Abstract: The operation of a cyclically moving, e.g. rotating, power generating and power transmitting element (10, 12) is monitored by monitoring the motion of the element at two points (14, 16) thereof. A measuring signal representing the actual differential motion of the element is provided in a differential circuit (26) and, through appropriate signal processing circuits (28, 30), input to a comparator (36), which further receives a reference signal representing a reference differential motion of the element from a reference store (34). In case the comparison of the reference signal and the measuring signal within the comparator (36) results in a divergence exceeding a predetermined limit, an anomalous signal representing an anomalous operation of the element is generated in a generator (38) which supplies an anomalous signal to an indicator block (40).