The Technische Universität Dresden (TU Dresden) and the German Aerospace Center (DLR) have cooperated in developing a novel laser sensor which measures two quantities that are important for turbine engineering more precise than ever before: firstly, the tip clearance between the rotating blades and the turbine casing over and, secondly, the vibrations resulting from the blades’ supersonic velocity. The novel laser Doppler profile sensor, which has been developed by Professor Jürgen Czarske and his team at the Department of Metrology at the Faculty of Electrical Engineering and Information Technology, exploits the advantages already known from laser technology but, in addition to velocity, is also capable of measuring positions.
Due to mechanical and thermal influences, the tip clearance constantly changes in terms of micrometers; however, it has to be controlled permanently. Furthermore, the tip clearance is also important in economic terms as a clearance, which amounts to 0.5 mm on average, that is as precise and as low as possible can result in enormous cost reductions, given the turbines’ high energy consumption. Traditionally, the tip clearance has been determined via so called capacitive measurements, which are carried out through electrical sensors on the casing over. However, while these are less precise and can only be applied with metal, the novel laser sensor is especially suitable for turbine blades made of ceramics or synthetics. Thus, it is likely to be of great interest for the aircraft industry, which has increasingly employed lightweight construction.
The scientists applied the laser Doppler technology, developing a sensor which allows for the mathematical calculation of the tip clearance via the electronic signal processing of two different Doppler frequencies – instead of one – which are generated by two separate laser wavelengths. In the process, two laser wavelengths are sent via fibre optic cable to a measuring head which is situated on the turbine’s outside wall. Through a window, the laser beams get to the blades which in turn reflect the beams. The optic signals are then transformed into electronic signals and are analysed via a computer.
Prof Jürgen Czarske | alfa
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