Introduction

Abstract

Periodic reference and disturbance signals are widespread in engineering practice, as every rotating machine and repeated process involves periodicity. Periodic disturbances are for instance encountered in the track-following servo system of disk drives [24,25,26,101], steel casting [98], power electronics [12, 164,165], active air bearing systems [7,67], active noise control [11], satellite attitude stabilization [16,156], peristaltic pumps used in medical devices [64] and robotized laparoscopic surgery [50]. Furthermore, disturbances due to rotating unbalances or nearby combustion engines are dominantly periodic. On the other hand, periodic reference trajectories occur in noncircular machining [83,148], electronic cam motion generation [84] and robots performing repetitive tasks [80,119].