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Vibration control of vehicle's suspension system has been a very active subject of research recently, due to the fact that it can provide a very good performance for drivers and passengers. For a long time, efforts were done to make the suspension system works in an optimal condition by optimizing the parameters in the suspension system, but due to the limitation of passive suspension system, the improvement is effective only in a certain frequency range. Compared with passive suspension system, active suspension system, which consist only actuators to provide the damping force, can improves the performance of the suspension system over a wide range of frequency. Semi-active suspensions were only proposed in the early 1970s (Karnopp et al., 1974), this system can be nearly as effective as fully active suspensions in improving ride quality. The semi-active suspension system basically consist a spring and a variable-damping shock absorber. The advantage of the semi-active suspension system compared with the active suspension system is that, when the control system fails, the semi-active suspension can still work in passive condition.
1.1 Background of the problem
In early introduction of semi-active suspension, the regulating of the damping force can be achieved by adjusting the orifice area in the oil-filled damper, thus changing the resistance to fluid flow, but the changing of speed is much slow for using of mechanical motion. More recently, the possible applications of electrorheological (ER) and magnetorheological (MR) fluids in the controllable dampers were investigated by many researchers (Carlson, 1994; Boelter and Janocha, 1998). ER and MR fluids are two kinds of smart materials, which made by mixing fine particles into a liquid with low viscosity. The particles will be formed into chain-like fibrous structures in the presence of a high electric field or a magnetic field. When the electric field strength or the magnetic field strength reaches a certain value, the suspension will be solidified and has high yield stress; conversely, the fluid can be liquidified once more by removal of the electric field or the magnetic field. The process of change is very quick, less than a few milliseconds, and can be easily controlled. The energy consumption is also very small, only several watts. Both ER and MR fluids were initially developed independently in the 1940s (Winslow, 1947; Rabinow, 1948). Initially it was ER fluids that received the most attention, but were eventually found to be not as well suited to most applications as the MR fluids. In their non-activated or "off" state, both MR and ER fluids typically have similar viscosity, but MR fluids exhibit a much greater increase in viscosity, and therefore yield strength, than their electrical counterparts. For ER fluid, the maximum yield stress is about 10 kPa; but for MR fluid, the maximum yield stress can reach about 100 kPa.
1.2 Statement of the problem
Normal suspension system or passive suspension system only provides a limited ride comfort to the vehicles. If the passive suspension system faces a case where it need to act beyond its design specification such as terrible road harshness, vehicle's ride comfort will reduce. Vehicle manufacturers typically compromised vehicle's ride comfort for a better vehicle handling. Typically, the suspension settings was designed by emphasizing the handling performance and followed by the ride comfort. Semi-active suspension system, which utilize magnetorheological damper, has the ability to overcome this problem since it is operating based on a control system, which can be designed to face a wide range of road harshness. The ability of semi-active suspension system made the vehicle's ride performance can be maintained under various circumstances. A MR damper need to be designed so that it can work well with the vehicle's suspension system during its on-state and off-state while the same time providing the best performance of ride comfort and handling performance to the vehicle.
1.3 Objectives of the study
The objectives of the study are to:
To design and develop magnetorheological damper
To characterize the characteristic of the developed MR damper.
To design control systems for the MR damper in order to control its operation.
To study the performance of vehicle's ride and handling model with semi-active magnetorheological damper.
To study the effect of different controllers in improving vehicle's ride and handling.
1.4 Scopes of the study
The scopes of the study are as stated below:
Modelling of vehicle's ride and handling model using Matlab/Simulink
Validating the ride and handling model with the experimental results.
Design and fabricate the prototypes of magnetorheogical damper for passenger vehicle
Designing controller for the developed magnetorheological damper
Implementation of magnetorheological damper characteristic and its controller onto simulation model.
Performance comparison of semi-active suspension system controlled by different controllers with passive suspension system.
Modification (when necessary) on the simulation model.
1.5 Significance of the study
Semi-active suspension system has the potential to be applied in Malaysia's production vehicles due to its better performance compared to the passive suspension system on the vehicle, which it was widely applied recently. The studies on how to design and control the MR damper for semi-active suspension system as well as investigating its performance in terms of improving vehicle's ride and handling performance will be an interesting and challenging research works to be performed.
Designing semi-active suspension system and its control system were now becomes a great concern among the automotive researchers and engineers. The next section of this proposal will describe briefly about the research done related to semi-active suspension system.
2.1 Relevant Research Reviews
The researches on semi-active suspension system are related to designing MR damper, modelling and identification of the MR damper, as well as designing the control system to control the operation of the MR damper. The next sections will be discussing the progress on these matters.
