Design of Tracking Controller for Holographic Digital Data Storage

In this paper we present a new tracking control method for holographic digital data storage systems. Holographic data storage systems that use disk media need a different tracking method from those of optical and hard disk drives. Here we present the design and implementation of the controller. Due to the intermittent characteristics of the tracking error signal (TES), the tracking controller is designed to regulate the tracking error only while valid TES is generated. Since the valid TES appears intermittently, we use a feedforward controller while the TES is not valid. Furthermore, the feedback control and feedforward control are mode switched depending on the radio frequency sum signal level. We also show the stability of the closed loop system employing the mode switching controller. Experimental results show the effectiveness of the proposed tracking control algorithm. For a plusmn100 mum repeatable runout at 1 Hz, we were able to reduce the TES to 3.13 mum (1sigma) with feedforward mode switching control.