Openloop Gain Criteria of Sampled-data Control Systems with Mechanical Resonant modes above the Nyquist Frequency

For some sampled-data systems such as hard disk drives, which have mechanical resonant modes above the Nyquist frequency, suppressing the gain of these modes is a trade-off issue in mechanical/control design. In this paper, we propose a new method to determine openloop gain criteria above the Nyquist frequency. It clarifies the upper limit of the gain so that helps to design less conservative controllers. The method is derived from a frequency response analysis of multi-rate sampled system using frequency response matrix. The validity of the method is demonstrated by numerical simulations

1. INTRODUCTION

Head positioning control of hard disk drives (HDDs) is known as a typical sampled-data system, usually having mechanical resonant modes at high-frequency range. Fig. 1 shows an example of the frequency response of HDD head positioning actuator. For mechanical/control design of such systems, handling these modes is a complicated trade-off issue of performance/stability/cost, so it is important to analyse the frequency response of the system.