The research methodology applied during this project is listed below in step se- quence:
1. The nonlinear half-car AVSS is modelled mathematically, based on application of Newton’s 2nd law. The model includes nonlinear springs, dampers and hydraulic actuator dynamics. Input disturbance models are also developed. 2. Model parameters and controller specifications are selected.
3. The following control systems, with two control loops, are developed: PID, MPC and NNFBL in the outer control loop. All have PID controller based hydraulic actuator force feedback in the inner control loops.
4. Simulation is carried out in the MATLAB/Simulink R
environment, the results are collated and evaluated. Controllers are redesigned if their performance is found to be poor.
5. Each controller’s performance is analyzed individually and then compared with each other.
6. Conclusions are then drawn and recommendations made.
The research methodology applied in the design of the control systems for the half- car servo-hydraulic AVSS is illustrated in Figure 1.1.
1.6
Research Contributions
This research work makes two contributions:
• The main contribution of this study is the comparison between conventional (non-intelligent) PID and MPC with Intelligent (Neural Network based) FBL applied to a nonlinear half-car AVSS. All these control methods are applied in conjunction with PID control force-feedback of the hydraulic actuator force. Nonlinear springs, dampers and servo-hydraulic actuator dynamics as well as suspension travel and actuator input voltage constraints are considered in the system’s mathematical model development.
• A secondary contribution is the application of Conventional MPC and NNFBL to a nonlinear half-car AVSS. MPC techniques are noted for the ability to han- dle Multiple Input Multiple Output (MIMO) systems with input and output constraints. FBL enables linearization of nonlinear systems, allowing applica- tion of linear control methods thereafter (Deng et al., 2009).
1.7
Dissertation Outline
This dissertation is structured as follows:
A review of the current literature in the subject area is given in Chapter 2, begin- ning with a general overview of AVSS and the force actuators they use, followed by a look at the road input disturbances that are used to evaluate suspension system performance. Half-car AVSS are then introduced and an investigation of the control techniques applied to their control is presented. A further study into conventional and intelligent control systems is also provided.
The physical and mathematical models of the nonlinear half-car AVSS are derived in Chapter 3 starting with the modelling assumptions and a look at the physical model. The nonlinear spring and damper models are developed, followed by the tyre force model, the road disturbance inputs and hydraulic actuator dynamics. A nonlinear half-car active vehicle suspension with hydraulic actuator dynamics is then formed based on the models developed in the previous sections.
Before beginning controller implementation, the performance specifications used throughout this work are given in Chapter 4. Chapters 5 to 7 contain controller implementations of the PID suspension travel controller with PID force feedback, MPC suspension travel controller with PID hydraulic actuator force feedback and the NNFBL suspension travel controller with PID force feedback, respectively. Each chapter begins with an introduction, description of the controller design method, presentation of the simulation results with a discussion and finally a conclusion. Simulations are performed in the time domain testing the AVSS and PVSS ro- bustness to variations in vehicle speeds and along with variations in sprung mass loading. For uncertainty analysis, a frequency sweep is performed on both the AVSS and PVSS with the aid of a chirp road input disturbance signal; the chirp signal’s frequency varies with time. A spectral analysis of all model outputs is performed and the results plotted in the frequency domain for variations in sprung mass loading and pitch moment of inertia, suspension spring coefficients and suspension damping coefficients.
The appendices contain MATLAB/Simulink R implementation of the half-car active
2
LITERATURE REVIEW
2.1
Introduction
A review of the literature related to AVSS is presented in this chapter. Beginning with the functions of suspension systems, their classification and the types of math- ematical models used to design suspension systems in Sections 2.2, 2.3 and 2.4. The different suspension system actuator/spring and damper setups are presented in Sec- tion 2.5, followed by a look at the causes of nonlinearity in vehicle suspension systems 2.6. Types of actuators commonly used in AVSS are given in section 2.7 and the need for force feedback is highlighted in Section 2.8. Sections 2.9 to 2.10.4.2 present in detail the control methods that have been applied to AVSS, before concluding the chapter in Section 2.11.
2.2
Suspension Systems
As introduced in Section 1.1 of Chapter 1, Vehicle Suspension Systemss (VSSs) consist of shock absorbers (dampers), springs and mechanical linkages (wishbones) that connect the vehicle wheels to the vehicle body (Fischer and Isermann, 2004). A vehicle suspension system serves the following functions:
1. Supporting the vehicles static weight.
2. Improving vehicle ride quality (comfort) by isolating the vehicle body from road disturbances.
3. Minimizing loss of traction between the tyre and the road (maintaining road holding) which contributes toward improving vehicle handling. Vehicle han- dling is the ability of the vehicle to respond to the drivers commands. It is a function of road holding and the vehicles physical properties.
4. Ensuring that suspension travel does not exceed suspension workspace limits.
A vehicle suspension’s effectiveness at carrying out these functions is used as a measure of performance. Vehicle suspensions also have the additional function of reducing road surface damage caused by vehicles driving over it (Williams, 1997a).