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1. Nanjing Research Institute for Agricultural Mechanization, Ministry of Agriculture, Nanjing 210014, China; 2.Haila’er Agricultural Settlement(Group) Limited Haila’er Bureau of Farm & Pasture, Hulunbeir 021008, China

AbstractFirstly, this paper introduced the special requirements of pesticide application technical equipment for the facility agriculture. Secondly, it analyzed domestic situation of pesticide application technical equipment for the facility agriculture. Then, it discussed existing controlled droplet pesticide sprayer from operating principle, structural characteristics, and innovation points, and pointed out existing research findings and future research modules of the controlled droplet pesticide sprayer. Finally, in line with main research points and development direction of controlled droplet pesticide sprayer for the facility agriculture, it came up with pertinent recommendations and measures. It is intended to provide references for the application and development of pesticide application technical equipment for the facility agriculture in China.

Key wordsFacility agriculture, Pesticide application technology, Controlled droplet, Centrifugal atomization

1 Introduction

Facility agriculture is a remarkable mark of the development of modern agriculture. To promote the development of facility agriculture is an important task for the realization of agricultural modernization. In recent years, both the central and local governments show great concern, and China’s facility agriculture has entered a stage of rapid development. However, due to little attention to the supporting machinery and equipment in the greenhouse, the overall development level of China’s pesticide application technical equipment for facility agriculture is not high, and the mechanization level is relatively low. Within the greenhouse, the space is confined, the sunshine is not sufficient, both the temperature and humidity are high, in addition to few natural enemies, the resistance of crops to diseases is low, crops cultivated in greenhouse are more vulnerable to those cultivated in the open air[1]. Products of facility agriculture are generally required to meet the pollution-free or higher quality standards of agricultural products. In the production process, the control of pollution-free agricultural products lies mainly in the use of pesticide, including the control of pesticide types and the control of safety interval. For plant protection, at the same time of preventing and controlling insect pests and diseases, it is required to rapidly and effectively control insect pests and diseases, prevent production loss, ensure the product quality, and ensure the product safety. Due to its special ecological environment and characteristics of continuous production and high-quality products throughout the year, the facility agriculture imposes more strict requirements on environment, and requires more stringent control over agricultural non-point source pollution that focuses on pesticides. Therefore, the prevention and control of insect pests and diseases for crops cultivated in greenhouse need higher frequency and the difficulty is higher, and it needs specialized prevention and control equipment. The controlled droplet pesticide application technology is a low-volume spraying technology rapidly developed in developed countries; centrifugal atomization is an internationally recognized the most suitable method for obtaining uniform and fine droplets, and its application can not only achieve the purpose of effectively preventing and controlling agricultural insect pests and diseases, but also reduce the pollution to the surrounding environment[2-3]. This technology can control the droplet diameter through the change of the rotational speed of the centrifugal atomizer, to meet the requirements of the droplet for different operating objects and meteorological conditions. At the same time of increasing the attachment rate and uniform distribution of pesticide droplets on target crops, this technology greatly reduces the number of ineffective droplets, so as to increase the pesticide utilization efficiency and accordingly save water and pesticide.

2 General situation of pesticide application technical equipment for facility agriculture

In facility agriculture, the types of pesticide application technical equipment mainly include knapsack manual (electric) sprayer, electrostatic sprayer, knapsack power sprayer, stretcher (trolley) type power sprayer and room temperature cold aerosol sprayer.

2.1 Knapsack manual (electric) sprayer

Knapsack sprayer is a traditional pesticide sprayer with largest number of use in China. Characterized by low cost, simple operation and wide adaptability, it is mainly suitable for wheat, maize, cotton, rice, fruit trees, greenhouse, grape, tea trees, flowers and gardening and horticultural crops[9], as shown in Fig.1. However, such equipment has low application efficiency, difficult for large-scale prevention and control, the atomization quality of liquid drug is poor, the rate of attachment of liquid drug to plants is low, the spray distribution is not uniform, and overall prevention and control effect is not ideal.

