Thorsten Pfister

Turbo machine tip clearance and vibration measurements using a fibre optic laser Doppler position sensor

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.

Laser Doppler profile sensor with sub-micrometre position resolution for velocity and absolute radius measurements of rotating objects

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.

Monochromatic heterodyne fiber-optic profile sensor for spatially resolved velocity measurements with frequency division multiplexing

Investigating shear flows is important in technical applications as well as in fundamental research. Velocity measurements with high spatial resolution are necessary. Laser Doppler anemometry allows nonintrusive precise measurements, but the spatial resolution is limited by the size of the measurement volume to approximately 50 microm. A new laser Doppler profile sensor is proposed, enabling determination of the velocity profile inside the measurement volume. Two fringe systems with contrary fringe spacing gradients are generated to determine the position as well as the velocity of passing tracer particles. Physically discriminating between the two measuring channels is done by a frequency-division-multiplexing technique with acousto-optic modulators. A frequency-doubled Nd:YAG laser and a fiber-optic measuring head were employed, resulting in a portable and flexible sensor. In the center of the measurement volume of approximately 1-mm length, a spatial resolution of approximately 5 microm was obtained. Spatially resolved measurements of the Blasius velocity profile are presented. Small velocities as low as 3 cm/s are measured. The sensor is applied in a wind tunnel to determine the wall shear stress of a boundary layer flow. All measurement results show good agreement with the theoretical prediction.

Laser Doppler distance sensor using phase evaluation

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.

Fiber-optic laser Doppler turbine tip clearance probe

A laser Doppler based method for in situ single blade tip clearance measurements of turbomachines with high precision is presented for what we believe is the first time. The sensor is based on two superposed fanlike interference fringe systems generated by two laser wavelengths from a fiber-coupled, passive, and therefore compact measurement head employing diffractive optics. Tip clearance measurements at a transonic centrifugal compressor performed during operation at 50,000 rpm (833 Hz, 586 m/s tip speed) are reported. At these speeds the measured uncertainty of the tip position was less than 20 microm, a factor of 2 more accurate than that of capacitive probes. The sensor offers great potential for in situ and online high-precision tip clearance measurements of metallic and nonmetallic turbine blades.

Interferometric Sensor System for Blade Vibration Measurements in Turbomachine Applications

In order to improve the safety, lifetime, and energy efficiency of turbomachines, the dynamic behavior of the rotor has to be analyzed. Blade vibrations have to be monitored during operation to optimize the rotor design and to validate numerical models. However, measuring the vibration amplitude and frequency of the blades is a challenging task for metrology, since the blades to be measured are rotating quickly, and noncontact measurements are demanded. To solve this problem, we present a measurement system consisting of four laser Doppler sensors that have been mounted around the circumference of the rotor. These sensors measure simultaneously and contactlessly the in-plane velocity and the out-of-plane position of laterally moving objects. By analyzing the variation of the blade tip velocities, the vibration amplitude and frequency of the blades were estimated. Blade vibration measurements down to amplitudes of only 20 μm in tangential direction have been carried out. We achieved a standard uncertainty of approximately 400 nm for these experiments.

Cramér–Rao lower bound of laser Doppler measurements at moving rough surfaces

Laser Doppler techniques are widely used for measuring both fluid flows and moving solid surfaces. The measurement uncertainty of laser Doppler sensors is fundamentally limited by the uncertainty of the Doppler frequency estimation. Generally, the minimum achievable uncertainty of any unbiased estimator is given by the Cramer–Rao lower bound (CRLB). While the CRLB is well known for laser Doppler burst signals of single tracer particles used in flow research, no analytical expression for the CRLB has been known up to now for scattered light signals of rough solid surfaces where speckle effects occur. Therefore, the aim of this paper is to close this gap and to provide a simple analytical expression for the CRLB for the Doppler frequency estimation from scattered light signals of moving rough solid surfaces for the first time. A comparison with experimental data demonstrates the validity of the derived analytical CRLB formula, which is also proven to be consistent with previous works. The progress for science is that this analytical CRLB formula enables both an easy estimation of the minimum achievable uncertainty of laser Doppler measurements at moving rough surfaces and a direct analysis of the influences of certain system and signal parameters on the measurement uncertainty. This reveals specific measuring features and capabilities of different laser Doppler techniques. In addition, the CRLB is a valuable tool to evaluate the efficiency of applied signal processing techniques.

Distance measurement technique using tilted interference fringe systems and receiving optic matching.

The precise measurement of the distance of fast laterally moving rough surfaces is important in several applications such as lathe monitoring. A nonincremental interferometer based on two tilted interference fringe systems and a precise phase-difference estimation has been realized for this task. However, due to the speckle effect, the two scattered light signals exhibit different phase jumps and random envelopes causing small correlation coefficients and high uncertainties of the phase difference as well as the distance. In this Letter we present for the first time a method to enhance the signal correlation coefficient significantly. The interference signals are generated by scattered light of a rough surface from two different directions. A matching of illumination and receiving optic is performed. By this novel method, distance measurements with an uncertainty down to 1.2 μm at about 10 m/s lateral moving velocity have been achieved. Together with the simultaneously measured lateral velocity, the shape of rotating objects can be precisely determined.

In-process shape and roundness measurements at turning machines using a novel laser Doppler profile sensor

We have investigated the application of a laser Doppler profile sensor for in-process shape and roundness measurements at turning machines. This sensor is an extension of a conventional laser Doppler velocimeter (LDV), where two interference fringe systems with contrary fringe spacing gradients are generated inside the same measuring volume using wavelength division multiplexing. Scattering objects passing the measuring volume generate scattered light signals with two different Doppler frequencies, from which the velocity as well as the position of the objects can be determined via a proper calibration function. Hence, the radius and the tangential velocity and, thus, the shape of rotating work pieces and components, e.g. turbine blades or turning parts, can be measured absolutely and with only one single sensor. Two-dimensional and three-dimensional measurements of shape, excentricity, and roundness on quickly rotating cylinders inside a turning machine are presented. The results are compared with tactile measurements conducted with a coordinate measuring machine.

Displacement, distance, and shape measurements of fast-rotating rough objects by two mutually tilted interference fringe systems

The precise distance measurement of fast-moving rough surfaces is important in several applications such as lathe monitoring. A nonincremental interferometer based on two mutually tilted interference fringe systems has been realized for this task. The distance is coded in the phase difference between the generated interference signals corresponding to the fringe systems. Large tilting angles between the interference fringe systems are necessary for a high sensitivity. However, due to the speckle effect at rough surfaces, different envelopes and phase jumps of the interference signals occur. At large tilting angles, these signals become dissimilar, resulting in a small correlation coefficient and a high measurement uncertainty. Based on a matching of illumination and receiving optics, the correlation coefficient and the phase difference estimation have been improved significantly. For axial displacement measurements of recurring rough surfaces, laterally moving with velocities of 5 m/s, an uncertainty of 110 nm has been attained. For nonrecurring surfaces, a distance measurement uncertainty of 830 nm has been achieved. Incorporating the additionally measured lateral velocity and the rotational speed, the two-dimensional shape of rotating objects results. Since the measurement uncertainty of the displacement, distance, and shape is nearly independent of the lateral surface velocity, this technique is predestined for fast-rotating objects, such as crankshafts, camshafts, vacuum pump shafts, or turning parts of lathes.