62 mounts, and we can put the unit under test (UUT) on top; for example, a steering motor. We can then measure the structural noise from the unit at the same time as applying signal to the unit, and create a load condition of the environment using the HIL, so we can use it as a simple dynamometer or to recreate a particular situation in the labs where it is much easier to investigate particular factors of operation.” “For us, it has been a really useful facility, as with the HIL rigs you can do lots of tests, automate them and simulate parts of the system you haven’t got available early in the development cycle. Having the HIL rig allows us to do that. We have the models in the rig, creating the situations that the UUT can then see. It allows us to get insights that would be very difficult to do otherwise.” The models are developed and maintained by the HIL tooling team in Dusseldorf, Germany, and they maintain all the models for different types of HIL, from the ECUs to the motors. “Often, the models are developed in advance from the design requirements, so the models might be used for virtual ECUs, MIL or HIL. The crossover between HIL and MIL is very strong and the level of fidelity depends on the test case. For the HIL rigs, it’s about the power in and out, modelling the losses and the non-linear effects or fault conditions and the dynamic stability. “One of the use cases in calibration is to look at a wide range of situations, and apply the method repeatedly to drive the excitation to the load to look and see what’s happening, and get the best calibration,” says Langley. “Where we are putting together our models we try to do so with standard interfaces, which helps the interoperability for a library as we assemble tests. At the level we work at for our servo drives we tend to have an interface that works for us rather than a generic version. In r&d what we are finding is that customers have a particular interface and requirements in much the same way as a physical product. “We provide the servo drive to the system team and they integrate that into the higher-level system model, and they interact with the completely virtual systems team elsewhere in ZF,” says Vishal Teewari, senior engineering supervisor for servo drives at ZF. ZF has been simulating the NVH aspects of the drives. “We were bringing the NVH aspects into the models, and comparing them with the results from the HIL rig to get more convergence of the MIL and HIL,” says Langley. Test environments The models and data from the r&d stage can be used throughout the production process. “This gives us the ability to reuse hardware and software, to have the same blocks of code on production lines and HIL environments ready validated,” says Langley at ZF. “It’s not just standalone testing in the lab, but it’s the ability to incorporate modules for analysis, for test sequences that are used elsewhere, and that helps to bring test sequences to fruition quickly.” This also allows comparison between what is used on the HIL rig and on the end-of-line test to compare the data. “We get asked for correlation testing a lot, so having those routines with the test cases and automation tests, it makes validation so much easier using the same code,” says Teewari. “If there is an issue with NVH on a vehicle, we can take data from the prototype vehicle to see if the problem comes from the servo drive, or elsewhere, or for NVH analysis to identify the harmonics coming from the servo drives.” Conclusion Building an end-to-end development process for a digital twin with all the models needed for e-mobility is a huge job that is taking years to achieve. HIL allows the transition of the digital twin into a real-world version long before a physical vehicle needs to be assembled. This is allowing the physical design to be tuned before it hits the road, and provides key data on how the platform will perform both now and in the future. This allows the detailed models and code used for the development to be reused throughout the manufacturing process in ECUs, and for end-of-line testing, again saving development costs and time, and avoiding the need to redevelop the code for production testing. Deep insight | HIL testing An e-axle can be tested in production with the same code from the HIL system (Image courtesy of ZF) March/April 2025 | E-Mobility Engineering
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