Deposition experiment of air-blast variable-rate spray with constant-pressure control in orchard

The traditional sprayers adopt large-area uniform pesticide application, resulting in a low effective utilization of pesticide and a harmful effect to the natural environment. Variable-rate spray is the key point to precision chemical application. However, it is inevitable that the spray pressure sharply fluctuates during variable-rate spray, which will definitely influence the spray characteristics, such as spray droplet sizes, spray angles, spray droplet velocities, etc., and reduce the efficiency of pesticide applications. Therefore, the research on how to keep the spray pressure constant during the process of variable-rate spray has practical significance to Precision pesticide Applications. In order to achieve the stability of spray pressure for variable-rate spray, a sprayer with constant-pressure control was set up using a closed-loop PID controller of constant-pressure water-supply which employed the techniques of single-phase AC Chopper Variable-Voltage Control and PID Feedback Regulation. Using hollow-cone nozzles, the spray volume was changed by adjusting spray pressure, frequency, and duty cycle of electromagnetic valve switching. An air-assisted variable-rate sprayer with the constant pressure control was set up and its deposition feature and spray-field simulation were studied in the research. The conclusions are as follows: (1) By the factorial experiment of spray droplet deposition, combination of influence factors in the test which the smallest coefficient of variation is: the travelling speed of 1.1 m / s, the frequency 5, and the duty cycle 0.5. (2) Set the pressure of the constant pressure controller at 0.3MPa, the spray distributions on the front side, the back side, and the shadow of canopy were compared in two conditions of the open nozzle unilateral and bilateral operations. The results indicated that the spray pressure kept constant with the different number of open nozzles and the regional distribution of spray deposition was consistent. (3) The simulation of spray field with CFD showed that the wind speed in the middle region was obviously larger than that in the upper and lower regions of spray field and the wind speed in upper region was larger than that in the lower region. Therefore, the spray droplets in the middle of the spray field are apt to penetrate into the center of the thickest part of the canopy so as to increase the deposition rate in central and lower parts in the tree. The more, the droplets in the upper spray field can easily be sent to the top of the fruit trees so as to improve the uniformity and coverage of the spray droplet distribution.