The FOCISTM system can determine the axial position of a rotor by measuring the position of the blades. This indirect measurement of axial position is performed by measuring the blade width.
A probe is looking at the blade tip close to the leading edge. At the nominal rotor position, the width of the blade is recorded. As the rotor changes axial position, the blade shifts relative to the probe so that the probe sees a different blade width. By knowing the relationship between blade width and axial position, the axial position of the rotor can be determined.
This method works for blades which have a large variation in thickness along the chord. It allows precise detection of the width, but users must be attentive to other phenomena which can be mistaken for axial shift of the rotor, such as blade untwist which changes the perceived width of the blade as well.