Page - 122 - in Joint Austrian Computer Vision and Robotics Workshop 2020

PowderBedAnalysis inAdditiveManufacturingUsingImageProcessing FlorianRecla,MartinWelk UMIT–PrivateUniversity forHealthSciences,Medical InformaticsandTechnology florian.recla@edu.umit.at, martin.welk@umit.at Abstract. Systems for additive manufacturing are experiencing an enormous upswing in the industry. In thispaperamethodfor theopticalcontrolofpow- derbeds ispresented. Thesystemisbasedonacam- eraanddirectionallightingandissuitablefordetect- ingtwotypesofdefects, including(i)areaswheretoo little/toomuchpowderhasbeenapplied, and (ii) ar- eas with different porosity. The system is evaluated forboth typesof errors. 1. Introduction Binder-Jetting is a popular method for additive manufacturing of high-resolution components. In this process, powder is applied in layers, which is then selectively cured by a binder [3]. In order to prevent dead times and production downtimes in powder-bed-based additive manufacturing, a system was sought that would reliably find defects in the in- dividual powder layers. Defects in the powder bed can occur either in the form of excess/missing pow- der, or as insufficient porosity of the powder. If such defects are not detected, components may be pro- duced which do not achieve the expected strength values or contain predetermined breaking points in- side. Theanalysis systemshouldbesimple indesign and reliable in operation. Three different types of optical analysis are used in existing plants: Laser triangulation [2], a cam- era with structured illumination [4] or a camera with directed illumination [1]. All approaches aim at creating a geometric image of the powder surface. Laser triangulation or structured illumination can be usedtocreate three-dimensionalmodelsof theplane, while directed light can only be used to find qual- itative deviations from the plane. Since a qualita- tiveevaluationof thesurfaceissufficient, themethod withdirected light is chosen. Top-light Side-light Shadows Figure1. Side-light causes shadowsatdefectivepositions 2. ImagingSystem The prototypical image system is installed in an existingmachine for additivemanufacturing. The system consists of a camera and two lights. One light source illuminates the powder bed verti- cally fromabove toachieve themostuniformillumi- nation possible (top-light). The second light source shines on the powder bed at a very flat angle (side- light). Theside-lightcreates shadowswhen thereare differences in height in the powder bed, which the camera captures from above. These shadows are not created when using the top-light (Figure 1). After the creation of each powder layer, two images are acquired, one using the top-light, and one using the side-light. By subtracting the two images from each other, the shadows are extracted and evaluated. The top-lightensures thatcolordifferences in thepowder arenotmisinterpretedas shadows. 3. ImageProcessing After acquiring the images of one layer, the pro- cessing of the images is done. The two images arehigh-passfilteredtominimiseglobal illumination differences. (Aconstantcorrectionisnotpossiblebe- cause differences in the powder mixture lead to dif- ferent reflection properties.) Afterwards the images are subtracted from each other to extract the shad- ows. A powder layer without defects thus produces an image with very low grey values, as there is no difference between top-light and side-light. Defects 122