The distance between a turbine blade and the turbine casing is a critical measurement for engineers. It is, however, a difficult measurement because of the typically high temperatures and rotational speeds involved. It’s usually difficult to locate electronic instruments close to the measurement and cables must be routed through a hostile environment before arriving at the electronic equipment’s location.
Fylde have a long history of supplying electronic equipment to make tip clearance measurement using triaxial capacitance probes. Probes are mounted in the casing and measure the capacitance formed by each blade tip as it passes the probe.
Techniques include using the capacitance between blade tip and probe to change an oscillator frequency (Frequency Modulation) or using the capacitance at a fixed frequency to vary the amplitude of an oscillator (Amplitude Modulation.)