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Multiscale Geometry Inspection of Joining Areas (SFB 1153 - C5)

Figure 1: Laser beam deflection due to refractive index field.
Figure 2: Laser beam deflection as a function of triangulation angle and workpiece temperature.

The objective of this sub-project ist the research on measurement and evaluation techniques for the inline condition-monitoring and assessment of joining areas of rotationally symmetric Tailored Forming-workpieces in variable scale ranges.

For a manufacturing-process adjustment and control of tailored forming-workpieces, the complete inspection of geometric features of the joining areas is of vital importance. The inspection of the joining areas has to be implemented after each single process step (joining, forming, tempering, finishing). Hereby advantages can be derived – such as early error detection, reduction of production costs and real-time process control.

During the first sponsorship period a fast, optical and extensive measuring multi-scale sonsor measurement system using the triangulation technique will be developed, constructed and investigated. The system will acquire the geometry shape on macro level as well as detect and characterize surface defects (e.g. shrinkage cavities, surface cracks) on meso level. To stich the measurement data of the variable scale ranges, it is necessary to investigate methods to calibrate the differently resoluting sensors to each other. To be able to characterize the joining areas with regard to the manufacturing process, tailored forming-workpiece adapted methods need to be developed to describe and quantify the geometry deviation compared to a reference geometry.

Special research emphasis will be laid on the development of measuring methods to characterize the joining areas of wrought-hot tailored forming-workpieces directly after the forming process. Due to the heat transfer of the wrought-hot workpiece to the ambient air, the air will be heated up. The hot air results in a density gradient field around the workpiece which again causes a threedimensional, inhomogeneous optical refraction index field.

Figure 1 displays a simplified simulation of the refraction index field supposing a linear temperature curve (and thereby also a linear curve of the mass density of air) from the surface temperature to the ambient temperature. The corresponding optical density curve is also linear. The boundary layer thickness is assumed to be 300 mm. A considerable beam deflection of some 10 micrometers can be observed (fig. 2). The described effect applies to all kinds of optical triangulation techniques, such as laser light section, fringe pattern projection and stereo-photogrammetry. The beam deflection exceeds clearly the pursued resolution. A consideration and compensation of the deflection is therefor reasonable. Sensors with higher geometrical resolution, for instance in white-light interferometry or focus variation, also underlie the influence of this effect, as the optical path changes compared to the geometric path due to the variation of the refraction index.

Contact person(s): Dipl.-Ing. Rüdiger Beermann, Dr.-Ing. Dipl.-Phys. Markus Kästner

The intelligent operation room

Laboratory: OR- Table with lights

Today's operating room (OR) is an environment for high tech equipment, used for therapeutic and diagnostic applications. The equipment is applied to treat the patient with new and better surgical techniques. This causes an increasing variety and complexity of surgical procedures which poses a further requirement on the staff. This is taken into account through intelligent functions, which should decrease the workload. Processes within the OR are new designed and optimized, to ensure the best patient treatment.

Within the shown laboratory, approaches in human machine interaction, patient positioning and collision control are evaluated. 3D sensors are used to observe the scene and algorithms of digital image processing are applied. Based on the results, the OR should achieve information about the OR state and situation in order to interact with the staff. Besides assisting functions, active manipulation of OR equipment through the interface is possible.

Our technical focus is in the development and implementations of sensor array systems. Because the OR- environment is very complex, due to many people and objects within a small room, the scene is observed from various viewpoints. Through this, the complete segmentation of objects becomes possible. This causes a lot of challenges in the fields of real time applicability, data fusion capability and consideration of complex picture failures, like reflection, shadowing and interfacial effects.

Contact person: Dipl.-Ing. Stephan Schröder

Fast Measurement of Complex Geometries (SFB871 - C2)

Geometric measurement of the workpiece is an essential element of product regeneration. This yields for the decision of reparability as well as for planning and surveying various steps of the regeneration process. The project C2 of the SFB871 aims at measuring the actual shape of the workpiece and comparing it against a CAD model to analyze geometric deviations.

The inverse fringe projection technique is, in contrast to the conventional fringe projection technique, a very efficient method for the direct acquisition of geometric deviations. It is possible to come by a set of several million three dimensional data points of the geometric workpiece surface within a few seconds. For that purpose, however, a workpiece- and measuring system-adopted inverse fringe pattern needs to be projected onto the workpiece’s surface and its diffuse reflection be monitored by a digital camera from a triangulation angle β. Contemporarily, the complexity of the generation of the inverse fringe pattern prevents industrial establishment of the inverse fringe projection technique.

The research efforts of this project cover new methods for the generation of inverse fringe projection patterns by means of raytracing simulation techniques using a virtual fringe projection system. The virtual fringe projection system can, additionally, be utilized prior to the real measurement in order to generate data which allow for speeding up the quantitative calculation of geometric deviations of the workpiece’s shape.

A newly developed borescopic fringe projection system adds the capabitly of full and fast geometry inspection of complex geometries like blisks (blade integrated discs) as show in the picture above.

