Robert Kuschmierz

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.

On the speckle number of interferometric velocity and distance measurements of moving rough surfaces.

The minimum achievable systematic uncertainty of interferometric measurements is fundamentally limited due to speckle noise. Numerical and physical experiments, regarding the achievable measurement uncertainty of Mach-Zehnder based velocity and position sensors, are presented at the example of the laser Doppler distance sensor with phase evaluation. The results show that the measurement uncertainty depends on the number of speckles on the photo detectors. However, while the systematic uncertainty due to the speckle effect decreases, the random uncertainty due to noise from the photo detector increases with increasing speckle number. This results in a minimal total measurement uncertainty for an optimal speckle number on the photo detector, which is achieved by adjusting the aperture of the detection optics.

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.

Optical dynamic deformation measurements at translucent materials

Due to their high stiffness-to-weight ratio, glass fiber-reinforced polymers are an attractive material for rotors, e.g., in the aerospace industry. A fundamental understanding of the material behavior requires non-contact, in-situ dynamic deformation measurements. The high surface speeds and particularly the translucence of the material limit the usability of conventional optical measurement techniques. We demonstrate that the laser Doppler distance sensor provides a powerful and reliable tool for monitoring radial expansion at fast rotating translucent materials. We find that backscattering in material volume does not lead to secondary signals as surface scattering results in degradation of the measurement volume inside the translucent medium. This ensures that the acquired signal contains information of the rotor surface only, as long as the sample surface is rough enough. Dynamic deformation measurements of fast-rotating fiber-reinforced polymer composite rotors with surface speeds of more than 300 m/s underline the potential of the laser Doppler sensor.

Camera-based speckle noise reduction for 3-D absolute shape measurements.

Simultaneous position and velocity measurements enable absolute 3-D shape measurements of fast rotating objects for instance for monitoring the cutting process in a lathe. Laser Doppler distance sensors enable simultaneous position and velocity measurements with a single sensor head by evaluating the scattered light signals. The superposition of several speckles with equal Doppler frequency but random phase on the photo detector results in an increased velocity and shape uncertainty, however. In this paper, we present a novel image evaluation method that overcomes the uncertainty limitations due to the speckle effect. For this purpose, the scattered light is detected with a camera instead of single photo detectors. Thus, the Doppler frequency from each speckle can be evaluated separately and the velocity uncertainty decreases with the square root of the number of camera lines. A reduction of the velocity uncertainty by the order of one magnitude is verified by the numerical simulations and experimental results, respectively. As a result, the measurement uncertainty of the absolute shape is not limited by the speckle effect anymore.

Measurement uncertainty of non-incremental, non-contact, in-situ shape measurements

Abstract Optical measurement systems work fast and non-contact and can achieve sub-micron precision. Thus they appear to be well suited for in-situ shape measurement of fast rotating objects such as cutting processes in metal working lathes. Most optical measurement systems, however, allow an axial position measurement only. In order to retrieve the shape of the object from a distance measurement, the distance between the sensor and the centre of the object has to be known. Otherwise, deviations of this distance, for instance due to temperature effects or vibrations, will result in a measurement deviation. In order to allow an absolute shape measurement, which is independent of the sensor position, the mean radius of the rotating object can be retrieved from the objects circumferential velocity. The laser Doppler distance sensor with phase evaluation (P-LDD sensor) allows a simultaneous velocity and distance measurement with high temporal resolution. Thus, the P-LDD sensor allows to measure the mean radius as well as the spatially resolved deviation of the radius independently of the sensor position. In order to quantify the achievable measurement uncertainty, and especially the influence of the temperature the measurement uncertainty budget is derived and considers random as well as systematic errors. It is shown that the P-LDD sensor allows an absolute, three-dimensional shape measurement of fast rotating objects with sub-micron uncertainty. The systematic measurement uncertainty of the absolute shape due to the temperature amounts to only 200 nm/K. Thus the P-LDD sensor is not dependent on temperature-controlled laboratories but can be employed directly in the production process (in-situ or in-process).

In-process, non-destructive multimodal dynamic testing of high-speed composite rotors

Fibre reinforced plastic (FRP) rotors are lightweight and offer great perspectives in high-speed applications such as turbo machinery. Currently, novel rotor structures and materials are investigated for the purpose of increasing machine efficiency, lifetime and loading limits. Due to complex rotor structures, high anisotropy and non-linear behavior of FRP under dynamic loads, an in-process measurement system is necessary to monitor and to investigate the evolution of damages under real operation conditions. A non-invasive, optical laser Doppler distance sensor measurement system is applied to determine the biaxial deformation of a bladed FRP rotor with micron uncertainty as well as the tangential blade vibrations at surface speeds above 300 m/s. The laser Doppler distance sensor is applicable under vacuum conditions. Measurements at varying loading conditions are used to determine elastic and plastic deformations. Furthermore they allow to determine hysteresis, fatigue, Eigenfrequency shifts and loading limits. The deformation measurements show a highly anisotropic and nonlinear behavior and offer a deeper understanding of the damage evolution in FRP rotors. The experimental results are used to validate and to calibrate a simulation model of the deformation. The simulation combines finite element analysis and a damage mechanics model. The combination of simulation and measurement system enables the monitoring and prediction of damage evolutions of FRP rotors in process.

