When it comes to hybrid powertrains, Japan's Mazda Motor Corporation has long been a pioneer in the pursuit of unique driving experiences. It has now introduced two different drive variants for its CX-60 model: a plug-in hybrid system and a 48 V mild hybrid system. Both variants were put through their paces in advance using dSPACE HIL systems to ensure the best possible performance and maximum safety. Yasushige Nagira from Mazda's Powertrain Development Division implemented dSPACE HIL for the development and verification of their inverters in the different drive variants.
Two Very Different Drives
The CX-60's hybrid system efficiently switches between driving modes depending on the state of charge of the onboard battery and driving requirements. The plug-in hybrid system uses electric vehicle (EV), hybrid electric vehicle (HEV), and engine modes. In contrast, the 48 V mild hybrid system uses the engine mode as the main drive and the HEV mode as a supplement. Both systems operate with an 8-speed automatic transmission.
The plug-in hybrid system (above) delivers up to 327 hp. The diesel engine with the 48 V mild hybrid system delivers 200 hp. Picture Credit: © Mazda
Time as a Challenge
When developing the inverter for the CX-60, there were several challenges in terms of safety, required power, and available installation space. Time to market also had to be as short as possible. For example, component testing had to be performed for early validation of sub-functions to make the overall integration more efficient.
In addition, verification of fault diagnosis had to be performed under safe conditions, so signal-level testing was used. The advantage of signal-level HIL testing is that the simulation can be performed without the real power module or motor. As no real currents or voltages are present, tests can be focused on control software, communication, and system integration. In addition, the partial functions can be validated before the complete ECU is available.
The Inverter as a Brain
The inverter plays an extremely important role in a system with electric motors. It is the brain of the system, so to speak, because it converts the direct current from the battery into alternating current to power the electric motor. Depending on the operation mode, it can also be used to charge the battery by transforming kinetic energy from the vehicle into electrical energy that can be stored in the battery.
The inverter controls the motor torque and ultimately the vehicle speed. This is why its trouble-free operation is so important, especially from a safety point of view. For example, a sudden and uncontrollable drop in power could cause a life-threatening collision. Aspects such as higher efficiency and more driving pleasure also depend on the optimal function of the inverter.
Simulation as the Key to Success
In order to meet the tight schedule as well as the extremely high safety requirements, Mazda opted for software verification in a simulation environment. In this case, more than 100 fault patterns had to be verified for the inverters of the two drive systems. In contrast to simulation with real devices, this saved a lot of time and thus costs.
Close to Reality
Nagira met this challenge by developing a HIL environment that enables simulation-like verification. The purpose of this HIL environment was to achieve the following three goals in the simulation environment before the software was released:
- Conduct preliminary testing as a vehicle system to ensure safety and enable timely development.
- Verification of safety requirements, including through simulation of failure conditions.
- Optimize staffing for preparation and verification with real products versus simulation environments.
When developing the HIL environment, there were many things to consider in order to accomplish our task. We decided to approach our problems in a step-by-step manner, by first optimizing the developed environment for each verification purpose. Then we developed an environment that enables high-accuracy verification using a model-based implementation.
High-precision verification using a model-based implementation was easily possible in the HIL environment.
HIL Environment Development
The following questions and requirements were considered in the development of the HIL environment:
- How can two different verification environments for the two different drive systems be set up in a short period time?
- What mechanisms exist to efficiently realize the many verification patterns or the different modes based on the test objects?
- How can an environment be realized in which the verification accuracy for the vehicle system is guaranteed?
- The HIL environment must allow the simulation of virtual driving cycles.
- The fault simulation must be model-based from a safety perspective
- The simulation environment had to provide all necessary dynamic signals, both bus communication and dynamic model variables, to run the connected ECUs with production software in a virtual environment.
A number of real ECUs had to be connected to the HIL environment in order to simulate the operation of the vehicle and the components corresponding to the real vehicle.
The complexity of the simulation increased with the number of ECUs integrated. Additionally, there were concerns that realizing the connected ECU in a simulation would reduce reproducibility and raise doubts about the verification accuracy. Therefore, a second solution was needed. For this purpose, an optimized environment was developed that enabled highly accurate verification using a model library.
Further Development in a MIL Environment
Mazda has developed a MIL environment for simulation and uses it for verification. Each component of this MIL environment was developed with a model that is close to the real part, built on a model-based idea, using a high degree of realism. Nagira believed that this could achieve a certain level of verification accuracy as long as certain conditions were met:
- Using a MIL environment based on a realistic model
- Development of a HIL for verification of a single inverter using the library.
- Combining a MIL environment for the verification of a vehicle system and a HIL for the verification of a single inverter.
Nagira decided to first develop a stand-alone HIL environment to be used for unit testing. He then replaced the developed HIL environment with the corresponding part of the MIL environment. The stand-alone HIL environment for the inverter was developed using the library. The model part of the HIL environment uses dSPACE's XSG Electric Library, which enables high-speed processing in an FPGA-based simulation environment. For the power-stage module parts, such as the IGBT, a FPGA-based inverter model was used. The inverter models provided by dSPACE were adapted to the customer requirements, to enable efficient and safe fault simulations.
Successful Combination
By combining the model-based MIL environment with the stand-alone HIL environment, the following goals were achieved:
- Realization of an interaction environment for vehicle model, virtual ECU, drive model, and real ECU.
- Realization of a model for fault simulation using the XSG Electric Library and customer-specific extensions.
- Realization of the targeted verification accuracy with a minimum of only two real ECUs.
Using two separate solutions, Nagira developed an environment that met both verification goals and verification accuracy. By using models and various libraries, an environment was realized that enabled efficient verification with a minimum amount of required hardware. In terms of developing internal verification environments, Nagira noted that "even without experience in HIL development, we were able to develop each system in about a month." "Verification of electric vehicles requires several hours for charging and other preparations," he explained. "Since the HIL environment is based on dynamic models, we were able to quickly adapt the environment by, for example, changing the state of charge and reinitializing, so we could do the verification in about three minutes."
Simulated Verification of Faults
For checking open faults in insulated gate bipolar transistors (IGBTs), which are power devices, it was estimated that using real devices would cost several hundred thousand Yen just to prepare the fault module. Preparation would take an additional hour. However, with HIL, it was possible to simulate faults using the fault simulation function of the inverter model for power devices.
Future Developments
Looking to the future, Mazda Motor Corporation aims to automate verification by defining all causes that lead to faults. For example, verification reports will be generated automatically using a common format for configuration files, and verification time will be used in parallel with other tasks.
Courtesy of Mazda Motor Corporation
dSPACE MAGAZINE, PUBLISHED JANUARY 2025
