Silicon Laboratories Inc. (SiLabs) has recently introduced a wireless sensor solution very efficient from an energy point of view, powered by a solar energy harvesting source.
This energy harvesting solution enables designers and developers to create low-power self-sustaining wireless sensor networks, particularly suited for applications such as:
- home and building automation networks
- security systems
- industrial sensor networks
- medical monitoring systems
- agriculture monitoring systems
- wireless sensor nodes
- infrastructure sensing systems
- asset monitoring/tracking devices
SiLabs's solution, called Si10xx, is based on a single-chip microcontroller (MCU) and on a wireless transceiver, and can therefore guarantee both control and wireless functionalities at an extremely very low power consumption. A solution like this, based on energy harvesting, has several significant advantages, including:
- it is environment friendly
- it is virtually inexhaustible (being self-sustaining from the energy point of view)
- it is advantageous from the economic point of view (avoiding the replacement of batteries)
- it is a suitable alternative to traditional batteries
Do not forget the batteries still involve a cost, and they have the disadvantage of requiring a periodic replacement (this is very uncomfortable when dealing with a network of wireless sensors located in places not easily accessible), plus the fact that they become unreliable in conditions of extreme temperature. The following image shows the block diagram for a typical energy-harvesting system.
SiLabs reference design (RD) based on energy harvesting includes software for network and wireless USB interface management, a complete circuit with radio frequency (RF) layout, bill of materials, schematics and Gerber files. The elements of the RD are as follows:
- a wireless sensor node powered by solar energy (energy harvesting), which measures temperature, light intensity, and the level of battery charge. These functions are performed by the wireless MCU Si10xx which monitors the sensor system and transmits related information via radio. There is also a thin-film battery (TFB) to store energy recovered by the harvesting technique
- a wireless USB adapter (dongle) that connects the wireless sensor node to a PC, which in turn graphically displays the received values. The adapter is based on the Silicon Labs Si4431 EZRadioPRO transceiver, with an MCU equipped with USB-HID class and EZMac software stack
- a graphical user interface (GUI) running on a PC where you can view the information gathered by a wireless sensor network (up to 4 sensor nodes are supported)
The used thin-film battery has a capacity of 0.7 mAh. This means that, under conditions of direct sunlight, full charge can be performed in less than 2 hours. When the device is in sleep mode, the batteries can be maintained for 7,000 hours. If instead the wireless device transmits data continuously, is granted a period of about 3 hours of continuous operation, even if the system is designed to allow for constant charging of the batteries during operation and thus prevent this from going dead. Usually, these type of systems are based on a programmable timer (RTC) that periodically "wakes up" the CPU and triggers the data acquisition and transmission via radio, so you get a perpetual operation of the system. The system is also configurable with regard to the source of energy harvesting. It is therefore possible to bypass the solar cell (solution chosen by default), and coonect to an auxiliary input other forms of energy harvesting, such as piezoelectric (vibration), thermal, or RF. Transmission between wireless nodes can be up to a distance of 300 feet (about 100 meters). SiLabs solution is currently avaialble at a price of U.S. $ 45. The following image shows the conceptual scheme of SiLabs energy harvesting reference design.
The wireless MCU
SiLabs Si100x/1x wireless MCU family is today the integrated single-chip solution with the lowest power consumption in absolute terms, combining the versatility of a microcontroller with the wireless feature of a RF transceiver for bidirectional communication. The Si100x/1x MCU have the lowest absorption value of all similar devices currently on the market, in all modes of operation: active, sleep, deep sleep. The following video provides an example of this technology, and refers to a M-bus measurement application: two boards are here used to establish a wireless communication link between them.
Here we can see the block diagram related to a Si100x/1x MCU series: note the presence of a number of blocks needed to realize a complete wireless solution for energy harvesting, such as ADC, oscillators, timer, radio interface, integrated temperature sensor. The MCU is provided with a high speed core based on the "historical" 8051, able to achieve a peformance of 25 MIPS at a frequency of 25MHz. The CPU architecture has also an instruction pipeline capable of performing 70% of instructions in 1 or 2 clock cycles. Power supply is between 1.8 and 3.6V, the current consumption in sleep mode is less than 0.1 microA (the contents of RAM memory is preserved even in this state), and the wakeup time is less than 2 micro seconds. There is an on-chip block to allow the debug: this allows to perform a non-intrusive debugging while the CPU is executing the application at full-speed (therefore it is not necessary to use an emulator). The transceiver operates in the 240-960 MHz frequency range (sub-GHz), has a maximum power output equal to +20 dBm, supports FSK, GFSK and OOK modulations, can transmit at speeds up to 256 kbps, and has a current consumption of 18.5 mA at +20 dBm.