Method and apparatus for decomposing drive error signal noise sources

Abstract

A process and apparatus is described to break down the Position Error Signal (PES) of a magnetic or optical disk or tape drive to its contributing components. (In the case of the optical disk drive, the method is actually two different PESs, Focus Error Signal (FES) and Tracking Error Signal (TES).) This method is based on three concepts: an understanding of how Bode's Integral Theorem ties into noise measurements, a measurement methodology that allows for the isolation of individual noise sources, and a system model that allows these sources to be recombined to form the drive's Position Error Signal. The method measures frequency response functions and output power spectra of each servo system element. Each input noise spectrum can then be inferred and applied to the closed loop model to determine its effect on the PES uncertainty. The method allows the user to identify and rank the most critical noise sources in the positioning mechanism of a drive. This allows for optimization of the drive positioning control loop(s) by suitable design choices. Such choices may include--but are not limited to--optimization of the position encoding on the data storage surface (such as groove dimensions in an optical drive), optimization of the position signal detection method (such as the demodulation method on a magnetic disk or tape drive), and optimization of the actuator design. Such optimization is far more difficult without this invention because it is very difficult to identify which improvements are most helpful.