Studies On Online-Measuring And Controlling System For Nutrient Solution Of Soilless Culture

Facility agriculture, a crucial component of modern farming, plays significant roles in promoting the development and enhancing the comprehensive productivity of agriculture. As one of the main cultivation methods, detecting and controlling the nutrient solution automatically are key technologies to widely spread, high-yield and high-efficiency of soilless culture. However, there is not a mature way which can both balance the concentrations of multiple ions and can control the nutrient solution automatically.By Modular design method, the structure is planned. The system consists of three parts: the automatic calibration module, the detection module and the nutrient control module. The Programmable Logic Controller (PLC) is selected as the core controller, and the calibration of electrode is depended on motor rotation with step instructions. In PLC system, the output voltage of ion selective electrode (ISE) is collected by analog expansion module; the open level of solenoid value is calculated by PID instructions. In addition, KingView is applied for the human-computer interaction, the working process monitor and the data display and storage. Three modules are cooperated with each other. Thus, the on-line detection is finished, as well as automatic regulation of the nutrient solution.In the automatic calibration module, the screw is chosen to realize the moving of ISE, in order to obtain the voltage automatically; Then based on the Nernst Equation, least squares is fitted for the relationship between concentrations and output voltages. Finally, the automatic calibration of ISE is realized, which provides technical supports for online detection.In the detecting module, based on the redundant control principle, the data fusion method of double-sensors is used to detect the concentrations of NO3-, K+ and Ca2+ online. At the same time, the main ion concentrations which cannot be measured directl yare predicted using soft measurement. Thus, six important ion concentrations (nitrogen, phosphorus, sulfur, potassium, calcium, magnesium) can be displayed in the PC, which makes it possible to supply and control nutrient solution timely.In the control module, PID control principle is applied to calculate the deviation of the online detection value and target value. The results are then converted into open position of solenoid valve, realizing the control requirements about regarding pH as the core, and taking into account the dynamic balance of nutrient solution, according to the control parameters managers set.In conclusion, two experiments are designed to verify the accuracy of automatic calibration module and the reliability of detection module in the actual production. Results show that the system can detect the Electric conductivity (EC) value, the pH value, and the main ion concentrations online, and can further supplement the nutrient solution automatically according to specific requirements.