The Use of Eddy Current Sensors for the Measurement of Rotor Blade Tip Timing: Development of a New Method Based on Integration

The advent of tip-timing systems makes it possible to assess turbomachinery blade vibration using non-contact systems. The most widely used systems in industry are optical. However, these systems are still only used on developmental gas turbine engines, largely because of contamination problems from dust, dirt, oil, water etc. Further development of these systems for in-service use is problematic because of the difficulty of eliminating contamination of the optics. Eddy current sensors are found to be a good alternative and are already being used for gas turbine health monitoring in power plants. Experimental measurements have been carried out on three different rotors using an eddy current sensor developed in a series of laboratory and engine tests in-house to measure rotor blade arrival times. A new tip-timing algorithm for eddy current sensors based on integration has been developed and is compared with two existing tip-timing algorithms: peak-to-peak and peak-and-trough. Among the three, the integration method provided the most promising results in the presence of electrical noise interference. INTRODUCTION Maintenance of modern aircraft and power plants is an important area of research. Modern aircraft engines and gas turbine power plant performance are increasing day-by-day and so is the maintenance cost. In order to reduce costs, active health monitoring systems are needed. Active health monitoring of gas turbine components ensures detection of potential failures and unnecessary down time, thus allowing timely maintenance. Gas turbine components especially the blades are subjected to vibrations caused by dynamic loads. These loads are generated by various mechanisms such as rotor imbalances, varying blade tip clearance due to non-concentric casings, distortions in the inlet flow (caused by irregular intake geometries). The damage on the blade can lead to aerodynamic forcing causing high cycle fatigue. This has a major impact on safety and whole life costs. Detection of the changes in blade vibration modes and levels due to damage or deterioration would allow improvements to the inspection, repair and replacement process. Non-Contact Strain Measurement System (NSMS) or Blade Tip-Timing (BTT) is routinely used in monitoring engine blades for research and development purposes. There are various methods to detect BTT that comprises, optical probes, eddy current, capacitive, Hall effect sensors etc [1]. All these sensors measure the arrival time of the blades. In the absence of any structural vibration, the time for the tip of a particular blade to reach the probe is dependent on the rotational speed alone. However, when the blade is vibrating, the blade arrival time will depend on both the amplitude and frequency of the vibration. The eddy current sensor is the most robust and immune to contaminations when compared with industry standard optical probes. They are already being used in gas turbine power generation for in-service health monitoring on first stage compressor blades and on the last stage of steam turbine blades, whereas, the 1 Copyright c 2016 by ASME other type of sensors are still used on developmental engines due to their limitations. Accurate determination of the blade arrival times is crucial to assess blade vibrations. General operating conditions of gas turbines result in blade tip speeds and amplitudes of the order 400 m/s and 0.1-0.3 mm. This means that a vibration mode occurs within 1 μs and to measure this, an accuracy of at least an order of magnitude greater (i.e 100 ns) is necessary [2]. The aim of this research is to compare and show the performance and accuracy of a newly developed tip-timing algorithm that is based on integration with existing tip-timing algorithms: Peak-to-Peak and Peak-and-Trough.