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.
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.
The high stiffness to weight ratio of glass fibre-reinforced polymers (GFRP) makes them an attractive material for rotors e.g. in the aerospace industry. We report on recent developments towards non-contact, in-situ deformation measurements with temporal resolution up to 200 µs and micron measurement uncertainty. We determine the starting point of damage evolution inside the rotor material through radial expansion measurements. This leads to a better understanding of dynamic material behaviour regarding damage evolution and the prediction of damage initiation and propagation. The measurements are conducted using a novel multi-sensor system consisting of four laser Doppler distance (LDD) sensors. The LDD sensor, a two-wavelength Mach-Zehnder interferometer was already successfully applied for dynamic deformation measurements at metallic rotors. While translucency of the GFRP rotor material limits the applicability of most optical measurement techniques due to speckles from both surface and volume of the rotor, the LDD profits from speckles and is not disturbed by backscattered laser light from the rotor volume. The LDD sensor evaluates only signals from the rotor surface. The anisotropic glass fibre-reinforcement results in a rotationally asymmetric dynamic deformation. A novel signal processing algorithm is applied for the combination of the single sensor signals to obtain the shape of the investigated rotors. In conclusion, the applied multi-sensor system allows high temporal resolution dynamic deformation measurements. First investigations regarding damage evolution inside GFRP are presented as an important step towards a fundamental understanding of the material behaviour and the prediction of damage initiation and propagation.
Temperature drifts, tool deterioration, unknown vibrations as well as spindle play are major effects which decrease the achievable precision of computerized numerically controlled (CNC) lathes and lead to shape deviations between the processed work pieces. Since currently no measurement system exist for fast, precise and in-situ 3d shape monitoring with keyhole access, much effort has to be made to simulate and compensate these effects. Therefore we introduce an optical interferometric sensor for absolute 3d shape measurements, which was integrated into a working lathe. According to the spindle rotational speed, a measurement rate of 2,500 Hz was achieved. In-situ absolute shape, surface profile and vibration measurements are presented. While thermal drifts of the sensor led to errors of several mµm for the absolute shape, reference measurements with a coordinate machine show, that the surface profile could be measured with an uncertainty below one micron. Additionally, the spindle play of 0.8 µm was measured with the sensor.
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.
Precise measurements of distance, eccentricity and 3D-shape of fast moving objects such as turning parts of lathes, gear
shafts, magnetic bearings, camshafts, crankshafts and rotors of vacuum pumps are on the one hand important tasks. On
the other hand they are big challenges, since contactless precise measurement techniques are required. Optical techniques
are well suitable for distance measurements of non-moving surfaces. However, measurements of laterally fast moving
surfaces are still challenging. For such tasks the laser Doppler distance sensor technique was invented by the TU
Dresden some years ago. This technique has been realized by two mutually tilted interference fringe systems, where the
distance is coded in the phase difference between the generated interference signals. However, due to the speckle effect
different random envelopes and phase jumps of the interference signals occur. They disturb the phase difference
estimation between the interference signals. In this paper, we will report on a scientific breakthrough on the
measurement uncertainty budget which has been achieved recently. Via matching of the illumination and receiving
optics the measurement uncertainty of the displacement and distance can be reduced by about one magnitude. For
displacement measurements of a recurring rough surface a standard deviation of 110 nm were attained at lateral
velocities of 5 m ∕ s. Due to the additionally measured lateral velocity and the rotational speed, the two-dimensional
shape of rotating objects is calculated. The three-dimensional shape can be conducted by employment of a line camera.
Since the measurement uncertainty of the displacement, vibration, distance, eccentricity, and shape is nearly independent
of the lateral surface velocity, this technique is predestined for fast-rotating objects. Especially it can be advantageously
used for the quality control of workpieces inside of a lathe towards the reduction of process tolerances, installation times
Shape measurement of moving, especially rotating objects is an important task in the field of process control. The Laser Doppler Distance Sensor was invented for this purpose. It is realized by two tilted interference fringe systems and enables the simultaneous measurement of the surface velocity and profile. The distance is coded in the phase difference between the generated interference signals of two photo detectors. In order to achieve a distance uncertainty of below 1μm a steep calibration function is necessary. This can be achieved by increasing the tilting angle. However, due to the speckle effect at rough surfaces, random envelopes and phase jumps occur disturbing the phase difference estimation with increasing tilting angle. This problem was overcome recently by employing a receiving optics matching reducing the distance uncertainty by about one magnitude. By evaluating the Doppler frequencies of the two fringe systems the surface velocity and thereby the objects mean diameter can be calculated as well as angular misalignment of the sensor can be detected.