Researchers have developed a prototype battery inspection system, using photometric stereo techniques. A Mikrotron EoSens CoaXPress camera has been utilised to make this possible.
Verifying the quality of batteries during manufacturing is crucial. Batteries are not only necessary for electric car power but also for emerging smart grid technologies such as energy distribution and sustainable energy storage. During the battery manufacturing process, a slurry of electrochemically active material is applied onto a metal substrate foil, typically nickel manganese cobalt onto an aluminum substrate. Next, it is dried and mechanically compressed. Defects occurring during the electrode manufacturing process can result in decreased battery efficiency and dangerous short circuits in the finished cells.
Photometric Stereo Inspection System Developed
An inline battery inspection system based on photometric stereo techniques was recently developed by researchers at the University of Technology Vienna (Vienna, Austria). It can identify flaws on coated electrodes at up to 2000 mm/second (78.75 inches/sec) including pinholes, agglomerates, and missing coating. Photometric stereo techniques assisted the researchers in resolving shadowing and other imaging issues encountered during the optical inspection of the dark gray to black battery coating materials.
In creating a system architecture to inspect the quality of applied coatings, the researchers designed a sensing subsystem comprised of a monochrome industrial camera from Mikrotron and four line light sources that illuminated a series of purposefully defective anode and cathode foils on a motorized roller. This subsystem was connected to an industrial PC running Windows 10 that managed the image data and calculated the foil surface representation. An FPGA-based trigger hardware synchronized the strobing of the four lights with the camera’s acquisition from four different angles. To have enough light for a proper signal, the light sources were strobed at 10 kHz. The illuminance in the object plane, generated by a single line light, was approximately 500,000 lux. Flat field correction (FFC) calibration was performed before scanning.
The camera, a Mikrotron EoSens 4.0MCX6-CM featuring a four-megapixel sensor with 2336(H) × 1728(V) resolution, 563 fps speed via CXP-6, and 7 x 7 µm pixel size, was equipped with a 2.0/28 C-mount lens featuring 29.29 mm focal length, mounted at a 226 mm working distance for a magnification of 0.14. This setup realized an optical resolution of 50µm/px equal to the resolution in the transport direction. It also achieved a Field of View (FOV) of approximately 116 mm perpendicular to the transport direction and 200 µm in the transport direction. The FOV corresponds to a region of interest of 2336 × 4 pixels configured in the camera. The FOV, together with the resolution of 50µm/px, acquired the whole width of the material and the transition from the substrate to the material at a resolution suitable for the expected defect’s size.
Image data from the Mikrotron camera was transmitted via a CoaXPress four-lane frame grabber at 25 GBit/second. Next, photometric stereo results were processed and stored on the PC’s hard disk for further analysis. A graphical user interface enabled the operator to review the results in real time.
Samples used in testing were selected to provide a good overview of real-world defects in foil coatings, such as missing coating, coating inhomogeneities, pinholes, agglomerations, cavities, and cracks. The researchers found that using photometric stereo techniques for surface reconstruction enabled their system to capture and analyze highly detailed information on the dark surface structure of battery electrodes.
While the researchers focused on electrodes in this test, they believe the system can be flexibly repurposed for defect detection for other materials, for instance, for defects of steel surfaces.
Learn more about Mikrotron cameras at www.svs-vistek.com