Dual-rate iterative learning control for spiral servo track writing

In this paper, we present a preliminary result on dual-rate iterative learning control which is applied to spiral servo track writing. Plant inversion iterative learning control (PIILC) is used to enhance tracking performance of conventional servo track writing. Since the discretized plant becomes a non-minimum phase system we used zero phase error tracking controller (ZPETC) for PIILC. In the presence of periodic disturbances, PIILC itself does not have capability to reject them unless the period of the desired trajectory is a multiple of the period of the disturbance such as repeatable runout (RRO). Although adding another ILC to the single ILC system removes the condition, the dual-rate ILC, however, does not meet identical initialization condition (IIC). To solve this problem, we use a switching operation to satisfy IIC of ILC for disturbance rejection. From simulation results, we show that the tracking performance and disturbance rejection are improved compared with conventional feedback controller.