and is affected by random noise as well as by noise related to other nearby rotary members more specifically and with reference to fig3 the method comprises an initial step in which a variable f explained later on with reference to fig5 is set to a first predetermined value e g 0 block 20 signal s t and signal s1 t are then acquired block 21 and a sampling frequency fs is calculated by multiplying the frequency of azimuth sensor signal s1 t by a memorized coefficient kt equal to the ratio between two prime whole numbers and in particular correlated to the transmission ratio between the azimuth sensor shaft and the shaft monitored by the sensor in question so that the resulting sampling frequency fs is correlated to the azimuth sensor frequency and is such as to supply exactly nj points where nj is a power of 2 of signal s t for each revolution of the monitored shaft signal s t is then sampled and filtered by interface unit 151 to remove the random noise and nonsynchronous periodic components block 22 and obtain a filtered signal t 1m i defined by a number of samples signal s t is preferably sampled and filtered using signal s1 t and the sequence of steps shown in fig4 and described in detail later on.