Abstract
In the many magnetic disk drives, several servo controllers are switched from one to another to perform one servo action. This type of servo controller is called a mode switching controller and obtains fast access and accurate positioning. The authors previously proposed a basic idea of applying initial value compensation method to the mode switching controller at switching in order to improve transient characteristics.
In this paper, an actual design method for applying to a head positioning servo of magnetic disk drives are shown. Zeros of the transfer function between state variables of the plant and a controlled variable (head position) can be relocated by injecting a suitable values to the state variables of the compensator at mode switching. These values are calculated by using detected plant state variables at switching. Therefore the relocated zeros can cancel some undesirable poles and this improves transient characteristics with no influence on a closed loop stability. In the case that the detected plant variables have some errors, a complete pole-zero cancellation is not obtained. In the practical way of avoiding this problem, a specified servo mode is added between fast access mode and accurate positioning mode, whose poles are assigned not to include slow modes. By combining this mode with the initial value compensation method, fast transient with less overshoot can be obtained. Finally some experimental results are shown.
