One for the road

Graduate engineers from Imperial College London’s Racing Green Endurance (RGE) team have raced their battery-powered Radical SRZero car 26,000 km along the Pan-American Highway to prove that all-electric vehicles are more than a fad.

The eye-catching Radical SRZero electric car is claimed to be the only road-legal car with no mechanical differential. In its place, the car uses a 54kWh lithium-ion phosphate battery pack placed along each wing of the car to power two motors mounted on its rear wheels.

’As simple as it sounds, this was not a trivial design task, partly because we had to deploy a 600V power bus to optimise the efficiency of the electrical drive system, with the result that two bespoke inverters had to be developed to drive the AC motors,’ said Alec de Zegher, chief software architect of RGE.

Not only that, but the motors also had to be custom wound to ensure that they worked at 95 per cent or greater efficiency at the 40-65mph average cruising speed of the car. Since no battery-charging management system existed to ensure that the batteries were being charged optimally, the team also developed their own hardware for that task too.

Radical SRZero is claimed to be the only road-legal car with no mechanical differential

To simplify development, RGE’s de Zegher built the car’s control system around National Instruments’ LabVIEW graphical design software and deployed it on NI’s CompactRIO hardware placed underneath the dashboard of the car.

CompactRIO is an industrial controller that sports a real-time embedded processor and a dedicated Field Programmable Gate Array (FPGA), an arrangement that enables designers to partition the key computational software functions between the controller and the FPGA.

That’s exactly what de Zegher did, deploying a traction-control algorithm for the drives, the systems’ data-logging software and the user interface in the embedded controller while programming time-critical tasks, such as digital signal filtering, battery management and a hardwired mechanical differential back-up control algorithm, in the system’s FPGA.

To interface the embedded processors to the electrical and electronic subsystems, de Zegher chose to plug five hot-swappable I/O modules into the CompactRIOchassis - a digital CANbus module, an analogue input module, a digital input module, a digital output module and a mechanical relay module. Of all the I/O modules, de Zegher believes that the choice of the CANbus card was the most crucial. ’The CANbus network simplifies the wiring on the car, enabling the inverters, the bespoke battery-management control system and an inertial measurement system to be daisy chained across a three-wire interface,’ he said.

The battery-management control system is one of the more innovative aspects of the car’s design. ’Under the control of the CompactRIO FPGA and over the CANbus, the battery-management system controls a number of passive slave devices distributed throughout the car that contain an array of resistors wired to each fuel cell,’ said de Zegher. ’When the battery-management system detects that one cell is fully charged, it instructs the slave to “short” the power delivered to that cell across a resistor, allowing all the cells in the car to be charged to their optimal voltage at all times.’

The analogue input module is responsible for monitoring several sensors in the car that measure the angle of the steering wheel, the front and rear brake pressure and the ambient temperature of the car for data-logging purposes.

For its part, the digital input module monitors the gear switch for front, neutral or reverse positions, the speed from two Hall-effect sensors mounted in proximity to both driven wheels and detects the presence of the key in the car’s ignition.