A process for obtaining the periodic component in a data vector of data taken of a sensed property of an element driven by one or more rotary components over several rotations. The procedure comprises first forming the data vector asynchronously with respect to rotation of said one or more rotary components. A series of marker signals representing the rotation of the one or more rotary components is also established. From these the periodic component of data in said data vector is determined as a function of the data in the data vector and the marker signals.