Abstract
Recently there has been an increased demand for large-capacity magnetic hard disk drives. To meet this demand, a fast and precise head positioning servo system is required. A mode switching control (MSC) system, which includes several different servo structures and a switching function, is widely used to meet this requirement. In an MSC system, each servo system can be optimally designed to satisfy a certain performance index. On the other hand, a design method for the switching function has not yet been perfected.
In this paper, the initial value compensation method has been applied to this problem. When the servo mode is switched from one mode to another, certain initial values are input for the state variables of the compensator in the new servo mode. Two approaches to determining the initial values are proposed. One is to minimize a linear quadratic function of state variables, which is called the J-min method in this paper, and the other is to cancel the unfavorable poles of the closed loop with zeros, which is called the zero-assign method.
In actual systems, several filters are used to compensate for mechanical resonance gain peaks. Since applying the initial value compensation method to the filters is fairly complicated, a 1st-order approximate time delay model can be applied to represent the filters. The initial values in the compensation method are chosen for this. Several simulation results show that this method, especially the zero-assign approach, is an effective way of settling the head swiftly, and provides robustness for velocity fluctuations during mode switching.
