Today, automobiles are equipped with an enormous number of ECUs to manage expanded functionality and advanced controls in the vehicle. In a hybrid vehicle, the motor ECU plays an even more complicated role as it manages the interaction between the conventional engine and the electric motor, along with its power systems.
Fuji Heavy Industries, parent company of Subaru, set out to develop its first hybrid vehicle—the Subaru XV Crosstrek Hybrid.
Tomohiro Morita of FUJI Heavy Industries explained, "Our engineers had developed a motor ECU for an earlier hybrid prototype, but the component did not meet the rigorous requirements to take a vehicle to market. For the production model vehicle, the ECU needed various control functionalities to prevent damage to the vehicle body and to ensure driver and passenger safety under various operating conditions, even scenarios that would be impossible or impractical to test on physical hardware."
A new approach
FUJI engineers connected the ECU to a real-time electric motor simulation to test and verify a variety of conditions, including the extreme outliers that might break the system in traditional mechanical testing. They developed a mechanism to sufficiently confirm this software simulation approach with three primary goals for successful testing:
• Verify ECU functionality in various conditions, including extreme environments not easily created or replicated
• Map test cases to requirements to ensure complete test coverage
• Perform regression tests with ease to quickly validate design iterations
The new verification system consists of a real motor ECU and the PXI-based hardware-in-the-loop (HIL) system that simulates motor operations. The HIL system can represent any operating condition of the motor by setting physical parameters such as inductances or resistances. It can also set parameters of the power electronics, including fault conditions or test scenarios such as combinations of load torque and desired rotating speed. By simply changing a parameter in the middle of the test, the HIL system can easily simulate complex test scenarios.
"Because the computational performance required for this process was so high, we felt National Instruments was the only supplier that could meet these requirements" explained Morita. "We chose core system hardware based on NI FlexRIO, FPGA-based PXI modules. The modules executed a model representing the simulated operation of the motors, with all deployed programs using NI LabVIEW system design software."
In the HIL system, the simulation loop rate, equivalent of the temporal resolution in simulation, was a critical factor. For the motor ECU, the loop rate needed to be 1.2 µs or less for the simulator to work.
Morita commented that, "Most simulation platforms from other suppliers use CPUs for computation, resulting in a loop rate in the range of 5 µs to 50 µs. NI FlexRIO used the FPGA for control and computational purposes to meet the required simulation rate of 1.2 µs. Additionally, because the NI FlexRIO has a high-capacity, built-in dynamic random access memory, we could use the JMAG-RT model provided by JSOL Corp.'s JMAG software tool chain. This made it possible to represent the highly non-linear characteristics closer to the real motor."
"Moreover, our engineers could program the FPGA on the NI FlexRIO device graphically with the NI LabVIEW FPGA Module, which made it possible to develop a system with FPGA technology in a short time frame without using a hardware description language."
Morita continued, "All the test patterns developed can be run automatically in only 118 hours. Performing all the tests manually would take an estimated 2,300 hours. The HIL system delivered additional time-saving advantages that included a significant reduction in the number of setup procedures, such as preparing a motor bench and a test vehicle, and it removed the need for test personnel to be qualified to handle high-voltage equipment."
Jeremy Twaits is Automated Test Product Manager, National Instruments UK & Ireland.