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In order to ensure the highest levels of safety, reliability and environmental protection, leak testing is of great importance to the automotive sector in the production of fuel cell electric vehicles (FCEVs). Pfeiffer Vacuum has been helping automakers develop hydrogen-powered vehicles since the early days of this technology and is a key partner in providing leak detection solutions. Safety, reliability and the environment Leak testing is particularly important for the FCEV market to ensure that strict safety and operational standards are met.  For example, it is mandatory that the fuel tank, which stores hydrogen, and the fuel cell stack, which converts hydrogen and oxygen into electricity through cold combustion, are tested for leaks. But it is not just the tank and stack that are tested several times during the production of an FCEV. This applies both to the components in the hydrogen circuit that carry the operating and cooling media and to the battery components that are also present in all FCEVs. Finding the right leak detection method Hydrogen fuel cell stacks, as the heart of an FCEV, present particular challenges for leak testing. The length of all the seals in a 120 kW fuel cell stack can be up to 1 km and must be fully tested. This process is further complicated by the fact that the dimensions of functionally relevant leaks are no longer visible to the human eye. A simple visual inspection is no longer sufficient. Repair is only possible once existing leaks have been located. Due to the risks associated with the use of hydrogen as a flammable or explosive medium, leak detection is considered a safety-relevant aspect of the fuel cell manufacturing process. A variety of leak detection methods and test equipment are currently used in the industry. Standards in the field of leak testing, such as DIN EN 1779 or DIN EN ISO 20485, provide assistance in the selection of methods by identifying different leak testing methods and providing procedural instructions. Methods range from leak testing with air (pressure change and flow methods) to tracer gas methods using hydrogen, helium or mixtures of the two gases to be selectively detected.  “The great merit of DIN EN 1779, first published in 1999, is the systematic classification of the most important leak test methods used in industry and the decision support based on three clear criteria” explains Dr. Rudolf Konwitschny, leak detection expert at Pfeiffer Vacuum. These criteria are: 1. In which direction does the gas normally flow when it escapes from a leak? Out of the object or into the object? 2. Do I test only components of a test object or the test object as a whole? 3. Do I test integrally or localizing? Integral testing provides information on whether or not leaks are present. Localizing methods can determine where the leak is located. After applying these questions, the standard leaves seven quantitative integral leak test methods based on air or tracer gases that are potentially suitable for testing a pressurized component. Figure 1: Selection criteria of the leak test methods according to DIN EN 1779 According to this pre-selection, both air and tracer gas methods can be considered for integral leak testing. This decision is influenced by a number of factors. The use of air for leak testing has the advantage of requiring little equipment, air is cheap and readily available compared to tracer gases, and inexpensive test equipment can be used. Disadvantages are limited minimum detectable leakage rate (standard leak rates down to a minimum of 1E-4 mbarl/s) and influencing factors such as temperature and volume. The strengths of air leak testing lie primarily in applications under isothermal conditions and in small volumes. “A number of leak tests can be performed, for example, using the differential pressure method (which measures how much air is lost from the unit under test compared to a reference volume) or flow methods,” says Konwitschny. ISO 22734, which describes leak testing of electrolyzers among other things, states that the cell stacks must be subjected to a common pressure test in which the oxygen and hydrogen sides of the individual stacks are connected to a common pressure source and tested simultaneously.  Test conditions are also specified in the standard: The pressure should not be less than the maximum operating pressure and the test duration should be at least two minutes. Exact temperature conditions are also specified, but there is no explanation of the influence of these parameters on the test result. According to Konwitschny, this is an important aspect for manufacturers to consider. Basically, air-based leak testing methods have physical limitations. Temperature is one of the most important environmental parameters. Depending on the size and volume of the part being tested, it can have a significant effect on the measured value of a pressure transducer. Konwitschny explains: “For parts such as the bipolar plate of a fuel cell, we are talking about temperature constancy in the range of 0.1 °C or even less. This is one of the reasons why, in comparative measurements in our application laboratory, we have found a wider spread of measured values with Micro-Flow methods than with test gas methods. In our experience, the instrumentation and process capability of test gas methods are superior. The test gas helium or mixtures containing helium give even better results than hydrogen due to the lower and more constant background signal. The impossibility of perfect control of the test environment, together with the requirement for detection limits below 1E-4 mbarl/s, makes the use of test gases mandatory in many applications. Tracer gases such as helium are more expensive than air, but have the advantage of allowing lower detection limits and often shorter cycle times. The decision between air and tracer gas should therefore be made in the context of the prevailing conditions and the leak test requirements. Another important factor in choosing a leak testing method is the investment and running costs. “If you are starting your production with low volumes, the initial cost of the test

