Novel laser sensor allows for precise tip clearance and vibration measurements of turbo-machines

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.

The experiment was carried out on a turbine at the Institute of Drive Engineering at the DLR in Cologne. The researchers constructed an optical measuring head which is as compact and as robust so as to overcome the vibrations resulting from the turbine blades’ supersonic velocity. The measuring head was especially designed in Dresden and contains an integrated cooling system so that measurements are even possible at a temperature of 300°C. During the experiment the blades passed the measuring point 22.000 times per second. The tip clearance could be determined accurate to 20 µm – a new record! Until then, the measurement uncertainty amounted to 50-100 µm.

Apart from the changes of the tip clearance, the accurate detection of the vibrations presented an unplanned but welcome result. This became possible as each blade could be allocated to an exact measurement and, thus, the temporal changing of the position was determined. Professor Czarske relates the research team’s success to the synergy effects following the cooperation of electrical engineers, mechanical engineers and physicians.

The laser sensor, which has already been patented, permits an online control of the tip clearance between the turbine blades and casing over. On behalf of the Bosch GmbH it is being applied to the development of an electric motor. Also, machine tools present another major area of application of the laser sensor, allowing for the in-process measurement of both absolute radius and shape of an object. Currently, Professor Czarske and his team are looking for industrial partners in order to continue with their research, for example the laser sensor is supposed to be further miniaturised and refined so it can be transferred into mass production.

The project was promoted by the German Research Association.