Development of an autonomous plant monitoring system based on Plant Microbial Fuel Cells

The objective of this thesis is to propose an autonomous receiving system, which is part of a sensor node for plant health monitoring. This latter is meant to be used in the Internet of Things (IoT) for collecting data in agricultural applications, so that the farmers can use the real-time information from the fields to make the best decision for the optimization of the resources having as objective the enhancement of the sustainability and the efficiency in the management of the crops. The energy harvesting from environmental sources can replenish the sensor nodes with the energy required to ensure a long-term autonomous operation. In this case, the system is based on Plant Microbial Fuel Cells, a green technology that exploits the microbial activity at the plant's roots (rhizosphere region) to produce bioelectricity. Thanks to the addition to the basic technology (i.e., microbial fuel cell) of the plants, which continuously provide organic substrate to be converted into electricity, the amount of power extracted from PMFC only recently has been demonstrated to be sufficient to sustain the operation of ultra-low-power sensor nodes. An ultra-low-power receiving system has been designed and consists of a power management system (PMS) to extract the energy from the PMFC and manage the storing of this energy in a supercapacitor, a microcontroller, a LoRa wireless transmission unit, and a signal conditioning unit. A power management strategy has been adopted to increase the node's lifetime, which consists of exploiting the microcontroller's low-power modes. Indeed, the receiving system measures the frequency of an oscillating signal sent to the microcontroller by a transmitting system implemented in one previous work of the MiNES group. According to the study on which that work is based, the frequency at which the signal propagates along the plant stem (proportional to its electrical impedance) should give information about the plant's watering status, allowing optimized water use. Since the plant status changes slowly during the day, frequent measurements are not needed. Thus the microcontroller is normally in stop mode, and it is awakened by the interrupt of the GPIO pin when a signal is received and thus a measurement must be performed, or by a timing system when a packet of data must be sent wirelessly through the LoRa transceiver. To this purpose, the microcontroller executes a C code developed from the firmware provided with the Murata module. As a final step, the system's power performances are analyzed through real measurements in each operating state, and considerations about the possibility of powering it with the energy level extracted from the PMFC are given.