“…However, the minimal clearance existing between the moving blades with high speed and the static cabinet is not able to be directly measured by conventional approaches. Recently, quite a number of novel methods for fast and accurate BTC measurement have been developed, such as optical methods [ 13 , 14 , 15 , 16 ] based on the intensity modulation of the reflected light, capacitance method [ 17 , 18 , 19 , 20 ] by constructed capacitance between the cabinet and blade tip, eddy current method [ 21 , 22 , 23 ] by installing the sensor coil probe in the cabinet to form an eddy current on the blade tip surface, and the microwave method [ 24 , 25 , 26 ] based on the principle, which is similar to a short range radar system. At present, optical methods and capacitance methods are widely used [ 27 , 28 , 29 , 30 ], generally at the R&D stage of engine.…”