Optimal design and experiment of fertilizer EC regulation based on subsection control algorithm of fuzzy and PI

Precise control of EC for fertilizer solution is one of the key technologies to realize smart irrigation in the production of protected vegetable.Serious lag and instability in EC detection will happen during the mixing of fertilizer and water due to differences in mother liquor concentration, fertilizer suction method and irrigation flow etc.To solve this problem, a new EC sensor was designed to measure the EC value of fertilizer solution indirectly by measuring the frequency based on different EC values leading to different equivalent resistances, and the smart irrigation system was developed as microcontroller (MSP430) was the master controller.Meanwhile, a subsection control strategy was introduced to regulate and control the EC value of fertilizer solution, and the opening or closing of electromagnetic valve for the suction of fertilizer solution was controlled by outputting corresponding duty cycle.When the difference between the target EC value and the measured value was large, PI control was adopted to narrow the gap as fast as possible.In this study, the threshold is defined as 0.2 mS/cm according to practical knowledge, therefore, fuzzy control with good robustness and high adaptability was adopted when the difference was smaller than 0.2 mS/cm.In fuzzy control stage, triangle function and centroid method were selected as membership function and defuzzification calculation with 63 rules and the output language variable value was divided into 13 levels.A compound venturi was designed as fertilizer solution suction device, with the diameter of inlet and outlet pipes was both 40 mm, and a closed-container (10 cm×10 cm×6 cm) was installed at the throat part to connect the inlet and outlet pipes, and 7 suction interface were located at the bottom of the container to link with the electromagnetic valves.The main pipe of the irrigation system was controlled by 24VDC electromagnetic valve and the drippers of the branch pipes were inserted into the substrate near plant roots.The filter was installed between inlet water and the irrigation system to avoid blocking venturi and drippers.An experiment including 4 different target EC values and 4 different suction pipelines was carried out to test the effect of the designed control system.The results showed that the frequency measured by our EC sensor had a significant linear correlation with the measured EC value with R2 is 0.999 5.In the case of subsection control, when the target EC value was 2 mS/cm, the steady response time was 122 s and maximum overshoot is 0.34 mS/cm when 4 channels were used for fertilizer solution suction, which performed much better than only PI control with steady response time 180 s and maximum overshoot up to 0.62 mS/cm.In addition, the higher the target EC value set, the more accurate the steady EC value was, however, the steady response time became longer and overshoot was more serious.The subsection control strategy used in this study can effectively overcome excessive overshoot, also the time for mixing fertilizer solution and the measured EC value can both meet the requirements for the practical production of protected vegetable.