Analysis, modelling and robust control of automotive HID headlight systems

Abstract

Automotive high intensity discharge (HID) lamps have received attention recently because of its significant improvement in vision and high luminous efficacy. The automotive headlight system usually consists of Xenon lamp and HID ballast. The modelling of Xenon type lamp requires programming of complex physical characteristic. The characteristic is nonlinear and highly dependent on temperature. Ageing study of the lamp allows the improvement in ballast design and illumination compensation. An ageing model is therefore necessary, but it requires the detailed study under the long term time-monitoring of the lamp. Another issue is the dimming control as it provides a flexible control of the lighting output. This thesis is focused on these two aspects: development of the electronic ballast circuit, and the new control methods to optimize the performance of the system. The study first begins with the comparison of a commerical available controller UCC2305 for the electronic ballast and a microprocessor controlled electronic ballast. The design method, theoretical analysis and experimental results are presented. A unified controller is then developed based on a flexible control platform dSPACE and it is realized by adaptive control method and PI control method. The adaptive control parameter is then examined and compared with the conventional PI control method. Simulation and experimental results show that the system is stable and the nonlinear adaptive control is more robust to system parameter variation. A 2-D spline interpolation model has been developed to present the characteristics of lamp. The model can then accurately represent the transient and steady states under various temperature control schemes. It provides a platform using the spline method to develop other HID lamp models with limited data. In order to further examine various control methods, especially under constant power control for the lamp, Passivity-based Control (PBC) controller of HID electronic ballast is examined. This controller uses the nonlinear equations of the system to avoid the nonlinearity of plant, and to guarantee the global stability and the asymptotic convergence of all state errors. Using a power function, this indirect PBC controller is realized by reshaping the energy of the system and injecting the required damping. The load estimator is derived to insure that the controller is effective in large load disturbance. The simulation and experimental results are then used to verify the model and the nonlinear controller. The dimming control for the HID lamp ballast is further investigated to include the relationship between the lamp power and various control parameters. A power-dependent lamp model is first established. Two dimming control methods, namely variable duty-ratio control and variable bus voltage control, are then compared and developed. The summation method and multiplier method versions of variable bus voltage dimming control are compared for theoretical calculation and simulaion. It is found that the summation method has a higher performance in constant power control under lamp voltage variation. The variable duty-ratio control is then examined. Three different duty-ratio dimming control methods, power tracking method, constant voltage power tracking method and constant current power tracking method, are compared. The simulation results show the power tracking method is better and more applicable to a practical lamp. The experimental results are used to verify the power tracking method and summation method. Lastly, the lamp ageing measurement including voltage, current and lumen were made. Two groups of lamp each consisting of 5 lamps have been used. The results provide useful data for the understanding of the ageing. The lamp resistance and life time study are provided. A compensation has been proposed for the aged lamp in order to provide a consistent lumen output. The result forms a useful application for practical lamp implementation.