Analysis of Unobservable Oscillations in Sampled-Data Positioning Systems

Unobservable oscillations of the controlled variable of sampled-data positioning control systems may degrade the performance and reliability of the mechatronic products. The degree to which these oscillations are unobservable or observable depends on the sampling frequency and the oscillation frequency. In this paper, we study the relationship between the oscillation frequency and the sampling frequency. We define the index of unobservable position based on its rms amplitude to estimate the effect of the unobservable oscillations of mechanical resonances in positioning control systems. This index shows that a sampling frequency and a number of samples of measurement data for estimating the unobservable magnitude of oscillations are important parameters which designers must choose carefully. To show the use of the proposed index, we have analyzed unobservable magnitudes of oscillations for a head-positioning control system in a hard disk drive.