Internal model control (IMC) approach for designing disk drive servo-controller

This paper presents a design procedure for disk drive servomechanism using an internal model control (IMC) structure. A typical disk drive actuator can be modeled as second order dynamics for low frequencies. However the response at higher frequencies shows resonant behavior which is difficult to model. We discuss the use of IMC structure for designing servo-controllers for disk drives. In this method, a second order nominal model of the plant is used to design an H ₂-optimal controller to attain minimum integral-error-square (ISE) performance. Then to maintain robust stability at higher frequencies, sufficient roll-off at such frequencies is provided by an H _∞ optimization procedure. Here, the H₂-optimal control is augmented by a low pass filter with sufficient high-frequency roll-off to ensure robust stability and robust performance. A multiplicative uncertainty bound is defined using the data of the disk drive servo plant´s frequency response and the response of the nominal model, and this is then used to decide robust stability and robust performance bounds. Tuning of only one parameter of the IMC filter makes this design method easy and convenient. Simulation results for the designed controller are presented