Research progress on precision control methods of air-assisted spraying in orchards

Abstract: Orchard air-assisted spraying technology and equipment are incorporating intelligent technologies to achieve higher precision. Liquid application rate and air supply rate are the two manipulated outputs of an orchard sprayer control system, which should be simultaneously controlled precisely at all times. For a target-oriented precision spraying, which aims to reduce off-target deposition or drift to keep environmental pollution to a tolerable limit, an orchard tree detection is indispensable. In this paper, the research progress analysis focuses on three methodologies: orchard target detection method, spraying dose control method and airflow control method. The orchard target detection method provides characteristic information of a target including tree position, canopy profile, canopy volume, canopy internal structure, canopy density, and canopy pest/disease level. Orchard tree positions are obtained by detecting tree canopies using ultrasonic sensors or optical sensors at different heights, or by sensing tree trunks using photoelectric sensors. Tree canopy profile and internal structure, which are used to estimate the volume and density of the canopy, can be detected based on ultrasonic sensing, LIDAR and machine vision. Spectroscopy, machine vision and electronic nose technologies are applied to evaluate the canopy pest/disease levels. Flow rate control methods through the sprayer plumbing and nozzles were reviewed. The plumbing flow rate can be adjusted by controlling spray pressure with electric regulating valves using hysteresis switch control, PID control, fuzzy control and artificial neural network. Regulating the injecting flow rate is another effective way to control pesticide application rate using an online mixing system in a sprayer. Major breakthroughs were obtained in plumbing design, pesticide mixing methods and strategies for plumbing flow rate control. There are also a large number of achievements in individual nozzle flow rate control based on PWM (pulse-width modulation) technology. Control systems of plumbing and individual nozzle flow rate regulation are commercialized, and some sprayers with these systems are available on the market. The theoretical principle of the air speed and air volume demand, which is the basic information for airflow control in orchard air-assisted spraying, is summarized. Air and droplet field modeling, airflow adjustment methods, and equipments are discussed. CFD (computational fluid dynamics) simulations combined with laboratory/orchard tests using special airflow and droplet deposition measuring systems become a viable way to establish spray spatial dynamic models. To adjust airflow of the air-assisted sprayer, the three key factors, including air direction, air speed and air volume, should be focused on. Air direction control mainly adopts the rotation of sprayer bellows and angle adjustment of air deflectors in the bellows, and the air speed and air volume are controlled mainly by changing the air inlet area, air outlet area and fan speed of an air-assisted sprayer. Finally, the obstacles and challenges in the current research related to the methods of orchard tree detection, spraying dose control and the airflow control are discussed. The obstacles and challenges include precision sensing methods exploration for canopy density and pest/disease level detection, establishment of universal models of orchard target air speed and air volume demand, investigation of airflow modeling and control base on the airflow demand, and integration of orchard precision spraying systems. The future development of precision control methods for air-assisted spraying in orchards was presented: 1) Orchard canopy density and pest/disease level online detection method is becoming a new research topic; 2) The study on the basic theoretical principles of air speed and air volume demand, airflow modeling, and control methods are an urgent need; 3) With the new advanced technology, scientific research institutions and companies will have great opportunities to optimize the design and development of new spraying dose and airflow control systems and integrate it with orchard precision sprayers.