements as well as the microprocessor (control unit) and acceleration sensors. With the similar work structures to active systems, semi-active suspension systems are particularly releated. A variable damper works parallel to the spring at the vehicle suspension. Commands to the variable damper come from the microprocessor, which gets input signals from the sensors based on different points of the vehicle. Because of it is used in the fourth part, the relative control strategy and the other control - xvi - strategies (skyhook and extended skyhook strategies) of semi-active suspensions are explained. It has seen that semi- active suspensions achieve the functions of active suspensions. At the end of this section an " ambulance stretcher" example is given to compare the passive and active systems. In the fourth part, a simulation of a passive and semi-active system has done on the quarter car model, by means of the computer programmes written in Quickbasic language. With them are the dynamic tire loads, relative displacements, velocities and accelerations investigated. The result graphics, obtained after running the programs, are interpreted and compared in the fifth part. On the graphics it is possible to see the differences between pasive and semi-active suspensions. Also using semi-active suspensions would be more profitable than passive suspensions for more ride comfort. Finally a general evaluation of the suspension systems is made and the state of the transformation from passive to active is emphasized. It should be decleared that more mechanical events are studied than electronical operations or automatic control circuits.