We have developed a differential laser Doppler profile sensor for distributed one-component velocity measurements with high spatial resolution. Two Doppler frequencies are measured simultaneously in order to determine the position as well as the velocity of individual tracer particles passing through the measurement volume. In the centre of the measurement volume the obtained uncertainty of the position is about 1.6 µm. The profile measurement has the advantage that no mechanical scanning is needed to obtain flow velocity profiles over a length of 5 mm. The profile sensor thus provides a tool for highly resolved instantaneous measurements of shear flows, which have strong velocity gradients.

We have investigated the application of a laser Doppler profile sensor for simultaneous measurement of position and velocity on moving rough surfaces. It is shown that, with this technique, the shape of rotating workpieces and components, e.g., turbine blades or turning parts, can be measured absolutely and in-process with only one single sensor. Measurements on different surfaces with defined shape and roughness are presented. The obtained minimum uncertainty in the position is about 250 nm in the centre of the measurement volume. For the velocity, a relative statistical error of 0.02% was obtained. Furthermore, shading effects as occurring for example at triangulation are reduced since illumination and signal detection can be coaxial. Because the measurement occurs contactless and a high temporal resolution is achievable, this sensor can open up new perspectives in the field of real-time production metrology, for example controlling the turning and the grinding process or at tip-clearance measurements in gas turbines.

This paper presents a novel fibre optic laser Doppler position sensor for single blade tip clearance and vibration measurements at turbo machines, which offers high temporal resolution and high position resolution simultaneously. The sensor principle is based on the generation of a measurement volume consisting of two superposed fan-like interference fringe systems with contrary fringe spacing gradients using wavelength division multiplexing. A flexible and robust measurement system with an all-passive fibre coupled measurement head has been realized employing diffractive and refractive optics. Measurements of tip clearance and rotor vibrations at a transonic centrifugal compressor performed during operation at up to 50 000 rpm (833 Hz) corresponding to 21.7 kHz blade frequency and 586 m s−1 blade tip velocity are presented. The results are in excellent agreement with those of capacitive probes. The mean uncertainty of the position measurement was around 20 µm and, thus, considerably better than for conventional tip clearance probes. Consequently, this sensor is capable of fulfilling the requirements for future active clearance control systems and has great potential for in situ and online tip clearance and vibration measurements at metallic and non-metallic turbine blades with high precision.

This paper presents a novel optical sensor which allows simultaneous measurements of axial position and tangential velocity of moving solid state objects. An extended laser Doppler velocimeter setup is used with two slightly tilted interference fringe systems. The distance to a solid state surface can be determined via a phase evaluation. The phase laser Doppler distance sensor offers a distance resolution of 150 nm and a total position uncertainty below 1 microm. Compared to conventional measurement techniques, such as triangulation, the distance resolution is independent of the lateral surface velocity. This advantage enables precise distance and shape measurements of fast rotating surfaces.

In this paper we report on a differential laser Doppler velocimeter which offers spatially resolved measurements in the probe volume. For the first time, two laser wavelengths are employed to generate two interference fringe systems with identical fringe spacing which are slightly tilted towards each other. The relative phase shift varies linearly with the position along the optical axis, thus allowing spatially resolved measurements. In the centre of the measurement volume a spatial resolution in the nanometre range is achieved. The sensor was applied in a wind tunnel for the determination of the wall shear stress of a laminar flat-plate flow without mechanical traversing. Spatially resolved measurements of the Blasius velocity profile are presented. All results agree well with the theoretical prediction.