Swisswin Zynq 7015-powered Formula E car wins FS race

Formula Student is the world's largest engineering competition. Thanks to its innovative electric drivetrain based on FPGA modules, the AMZ student team in Zurich, Switzerland, won the event. The AMZ team's car was equipped with four inverters based on Enclustra Mercury ZX5 core boards (based on Xilinx Zynq 7015 SoC) and set the fastest lap time.

The Formula Student car competition has 18 races every year, with more than 600 student teams participating. The AMZ (Akademischer Motorsportverein Zürich) racing team is composed of students from the Swiss Federal Institute of Technology in Zurich and the University of Applied Sciences in Lucerne, Switzerland. In the history of this competition for more than ten years, due to the continuous improvement of concepts and the introduction of innovations, such as the use of FPGA core board modules to control the electric drive motor, it has set a world record for electric vehicles accelerating from 0 to 100 km/h in 1.513 seconds. In order to ensure competitiveness, the various components of the car must be coordinated and integrated into a reliable high-performance system. AMZ has developed most of the components itself to achieve this.

Road to the top

The goal of the Eiger (all the cars are named after Swiss mountains) is to score as many points as possible in the race, which is achieved by setting the fastest lap time. Through lap simulations, energy calculations and analysis of log data from past seasons, AMZ decided to use a purely custom four-wheel drive system, a carbon fiber reinforced polymer (CFRP) monocoque structure, computational fluid dynamics (CFD), a wind-proven aero assembly and hydraulic suspension.

Inverter based on FPGA core board

In the history of AMZ, this is the first time that the team has developed all the components of the powertrain in-house. The last thing to be developed was the inverter, based on the Enclustra FPGA core board. The inverter converts the direct current from the lithium battery into three-phase alternating current to run the permanent magnet synchronous motor.

Four self-developed inverters each control one motor; a self-developed direct torque control (DTC) modulator runs on the Enclustra Mercury ZX5 core board (based on the Xilinx Zynq 7015 system-on-chip). VHDL makes it possible to estimate the current in the motor and calculate the new switch position every 10 nanoseconds - this is impossible with a microcontroller or DSP-based system.

The customized 1200 volt SiC MOSFET module, with an on-resistance of only 10 milliohms, uses a self-developed intelligent gate driver and is water-cooled through a 3D-printed cooling fin, which reduces conduction and switching losses and increases switching speed, with the rise time down to 39 nanoseconds. Two additional 47 nanofarad DC link capacitors on the core board can reduce the power loop inductance. A hybrid DC circuit with 6 microfarad Ceralink ceramic capacitors and 240 microfarad film capacitors is used to reduce weight and reduce DC link voltage ripple. Two PCBs are designed with 1 mm copper inlets for the connection of the traction system to reduce the circuit board area. To control the motor, the three-phase current, DC link voltage and current, and phase-to-phase voltage are measured at up to 1 million samples per second. A resolver is used to determine the current position of the motor. Gigabit Ethernet and CAN connections ensure fast and safe communication between the car and the test bench. For the highest degree of customization, the entire inverter software is developed in-house.

Enclustra Mercury ZX5 SoC Core Board

The processing unit is a System-on-Chip (SoC). In most cases, the bare SoC is packaged in BGA, which is difficult to solder and requires a multi-layer PCB to lead the signal to the chip. SoC also requires many peripherals, such as memory, clock, interface, and complex power supply. The Enclustra Mercury ZX5 SoC core board provides all of the above functions on a small size PCB. The core board contains 1GB DDR3L SDRAM, 512MB Nand Flash, an Ethernet PHY and a power supply that provides all voltages. The core board can even power the circuits on the baseboard, minimizing the need for power converters.

Rich computing power

Because of the need for very low latency and high refresh rates, the modulator and all communications with peripherals are implemented on the FPGA. All critical safety functions are implemented on the FPGA, with a maximum delay of 1 microsecond for overcurrent protection and 2 microseconds for overvoltage protection. A multi-layer redundant safety system is implemented on the FPGA and processor, with the processor and FPGA monitoring each other and shutting down the inverter in the event of inconsistencies.

