*2.1. Working Principle of Liquid Manure Spreader*

The liquid manure spreader used in this study was towed using a tractor, and the liquid manure was spread to the field ground through 12 spray pipes. The operating breadth of the fluidity manure spout pipe was 6 m and the driving velocity of the tractor used for towing varied from 1 to 2.5 m/s. An important part of the system for controlling the change in liquid manure flow was the electric proportional valve, which was installed at the front end of the fertilizer distributor. The distributor used in this study was a mass-production product with a uniform distribution. We did not need to consider its distribution uniformity and blockage but instead only needed to consider the open–shut sizes of the electric proportional gate controlled via computer to achieve accurate control of the liquid manure distribution. Based on the requirements for the accurate application of liquid manure, the principle of system flow control was based on the demand for liquid manure in the field. Turning on the solenoid valve was transformed by vehicle speed. The application amount was monitored using a fluid meter. Then, the data were coupled back to the regulator. The control formed a closed–cycle coupled-back uniform machine by analyzing and comparing the actual time runoff, the magnitude of fertilizer called for by the current plot, and the current vehicle speed, achieving an accurate control of the current plot spraying amount. The response accuracy and stability of the closed–cycle coupled-back uniformity machinery were the main influences of the exact fluidity manure spray control system in this study.
