Desenvolvimento de sensor de teor de água do solo e de sistema de controle e automação em malha fechada para uso em irrigação

• Autor(a) principal: Rêgo Segundo, Alan Kardek
• Data de Publicação: 2010
• Tipo de documento: Dissertação
• Idioma: por
• Título da fonte: LOCUS Repositório Institucional da UFV
• Texto Completo: http://locus.ufv.br/handle/123456789/3563
• topic: Sensor de teor de água do solo; Sensor capacitivo; Automação; Sensor of soil water; Capacitive sensor sensor; Automation; CNPQ::ENGENHARIAS

Fresh water becomes increasingly scarce in the world, and for dealing with this problem, in this work it is proposed to develop a technology aiming to use it in a rational and intelligent way in irrigation systems. Both excess and deficit of water are factors that affect the productivity of irrigated crops and, consequently, the economic return of this production system. Therefore, it is interesting to establish an optimal point between the minimum amount of water applied and the maximum yield. In this work, there were developed sensors of soil water content, two types of measurement circuit, an electronic panel for controlling and management of the irrigation system, and a wireless network for transmission of data from the sensors to the panel. Therefore, the actuation on of the irrigation system (turn it on or off) will be done in closed loop, and in real-time, according to the evaluation of the soil water content. The sensors were constructed with three parallel stainless steel rods, supported by polyester resin at the top. Its working principle is based on the capacitance of a capacitor, with the dielectric been the soil material between the rods that form the sensors. Thus, when there is variation in the dielectric constant of the soil located between the rods, due to the presence of water, the capacitance of the sensor will also vary. Two types of coatings to insulate the sensors rods were tested: (i) insulating varnish of the type used in motor windings and (ii) epoxy paste. Two types of circuit for measuring the signal from the sensors were evaluated: (i) ac bridge and (ii) oscillator. Both circuits have a microcontroller, which provides its signal in digital format for the wireless network. The control panel was also based on a microcontroller, plus buttons, liquid crystal display, and solid state relays for actuation on the pump-motor set and on the valves that direct the flow of water of the irrigation system. The wireless network was configures using star topology. Thus, the panel communicates directly with each sensor, which has a fixed address. In his algorithm was implemented a master-slave system.Therefore, the panel (master) requests the data of a given sensor (slave) through its address. Then, the sensor responds to the request with the measurement of the water content of soil. Three requests attempts are made, returning fault signal in case they all fail. So, two vectors are temporarily stored in the memory of the panel microcontroller: (i) a vector containing the sensor data and (ii) a vector indicating a possible failure of the transmission. It was also developed in this algorithm the possibility of repeating the signal. In this case, for more remote sensors the request and response are retransmitted by the circuit of a sensor closer to the panel. The two types of sensors were calibrated using the standard oven method as reference. Three samples of Neossolo Quartzarênico soil material of known physical and chemical properties were placed in three recipients of known volume. These samples were saturated and then a sensor was inserted in each sample and its output signal was measured in the electronic circuit, and the mass of the sample was also determined. Then the samples were placed in an oven for drying. The same procedure was performed daily until the variation of the amount of water in the soil samples became practically constant. At the end of the experiment the mass of dried soil was obtained, and from these data the water content of the soil was determined for each measurement. Finally, a mathematical model was fitted to estimate the soil water content from the correlation between the data of the reference method (standard oven method) with the data measured by the electronic circuit. In the experiment using sensors with the rods covered with insulating varnish, it was obtained a coefficient of determination of 93.17% and precision in the measures of -1±0.010kgkg. In the experiment using sensors with rods coated with epoxy paste, it was not possible to fit a mathematical model to the experimental data. Based on these results, it was concluded that the sensor with rods covered with insulating varnish and its measurement circuit can be used in the automation of irrigation systems, along with the controller panel and wireless network for data transmission.