2.1.1 MR Damper Modelling
MR damper model can be divided into two groups; parametric model and non-parametric model. In parametric model, MR damper are represented by a mathematical function whose coefficients value are adjusted until the quantitative results of the model closely match the experimental data. Below are the lists of parametric models proposed by the other researchers:
Modified Algebraic model (Cesmeci and Engin 2010)
Bajkowski model (Bajkowski et. al., 2008)
Bingham model( Spencer et. al. 1997)
Extended Bingham model(Gamota and Filisko)
Three Element model(Powell 1994)
Bingmax model(Makris et al. 1996a and 1996b)
Bouc-Wen model(Spencer et al 1997)
Modified Bouc-Wen model(Spencer et al 1997)
Non-Linear Viscoelastic-Plastic model(Kamath and Werely 1997a)
Augmented Non-Linear Viscoelastic-Plastic Model(Kamath and Werely 1997b)
Bi-viscous model(Stanway et. al 1996)
Non-parametric model of MR damper are the model where its behaviour were entirely based on the performance of a specific MR damper device. For this model, an elevated amount of experimental data, obtained by observing the damper response to different excitation under varying operating conditions, is used to predict the device response to random excitations. Below are the lists of parametric models proposed by the other researchers:
Chebyshev Polynomial Fit (Ehrgott and Masri (1992)
Neural Networks (Burton et al., 1996; Boada et. al., 2008; Metered et. al., 2010)
Data mapping technique ( Abu Bakar et. al., 2008)
2.1.2 MR Damper Design
Mazlan et al., ( 2009) design a magnetic circuit for a squeeze mode experiment to investigate the compression and tension characterisitic of the MR fluid. The design of the magnetic circuit was made using FEM approach to determined the theoretical magnetic field distribution through MR fluid. The test equipment was design based on the parameters used in the FEM model. The results from the experiment shows that the compressive/tensile stresses of MR fluids increased as the applied current increased.
Karakoc et al., (2008) apply the magnetorheological fluid technology in building a brake system for automotive application. The brake consists of multiple rotating disks which immersed in MR fluid and enclosed with electromagnetic circuit. The braking torque produced when the electromagnetic circuit being activated creating shear friction between the rotating disk. Karakoc et. al., (2008) also emphasize on the design criteria that should be considered in designing a brake system utilizing MR fluid and those criterias are material selection, sealing, working surface area, viscous torque generation, applied current density, and MR fluid selection.
Gordaninejad and Kelso, (2000) designed a MR damper to be used on off-high-payload, off-road vehicles. The designed MR damper was tailored to cater for a wide range of dynamic loading with the capability to operate at a different rebound and compression. The design is based on the original shock absorber as the reference. The author used Bingham model to design its MR damper and also conduct experimental studies to verify the design. The results from the studies show that the experimental results demonstrate good correlations with the simulation results.
Ahmedian, (2000) from Advanced Vehicle Dynamics Laboratory, Virginia Tech has applied the MR fluid technology to design the MR damper for bicycle applications. The author used two approaches in order to study the effectiveness of MR damper in providing comfort for the bicycle users. The first method is by only retrofitting the MR valves inside the original damper while for the second method the author made a new design for the bicycle by considering the easiness of fabrication and assembly. The author also considered the new characteristic of the designed damper to envelope the original damper. The results from this studies are that, a properly designed MR dampers can provide significant dynamic improvement in terms of comfort, as compared with a conventional passive bicycle damper.
Ahn et al., (1999) had modified a conventional fluid mount and replaced the original fluid of the mounting with MR fluid. The dynamic behaviour of MR fluid when subjected to magnetic field was used to control the fluid flow inside the mounting. The behaviour of MR fluid was used as a switch to control the openings of fluid passage of the mounting. A simple control scheme was used to control the operation of the MR fluid and it was found that the application provides a good improvement for the mount's isolation effect.
Simon and Ahmedian, (1999) studied the effect of magnetorheological damper as a primary suspension in improving ride comfort on heavy truck vehicle. For this purpose, a set of controllable magnetorheological damper was fabricated. An embedded controller was used to determine the damping level for the semi-active suspension based on the skyhook control scheme.
Pare' (1998) has studied the effectiveness of magnetorheological damper in improving vehicle's ride comfort by testing the developed magnetorheological damper on a full scale quarter car model, constructed at the Advanced Vehicle Dynamics Laboratory at Virginia Technology. The author evaluated the response of the developed damper on the vehicle suspension using different control algorithms namely skyhook, groundhook and hybrid skyhook-groundhook. The comparison between semi-active system and passive system were also studied using the developed quarter car rig. Very promising results were shown in terms of ride quality improvement that can be produced by the semi-active magnetorheological damper.
