Two degree of freedom control for disk drive servo systems

This dissertation investigates two degree of freedom (2DOF) control and its application to the hard disk drive (HDD) servo system. In the HDD application, the 2DOF controller operates for both track seeking and track following modes, therefore eliminating the mode changes that may exacerbate the settling of the read/write (R/W) head.; Practical considerations for the HDD servo system are addressed in its design and optimization. A simple method is also developed to model the track mis-registration (TMR) sources from the position error signals (PES).; The 2DOF controller contains three major components. The first component is a feedforward controller, for which zero phase error tracking (ZPET) control or model following control can be used. A proposed feedforward control using embedded position in servo sectors shows 30% PES improvement by simulations. The second component is a reference generator providing aggressive but safe position profiles for track seeking. The third component of the controller is a robust feedback controller, for which a disturbance observer (DOB) or an adaptive robust controller (ARC) can be used. Simulations and experiments on its use for track seeking control show that both seek dine and tracking errors can be reduced.; HDD track density is determined by the TMR performance of the track following controller. Various tradeoffs have to be made for the required performance. This dissertation addresses the optimization of the servo controller with a fixed structure. The optimization problem is formulated to minimize the PES variance with constraints. The cost functions use weighting sequences from the TMR source models. The noise environment of the servo system is then accurately represented. Using the proposed method, a 2DOF controller with DOB and a conventional PID controller are optimized. Simulations show that the 2DOF controller achieves better stability margins, as well as better TMR performance.