Contact person: M.Sc. Jochen Schlobohm

Development of a newly image derotator

The analysis of the dynamic behaviour of rotary objects under operating conditions is a great challenge due to the overlayed objects rotation.

At the Institute of Measurement and Automatic Control (IMR) a novel image derotator with a specially designed mirror arrangement is developed, that can simplify the analysis of fast rotating objects. By means of a laser doppler vibrometer or image processing the analysis of vibration or deformation can be accomplished.

Contact person: Dipl.-Ing. (Univ.) Andreas Pösch

Derotator-Roller slippage measurment

At the Institute of Measurement and Automatic Control (IMR), Leibniz Universität Hannover, a self-developed optical image derotator is applied for the analysis of the dynamic behaviour of rotating objects under operational conditions. .If a rotating object is observed through the prism, which rotates with half of the rotational speed of the object and in the same direction, the object will appear stationary. To receive an optimally derotated image, the optical axis of the prism, the rotary axis of its drive and the rotary axis of the object should be identical. Using the combination of image derotator and a high speed camera or a laser Doppler Vibrometer in-plane and out-of-plane vibration of rotating components with a rotational speed of up to n = 10.000 min-1 can be measured. Within an EU project , the image derotator is applied to measure the dynamic slippage behaviour of roller elements in a roller bearing. Therfore , the dove prism rotates with half of the rotational speed of the bearing cage. Thus, it is possible to receive a stationary view of the cage and to observe the rotational movement of the roller elements around its own axes. Using digital image processing methods, the rotational behaviour of roller elements canl be calculated and analysed.

Contact person: Dipl.-Ing. (Univ.) Andreas Pösch

Inspection of microstructured surfaces in industrial conditions

Special microstructured surfaces can reduce the drag of components. The shape of this structure is motivated from skin of sharks and is called riblets. The field of application of this structure is found mainly on the outer skin of aircraft and compressor blades of aircraft engines. The structure consists of periodic elevations of the component surface. The period length of this structure corresponds to the prevailing flow conditions and is in the range of approx. 50-500 microns and the structure height is in the range of approx. 10-200 microns. The flow reducing effect of riblets has been known for some time, however, efficient production method of riblets are still in development.

In this project, new measurement systems are developed in order to make the manufacturing process of riblets efficiently. This includes in particular the development of measurement systems that allow microscopic inspections not only under laboratory conditions, but directly during the manufacturing process. These systems must firstly have a very high resolution, so that defects in the range of <10 microns can be found. On the other hand, the newly developed measurement system must be very robust against all manufacturing conditions. These include in particular impurities such as coolant during a grinding process or vibration.

The high resolution of the measuring systems for the inspection of riblets also leads to very low area ranges for a single measurement. In this project, possibilities are investigated to measure large, microstructured surfaces efficiently despite the small areas rates.

Surface metrology based on Scanning Electron Microscopy

Sketch of the structure to reduce SE3 yield with possible electron trajectories

Surface metrology based on Scanning Electron Microscopy (SEM) makes it possible to study materials up to the atomic level. To do so, a sample is irradiated point by point and the resulting radiant intensities are recorded. Classic studies limit themselves to various intensities which are applied to image processing or visualization methods. For many years, the Institute of Measurement and Automatic Control (IMR) has been researching a method of high-precision 3-D electron microscopy which, in addition to intensity values, also reconstructs the surface profile of the sample. This 3-D SEM measurement technique does not only represent a unique new measurement method, but also provides new insights into the microstructure of material surfaces, based on which it is possible to develop new analysis techniques in various areas of application. For the time being, these research methods are still being developed, but in several years they could be market-ready.
The measurement technique developed at our institute uses several detectors for secondary electrons (SE), so that it is possible to work with lower radiation energies and to obtain high precision of the surface reconstruction. To do so, imaging procedures apply directional characteristic, from which it is possible to calculate the surface slope using sophisticated material models and, therefore, create a surface profile. At one of the first stages of development two detectors placed at a 180° angle were used. The results were promising, so that the next stage of development is currently being expanded by four Everhart-Thornley detectors and an elaborate electron "lens". Simulations and the first results obtained at this stage of development also allow to expect that the measurement results have a novel quality in comparison to the existing imaging procedures. The only obstacle in this approach is created by backscattered electrons (BSE) which can distort the measurement results, because these high-energy electrons release further SE on the surfaces of the sample chamber.
To significantly reduce this effect, the IMR plans to develop an electron-trap system based on nanostructures. This way, it could be possible to intercept the unwanted electrons. Therefore, the IMR expects to further improve the triggering of measurements. It makes most sense to place the electron-trap system on the bottom of the electron gun, because the majority of BSE occurs here. The structure should collect high-energy electrons and intercept the so-called SE3 emission that occurs in it. In order for the electron-trap system to perform this way, it is necessary, in addition to finding a suitable material, to find an optimal surface structure for the electron-trap system. It is most probably a square wave shape, however, the measurements of squares or the depth of holes must be first determined by means of several series of experiments.

Contact person: Renke Scheuer