Standard nutrient-solution-based fish-tail-water-fertilizer machine: Design, control, and decision

• Proposed a new strategy of nitrogen substitution control in fish tail water. • Designed a fertilizer-dispensing robot with embedded substitution strategy. • Analyzed the performance of the fertilizer-dispensing robot. • Determined the optimal working parameters of the fertilizer-dispensing robot. Although using fish tail water as a replacement for nitrogen fertilizers has excellent application prospects, the nitrogen content of fish tail water fluctuates greatly and is uncertain. Therefore, realizing intelligent allocation and organic substitution under dynamic nitrogen changes in fish tail water is pivotal for its application. This study proposes a nitrogen-replacement-fertilizer-dispensing strategy for fish tail water by using a standard nutrient solution as the reference. To this end, an intelligent fertilizer-dispensing machine based on the dynamic intelligent fertilizer-dispensing requirements is designed. The effects of three parameters, i.e., the inlet pipe flow of fish tail water, concentration ratio, and suction flow rate, of the proposed machine on the electrical conductivity (EC) stability time and liquid-level target time of the nutrient solution are investigated using a response surface method, and a double-objective function decision model is established. Results show that the root mean square error and mean square error of the nutrient- and reference-solution EC values based on the configuration of the machine are less than 0.07246 ms/cm. This validates the proposed nitrogen-replacement strategy for fish tail water. Moreover, the EC stability time and liquid-level target time are significantly affected by the three parameters, among which the fertilizer absorption flow rate and fish tail water inlet pipeline flow are identified as the most critical parameters. Subsequently, the optimal working parameters of the intelligent fertilizer-distribution machine are obtained using the established response surface model, as follows: the inlet pipe flow of fish tail water = 1 m 3 /h, concentration ratio = 66, and fertilizer absorption flow = 102 L/h. Thus, this study aids in realizing the efficient use of fish tail water and organic substitution of nitrogen fertilizer.