Prof. Peter Lehmann
Universität Kassel, Germany
Title: Resilient high-resolution surface topography inspection techniques
for machine vision systems
Abstract:
Machine vision systems typically acquire two-dimensional (2D) images in order to obtain relevant surface texture features of an object of interest, and even if AI methods may help to extract 3D information from 2D images, optical 3D imaging techniques are still seen as more trustworthy. However, in machining environments conventional 3D imaging techniques such as fringe projection, focus variation, or confocal microscopy fail due to motion blurring and environmental vibrations. These difficulties get worse, if methods of highest sensitivity such as phase shifting or coherence scanning interferometry come into play.
This contribution shows, how well-established 3D measurement techniques can be modified in order to fulfill the requirements of machine vision. We show exemplary results of current research projects dealing with in-situ measurement of 3D surface topography.
One option to obtain 3D information from a single 2D image is based on single-shot fringe pattern profilometry (FPP). We demonstrate that FPP can be used to obtain 3D information of surface texture and damages of civil infrastructure devices such as wind turbines. The pulsed projection of a fixed Ronchi grating in combination with spatially resolved phase analysis of the fringe pattern captured by the camera enables the detection of small surface delamination and spalling defects on the leading edge of rotor blades of wind turbines. The phase analysis is based on a discrete short-time Fourier transform along the fringes of the captured image. We developed this technique in order to perform a drone-based in-situ inspection of rotor blades.
The second example demonstrates how one can adapt coherence scanning interferometry (CSI) in harsh machining environments. CSI requires a so-called depth scan, where an interferometer moves towards the measurement object in order to obtain an interferometric image stack. This is needed in order to reconstruct the surface topography requiring equidistant sampling of interferograms and appropriate signal processing algorithms. Due to the depth can CSI instruments strongly suffer from environmental vibrations. We integrated an interferometric laser optic distance sensor (IDS) into custom-made CSI systems, that records distance changes between the object and the interferometer with nanometer resolution and high data rates of several tens of kHz. Since the image acquisition by the camera is synchronized with the IDS distance measurement, the IDS data allow us to correct for vibration disturbances by rearranging the measured image stack and thus compensating for the vibration influences.
Biodata: Peter Lehmann studied physics at the University of Karlsruhe (KIT), Germany. He reached the PhD degree in engineering at the University of Bremen in 1994 with a thesis dealing with light scattering and speckle techniques for surface roughness measurement. In 2002 he finished his habilitation procedure with a thesis on Fourier optical analysis of laser-based in-process measurement techniques. From 2001 till 2008 he was employed at an industrial instrument manufacturer, where he coordinated the research activities related to optical metrology. Since 2008 he is a full professor and holds the chair in measurement technology at the faculty of electrical engineering and computer science of the University of Kassel, Germany. His main research focus is on interferometric, in-process, and fiber-optical measurement techniques including theoretical and experimental analysis and optimization of systems. Since 2009 he chairs the SPIE conference “Optical measurement systems for industrial inspection”, which is part of the SPIE Optical Metrology conference in Munich, Germany.
He authored or co-authored more than 250 publications including more than 100 journal contributions and has been listed as the inventor or co-inventor of roughly 30 patent applications.
Prof. Pascal Picart
Le Mans Université, France
Title: Application of holographic imaging to diagnostic in laser additive manufacturing
Abstract:
Through the last decades additive manufacturing (or also known as 3D printing) has become an essential technique for the production of small to large size structures. Additive manufacturing encompasses several processes that create a part by adding material layer by layer, unlike machining, which removes material. Today, a large diversity of methods is available: Material Extrusion, Vat Photopolymerization, Powder Bed Fusion, Binder Jetting, Material Jetting, Directed Energy Deposition or also Sheet Lamination. It turns out that those methods also require tools for in-situ diagnostics of the on-going process. In this paper, we aim at presenting holographic imaging a highly relevant approach for in-situ and real time diagnostic of the melt pool in Laser Powder Bed Fusion (LPBF). Digital holographic imaging has the advantage of being contact-less, non-intrusive, and yields full-field surface shape data without any requirement for scanning. We propose to discuss the principle of multi-wavelength digital holography with application to the in-situ investigation of the melt pool in LPBF. This paper describes the optical set-up and its installation in one LBM system. We present first experimental results for the in-situ diagnostic of the melt pool with several measurements realized with different LBM processing conditions.
Biodata: Pascal Picart is a Professor at Le Mans Université, France. He graduated from the École Supérieure d’Optique in 1992 and received his PhD in physics from the University of Paris XI, Orsay, France, in 1995. He joined Le Mans University in 1996. He is the author of 120 journal papers, 30 invited talks, more than 180 proceedings in international & national conferences, 7 book chapters, coordinated 4 books and co-founded one start-up. His research topics are connected with coherent imaging based on digital holography and its various applications to acoustics, mechanics and fluid mechanics. Pascal Picart is member of the OPTICA (formerly OSA), SPIE, Société Française d’Optique (SFO), and the European Optical society (EOS). Pascal Picart is Fellow OPTICA and Fellow SPIE.
