Research On DVD Focus Servo System

The DVD player yearly output of China mainland is more than 87 million, and reaches 80 percent of the world output. But, to use the advanced technologys about DVD, nearly 20$ per DVD should be paid to those companys who own the patents. Servo module of the optical disk drives (ODDs) is one of patents. Therefore, it is valuable and meaningful to study the servo system. Nowdays, ODDs should be capable of reading high density and hign speed disks, and need to be convenient to mass production. To meet these needs, this thesis focuses on DVD focus servo system, including system identification and controller design.In modeling of DVD focus servo system:A drive board of the DVD focus control system is constructed based on dSpace 1104. Then using the experimental data sequency, a mathmatical model is identified through the subspace identification method.To develope adaptive controllers for DVD servo system in the furture, an improved two-stage subspace method is proposed for closed-loop system identification. The method combines the two-stage method with orthogonal projection. Using QR decomposition, it constructs the Z sequence directly and identifys the model through PI-MOESP method. Simulation results show that the improved method can effectively reduce the computational burden.There are two kinds of disturbance in modern DVD servo systems. One is the periodic disturbance caused by the disk skew, the other is external shock when DVD plyaers work in a mobile device such as car or protable media player.In designing of controllers that can reject periodic disturbance caused by disk skew, the main contents of this thesis are:1,A compound controller is designed for DVD focus control system. The controller uses a disturbance observer to estimate the disturbance, and then feeds it back to the farward channel to realize the asymptotical rejection of the periodic disturbance. But, due to the posiitve feedback, the stability of the closed-loop system is sensitive to the plant uncertainty. To overcome this problem, a controller parameter turning method based on the H_∞theory is given.2,A repetitive controller is designed for DVD focus control system. Accroding to the internal model theory, this controller can reject the periodic disturbance asymptotically, since it contains the model of the disturbance in closed-loop. But, the time-lag part of the repetitive controller can cause complicated phase-frequency property, which will deteriorate the stability of closed-loop system. To overcome this problem, a controller parameter turning method based on the H_∞theory is also given. Another problem is that the performance of the repetitive controller is sensitive to the frequency error of the periodic disturbance. To overcome this problem, a so-called "high order repetitive controller" is proposed with the parameters turned by minimizing the H_∞norm of the "relative sensitivity function" .In designing of the anti-shock controller, the main contents of this thesis are:A variable gain controller is designed for DVD focus control system to reject external shock. In this controller, the loop gain is adjused dynamiclly accroding to the amplitude of the focusing error signal. Therefore, large-amplitude low-frequency focusing error which is induced by the external shocks excites high loop gain. While small-amplitude high-frequency focusing error induced by measurement noise (surface defects such as fingerprints, dusts, scratches etc.) excites low loop gain. Then the performances of low-frequency disturbance rejection and high-frequecny noise rejection are all improved. In addtion to this conclusion, a new defination which is called non-linear sensitivity function is proposed to evaluate the performance of the variable gain controller quantitatively.Till now, some methods have been successfully used in GuangDong TianYuan ecletronical limited company. The products have been exported to the United States, Europe, Japan, South-east Asia and other countries and regions. The total sales amounts to 100million US dollars.