In modern agriculture, the persistent challenge of weeds and pests significantly compromises crop yield and quality. Traditional pest control methods, such as manual or ground-based spraying, are often limited by terrain variations and crop diversity, leading to poor adaptability. Consequently, crop spraying drones, or spraying UAVs, have gained widespread adoption due to their flexibility and efficiency. However, conventional spraying UAVs frequently suffer from liquid drift, where pesticide droplets disperse beyond the target area, causing damage to adjacent crops and environmental contamination. To address this, we have developed a crop spraying drone that integrates high-voltage electrostatic technology. This system generates a powerful electrostatic field between the nozzle and the spray, imparting an electric charge to the liquid. As a result, the emitted droplets adhere uniformly to all parts of the crop, minimizing bounce and runoff. This innovation not only reduces pesticide waste but also mitigates environmental pollution. Additionally, the drone is equipped with high-precision millimeter-wave radar for precise obstacle avoidance, enhancing operational safety and efficiency. By automating spraying tasks, this crop spraying drone lowers labor demands and costs, contributing to sustainable agricultural practices.
The global research landscape for crop spraying drones reveals both advancements and challenges. Domestically, supportive policies, such as subsidies for agricultural machinery, have spurred adoption, with spraying UAVs accounting for approximately 18% of the market in regions like Xinjiang and Eastern China. Despite progress, issues like short battery life, limited payload capacity, and liquid drift persist, particularly under windy conditions that exacerbate off-target drift. Internationally, spraying UAVs have achieved higher levels of precision, incorporating real-time crop monitoring, autonomous path planning, and advanced navigation systems. These drones often carry multiple sensors for variable-rate spraying and image analysis. However, challenges remain in balancing operational efficiency with environmental impact, reducing manufacturing and operational costs, and increasing penetration in diverse agricultural sectors. Overall, both domestic and international efforts face hurdles that necessitate further innovation in spraying UAV technology.
The structural design of our crop spraying drone incorporates a拱形-inspired framework, which efficiently distributes external forces during flight, enhancing load-bearing capacity and stability. This design, combined with modular construction, simplifies assembly and maintenance, allowing for rapid module replacement in case of failures. The drone employs a hexacopter configuration, offering superior stability, maneuverability, and resistance to external disturbances compared to quadcopters. While octocopters may provide greater payload capacity, the hexacopter optimizes energy efficiency, ensuring prolonged operation in complex environments. Key components include integrated GPS/RTK positioning systems, sensors like LiDAR and inertial measurement units, and intelligent path-planning algorithms. These elements enable autonomous navigation, real-time environmental perception, and adaptive flight adjustments, ensuring safe and efficient spraying operations without human intervention.
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Millimeter-wave radar is a critical component of our spraying UAV, enabling precise obstacle detection and avoidance in varied agricultural settings. Operating in the 30–300 GHz frequency range, with wavelengths of 1–10 mm, this radar emits modulated signals—typically linear frequency-modulated continuous waves (FMCW). When these signals encounter obstacles, reflected echoes are captured and compared to the transmitted signals to calculate distance. By analyzing frequency shifts, the radar determines relative velocities of objects, identifying whether they are approaching or receding. Multiple antennas facilitate three-dimensional spatial detection, providing comprehensive data on obstacle position, direction, and speed. A typical millimeter-wave radar system consists of a synthesizer generating signals split into two paths: one transmitted via an antenna and the other reserved as a reference. The reflected signal mixes with the reference in a mixer, producing an intermediate frequency (IF) signal. This IF signal is filtered, digitized, and processed for real-time decision-making. The relationship between signal amplitude and time can be described mathematically. For instance, the transmitted signal $s_t(t)$ and received signal $s_r(t)$ are modeled as:
$$s_t(t) = A \cos(2\pi f_c t + \pi \beta t^2)$$
$$s_r(t) = A’ \cos(2\pi f_c (t – \tau) + \pi \beta (t – \tau)^2)$$
where $A$ is amplitude, $f_c$ is carrier frequency, $\beta$ is chirp rate, and $\tau$ is time delay. The IF signal $s_{IF}(t)$ is derived as:
$$s_{IF}(t) = s_t(t) \cdot s_r(t) \propto \cos(2\pi f_{IF} t)$$
with $f_{IF} = \beta \tau$. The distance $d$ to the obstacle is then calculated as:
$$d = \frac{c \cdot \tau}{2}$$
where $c$ is the speed of light. This enables the spraying UAV to dynamically adjust its flight path, ensuring accurate spraying while avoiding collisions.
