Sensors and Energy Harvesting on the Renesas/Eco Solar Breizh Solar Car
The next World Solar Challenge car race takes place in October 2013. The race involves purpose-built vehicles being driven from the north to the south coast of Australia powered by energy which must come from the sun or be recovered from the kinetic energy of the vehicle itself. Here is the second post in a series charting the development of one entrant sponsored by Renesas Electronics.
The Modular Sensor/Control Boards
A CAN serial communication network links together ‘intelligent’ sensor and actuator modules all around the car eliminating the need for a complex and bulky wiring loom. A suitable microcontroller provides this intelligence together with the necessary CAN interface and analogue input/output.
Why choose the R8C microcontroller?
The Renesas R8C 16-bit microcontroller offers all the functionality needed for a versatile peripheral module. It needs to be versatile so that a single hardware design can be used for all the various peripheral functions required. For example, some modules measure temperatures (batteries and photovoltaic panels) while others control the solar vehicle lights. All must manage CAN communication. The on-board 10-bit ADC, multiple timers and CAN bus interface of the R8C provide this flexibility. The main computer (Renesas RX-based, see last post) controls all these peripheral modules via the CAN bus and is able to re-program (re-Flash) them via the same bus. Hence firmware changes do not require any components to be removed from the car.
Modular R8C microcontroller board architecture
Zero-power Sensor Modules with Energy Harvesting
The constraints sensors (which provide data on physical forces applied to the structure) are powered using energy harvesting techniques: specifically kinetic energy from suspension movement is converted to electrical energy. The energy levels are very small requiring the use of a very low power microcontroller with sophisticated energy-saving modes of operation.
Why choose the RL78 microcontroller?
Development is confidential and on-going, but only possible with an RL78/G13 using its innovative ‘Snooze’ mode. With this feature, the microcontroller can be set up to perform a periodic ADC conversion or a UART serial port operation while keeping the CPU in standby mode, thus lowering the overall current drain. In this system concept, there is no battery and the only source of energy is the suspension itself via mechanical vibration. The RL78 current consumption is so low in both snooze and active modes that it allows an autonomous system to be realised. The 10-bit ADC measurements of constraints sensors are triggered every second by the RTC timer without any need to ‘wake up’ the CPU. The ADC value is then compared with preset upper and lower limit register values using a window comparator. The CPU is ‘woken up’ with an alarm signal only if an ADC output lies outside these limits which occurs as a result of a body-overstress condition. Hence it achieves 1/10th of the power consumption measured during standard Run mode. For example, Snooze mode uses 0.5mA as against 5mA in Run mode (using ADC).
A further advantage of the RL78 is its low-voltage capability, with a power supply range from 1.6V to 5.5V. It allows the RL78-based subsystem to run even if the harvested voltage is extremely low. There is also an internal analogue reference voltage (1.4V) which allows analogue measurement independent from supply voltage. An internal temperature sensor in the RL78 is used by the team to record ambient temperature.
A very low-cost evaluation kit for the RL78/G14 microcontroller can be obtained from RS Components, Product Code 767-6360.