Poynor (2001) has researched several different applications of magnetorheological fluid technology. The developed magnetorheological dampers were for automotive suspension system application and for military purposes. The dampers developed for the automotive suspension system were mono-tube based damper and hybrid-tube based. In the military application, the magnetorheological dampers was developed to be used in gun recoil system in order to reduce the impact of back-thrust during firing.
Another research work on the development of magnetorheological fluid technology is by Gravatt, (2003). In his master's thesis, the magnetorheological dampers were developed to be used in super bike's suspensions system. The developed MR dampers were designed and fabricated based on the original manufacturers dampers and installed in the super bike's suspension system. The author used skyhook control algorithm to control the operation of the developed damper in order to improve the super bike's ride comfort and results from the installation are very promising.
2.1.3 MR Damper Control System
The most common control strategies used to control the operation of variable damping damper in semi-active suspension system are skyhook control strategies, groundhook control strategies and hybrid skyhook-groundhook control strategies.
Skyhook control system is the most basic and most common algorithm used in semi-active suspension system for disturbance rejection control. The algorithm was introduced by Karnopp et al., (1974). In skyhook control system, an imaginary damper is inserted between the sprung mass and the stationary sky as shown in Figure 2.18 as an effort to reduce or eliminate the motions of sprung mass when the vehicle is subjected to road inputs such as road harshness or bumps. The equation governing skyhook control is given by:
If then (2.28)
Figure 2.18 Skyhook control system
As for the groundhook control system, it was proposed by Novak and Valasek (1996) in order to eliminate the excessive unsprung mass motion and improves tyre force dynamics. In ground hook control, an additional fictitious damper is added between the unsprung mass and the ground. If in skyhook control system, the improvements are focused in reducing the sprung mass oscillations and isolate it from the base excitations, the ground hook control system is focused in improving unsprung mass oscillations from base excitations. Groundhook control system is more suitable to be used to control semi-active force in semi-active suspension system for heavy vehicle because the algorithm in Groundhook control improves tire force dynamics which will reduce the effect of road damage which might possibly cause by the heavy vehicle's suspension system (Valasek and Kortum, 1998a and 1998b). The configuration of Groundhook control is shown in Figure 2.19. The equations governing Groundhook control are as follows:
If then (2.29)
Figure 2.19 Groundhook control system
Hybrid skyhook-groundhook has been proposed by Ahmedian (1997). The control system which takes benefit both of skyhook and groundhook systems gives the users the ability to specify how closely the controller emulates the skyhook or groundhook. The hybrid skyhook-groundhook system has two dampers connected to some inertial reference in the sky and in the ground as depicted in Figure 2.20.
The hybrid control strategy is a linear combination of skyhook control system and groundhook control system and can be written as
where and are the skyhook damping force and groundhook damping force respectively. The parameters and are the skyhook and groundhook damping constant. The variable is the relative ratio between the skyhook and groundhook control, and G is a constant gain.
Figure 2.20 Hybrid Skyhook-groundhook Control System.
220.127.116.11 Advanced Control Strategies For Semi-active Suspension System
In this section, brief descriptions on alternatives control strategies that are used to control the operations of the variable damper are discussed with most of the control strategies involve complex algorithms.
a Sliding Mode Control
Lam and Liao (2001) and Yokoyama et al., (2001) studied the application of sliding mode control (SMC) with semi-active magnetorheological damper. The controller which considers the loading uncertainties and the undesirable non-linear properties of magnetorheological damper was reported to reduce significantly the sprung mass acceleration. It was also reported that the use of SMC with magnetorheological damper also improves sprung mass peak response and the root-mean-square values of the studied parameters (i.e. acceleration, vertical displacement).
b Fuzzy Logic Control
Fuzzy Logic control is known to be very effective in disturbance rejection control of semi-active suspension system (Brown and Harris, 1994). The concept of a fuzzy logic set is introduced by first defining membership functions. In order to develop a fuzzy logic system for control application, a functional form of the membership function is needed. This membership function describes the degree of certainty that an element belongs to a fuzzy set.
There are many shapes of membership functions proposed, and the most widely used membership functions are the triangular-type, trapezoidal-type, Gaussian-type and the singleton membership function. The mathematical form of a standard rule-based fuzzy system is given by
where and represents the membership function, fuzzy parameter, number of rule and output membership function for i-th rule respectively.
