Object digitalization using a scanning fringe projection system
The advantage of optical 3D measurement systems like triangulation based fringe projection systems compared with tactile working systems is their contactless and fast 3D surface data acquisition. The results in the form of point clouds can be processed regarding a mathematical surface description for CAD data generation. In many cases the optical 3D surface data acquisition of complex workpieces cannot be achieved in one measuring process so that it is necessary to merge different measuring fields. This approach is essential when the object is larger than the measuring range or the object cannot be measured in one measuring process in consequence of reflections or shadings. According to these problems a fringe projection system was integrated in a coordinate measuring machine with 5 axes (3 linear axes and 2 rotation axes). By using these axes, every region of the workpiece can be reached with the optical 3D sensor for digitalization. To get the complete point cloud of the object surface the different measuring fields must be transformed in one coordinate system by using the relative axes positions of the coordinate measuring machine. Results concerning Reverse Engineering applications and considerations of the measurement uncertainty of the scanning fringe projection system are presented.
Automatic registration method for hybrid optical coordinate measuring technology
Abstract A method to create holistic data models of complex, partly multiscale, workpieces is presented and verified on cutting tools. Different optical sensor principles are applied and the registration strategy is developed to transform partial views taken with single sensors into one coordinate system. Focus of the work is the implementation of automatic registration procedures, based on correlation and segmentation. These novel approaches in coordinate measuring technology are tested and compared. The method is completed by fusing all datasets to one holistic data model, considering the geometry of the workpieces by applying a curvature based segmentation.
Active profiling and polishing for efficient control of material removal from large precision surfaces with moderate asphericity
We present the development of an active fabrication technology for controlling material removal on large precision surfaces of moderate departure from a sphere. The underlying philosophy was established as an efficient solution to the challenging problem of fabricating secondary mirrors of up to nominally 2.5 m in diameter for modern 8 m class telescopes and beyond. The facility described comprises a CNC profiler, two contact profilometers, and a full size active polisher. The trial work-piece was a convex surface of 830 mm in diameter on a zero-expansion ceramic (`Cervit') blank – a 1/3 scale hyperbolic mirror for a proposed 2.5 m diameter f/7 secondary mirror for the 8 m Gemini telescope. Using software error-correction from profilometric metrology data, a factor of 2 improvement in generating the convex aspheric profile was achieved. An active loose-abrasive polishing process is also described, in which a full-size tool is configured to deliver variable, edgeless, sub-diameter removal-footprints. Real-time monitoring of process variables is described, and approximately 10% convergence in each polishing pass is reported.