Control design of concentric self-servo track writing systems for hard disk drives

Modern hard disk drives (HDDs) generate the head position error signal (PES) by reading special magnetic patterns called servo patterns, which are written in designated areas on the disk surface known as servo sectors, and measure the position of the head relative to the center of the data track. Servo sectors are typically created during the HDD manufacturing process and are not subsequently overwritten or erased. The process of writing servo sector patterns is known as servo track writing (STW). The accuracy and precision of the servo track writing process plays a crucial role in dictating the ultimate track density and areal storage density of the HDD. There are several mechanisms for performing servo track writing. Conventional servo track writing processes use additional sensing and positioning equipment external to the HDD. In contrast, self-servo track writing processes only use the HDD´s reading and writing heads and servo system; thus avoiding many of the manufacturing cost increases and productivity losses associated with conventional servo track writing. This paper presents two novel controller synthesis methodologies for performing concentric self-servo track using a feedforward control structure. In the first methodology, it is assumed that a conventional track-following causal feedback compensator has been designed. A non-causal feedforward compensator, which utilizes the stored error signal that was created while writing the previous track, is subsequently designed using standard H_∞ control synthesis techniques, in order to achieve good disturbance attenuation while preventing the propagation of tracking errors from previous tracks. In the second methodology, both the track-following feedback compensator and the feedforward compensator are simultaneously designed via a mixed H₂/H_∞ control scheme, which involves the solution of a set of linear matrix inequalities. Simulation results confirm that the tw- proposed control synthesis methodologies prevent error propagation from the previously written tracks and significantly improve self-servo track writing performance.