Optimization of spraying quality and drift risk in unmanned aerial spraying systems (UASS) based on Multi-Gradient droplet size control

Unmanned aerial spraying systems (UASS) are widely used in agriculture; however, spray drift remains a significant barrier to their broader adoption. Conventional drift-control measures—such as nozzle optimization and adjuvants—primarily act by altering droplet size. This study introduces a dynamic droplet-size control approach for UASS equipped with centrifugal nozzles to balance spray quality and drift risk. We established the relationship between nozzle rotational speed and droplet size and developed an embedded, variable droplet-size UASS. The system utilizes differential RTK to acquire real-time UAV position data, enabling dynamic adjustment of droplet size during operation. Field trials demonstrated the system’s stability and reliability: the UASS responded promptly to ground commands for droplet size changes and accurately logged the corresponding adjustment locations. Data analysis indicated that increasing droplet size markedly reduces drift volume and droplet density in the downwind drift zone. Relative to a baseline without droplet-size control, a three-stage adjustment strategy reduced the drift ratio by 89.18%, shortened the 90% drift distance to 3.36 m, and delivered the highest drift-mitigation rate. To minimize drift while maintaining effective penetration, we propose using the very fine (VF) droplet size (82.4 μm) for the first two flight paths and increasing to 300–350 μm for the third. These findings demonstrate that dynamic droplet-size adjustment via pulse-width modulation can effectively reduce drift. External factors, such as wind and terrain, continue to be influential, underscoring the need for further research to refine and optimize drift-control strategies under diverse operating conditions.