In-situ measurement of distances and shapes as well as dynamic deformations and vibrations of fast moving and
especially rotating objects, such as gear shafts and turbine blades, is an important task at process control.
We recently developed a laser Doppler distance frequency sensor, employing two superposed fan-shaped interference
fringe systems with contrary fringe spacing gradients. Via two Doppler frequency evaluations the non-incremental
position (i.e. distance) and the tangential velocity of rotating bodies are determined simultaneously. The distance
uncertainty is in contrast to e.g. triangulation in principle independent of the object velocity. This unique feature allows
micrometer resolutions of fast moved rough surfaces.
The novel sensor was applied at turbo machines in order to control the tip clearance. The measurements at a transonic
centrifugal compressor were performed during operation at up to 50,000 rpm, i.e. 586 m/s velocity of the blade tips. Due
to the operational conditions such as temperatures of up to 300 °C, a flexible and robust measurement system with a
passive fiber-coupled sensor, using diffractive optics, has been realized. Since the tip clearance of individual blades
could be temporally resolved an analysis of blade vibrations was possible. A Fourier transformation of the blade
distances results in an average period of 3 revolutions corresponding to a frequency of 1/3 of the rotary frequency.
Additionally, a laser Doppler distance sensor using two tilted fringe systems and phase evaluation will be presented. This
phase sensor exhibits a minimum position resolution of σz = 140 nm. It allows precise in-situ shape measurements at
grinding and turning processes.