Electrostatic spray technology is central to minimizing drift in our crop spraying drone. The process begins with hydraulic atomization, where liquid pesticide is pumped from a tank through a conduit to the nozzle. As the flow cross-section narrows, pressure and velocity increase, ejecting the liquid as a sheet that breaks into filaments and droplets. Atomization overcomes surface tension and viscous forces, with further refinement by aerodynamic forces. Droplet size is a key factor in drift reduction, influenced by nozzle design, environmental conditions, and liquid properties. The volume median diameter (VMD) and number median diameter (NMD) are critical metrics; a VMD/NMD ratio close to 1 indicates uniform atomization. Based on international standards, droplet sizes are classified as follows:
| Chinese Classification | VMD (μm) | WHO Classification | VMD (μm) |
|---|---|---|---|
| Aerosol | <50 | Smoke | <15 |
| Medium Mist | 50–100 | Fine Aerosol | <25 |
| Fine Spray | 100–200 | Coarse Aerosol | 25–50 |
| Medium Spray | 201–400 | Mist | 50–100 |
| Coarse Spray | >400 | Fine Spray | 100–200 |
| Medium Spray | 200–300 | ||
| Coarse Spray | >300 |
Research indicates that droplets smaller than 100 μm are prone to drift, with some studies highlighting risks below 50 μm. Larger droplets reduce drift but must balance evaporation losses. For instance, under conditions like temperatures above 25°C and low humidity, smaller droplets evaporate rapidly, leading to secondary drift. The electrostatic charging of droplets employs induction, contact, or corona methods. In our spraying UAV, induction charging is used, where a high-voltage electrostatic generator connects to electrodes, creating a field that imparts a unipolar charge to the liquid film. As the film ruptures, droplets carry the charge, enhancing adhesion to crops. The charge $q$ on a droplet can be approximated by:
$$q = k \cdot E \cdot r^2$$
where $k$ is a constant, $E$ is the electric field strength, and $r$ is the droplet radius. This ensures that droplets are attracted to plant surfaces, reducing rebound and runoff.
Nozzle selection is crucial for optimizing spray distribution and minimizing drift in a crop spraying drone. We compared fan and cone nozzles, finding that fan nozzles produce a uniform flat spray pattern with better horizontal distribution at heights of 30–80 cm. Cone nozzles show improved uniformity at greater heights but result in longer drift distances under various wind speeds. Thus, our spraying UAV incorporates symmetrically arranged fan electrostatic nozzles. The electrostatic spray system includes a power source, spray tank, diaphragm pump, nozzle mast, connecting pipes, electrostatic nozzles, and a high-voltage electrostatic generator. The power supply drives the pump to transport pesticide to the nozzles, which are connected to the generator’s anode and cathode via wires. This design separates positive and negative charges in the system, enhancing safety. The fan nozzle emits a wide, dispersed扇形 spray, with parallel electrodes inducing charges via electrostatic induction. The nozzle structure, made of insulating materials, minimizes droplet adhesion to electrodes and reduces airflow interference. The system’s efficiency is quantified by the deposition rate $D_r$, given by:
$$D_r = \frac{m_d}{m_t} \times 100\%$$
where $m_d$ is the mass deposited on the target and $m_t$ is the total mass sprayed. Experimental data show that electrostatic spraying increases $D_r$ by approximately 30% compared to conventional methods.
In prototype testing, the crop spraying drone demonstrated exceptional performance. With dimensions of 0.55 m in length, 0.5 m in width, and 0.4 m in height, and a total weight of 22.5 kg (including a 10 kg pesticide payload), the spraying UAV executed a predefined 20 m path followed by a serpentine trajectory. It oscillated laterally by 1 m, advancing 0.5 m per cycle to ensure full coverage. Over 15 minutes, the drone maintained stable operation, with electrostatic nozzles producing uniformly charged droplets that adhered to both sides of crop leaves. Upon completing the route, the spraying UAV returned linearly, with a landing deviation of less than 1.5 m, achieving high precision in navigation and spraying. These results validate the integration of electrostatic spray technology and millimeter-wave radar in enhancing the reliability and accuracy of crop spraying drones.
The adoption of electrostatic spray-based crop spraying drones represents a significant advancement in precision agriculture. By improving droplet adhesion and reducing drift, this spraying UAV increases pesticide deposition efficiency by around 30%, cuts chemical usage by approximately 20%, and minimizes environmental impact. The combination of electrostatic technology and advanced radar systems enables sustainable farming practices, promoting green and efficient agricultural development. As spraying UAVs evolve, they will play a pivotal role in smart agriculture, driving future innovations in crop protection and resource management. This design not only addresses current limitations but also sets the stage for broader applications of crop spraying drones in diverse agricultural contexts.