2.2 Knapsack power sprayer and stretcher (trolley) type power sprayer

Such sprayers adopt high-pressure spray method, the atomization effect is excellent, the liquid drug droplets have high penetration in the crop canopy, so the prevention and control effect of insect pests and diseases are significant and the operating efficiency is high. Knapsack power sprayer and stretcher (trolley) type power sprayer are mainly applied in prevention and control of insects and diseases in rice, orchards, flower nursery seedlings, vegetable base, and greenhouses. Due to the large diameter of the sprayed droplets and low rate of attachment to crop plants, much pesticide is lost on the ground; during spraying, water consumption is high; thus, after spraying, the humidity in greenhouse will increase; at the same time of preventing and controlling insect pests and diseases, it creates a condition for another insect pest and disease, leading to a vicious circle; besides, pesticide application operators spray the pesticide in a confined space, the labor intensity is high, and easily leads to poisoning and cause harm to human health. YS-100T stretcher (trolley) power sprayer is shown in Fig.2.

2.3 Cold aerosol sprayer

The cold aerosol sprayer (Fig.3) was introduced from Japan during the 9th Five-Year Plan period, and popularized and applied in prevention and control of insect pests and diseases in the facility agriculture. The average diameter of the sprayed droplets is about 20 μm, and the flow rate of spraying is in the range of 50-70 mL/min. Characterized by fine droplets and low-volume spray, this sprayer effectively raises the operating quality, consumes little water, does not increase the humidity in the greenhouse greatly, avoids inducing secondary insect pests and diseases, so the effect of prevention and control of insect pests and diseases within confined greenhouse is significant[13,17,24]. During the pesticide application, operators do not need entering the greenhouse, so it provides higher guarantee for safety of operators; however, compared with knapsack power sprayer and stretcher (trolley) power sprayer, the operating efficiency of cold aerosol sprayer is low. In addition, due to the small pore of the spraying component of the cold aerosol sprayer, and the gas-liquid two-phase flow atomization mechanism, the requirements are high for pesticide formulation and filtration condition of the atomization system. When the filtration of pipeline system for spraying component is not up to standard, it is easy to lead to blockage of spraying port, then it may reduce the activity of biological pesticide due to small spraying pore[23].

2.4 Trolley centrifugal atomization sprayer

In view of the special requirements of prevention and control of insect pests and diseases for greenhouse crops in facility agriculture, taking centrifugal atomization technology as the core technology, and integrating the air-assisted low volume spraying technology and electronic remote control technology, Nanjing Research Institute for Agricultural Mechanization of Ministry of Agriculture developed the controlled droplet pesticide sprayer with new centrifugal atomization spraying component for prevention and control of insect pests and diseases for greenhouse crops in facility agriculture. This pesticide application equipment is based on the theory of biological optimum droplet size (BODS). According to this theory, organisms have the selective capture ability for different fineness of the droplets, or specifically, a droplet size range most easy for them to capture. This sprayer can effectively control the droplet diameter according to different requirements of crop varieties, prevention and control objects, pesticide formulation, and operating environment in greenhouse, to obtain better prevention and control effect, and reduce the pesticide consumption[7].

3 Controlled droplet pesticide sprayer for facility agriculture

The controlled droplet pesticide application technology is a low-volume spraying technology rapidly developed in developed countries; centrifugal atomization is an internationally recognized the most suitable method for obtaining uniform and fine droplets. Researches have shown that its application can not only achieve the purpose of effectively preventing and controlling agricultural insect pests and diseases, but also reduce the pollution to the surrounding environment[5,7]. This technology can control the droplet diameter through the change of the rotational speed of the centrifugal atomizer, to meet the requirements of the droplet for different operating objects and meteorological conditions. It is a controlled droplet pesticide application technology with internationally recognized uniform droplet size and narrow range of droplet spectrum, the droplet diameter after centrifugal atomization is only 50-100 μm. There are mainly two types of centrifugal atomizers: rotary disc type and rotary cage type[25]. When the rotary disc type centrifugal atomizer works, the liquid is delivered to the disc center; in the action of centrifugal force, it is thrown to the disc edge and forms the liquid film, and forms droplets when it approaches to or disc edge; through changing the rotational speed of the atomizer, it is able to effectively control the droplet diameter. When the rotary cage centrifugal atomizer works, the liquid flows out from the central hole; in the action of centrifugal force generated from high speed rotation of the cage, the drug liquid is thrown onto the cage, then it is first roughly screened by the first layer of the screen mesh, and forms fine droplet after screening of the second layer of the screen mesh, and the droplet size is determined jointly by the working speed, diameter and mesh size of the case[19, 21-22]. Centrifugal atomizer is widely applied in insect pest prevention and control and health epidemic prevention of crops. On average, it can save water more than 50% and save pesticide about 15%-20%, so it is suitable for prevention and control of insect pests and diseases of crops in the middle and later period[19]. In recent years, centrifugal atomization technology has been applied to the field of ultra-low-volume aircraft spray. The remote-control type variable air-assisted controlled droplet sprayer is shown in Fig.4.

