Tracking filter apparatus for wheel monitoring systems

A wheel monitoring system comprising a wheel monitoring device that is mountable on said wheel and configured to generate a first signal that is indicative of a characteristic of said wheel, the wheel having, in use, a variable rotational speed, said first signal having a main frequency that is dependant on said variable rotational speed, wherein said wheel monitoring system further comprises a tracking filter apparatus, the tracking filter apparatus comprising:

• an input configured to receive said first signal;
• an adjustable filter arranged to receive said first signal and to produce a filtered first signal, said adjustable filter having an adjustable cut off frequency;
• and
• a filter controller arranged to measure at least one characteristic of said filtered first signal and to compare said at least one measured characteristic against a reference value, the filter controller being co-operable with said adjustable filter to adjust said cut off frequency if said at least one measured characteristic differs from said reference value by an amount that exceeds a threshold value.