A passive blade detection sensor exploits the electrically conductive trait of
typical turbo machinery components such as fan, compressor and turbine blades.
A permanent magnet is placed strategically adjacent to a wire coil to generate
a single pulse/antipulse signal when a blade passes in front of the sensor. The
electrically conductive blade allows eddy currents to flow briefly as it passes
through the magnetic field, the local reluctance is momentarily increased, and
the magnetic field is momentarily changed. The coil is sensitive only to the portion
of change through its cross section, so that a simple voltage pulse, useful for
precise timing of blade passage, is produced. Since the device is only sensitive
to fast changes in local conductivity, blades can be detected through non-ferrous
barriers such as typical engine case walls.
