Renesas Europe is providing electronics and technical support to a French company Eco Solar Breizh who are building a car for the race. DesignSpark will be following the progress of the team over the coming year and reporting on some of the technical aspects of the vehicle control system. Renesas have been a dominant force in automotive electronics in the Far East for many years and are looking to expand into Europe. The solar-powered car project offers an ideal opportunity to demonstrate their microcontroller products working in the hostile environment of the Australian Outback and by necessity consuming as little power as possible while doing so. The power requirement of some microcontrollers is now so low that suitable converters can harvest sufficient energy from, say road spring movement to power a complete wireless sensor module.

There are multiple technical constraints imposed on the design. For example, the carbon-fibre structure of the chassis does not allow easy wireless communication. It’s why the decision has been taken to provide multiple back-up solutions for each task or system. The man to machine interface (MMI) for the driver is possible via an Android smartphone or tablet displaying information like speed, battery state-of-charge, etc… and controlling the lights. In parallel, control of the lights via panel buttons has been taken into consideration during the development of the central computer system.

In addition, vehicle size and weights limits are important parameters to be considered. A heavier vehicle will have worse acceleration, braking, cornering and energy efficiency than a lighter one. The amount of energy available to power the motors is severely limited: the on-board vehicle battery has a limited capacity, while the solar cells generate a limited amount of power. On top of that, the race environment is harsh, with high ambient temperatures which limit the power which can be used without components overheating.

The team is today mainly focused on the electronic architecture, especially the implementation of a CAN serial communication bus which will convey crucial information to the driver.

Why the choice of CAN bus?

The car features distributed ‘intelligence’ with sensor and controller units having local data processing capabilities. The CAN bus provides a robust single communication channel between these dispersed modules and the central computer. The advantages are a significant reduction in wiring, greater modularity of electronic systems (easy replacement of faulty electronic modules) and also an increase in the security of the information transmitted. A CAN structure allows flexibility in the car development, with design changes implemented with a minimum of disruption. The CAN bus is widely used in production cars and illustrates the idea that the solar vehicle could be a "real" car.

What are the measurements and data transmitted to the driver?

The main measurements include for example, currents and temperatures of the photovoltaic (solar) panels, vehicle speed, engine load (electric BLDC motors), the battery and motor current. Much of the information is received by the driver through the CAN bus and is displayed on a touch screen. The communication between the main computer (Renesas RX-series microcontroller) and the Android tablet is today established via a Bluetooth wireless link, but the team is working on a back-up solution to implement USB connectivity which will allow charging of the tablet batteries.

Why choose the RX microcontroller?

A key component of the vehicle’s central ‘brain’, the 32-bit RX microcontroller offers the best compromise between performance and current consumption. The architecture delivers 1.65DMIPS/MHz with an FPU (Floating-point maths unit) and DSP (Digital Signal Processing) features. This provides up to 165MIPS with only 50mA current consumption at the highest CPU clock frequency. The main RX peripherals & interfaces used in the Eco Solar Breizh system are CAN, UART, USB, Timers and an SD card interface. As described previously, the CAN bus is used for the communication between the central computer and the remote sensor/controller modules, the DC/DC-Boost Maximum Power Point Tracker (MPPT) and the two electric motor controllers. Data frames received from each peripheral board may be monitored through the central computer via an RS-232 channel during vehicle debug tests. The timers are used to regulate the sending of data to a smartphone via Bluetooth and also to trigger data recording into a data logger. All measurements are stored on an SD memory card to allow off-line analysis after a test run or race.

What about the vehicle itself? And the team?

Today, the prototype is being mechanically assembled in Brittany. The carbon-fibre structure is nearly finished; the solar panels have been assembled and mounted. Innovation and Research are the key elements which unify the team. Since the beginning of the project in 2010, more than 75 students, providing 60 man-months of effort, have participated in various developments, supervised by academic and industrial partners like Renesas Electronics.

In the next post…

The development of the intelligent sensor modules will be discussed together with the Mechanical-Energy Harvesting system.