In the mass production of battery cells, even the smallest time or energy savings in individual production steps are crucial. When added together, these optimizations result in a significant increase in the output of a production line, and can even have a positive effect on costs and carbon savings. In order to make fully automated battery production as efficient as possible, it is worth taking a closer look at the control technology used in machines and systems: the shorter the cycle times, the higher the output. This can be illustrated by a simple example: If a line scan camera can record a 5 mm strip in one cycle, it achieves a speed of exactly 5 mm/s with a control technology cycle time of 1 second. With PC-based control technology, on the other hand, the 50 µs cycle time achieves a 20-fold higher speed while maintaining the same information density. PC-based control technology from Beckhoff runs all control functions on a central PC platform, which allows it to offer the highest possible production speeds. The programming and control of all functions are carried out in a single software system running on an industrial PC. This ensures efficient interaction of all components as well as maximum synchronization, since all information is available with a common time reference. It also avoids friction losses or latencies, such as those that tend to occur during communication between different systems. When it comes to multiplying modern gigafactories quickly in the future, battery producers have recognized the importance of correlating process data – i.e., formulations – with the corresponding control parameters of the plants. Jörg Rottkord, Automotive Industry Manager, Beckhoff Automation The high performance of modern processors, which form the core of the PC-based control platform, allows for the centralized execution of computationally intensive tasks. This means that a large number of axes, such as those required for winding battery cells, can be synchronously controlled via an industrial PC, enabling maximum precision and speed in even highly sophisticated processes. Vision applications and machine learning scenarios can also be integrated directly into the control system using powerful PC-based systems. Due to their easy scalability on both the software side (adding modular software blocks) and hardware side (processors with higher performance, multi-core), PC-based automation solutions provide an optimal foundation for machine control in battery production lines, regardless of whether this involves pouch, round, or prismatic cells, and even when considering future requirements. PC-based control technology not only enables maximum efficiency and speed, but also a completely new approach to production processes. High-performance industrial PCs can be used to control the intelligent Beckhoff XTS and XPlanar transport systems, for example, which facilitate the individualized transport of each battery cell: Individual movers transport the cells either along a linear transport rail or by floating two-dimensionally over a field of application-specific magnetic tiles. These individualized transport routes minimize the plant footprint, enabling a maximally flexible production environment and permanent, uninterrupted product tracking. PC-based control for cell production PC-based control offers advantages for all battery production processes. These are particularly evident in the stacking and winding processes for pouch or round cells, where the requirements for precision and speed are exceptionally high. The rolling process used in round cell production involves winding a jelly roll from an anode strip, a cathode strip, and two separator strips. This is subsequently inserted into a metal housing as part of the production process. Accurate web edge control is critical when winding the jelly roll to ensure precise positioning of the various belts. This requires perfect coordination between the vision system and the NC on the control side. PC-based control technology allows for the direct integration of image processing into the control platform, thereby optimizing synchronization with the motion control. The EtherCAT high-speed communication system offers a further advantage, as its ultra-fast data transmission with exact timestamp function means no time is lost during communication with sensors, actuators, or the vision hardware. With XFC technology (eXtreme Fast Control Technology), even response times of under 100 µs can be achieved to ensure not only precise winding accuracy, but also high output. Pouch cells are produced in a stacking process: Electrode sheets of cathode or anode material are placed between separator layers – for example, with a Z-fold. Similar to the winding process for round cells, a high level of accuracy and speed are also crucial in Z-folding. By consolidating all control functions into a powerful PC platform, even highly sophisticated motion tasks in the stacking process of battery cells can be executed with real precision. This makes it possible to control a high number of axes both centrally and synchronously. Robot kinematics can also be seamlessly integrated into the control system. Additional software functions for motion control include the rapid and accurate synchronization of continuous to clocked processes. Even on the hardware side, new automation solutions offer advantages with regard to the assembly of pouch cells: Traditional drive technology in the Z-fold can be replaced by decentralized servo drive systems, for example. These integrate the servo drive directly into the motor to save valuable space in the control cabinet. EtherCAT P is used to reduce the machine footprint further still, with this solution combining power and feedback lines into a single cable. www.beckhoff.com/battery-production