Some advanced controls, such as speed control and traction control, are implemented on one core of the ARM Cortex-A9 processor; another core is responsible for communicating with the vehicle control unit (VCU) or control computer and is also responsible for data logging.

High bandwidth interface

The compiled firmware along with the bitstream is copied to an SD card and inserted into the inverter baseboard. At startup, the bootloader copies the firmware to memory and loads the bitstream into the FPGA fabric.

The FPGA processes all current measurements at a sample rate of 1 million per second (1MSps) and voltage measurements at a rate of 500,000 per second (500 kSps). These components are accessed via an SPI-based protocol. The motor position is measured via a resolver with a 33 kSps parallel interface. In addition to being used directly by the modulator, the data is transferred to the processor via an integrated AXI PL-PS interconnect. Using this technology, the processor can simply change configuration data and read values ​​from the FPGA via memory access instructions.

In addition, the DDR3 RAM of the Mercury ZX5 core board can be accessed directly from the FPGA fabric. This allows large amounts of log data to be transferred to RAM without using the processor. This data is then stored to an SD card for offline analysis before the inverter is shut down.

The temperatures of the semiconductors and the output filter are measured with the built-in XADC of the SoC and used directly on the processor. In the car, the inverter is connected directly to the processing system with the VCU via a CAN interface. To run the inverter on the test bench and connect it to the computer, an Ethernet interface is used.

Simplified Power Supply

The Mercury ZX5 core board is powered by a single 5~15V power supply. The onboard DC/DC converter provides all the internal required voltages, and the converted voltages on the board are also led out to the connector. These 3.3V and 1.8V are used to power the analog and digital circuits on the inverter baseboard. Because the above power supply is already provided, the work that users need to do in terms of power supply is minimized when developing based on the Enclustra core board.

Extensive design support

To simplify the integration of its modules, Enclustra provides all the necessary hardware, software and support materials, in addition to user manuals, schematics, 3D models, PCB footprints and differential I/O length tables, detailed documentation and reference designs make it easy to get started. Because of this, the risk of pin calibration errors is minimized.

Enclustra Build Environment (EBE) can be used to compile Enclustra SoC core boards with integrated ARM processors, which is very smooth. The core board and baseboard are selected through a graphical interface; after that, EBE downloads the appropriate bitstream, the first stage boot loader (FSBL) and the required source code; finally compiles U-Boot, Linux and BusyBox-based root file systems.

With the free Module Configuration Tool (MCT) provided by Enclustra, the core board and the base board can be configured via USB without any additional hardware. Through the USB connector on the base board, users can program the core board's FPGA core SPI Flash, read the core board's EEPROM, and configure peripherals. All problems encountered by AMZ during the development of the inverter can be quickly solved with the support of Enclustra.

The next generation of racing evolution

The new inverter for the next generation AMZ race car Mythen is again based on the Enclustra Mercury ZX5 core board. With this new inverter, a fiber optic connection between two Mercury ZX5 core boards is implemented in the race car. For this purpose, a Gigabit transceiver is used. The smaller Mars ZX2 core board was also evaluated by AMZ, but its I/O count did not meet the requirements.

The Mythen drivetrain concept changes from 4 inverters (1 inverter controls 1 motor/wheel) to 2 inverters (each inverter controls 2 motors). Thanks to this new concept, many auxiliary circuits can be merged, the complexity is reduced, and some valuable space is saved. In addition, it opens the possibility to implement more advanced control algorithms that act on multiple motors.

About Formula Student

Formula Student is the world's largest competition for engineers and was founded in 1981. The aim of the competition is to introduce future engineers to the development, production, assembly, testing and racing of electric or gas-powered racing cars over a one-year period. The winner is not necessarily the team with the fastest car, but the one with the best combination of structure, performance, financial planning and sales arguments. In 2010, in order to train potential young engineers, prepare them for future technologies (such as electric drivetrains) and advance the innovation process, the competition committee also opened a separate competition unit for electric vehicles.

About Enclustra