HIL Testing Vital to Aerospace Innovation
With its use of artificial intelligence, real-time sensor tracking, and networked digital technology, the world of aerospace innovation is beyond the point where conventional flight testing will do. To ensure that all components work together as planned requires realistic testing on the ground, such as the Hardware-in-the-loop (HIL) testing method.
“HIL testing is used in the development and testing of embedded systems,” explained Dr.-Ing. Arne Brehmer, Senior Aerospace Manager at Vector, a provider of tools and services for testing aviation embedded systems such as Software Unit, Functional Software and Software Integration testing through to Component and System Integration test rigs. “Real signals from electronic control units are connected to a test system that simulates the complete environment of the control unit, sub-system or full system, including actuators, sensors, and loads. With HIL testing, you can easily conduct automatically thousands of possible test scenarios to fulfil the given test requirements and reach a high degree of test coverage.”
In plain language, “HIL tricks an embedded system into thinking it is in an assembled product,” said Noman Hussain. He is VP of Software and Strategic Business Development at Pickering Interfaces, a manufacturer of modular PXI (PCI eXtensions for Instrumentation) switching and simulation products primarily used in test and measurement. “To give you an example, in aviation we have an HIL testing model called Iron Bird. It is a ground-based test rig where an aircraft manufacturer can take all of the components, sensors, and systems used in an aircraft, assemble and connect them on the shop floor, and then use them to run realistic flight test scenarios.”
Opal-RT Technologies is a provider of real-time test facilities for the aerospace industry. “The real-time nature of our system allows our customer to run the plant model discreetly in a real-time domain, so one minute of dynamics applied to the model is calculated in the same minute,” said Mathieu Haineault, Account Manager in this company’s Aerospace HIL testing division. “This allows a virtual software model to deliver electrical inputs/outputs, signals, and communications for interacting with a real device, which is normally a controller that we call a ‘Device Under Test’ (DUT). The cycle of receiving control signals from the DUT, calculating the reaction of the control signal, and sending back the new status (feedback) of the model is called a ‘closed loop operation’, which is the foundation of an HIL system.”
“‘Hardware in the loop’ is exactly as the name implies,” concluded Francisco Flores, Business Development Manager for Aerospace and Defense at dSPACE, a provider of testing and validation solutions to many industries for over 30 years. “You put one piece of hardware — the actual controller under test — in a test bench and the rest of the system is simulated or emulated. This way you test your control software algorithms to make sure that it is in fact working within the parameters of the established requirements. This will also allow the hardware to be tested beyond its prescribed limits in a lab environment.”
A Testing Method for That Meets Many Needs
HIL testing successfully addresses four major areas of concern for the aerospace industry.
First, it provides a measurably reliable approach to testing aircraft systems on the ground. As such, HIL testing “increases system readiness before the completion of a new program,” Haineault said. “It also provides the necessary data to create digital twins of aerospace systems to support continuous integration and update programs. Once validated, an HIL testing system can be used to develop and help validate new updates on an existing aeroplane.”
Second, HIL testing helps aircraft manufacturers comply with government regulations, and document proof that they are doing so. “The development of embedded aircraft systems are regulated by standards like DO 178C or ARP 4754,” said Dr.-Ing. Brehmer. “To comply with these standards and meet the low and high level system requirements rigorous testing is needed.”
“As the complexities of avionics systems continue to evolve, the need to provide more sophisticated strategies and tooling to address the compliance will continue to grow,” he added. “The networked aircraft will require the ability not only to ensure that a single LRU functions correctly, but also that they all function correctly when the entire system is brought together. This means that the ability to isolate components at a software unit level, as well as an LRU level while simulating the remaining interfaces, will be critical to achieving the quality requirements of the avionics industry.”
Third, HIL testing is wonderfully practical. “For instance, products such as Linear Variable Differential Transformer (LVDT) transducers are used to control the flaps,” Hussain said. “When you are sitting on the shop floor and you are moving the steering, you should be seeing flap movement. By doing this kind of action in an HIL testing environment, you have the freedom to create what I would like to call a ‘really problematic aircraft’ where you can simulate different kind of faults in that system, see what happens when they occur in the HIL test environment, and then develop fail safe/preventative mechanisms to make your product better before it ever leaves the ground.”
