DUAL STAGE ACTUATOR MODELING AND CONTROL IN A CD-ROM DRIVE

Abstract

Development of high density and high speed rotating memory devices requires compatible performance from the servo system for track seeking and following. This thesis presents a robust servo design based on a Philips 4X CD-ROM drive, in which seeking and tracking are facilitated by a dual stage actuator consisting of the sledge and radial actuators. The optics are kept in focus by a separate actuator whose motion is orthogonal to the direction of seek. During seek or tracking, only the position of the radial actuator is measurable. The relative displacement between the sledge and the radial actuator is unknown. Thus the coupling between these two actuators cannot be neglected. As such the dual stage actuator is modeled and controlled as a multivariable (MV) system rather than two separate loops. With the servo presented in this thesis, both the tracking error and the seek time are reduced. High performance servo designs are based on precise knowledge of the various actuator dynamics and their uncertainty bounds. Frequency responses of the focus and radial actuators are measured and the corresponding transfer functions are obtained using a frequency domain weighted least squares method presented in this thesis. The model of the sledge actuator is set up by structural analysis instead of frequency domain modeling clue to the presence of nonlinearities. Coupling between the sledge and the radial actuator is included in the dual stage actuator model. Different CD-ROM carriages from the same manufacturer are measured to get the model variations. A multiplicative uncertainty model is extracted from the relative error between the identified model and the measurement data. Model nonlinearities as well as model variations are treated as model uncertainties, while the performance index is augmented into the system as a fictitious uncertainty [1-5]. Performance and uncertainty weighting functions are introduced to formulate the canonical robust control problem [4-5]. Stability and robust performance are sought by H? optimization on this augmented system [5]. Simulation results show that the tracking performance improves by 80%. A seek controller to reduce the access time is also designed in this thesis. An HP 10889B PC Servo-Axis Board is used for the implementation set-up. A reduced order controller for tracking was implemented with 10kHz sampling rate. Implementation results also show that the tracking performance improves.