Modeling and simulation of a digital focusing servo control system for an optical disk tester

摘要

The design of a digital focus servo control system for an optical disk tester is presented. A total focus misregistration budget is established for three primary disturbances: disk runout, shock, and system noise. The design of an analog servo system is established first to determine the required bandwidth. The analog design is transformed into a digital representation using pole/zero mapping techniques. The transformation to the digital domain includes the effects of the control law computation time, the D/A sample and hold, and the anti-aliasing filter. Modeling and simulation of the digital system in the frequency and time domain are used to obtain the phase margin and the transient response. The transient response overshoot is larger than that obtained from a second order model. The larger overshoot is a result of the additional phase delay produced in the digital system that reduces the gain margin drastically compared to the analog case.