Hitting a moving target: Designing a HILS test rig for an EV start-up
01 May 2022
Imagine the challenge: you’re working for an EV (electric vehicle) start-up, designing a hardware-in-the-loop simulation (HILS) test system. Everything is new: the hardware, the software, the tools, the testers & the test code. Yet, the system must be as accurate as possible in order to correctly simulate the operating environment.
This case study was originally featured in EPDT's 2022 PXI for T&M supplement in the May 2022 issue of EPDT magazine [read the digital issue]. And sign up to receive your own copy each month.
You’re also working on the ECU (electronic control unit), which monitors and controls the EV batteries. In order to test them thoroughly, the software will need to be stressed beyond its normal operating range. To thoroughly test the EV battery control algorithms, enhanced simulation with fault insertion is the only way to safely verify and validate the software before hitting the road in prototype vehicles. Here, Bob Stasonis, Technical Product Specialist at modular signal switching & simulation experts, Pickering Interfaces explains how a complete physical HILS system based around modular PXI switching and simulation modules helped accomplish this…
During development, HILS is typically employed to test the operation of the ECU in a simulated real-world environment in which the ECU will operate. Instrumentation is used to simulate an ECU’s sensor inputs, and capture and verify the ECU control outputs. Safety-critical controllers will usually require certification, where faults, including short and open circuits, are introduced, and the ECU’s response is analysed to check that it is performing in a predictable and, above all, safe manner. Automated fault insertion systems allow verification tests to be run efficiently in a controlled and repeatable way.
The automotive environment can often be very hostile, especially for sensors, with wide ranges and extremes of temperature commonly experienced. Failures can occur due to corrosion, ageing, damage or even faulty installation. Because of all the features and options now available in vehicles, especially as more electronic systems are being introduced for ADAS (advanced driver-assistance systems) systems, the ECUs are becoming very complex, so the accuracy of HILS is essential for a successful launch.
In an EV, the battery dominates, so the ECU that manages the battery must be highly accurate, efficient and guaranteed reliable to ensure safe operation.
Software testing a battery ECU in an EV startup
Pickering Interfaces provides products and services to streamline the design, deployment and sustainment of high-performance electronic test and verification systems. The company was approached by a team designing the software test systems for battery ECUs in an EV start-up. Both HIL test systems and benchtop testers were required.
The software test team faced three main challenges. Firstly, it found itself in a very dynamic and changing situation. As a start-up addressing a new market, the product range was still evolving as new requirements emerged. This meant that long-term test strategies with common test elements were difficult to plan and implement. Secondly, everything was new. The ECU software, hardware and sensors were new. Both the hardware and software needed debugging, and sometimes it was not obvious whether the issue was caused by a hardware fault, or something in the software. The battery configuration was also new. Indeed, the test team itself – although comprising highly experienced personnel – was also new. Finally, the test environment itself was potentially dangerous. To ensure operational safety limits, real EV batteries must be tested well above their specified ratings. High currents up to 2000A and high voltages in the hundreds of volts must be employed in a real EV battery for functional safety concerns. Therefore, lots of redundancy and extreme safety precautions were necessary in conventional testing.
Against this background, the team was constantly battling to ensure that the design of the HIL test system was functionally safe, accurate and repeatable, and was capable of expanding to dynamic hardware and software development cycles. In their own words, it was “a monumental challenge to wring out all the pieces simultaneously, while adhering to an aggressive schedule.”
Therefore, the software test platform needed to be expandable and very flexible. The decision to use instrumentation based on industry-standard PXI and LXI formats was therefore obvious. Still, the EV start-up also decided to use both a fully functional HILS system and flexible benchtop equipment in its software test strategy.
HIL systems must simulate the environment that the system under test will operate in not only for safety concerns, but to thoroughly instrument a comprehensive test strategy. This can be achieved using models and simulation, or by physical means. Accuracy and repeatability are vital. Models can be very space- and cost-effective, but accuracy or exact replication of the simulation environment might suffer.
With batteries, it is critical to control the temperature to prevent thermal runaway. Batteries for EVs can easily employ over 100 thermistors. The EV test development team preferred to use programmable resistors as a physical representation of the thermistors, since the behaviour of a programmable resistor more accurately represents that of a thermistor (which is, of course, a type of resistor). The programmable resistors will also give the full range of characteristics of the actual sensor, including variations with temperature – described by the EV company as the ‘thermal isotope’. However, with over 100 sensors to simulate, it is necessary to select programmable resistor cards with as many channels as possible to avoid making large test systems. This becomes a tradeoff to acquire enough channels, accuracy and range against cost and space. The customer commented that the test system ends up becoming “more complex than the actual system itself, in order to be able to simulate and test for all possible eventualities”.
Fault insertion is another consideration; for example, how do you replicate and respond to a broken wire? If driving a high voltage and current drive relay, in a worst-case scenario, control currents as high as 50A might be in circulation – basically as high as a welder – so all the circuitry must be able to handle 50A and respond in a specific time and shut off effectively.
The HIL Test System Team Lead was a long-time user of Pickering products from his previous employment, so the team approached Pickering to overcome the limitations of its existing HILS systems, which were not flexible enough to meet its needs for new test ranges and capabilities. Lack of support was also an issue. Pickering worked with the team to develop a roadmap for a HILS system that was more appropriate for their requirements.
A complete physical HILS system was based around the following Pickering PXI modular instrumentation products:
• PXI battery cell simulator (model 41-752)
• PXI programmable resistor modules (a customised version of their model 40-295)
• PXI fault insertion switching (model 40-190B)
• PXI high-density multiplexers (models 40-614C & 40-615A)
• PXI 14-slot chassis (model 40-914)
The HILS system was designed for fully automated operation, performing complete physical function tests overnight unattended. For more specific, granular test in the design flow, the EV company also specified benchtop systems based around similar products, housed in Pickering’s 2-slot LXI/USB chassis (model 60-104-001). Whereas with the complete HILS system, the emphasis was on the performance, scalability and flexibility of the Pickering solutions, small size was the key advantage for the benchtop tester.
In addition to these benefits, the EV test system development team commented that Pickering’s long history of delivering high-quality products that are easy-to-use and simple to program were significant reasons for them deciding to go with Pickering. “With all the changes that we were dealing with, it was nice to have something to rely on – something that I don’t have to worry about,” said Pickering’s customer.
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