Model and PID algorithm realization of nutrient solution control based on two-step fertilizer mixing system

Abstract: Closed cultivation is a new type of cultivation model, which is widely used in solar greenhouse, and it uses EC as the performance index of the regulation process of nutrient solution. In the study, a dynamic analysis was conducted on the regulation process of closed culture nutrient solution with two-step mixed fertilizer characteristics. PID control algorithm was used to control the regulation process of the nutrient solution, and the effects of 6 factors on control process were analyzed through comparative experiment. The results showed that it was difficult to use a precise model to characterize the fertilizer mixing process, since the output liquid flow rate, the fertilizer concentration and the PID parameters had great influence on the control effect of the two-step mixed fertilizer system. Experiments were carried out to find the effect of different output liquid flow rates, fertilizer concentration and the PID parameters, on PID control model. The results were obtained using the fixed variable method and orthogonal tests, under the target electrical conductivity (EC) of 2.3. The experimental results showed that the time of the system decreased with the decrease of the output liquid flow rate or the increase of the fertilizer concentration. Duration of system oscillation and overshoot amount increased with the decrease of the output liquid flow rate or the increase of the fertilizer concentration. Keeping the fertilizer concentration and the PID parameters (obtained under the condition that the output liquid flow rate was 4 m3/h and the fertilizer concentration was 4 channels) unchanged, and the target EC of 2.3, the duration of system oscillation was changed from 15 s to 125 s while the output liquid flow rate was changed from 10 m3/h to 4 m3/h, and, the stable time was changed from 180 s to 205 s and the overshoot was changed from 1% to 20%. Keeping the output liquid flow rate and the PID parameters (obtained under the condition that the output liquid flow rate was 4 m3/h and the fertilizer concentration was 4 channels) unchanged and the target EC of 2.3, the duration of system oscillation was changed from 40 s to 125 s while the number of the fertilizer opening channel was changed from 1 channel to 4 channels, the stable time was changed from 255 s to 205 s, and the overshoot was changed from 3% to 20%. According to the different executive agencies and environmental factors, it was necessary to adjust the PID parameter of the system to achieve better control effect. The closed cultivation system ran with high efficiency and stability using the adjusted PID parameters. According to the test results and analysis, the optimal output liquid flow rate and the concentration of the fertilizer solution in closed culture system with two-step mixed fertilizer were 6 m3/h and 200 g/L, respectively. Under this condition, the PID control parameters were adjusted, and the stability time of the system was 100 s, the overshoot was 3%, and the duration of oscillation was 25 s. In summary, the nutrient solution of the closed type cultivation system with double mixed fertilizer could be regulated by the PID algorithm to achieve stable and efficient operation, which provided a reference for the regulation.