Capacitance frequency modulation probe for blade tip clearance measurement

摘要

A gas turbine engine casing (10) is provided with a probe (12) having an electrode (16) which is adapted to cooperate with the tips (22) of successive rotor aerofoil blades (11) to define a series of capacitors, the capacitances of which are proportional to the clearance between the blade tips (22) and their surrounding casing (10). The thus formed capacitors constitute the frequency determining elements of the tuned circuit of an oscillator (23), the output frequency of which is consequently a function of the blade tip clearances. The output of the oscillator is connected to a guard electrode (21) within the probe (12) so that capacitance effects within the probe (12), other than those resulting from co- operation between the probe electrode (16) and blade tips (22), are substantially eliminated, by ensuring that the guard electrode (21) and lead (17) to electrode (16) are substantially at the same potential. IMAGE