Developing the Elektra Test System, a New End-of-Line Test Bench for Hybrid Inverters

Author(s):

Alessandro Andreoli, Project Manager - Loccioni
Alessandro De Grassi - Loccioni

NI Gold Alliance Partner Loccioni integrates ideas, people, and technologies to develop measurement and automated control systems to improve products, processes, quality, efficiency, and sustainability. We are committed to helping our customers perform measurements to make improvements and want to help all of our customers save time and money and respect the environment. Our clients and partners are world leaders in their markets, from automotive to home appliance manufacturing to health care.

New requirements for pollution reduction and better car performance push car makers to investigate new engine and car concepts. One of these new concepts is the hybrid vehicle for which two different power sources are used to move the vehicle. Normally, the power source is a traditional combustion engine and an electric engine. Currently, there are different solutions on the market for the connecting scheme of the two power sources (serial, parallel, and serial-parallel, for example). The electric engine system mainly comprises an electric motor, a battery system, and an electric power converter (inverter). This case study specifically describes a bench developed for testing an inverter for automotive applications.

Development History

Magneti Marelli needed to develop an automatic testing solution for its new inverter that could be mounted inside the new LaFerrari car. The customer also needed to be able to use the bench to test stand-alone subcomponents of the inverter. We had to start development before the complete definition of the product and its testing specifications were finished to reduce the delivery time of the test bench. To do this, we decided to start an R&D project in collaboration with Magneti Marelli and the University of Naples Federico II to define the test specifications for the new inverter. One University of Naples engineering student worked on the project for six months and used it as a starting point for his final thesis. The biggest task in this phase was to identify how to manage and test electric components that work with high voltages (600 V) and high current levels (800 A).

End-of-Line Test Bench for a Hybrid Inverter

To completely test the inverter, we worked with Magneti Marelli to divide the test into the following main parts:

• Unit under test (UUT) communications test—The bench powers the inverter CPU and verifies that the correct communication is occurring with the UUT. In this phase, some preliminary tests are performed, too. The communication with the UUT is facilitated using the controller area network (CAN) bus.
• Inverter functional test—We connect the inverter to a high-voltage battery simulation (450 V) to analyze the output on the U, V, and W components. We connect the U, V, and W outputs of the inverter to an inductive load that simulates the motor connected to the UUT in normal conditions. In this test, we also confirm that the UUT internal current measuring devices work as expected under different conditions.
• Overvoltage inverter functional test—We increase the high-voltage (HV) battery input of the UUT to verify that at 475 V, the UUT generates a fault.
• Undervoltage inverter functional test—We decrease the HV battery input of the UUT to verify that at 230 V, the UUT generates a fault.
• DC/DC functional test—We verify that the UUT can convert the DC voltage as required. We connect the inverter HV input to 450 V and we check that the DC output voltage levels are within the specified limits. In this test, we also verify the buck-boost converter functions.
• Overvoltage DC/DC test—We increase the HV input of the UUT to verify that at 475 V, the UUT generates a fault.
• Undervoltage DC/DC functional test—We decrease the HV input of the UUT to verify that at 230 V, the UUT generates a fault.

Hardware Configuration

To perform the tasks described above, we divide the bench into these main parts:

• Standard instruments cabinet
• High-power cabinet
• Test station where the UUT is inserted
• UUT cooling unit

The standard instruments cabinet includes all of the components responsible for the bench automation and the low-power test of the UUT such as communication tests with inverter electronics and internal diagnosis. The high-power cabinet includes all the instruments used for the high-power test of the UUT.

The control unit of the bench is based on a main PC that manages automation and measurements and a programmable logic controller (PLC) that manages the safety of the operator, which is required because of the high voltages and high current signals involved in the tests.

For the main PC, we equipped a standard industrial PC with a MXI bus extension that helps the user remotely control a PXI chassis and installed the following items:

• 3 NI PXI-2567 64-channel external relay driver modules  
• 2 NI PXI-6704 analog output modules
• 1 NI PXI-6143 simultaneous sampling multifunction DAQ module
• 1 CAN interface
• 1 NI PXI-GPIB high-performance GPIB controller
• 1 NI PXI-8430 high-performance 8-port serial interface
• 1 NI PXI-7841R multifunction RIO module

We also developed a series of custom electronic boards to adapt and protect the standard boards installed in the PXI chassis.

In the high-power cabinet, we installed all the instruments and devices needed for the high-power tests, including the following main components:

• 1 power supply for inverter electronics (30 V, 5 A)
• 1 main power supply (600 V, 25 A)
• 1 voltage electronic load (75 kW)
• 1 voltage electronic load (90 kW)
• 1 high-current power supply (510 A, 80 V)

We connected the high-power instruments to the UUT and between each other using a high-power matrix specially developed for this bench.

Liquid cooling of the UUT was required because of the high power used by the UUT. To perform this in the bench, we added a simple cooling device to ensure the UUT would not produce too much heat, which could be a symptom of problems in the devices. We also added a simple leakage test of the UUT on the circuits.

Test Station

We mounted the UUT on a specific pallet that helps the user mechanically adapt the bench to the different kinds of inverters produced. The pallets powered the transportation of the UUT between the end-of-line bench and the other test stations in the customer production line.

Software Configuration

We developed all the software internally to manage the bench described above and have better coding and more reliable software. Our software department divided the software into different modules. The main modules we developed were the automation module and the test module. To develop the inverter machine software, we started with a previous version of a Loccioni software solution that was used to test the electronic control units (ECUs) and infotainment products.

The automation module automated the management of the bench. This software comprised a user interface and a software PLC that managed all the activities of the bench. Submodules that managed different tasks such as user identification, alarm management, and utilities composed the software PLC. We used LabVIEW to write all the modules.

The test module created and executed the different steps necessary for a complete test sequence. To give the customer the most flexibility to create different sequences, our software department used NI TestStand as the test sequencer. We customized the NI TestStand environment by adding a step that allowed for the control of specific instruments. Each instrument had its own steps that were not connected with the others, which helped us to achieve better code reliability and maintainability and to preserve the concept of modularity in the development of the custom step type. We also developed some high-level steps that contained more single commands to optimize the test cycle time.

We used TCP/IP sockets for the internal communication between the automation module and the test module because it was easy to maintain a separation between the different software modules. Using NI TestStand as the test sequencer helped us easily expand the software for additional test stations.

Conclusion

The Magneti Marelli challenge to supply the new bench for the inverter of LaFerrari was a great opportunity for us to develop a new competence for the advanced high-power functional testing of the electronics components. Our well-established relationship with National Instruments was fundamental in reaching our goal of high modularity and flexibility with the hardware and software platform. Because of this experience, we consider National Instruments as a preferential supplier for the development of future platforms related to the strategic business of testing hybrid components.

Author Information:

Alessandro Andreoli
Alessandro De Grassi
Loccioni
Tel: +39 0731 8161
a.andreoli@loccioni.com
a.degrassi@loccioni.com