12 May Complete inline quality control of bipolar plates
Bipolar plates are key components in fuel cells. Up to six hundred of these components are stacked in a fuel cell; among other things, they provide the electrical connection as well as the distribution and discharge of reaction gases and water. The metallic bipolar plates are embossed on both sides from metal foils, some of which are less than a tenth of a millimeter thick. Slightly fluctuating process parameters during forming lead to typical defects such as tears, folds or bounce effects, which impair the function and longevity of the fuel cell. Until now, such defects could only be detected on a random basis by means of downstream quality control. As part of a collaborative project between the Fraunhofer-Gesellschaft and the German Research Foundation, the Fraunhofer Institute for Physical Measurement Techniques IPM, together with the University of Stuttgart and industrial partners Thyssenkrupp Automation Engineering and Chemische Werke Kluthe, has developed new sensor technology and simulation methods that form the basis for active process control and process optimization in the series production of formed bipolar plates.
Fine channels are embossed in the bipolar plates on both sides. Image: Fraunhofer IPM
Measuring large surfaces with micrometer precision
A digital holographic 3D sensor developed at Fraunhofer IPM records high-precision 3D data of the component surface and makes it available in real time for process control. In order to measure bipolar plates of different sizes, the researchers rely on a scalable system that can be expanded with additional measuring heads depending on the size of the component. A matrix sensor consisting of several sensor heads records the entire flux field of bipolar plates with a size of up to 400 mm × 150 mm in a single measurement pass – in less than a second and without moving the sensor. A stitching algorithm combines the measurement data from the individual measurement fields into an overall image. The high-resolution measurement images show forming errors from a size of a few micrometers. The system is currently being tested in a near-series environment at the Institute of Forming Technology IFU at the University of Stuttgart.
Digital multi-wavelength holography
Digital multi-wavelength holography is based on the principle of interferometry. The light from a laser is split into an object wave and a reference wave. While the object wave is scattered on the surface to be measured, the reference wave passes through a precisely defined optical path within the sensor. The object and reference waves are then superimposed in a camera. The resulting interference pattern contains the height information of the test object. Using numerical methods, the shape of a technical surface can be calculated from this interference pattern in fractions of a second. As both the intensity and phase of the object wave are recorded so precisely and holistically in digital holography, its propagation in space can also be calculated numerically. This makes it possible to measure a surface even if it has not been optically imaged sharply on the camera chip.
By using several lasers of different wavelengths, clear measuring ranges in the centimeter range and accuracies in the sub-micrometer range can be achieved simultaneously. LED illumination can also be integrated as an additional option. With bright and dark field arrangements, the smallest defects can also be detected and analyzed using conventional image processing methods – with a single optical sensor.
Fully assembled measurement image of a bipolar plate with an area of 296 × 152 mm². The measurement covers the entire flux field and contains around 512 million measuring points. Image: Fraunhofer IPM
Valuable data for process optimization
The measurement data recorded by the digital holographic sensor is not only intended to distinguish between good and bad parts, but also to improve the production process in the long term. To this end, a comprehensive simulation toolchain of the forming process was developed at the University of Stuttgart, into which the 3D measurement data flows. The aim is to systematically record and analyze recurring error patterns so that process parameters can be adjusted.
Source and image: www.ipm.fraunhofer.de
