Noncontact vibration measurements on compressor rotor blades

In turbine engine development, rotor blade vibration measurements are made to ensure the blades are sufficiently durable in later service. These measurements are conventionally taken using strain gauges or the frequency modulated grid. In recent years, a noncontact method of blade vibration measurement has become an increasingly accepted, low-cost alternative technique. This method uses a number of probes installed in the engine casing to sense the points in time at which the blades are passing the probes. When analysed, these blade passing times yield data on blade vibrations. This paper briefly describes the configuration of such a measurement system and the operating principle of two different probe types. An extended explanation is then provided of the various analysis methods in use at MTU. The methods are described by means of the essential equations and elucidated using comprehensive compressor test data.