Design of an indirect bio-impedance measurements system for plant health monitoring

Climate change is the greatest threat of the twenty-first century, endangering the well-being of present and future generations. As a result, climate-smart methods known as smart agriculture have been adopted in recent years. Proximity plant monitoring is one of several newly developed techniques for more sustainable agriculture. Standard crop and single-plant health tracking methods use sensors to measure environmental parameters such as moisture, lighting, and soil characteristics. This thesis proposes an alternative and novel approach to understanding plant health conditions by directly exploiting a signal that has been injected into the plant’s stem. It involves a low-cost device that aims to measure the attenuation state of a signal injected within the plant and correlate it to the water need. Since the final device requires energy autonomy it is battery-powered and consists of a transmitting and a receiving system. The former includes a signal generation block, which is responsible for producing the oscillating signal injected inside the plant. A ring oscillator was used for this purpose, which generates a square wave that is manually sent for the selected test period. Thus, the signal is injected on top of a tobacco plant and collected at the bottom thanks to two surgical needle electrodes, spaced almost 50 cm from each other. On the other hand, a signal-conditioning module and a microcontroller unit comprise the receiving part, taking at input the attenuated signal that traveled inside the stem’s plant. Specifically, the signal-conditioning module consists of an inverting comparator with hysteresis, whose feedback resistor is a digital potentiometer. By modifying the value of the potentiometer, the comparator’s thresholds (high-to-low transition and low-to-high transition voltage) change, until the signal’s estimated amplitude is identified and related to the watering condition. This is made possible by the microcontroller unit, which communicates with the potentiometer via I2C (Inter-Integrated Circuit) protocol and is in charge of estimating the amplitude via a C routine. In addition, a special focus is given to the microcontroller’s energy consumption by implementing its low-power modes. Lastly, the receiving system’s output is sent to the PC through serial communication for appropriate analysis and correlations with the plant’s water status.