## 3 Research Status of Fertilizer Discharge Devices
In an experimental study, urea fertilizer (equivalent diameter 2.05 mm, density 0.948 g/cm\({}^{3}\)) was selected, the length, angle and width of the inclined mouth were taken as the experimental factors, and the flow rate and coefficient of variation were taken as the indices. The results showed that when the rotation speed was 60 r/min, the variation coefficient of the amount of fertilizer discharged by the inclined spiral fertilizer discharger was 13.59%, and the fertilizer discharge performance was better. [PERSON] et al. [37] designed an inclined slot-type fertilizer discharge device, as shown in Figure 3.1 Through a theoretical analysis and model calculations of the fertilizer filling stage and fertilizer discharge stage of the fertilizer discharge device, the optimal parameter combination for fertilizer discharge was determined. In view of the lack of in-depth research on the related theory and analytical models of fertilizer dispensers, [PERSON] et al. [38] established a motion model of fertilizer particles in fertilizer dispensers. The quadratic regression orthogonal rotation combination test was used to determine the optimal horizontal inclination of fertilizer dispensers, and it was found that the angle is 30.4\({}^{\circ}\), the cone disc speed is 130 r/min, and the radial deflection angle of the push  
Figure 3.2 Force analysis before the slipping of the granular fertilizer. Figure 3.3 Test rig to optimize pin-roller parameters. 1. Hopper. 2. Metering device frame. 3. Left and right outer caps. 4. Left and right inner caps. 5. Lower flap 6. Bolt adjuster 7. Hexagonal roller shaft 8. Pin-roller 9. Inclined top flap. plate is 3.2\({}^{\circ}\). [PERSON] et al. [39] designed a screw-propelled fertilizer applicator which uses two sets of opposite spirals symmetrically arranged on the same axis and fertilizes two rows of crops at the same time. A test showed that the relative error of single-rotation displacement was 1.6% and the rotation speed and displacement were linear. To sum up, at present, the fertilizer discharge devices studied and designed at home and abroad mainly include outer groove wheel types, air delivery types, centrifugal types and spiral types, among which the outer groove wheel-type and spiral-type fertilizer discharge devices are the most widely used [40; 41; 42; 43; 44; 45]. The outer groove wheel-type fertilizer discharge device has a simple structure, easy operation and low price, and it can meet the needs of large fertilizer discharge [46; 47]. There are various types of grooved wheels in outer groove wheel-type fertilizer discharges: the straight grooved type, the inclined grooved type and the staggered grooved type [48; 49; 50; 51]. The traditional straight grooved pulley has a simple structure, but it has problems relating to broken fertilization bars and pulsation [52]. The interlaced straight grooved wheel type adopts the interlaced form of bilateral tooth grooves to discharge fertilizer, thus avoiding the discontinuous and pulsating phenomenon of fertilizer discharge caused by the grooves and groove teeth of the straight grooved wheel [53]. The inclined grooved wheel type can promote the continuous discharge of fertilizer particles through the continuous rotation of its spiral, which increases the uniformity of fertilization [54]. A spiral fertilizer discharge device has a simple structure and convenient operation. It is suitable for granular fertilizer with good fluidity. The linear adjustment of fertilizer discharge can be realized by changing the rotation speed of the fertilizer discharge wheel, which improves the accuracy of fertilizer discharge and results in good fertilizer discharge performance [55]. Spiral fertilizer discharge devices belong to various types, including the spiral propulsion type, the spiral combination type, the inclined spiral type, and the staggered symmetrical spiral groove type.