Toyota – Rapid X-ray imaging developed for Toyota’s Mirai

Rapid X-ray imaging developed for Toyota’s Mirai

Two major forces in Japanese manufacturing came together to solve the problem of particulate contamination of fuel cells for electric vehicles

When Toyota announced its new Mirai fuelcell electric vehicle the focus was on the design and performance features: longer cruising ranges, greater seating capacity and a stylish exterior makeover. What did not make the headlines were groundbreaking manufacturing innovations that will increase the number of zero-emission fuel-cell electric vehicles on the road and contribute to the slowing of global warming due to carbon emissions. Fuel-cell electric vehicles produce the electricity needed to power their electric motors via a chemical reaction that involves hydrogen and oxygen in the fuel-cell stack, the heart of fuel-cell electric vehicles. The fuel-cell stack in the second-generation Mirai contains 330 fuel cells. At the centre of each fuel cell are proton exchange membranes, which are sandwiched between bipolar plates, gaseous diffusion layers and catalysts. Oxygen drawn in from air outside the vehicle is fed to the cathode side of each bipolar plate, while hydrogen is fed to the anode side. The catalyst separates hydrogen into protons and electrons, of which the latter are forced into an external circuit, generating electricity for the motor. To make the fuel-cell stack in the new Mirai smaller, cheaper and more efficient, the proton-exchange membrane in the fuel cells was made thinner to enhance its proton conductivity. But that made the membranes more susceptible to metallic particles introduced during the manufacturing process. These particles can lower the membrane’s resistance to chemical deterioration, which, if left unchecked, can reduce the fuel cell’s fuel efficiency and power generation. To ensure the quality of fuel cells, it was vital to weed out any cells that had been contaminated with metallic particles during production. The engineering team thus needed a reliable and efficient way to check the cells for foreign matter. Since optical inspections were impractical, the team turned to X-rays, which have the power to see through surrounding structures. The concept sounded good in theory. “But it turned out that we had a major problem with X-ray inspection technology,” Shinya Takeshita, an engineer in the Fuel Cell Manufacturing Division of Toyota’s ZEV Factory said. “The foreign metallic matter was showing up as flat two-dimensional shadows in the X-ray images. To judge whether they would actually cause membrane deterioration, we needed to determine their sizes in three dimensions.” Three-dimensional X-ray computed tomography can grasp such fine detail, but a single measurement with this technology takes several hours — an impossible constraint for the mass production of vehicles. Results were needed in seconds, so he turned for help to Hitachi High-Tech Science Corporation, which manufactures measurement and analysis equipment. The two companies worked together to develop an X-ray inspection system that could rapidly and accurately infer the size of foreign particles. Mr Takeshita looked at some experimental data that showed a correlation between the two-dimensional size of foreign matter and the darkness of shadows in X-ray images, in addition to a correlation between the thickness of the foreign matter and the shadow darkness. While pondering the reason for this relationship, Hitachi High-Tech project leader Toshiyuki Takahara suggested it could be caused by diffraction, a phenomenon where light bends around the edges of objects in its path. The pair reasoned that because the darkness of the shadows on X-ray images intensifies with the amount of diffraction, the phenomenon could be used to estimate the three-dimensional size of foreign matter. Experiments confirmed this effect, and the two cooperated on developing a new inspection system that can determine the presence of detrimental foreign matter in a fuel cell in just a few seconds. After building a prototype, they gathered data on how to stabilize the system for a noisy environment like the production line and then put it to work. The result was a rapid, accurate and automated inspection system that can detect foreign matter in the cell production line in a few seconds.