I. Introduction
Periodic disturbances that have an underlying position domain nature are often encountered in positioning systems. Examples include; a system with cogging [2], a CD-player [3], or a wafer stage performing repeating motions [4]. In the present paper, a substrate carrier system, being used in the printing industry, is considered. It consists of a steel belt that rotates along two rollers. The rotational nature of the system induces a disturbance that depends on the roller angle. This disturbance is periodic in the roller position domain. However, in the time domain the disturbance period may change or become a-periodic depending on the roller velocity.