Fourth, HIL testing allows an aircraft’s entire suite of systems to be checked before it ever goes aloft. “In the world of aerospace — and I think that the FAA will most likely agree with me — the most important thing is to test an aircraft when the systems have been completed. For example, let’s say that you’re building a FADEC, which is a full authority digital engine control/ engine computer. The FAA would probably say, ‘well, what we want to do is just test it when it is built and completed.’ However, for manufacturers, that may prove to be too expensive and also requires a lot of hours. This is why HIL testing is so useful in aerospace manufacturing: It allows you to test within an environment where fault insertion and even testing critical paths can be done in a very safety conscious yet realistic environment.”
The benefits of HIL testing don’t stop here.
For instance, “HIL testing is usually fully automated and reproducible, hence it can be executed 24/7 in a lab environment,” said Olaf Strumberg. He is Head of Engineering Center Verification & Validation Solutions at Elektrobit, a vendor of embedded and connected software products and services that employs HIL testing extensively for its automotive industry clients. “HIL testing is also easily scalable, allowing tests to be run in parallel across several testing setups, and is typically much more cost efficient than a real system test.”
“The ability to automate and simulate these systems can greatly assist in reducing the overall effort, and hence implementation costs,” Dr.-Ing. Brehmer agreed. As well, HIL testing allows Original Equipment Manufacturers (OEMs) to test their suppliers’ equipment thoroughly,” said Flores. “You can pit suppliers’ products against each other, and see which ones work best to ensure the best possible combinations in the aircraft.”
“Overall, the benefits of using HIL testing are massive,” Hussain said. “The key benefit that we really talk about is the ability of HIL to create a truly realistic test environment. With HIL, you can do early validation of any of your equipment that’s been created. Safety is another one because you can test different kinds of scenarios on the shop floor, rather than in the air.”
In Widespread Use, and Growing In Popularity
With all the benefits that HIL testing has to offer, it’s not surprising that this method has taken the aerospace industry by storm.
“We are seeing more and more penetration of HIL testing across our industry,” said Haineault. “Due to the high cost and engineering skills required to do physical flight testing, I would say that Aerospace has been one of the industries to grasp the full potential of [the] HIL system for many years, and its usage is still growing. This is mainly due to the high price of aeroplane development, the complexity of testing, and the really high cost of undetected errors.”
“I will go as far as to say that almost every single OEM out there is using it or considering starting to use it,” Flores said. “HIL testing is especially widely used right now by the many newcomers into the world of aviation, such as electric aircraft. HIL is popular because it’s a good path to certification. You’re able to prove a lot of concepts before starting to go into wide scale manufacturing and rapid deployment. After all, there’s a lot of unknowns with respect to electric aircraft and their reliance on rechargeable batteries. Being able to test those batteries, and validate and verify that the battery management systems work properly using HIL guarantees that level of safety that the industry is looking for.”
“In fact, HIL testing is becoming a standard throughout the entire aerospace industry, from suppliers and manufacturers of small general aviation aircrafts, eVTOLS to helicopters and large civil/military aircrafts,” said Dr.-Ing. Brehmer. “It is also a state-of-the-art method in automotive, railway, agriculture, telecoms, defence industry, and manufacturing systems’ engineering,” Strumberg noted.
As for the future? Dr.-Ing. Arne Brehmer expects that “the constant growth in aviation systems complexity will require more automated testing using HIL.” At the same time, “tests on virtual platforms in software-only environments (SIL) will take a bigger share of project testing during the next couple of years depending on the availability of simulation models and the cost of computing performance,” said Strumberg. “From the current perspective, SIL testing will not replace HIL testing in the upcoming years, as there will still be scenarios which require real hardware to fulfil specific testing requirements. This being said, one very important factor of parallel SIL and HIL testing will be the possibility to seamlessly exchange test cases, vectors and stimuli between these two complementary environments to ensure comparability between tests in software and hardware environments.”
“I think there’s no limit to how far the aviation industry can go with HIL testing,” Flores said; “especially given the introduction of AI into the aviation systems process. With AI, it’s going to be very, very important for aviation companies to look at what the output is, how an AI system is making decisions and whether those decisions match the decisions made by a human being in the cockpit. I think that HIL will allow them to get those answers in a very safe environment, which will be initially in a lab, way before it is ready for flight testing.”
