An Iterative Learning Control design for Self-Servowriting in Hard Disk Drives using L1 optimal control

This paper presents an Iterative Learning Controller (ILC) design for Self-Servowriting (SSW) process in Hard Disk Drives (HDDs). In SSW, the position and timing information are written onto the disk surface by referring to the previously written servo information. This process is repeated until the whole disk is completely written. The main issue in this process is Radial Error Propagation (REP), which refers to the accumulation of the written-in errors as tracks are written sequentially. In this paper, an ILC scheme is designed by using the position error information in the previous track to prevent REP and improve the quality of servowriting. Analytic conditions for stability and monotonic convergence are discussed. The learning filter design is formulated as an L₁ (or induced L_infin ) optimal control problem, which is solved by optimization programming. Simulation results show that REP is contained and a good quality of servowriting is assured.