Design of ambient hybrid energy harvesting system for self-powered sensor node

Abstract: The agricultural internet of things (Ag-IoT), which promotes deep integration of modern information and agricultural technology, has become the focus in precision agriculture research. However, it is inconvenient to replace the battery for sensor nodes in real applications due to the facts of complicated farmland environment, large monitoring area, numerous sensor nodes and long crop growth cycle, which brings a big challenge to the power supply of Ag-IoT sensor node. Fortunately, with the rapid development of the electronic technology, the power consumption for the wireless sensor node decreases quickly, which has reached the μW level, and the ambient energy harvesting technology makes it possible for Ag-IoT sensor nodes to have self-power supply and free maintenance. The traditional energy harvesting devices are always limited to only one sort of ambient energy and have big size with poor reliability. This paper proposed a novel hybrid ambient energy harvesting integration system, focusing on the RF (radio frequency) energy and vibration energy which are rich in the environment. A new type of integrated hybrid ambient energy harvesting device was designed, and the RF energy and vibration energy could be harvested at the same time by the combination of the RF antenna and the piezoelectric ceramic effectively. The system consisted of antenna, matching network, piezoelectric module and rectifier. The antenna received the RF(radio frequency) electromagnetic wave and the matching network improved the power transmission from the antenna to the rectifier, and 2 Schottky diodes and 2 capacitances were used to form the double-voltage rectifier. To reduce the size and improve the conversion efficiency, the rectifier was designed to rectify both the RF electromagnetic wave and the low frequency vibration signal. The converted DC (direct current) power was saved in a super capacitance which had high quality factor with very low leakage current. The energy harvesting antenna was designed in an array with 4 antenna elements in order to collect more RF energy. It was simulated and optimized at the frequency of 1.9 GHz (3G band) by using 3D (three-dimensional) electromagnetic simulation tool HFSS (high frequency structure simulator), and the matching network was designed and optimized by using the ADS (advanced design system). The antenna was implemented by the common FR4 printed circuit board which had low cost. The energy harvesting system was measured by the VNA(vector network analyzer) and oscilloscope, and the measured return loss of the antenna was ?20.5 dB, which agreed well with the simulation results. The maximum output power of the harvested RF energy could reach 38 mW, and meanwhile, considering the typical low vibration frequency in farmland enviroment, we used 10 Hz as the vibration frequency for the vibration energy converting measurement. The measured maximum output power of the harvested vibration energy could reach 25 mW, which met the power requirement of the low power consumption sensor node. This device not only improves the reliability for the system power supply and the adaptive capacity to the environment, but also reduces the size of the device obviously, providing a new way for solving the problem of sensor node power supply during the rapid development of Ag-IoT.