Variable-rate control system for UAV-based granular fertilizer spreader

Variable-rate technology (VRT) is necessary for the site-specific application of fertilizer. VRT can be used not only to minimize the amount of fertilizer used and fertilizer pollution but also to improve work efficiency and reduce the cost of production. However, the utilization of technology has been limited to the existing ground machines, and studies on UAV-based variable-rate fertilization have rarely been reported. This paper proposed a variable-rate fertilizer control system (VRFCS) for a UAV-based granular fertilizer spreader (GFS) based on a prescription map. The VRFCS mainly consisted of a flight controller and spread controller. The control software was designed to realize variable-rate fertilization by modulating the metering apparatus with a fluted roller according to the real-time coordinates of the GFS. Laboratory and field experiments were carried out to evaluate the response characteristics and fertilization accuracy of the control system. The results as expected showed a specific relationship between the target discharge rate and rotational speed of the fluted roller motor, that is almost linear for the operational range. The control system could respond rapidly (in 0.1 s) and accurately to changes in the target discharge rates, whether in a fixed-rate or a variable-rate pattern. The error in the discharge rate between the average actual values and the target values in the fixed-rate pattern was 6.05%. During the accuracy test of the variable-rate pattern, the errors in rotational speed and the actual application amount were approximately 4.50% and 6.64%, respectively. The field experiment based on the prescription map showed an error of less than 6.07% and a high efficiency (average of 0.72 ha/h). This study demonstrated the possibility and adaptability of variable-rate fertilization by UAV-based GFSs using the developed control system.