High temperature eddy current sensor system for turbine blade tip clearance measurements

A new high temperature eddy current sensor has been developed for application in turbo-machinery tip-timing and tip clearance measurements to assess blade vibrations. Present industrial standard sensors used in tip timing systems such as optical, capacitance, Hall effect etc. are unable to operate at elevated temperatures > 400°C for long durations without active cooling and these sensors are not sufficiently robust to withstand the harsh environment. Eddy current sensors are found to be a good alternative and are currently being used for gas turbine health monitoring applications at low temperatures, for example in the first stage of compressors and in the low pressure section of steam turbines. The newly developed sensor is a modified version of the eddy current sensor that is able to op-erate at high temperatures of about 800°C. The sensor can be used to measure stator-rotor clearances in shaft seal applications, or in a turbine stage of a gas turbine engine where the temperatures are significantly higher. This paper presents the characteristics of the high temperature eddy current sensor, driving electronics and various validation results. The experiments were carried out on a rotor with blades and flat sectors simulating a stator/rotor seal to measure tip clearances. The sensor system is demonstrated at varying temperature intervals starting from room temperature to over 800°C. Comparisons are made against an existing industrial standard capacitance sensor system up to 250°C. The results show good agreement between the eddy current and the capacitance probe for the flat sectors over the temperature range, giving credence to the eddy current probe data, but that the eddy current probe achieves a more accurate, repeatable and reliable mea-surement for the blades, as well as extending the temperature range. The eddy current sensor is immune to dust, dirt, oil and water contamination and therefore is considered a better solution than the capacitance sensor.