Non-contact sensors may be used to detect the blades in a turbine by picking up an edge as the blade passes by. Blade tip timing (BTT) can be used to detect FOD (Foreign Objects Debris) in the turbine as FOD can affect the movement of the blade tips. BTT assumes that the movement of the blade tips is slower than the shaft rotation. If the time difference of the passing blade tips is not constant then there is “blade flutter”. Analysis of the blade fluttering can determine if there is a problem before a failure occurs.
Optical sensors are used regularly for blade tip timing, but they are prone to contamination and require regular cleaning. Eddy current based sensors can be a good alternative as they are relatively immune to contamination and can offer detection capability on a par with optical based sensors.
Kaman’s sensors can be used to pick up the turbine blade as it passes by. The KD-2306 measuring system has a 50 KHz frequency response making it suitable for most applications. Kaman also has sensors rated to 400°F and even to 1100°F.