Workpieces with decorative ground surfaces are used, for example, for covers in interior furnishings or armatures in automobiles. High requirements are placed on the appearance of the surface, which is defined by the customer using a test sample. After the decorative workpiece has been manufactured, the requirements are checked by means of a manual visual comparison with a test sample. This inspection process is based on the subjective perception of the production employees, so that no measurable quality features can be documented. On the one hand, the visual quality control makes it difficult to determine manufacturing tolerances and to trace the inspection process. In addition, the manual quality controls lead to high employee costs and a repetitive inspection process that places a considerable strain on the employees. By automating the manual quality control, costs can be reduced and processes can be made more transparent. In addition, the automation of the testing procedure is necessary to realise a fully automated process for the production of decorative ground workpieces.
Objective quality features must be defined for the automation of the quality inspection. In decorative applications, these features are often insufficiently known because only one test sample is provided by the customer. Moreover, due to the lack of defined features, no measurement principles have been investigated that allow a reliable determination of the features. Since the evaluation of the grinding process of decorative workpieces is not possible without measurable features, only insufficient process knowledge exists. A further challenge arises from the increasing number of variants of decorative workpieces, which require the generalisability of an automated measuring system.
In the research project "AuDeko", A&T Manufacturing GmbH, SHL AG and IFW are investigating the automated production of decorative workpieces. An essential part of the process automation is the research of a measuring system for quality inspection of decorative ground surfaces. At the IFW, the first step was to determine measurable quality features that objectively describe the decorative ground surfaces. For this purpose, the parameters cutting depth, number of grinding cycles, cutting speed and feed rate were systematically varied on a test stand and the subjective change in the ground surface was evaluated. The observed features are the gloss and waviness, as well as the length, depth and direction of the grinding marks. In the next step, a measuring system for recording the surface features was investigated. For this purpose, the ground features are captured with an industrial camera. The captured image data is converted into parameters using image processing algorithms depending on the grey value distribution.
The illustration above indicates that the industrial camera is guided by a collaborative robot at the test stand. This allows the measuring system to be adapted to the high number of variants of decorative workpieces. The industrial camera captures image data orthogonally and at a constant distance from the workpiece surface through appropriate path planning. The distance to the surface is chosen to be small so that the shape of the workpiece is not visible in the image data. The individual images are then combined to form a projected image. Due to the described path planning, the shape of the component is no longer recognisable in the projected image. This makes it possible to transfer the image processing algorithms from flat surfaces to all other surface shapes and thus make the quality inspection generalisable.
In the future, the robot-guided measuring system will be integrated into an automated production cell for the manufacturing of decorative workpieces in the project "AuDeko" in order to demonstrate its application-oriented use. For this purpose, sequences of actions in the production cell depending on the measurement results are defined. For the comprehensibility of the measurement data, a visualisation of the parameters is realised.
This Project is supported by the Federal Ministry for Economic Affairs and Climate Action (BMWK) on the basis of a decision by the German Bundestag.
Contact:
For further information, please contact Dr.-Ing. Carsten Schmidt, Institute of Production Engineering and Machine Tools at Leibniz Universität Hannover, by phone +49 511 762 4839 or by e-mail (geggier@ifw.uni-hannover.de).