The output of the fuzzy system is used to calculate the desired damping coefficients for semi-active suspension system (Al-Holou and Shaout,1994) or to calculate the desired force required by the active suspension system (Barr and Ray, 1996). It is reported via numerical studies that the use of fuzzy logic are able to improve the vehicle model's ride comfort (Sireteanu et al., (2001), Chen et al., 2001; Chen et al., 2003; Zhang et al., 2004). In real world application for semi-active suspension system, the implementation of fuzzy logic control had been performed by Fang et al., (1999), Craft et al., (2003) and Rui et al., (2004) and the results are very promising with the controller could effectively reduce sprung mass vertical oscillation and improve ride comfort and handling stability.
c H Control System
Hâˆž control system is known for its robustness when operating under different environment. It is a control method that considers uncertainties, including the model's uncertainties, model parameters and disturbances. H control system of semi-active suspension system with magnetorheological damper has been studied by Du et al., (2005). In this study, the dynamic behavior of magnetorheological was first simulated using a polynomials model based on the experiments carried out on the developed magnetorheological damper. Then the magnetorheological damper model was used in a quarter car model along with the H control system. The H controller used the suspension deflection and the sprung mass velocity as the feedback signals. The scheme was further studied via numerical simulation under random excitations. Simulation results showed that the designed H controller could provide good performance for the semi-active suspension system.
d Neural Network Control
Yiming and Xiangying (2004) investigated the performance of semi-active suspension system with magnetorheological damper controlled by neural network controller. The study was carried out via simulation model by using a quarter car model. Neural network control of magnetorheological with experimental assessment using a quarter car test rig was reported by Chen et al. (2000) and Zapateiro et. al., (2009). The results from both of these studies show that the neural network control system excel in improving the suspension performance.
e Linear Quadratic Regulator (LQR) Control
The LQR control system for semi-active suspension system has been studied by ElMadany and Abduljabar (1999) using a simple quarter car model. Hrovat (1991) studied the scheme on a full car model while Krtolica and Hrovat (1992) on a half car model. The strength and advantage of LQR approach is that the elements of the performance index can be weighted according to the designer's desires or other constraint. With this advantage, an optimal result can be achieved when all the criteria of performance are taken into account (i.e. body acceleration, relative velocity and dynamic tyre load). Gopala and Narayan (2009) used LQR to determine an optimal value of skyhook damping constant.
2.2 Work of Interest
The studies will focus on the efforts in fabricating a custom design of non-pressurized magnetorheological damper as well as developing the control system to control the operation of the developed MR damper. The developed MR damper will be characterized via an experimental work and its characteristic will be used in the simulation of ride and handling. Based on the literature review, most of the researches were too focused in improving vehicle's ride comfort with less efforts are done in improving vehicle's handling performance using semi-active suspension system. These studies will hopefully shows the effectiveness of magnetorheological damper in improving vehicle's ride and handling performance. This study will also investigate on how different controllers used to control the operation of magnetorheological damper in semi-active suspension system can give different ride and handling performance results.
3.0 Research Methodology
A 14DOF of vehicle's ride and handling model for Proton Waja will firstly be modelled using Matlab/Simulink software. The model will then be validated with the experimental results from the ride and handling test carry out on the selected passenger vehicle.
The next stage of this study is to do the simulation design or model-based design of MR damper for passenger vehicle by using the OE shock absorber used in that passenger vehicle's suspension system as the benchmark (in terms of geometric design and performance).
The model-based design will involve designing the geometric parameters and the electric circuit used in MR damper. Various controllers that can be used to control the operation of the designed MR damper will also be modelled and the selection of these controllers will depends on the ride and handling performance that it can provides.
The next stage is to carry out the simulation work between the designed semi-active suspension system (with the selected control algorithm) and the passive suspension system to determine the level of performance for these two systems.
Meanwhile the fabrications of the magnetorheological damper which is based on the model-based design will be carried out for the purpose of design verifications. The characteristic of the fabricated MR damper are determined by using Material testing System (MTS) machine which its characteristics are modelled into a simulation model.
Ride and handling performance test will be performed once the all the works related to the MR damper characterization and modelling as well as the controller design are done. The ride and handling test of magnetorheological semi-active suspension system will be done using various controllers to see the effect of controller in controlling the operation of magnetorheological damper and also the effect on vehicle's ride and handling performance. Figure 1 shows the work flow for the study.
All of the works; simulation model development, model validations, model-based designs of MR damper and its controllers, MR damper's prototype fabrication and semi-active performance via simulation works are expected to be finished within three years time. The scheduled for the work are as shown in Figure 2.
Figure 1 Study work flow
Table 1 PhD work plan
Expected Findings and Summary
All the works are hopefully will give a complete magnetorheological semi-active suspension system comprising of magnetorheological damper and its controller that can give the best ride and handling performance for the passenger vehicle. The study is also hopefully will create a standard guideline in designing MR damper to be used in vehicle's suspension system.