Berührungslose Messung des lokalen Schalldrucks mittels Hochgeschwindigkeits-Holographie / Non-invasive measurement of the local sound pressure using high-speed holography

Zusammenfassung Für die Lärmreduzierung von Flugzeugtriebwerken kommen perforierte Bleche mit dahinterliegender Kavität (sogenannte Liner) zum Einsatz. Das aeroakustische Dämpfungsprinzip dieser Elemente beruht auf der Wechselwirkung von Schall und Strömung. Bisherige physikalische Modelle der Dämpfungsprozesse erlauben jedoch nicht die notwendige Entwicklung effizienterer Liner, da die aeroakustische Wechselwirkung noch nicht vollständig verstanden ist. Daher ist die Kenntnis relevanter Größen, wie die lokale Schalldruckverteilung, entscheidend für das Verständnis der lokalen Dämpfungsprozesse. Dazu präsentieren wir ein kamerabasiertes Messsystem für die zweidimensionale nicht-invasive ortsaufgelöste Schalldruckbestimmung. Abstract In order to reduce the noise emission from airplane turbines perforated sheet with a cavity behind it (so called liner). The dampening principle of these elements are based on the interaction between sound and flow. Current physical models of this aeroacoustical sound absorption do not allow the development of more efficient liners, since the aeroacoustic interaction has not been yet fully understood. Therefore all relevant factors, like the local sound field distribution, have to be measured to fully comprehend the local sound absorption processes. For this purpose we present with this work a camera-based measuring system for the two dimensional non-invasive measurement of the local sound pressure with a high spatial resolution.

Holographic endoscope based on coherent fiber bundles and adaptive optics (Conference Presentation)

Coherent fiber bundles (CFB) are commonly used for endoscopic imaging, e.g. in biomedicine. Usually a CFB with several ten thousand cores is employed together with a lens system on its distal end. However, pixelation effects occur and the imaging plane can’t be scanned, limiting the field of application of CFBs. To circumvent these limitations, a spatial light modulator (SLM) is employed on the proximal side of a single-mode CFB. This enables creating arbitrary wave fronts at the distal fiber end, e.g. for instance for optical tweezers, endoscopes with tunable image plane or for exciting transgenetic nerve cells. However, of the shelf CFBs show phase distortions between individual cores (e.g. coupling between cores, speckle effect) which need to be calibrated and corrected at the proximal side. These distortions depend on the wavelength, temperature, polarization and most importantly on the bending of the CFB. Therefore an on-line calibration during bending variations is required. For this purpose a semitransparent mirror is employed at the distal fiber end, which allows to measure double the distortion at the proximal side by digital holography without the need for a guide star. For correcting the distortion the same SLM as above is employed. However, the distortion for a single transmission through the CFB commonly exceeds several 2 pi. Thus, an incremental phase measurement yields unambiguous results. To circumvent this problem, two approaches for on-line calibration are compared. 1st Multiple wavelength holography and 2nd initial calibration in transmission mode with subsequent tracking of distortion changes in reflecting mode.

3D interferometric shape measurement technique using coherent fiber bundles

In-situ 3-D shape measurements with submicron shape uncertainty of fast rotating objects in a cutting lathe are expected, which can be achieved by simultaneous distance and velocity measurements. Conventional tactile methods, coordinate measurement machines, only support ex-situ measurements. Optical measurement techniques such as triangulation and conoscopic holography offer only the distance, so that the absolute diameter cannot be retrieved directly. In comparison, laser Doppler distance sensors (P-LDD sensor) enable simultaneous and in-situ distance and velocity measurements for monitoring the cutting process in a lathe. In order to achieve shape measurement uncertainties below 1 μm, a P-LDD sensor with a dual camera based scattered light detection has been investigated. Coherent fiber bundles (CFB) are employed to forward the scattered light towards cameras. This enables a compact and passive sensor head in the future. Compared with a photo detector based sensor, the dual camera based sensor allows to decrease the measurement uncertainty by the order of one magnitude. As a result, the total shape uncertainty of absolute 3-D shape measurements can be reduced to about 100 nm.