Fig.3 LHW cold aerosol sprayer

Fig.4 Remote-control type variable air-assisted controlled droplet sprayer

Fig.5 Schematic diagram for controlled droplet pesticide sprayer

Note: 1. Centrifugal atomizer; 2. Centrifugal atomization spraying component drive motor; 3. Fan impeller end drive motor; 4. Fan impeller; 5. Drive motor support; 6. Air duct; 7. Air duct holder; 8. Variable frequency control cabinet; 9. Liquid supply system; 10. Frame.

3.1 Components of controlled droplet pesticide sprayer

The components of the controlled droplet pesticide sprayer mainly include signal sending and receiving device, control command actuating unit, drive motor, centrifugal atomization spraying compo-nent, low-pressure small flow submersible pump, pesticide box and pipeline system, drug liquid filtration and mixing system, axial-flow fan, and three-dimensional reversing mechanism, as shown in Fig.5. Among these, the centrifugal atomization spraying component is the core component of the sprayer.

Fig.6 Centrifugal atomization spraying component

3.2 Operating principle of centrifugal atomization spraying component

During operation, the drug liquid supply system delivers the liquid to the disc center, and in the action of centrifugal force generated from high speed of rotary disc, the liquid is thrown to the disc edge and forms the liquid film; when it approaches to or reaches the disc edge, it is split into liquid silk, and thrown out in dot form, collides with air and forms droplets, and droplets collide with atomization teeth and break into finer droplets. The centrifugal atomization spraying component is shown in Fig.6.

3.2.1Features of centrifugal atomizer. The large diameter and multi-bending structure of centrifugal atomizing rotary disc brings centrifugal atomization spraying component with large inertial force and spraying surface. Even at low rotational speed, it can form uniform liquid film, which is favorable for splitting and atomization of drug liquid. The multi-tooth structure for fixing the atomization disc makes the atomized droplets further collide and break into finer droplets, so as to effectively improve the atomization quality of the centrifugal atomizer.

3.2.2Effects of structural parameters of centrifugal atomizer on distribution of droplet spectrum. Some studies have shown that large diameter is the precondition for realizing low speed atomization; multi-bending structure increases the spraying surface of the atomizer, so it is helpful for improving the atomization quality at low speed; the atomization teeth can realize a secondary atomization of droplets that leave the disc, so as to obtain finer droplets and realize low flow spraying. The field to be further studied include: effects of existence, shape, quantity, and distribution of atomization teeth on the quality of secondary atomization, namely, effects on distribution of droplet spectrum (droplet size and droplet spectrum width) of collision type low speed centrifugal atomizer.

3.3 Innovation points of controlled droplet sprayer

3.3.1Precise and controllable droplet diameter. The working speed of this centrifugal atomization spraying component can be steplessly adjusted within a certain range, to reach the precise controllable average droplet diameter within the range of 50-200 μm. This changes droplet spectrum width and unadjustable droplet diameter of traditional pesticide sprayer. Besides, traditional pesticide sprayer can not satisfy requirements of different crops, prevention and control objects, pesticide formulation, and operating environment within the greenhouse for the droplet diameter. At the same time of increasing the attachment rate and distribution uniformity of liquid droplet on the target crops, this controlled droplet sprayer fully considers the theory of biological optimum droplet size (BODS), reduces quantity of ineffective droplets to a great extent, increases the pesticide utilization efficiency, reduces the pesticide consumption, and also effectively solves the problem of environmental pollution in the greenhouse and pesticide residue of agricultural products[11].

3.3.2Rapid, precise and controllable flow rate. Through the research on the technology of small flow hydraulic control, signal transmission, acquisition, analysis and processing, as well as the design and development of tube clamping mechanism, electric mechanism, and control circuit, it developed the electronic control tube clamping flow valve, tackled technical problems of difficult to make rapid and precise adjustment in small flow condition, and realized the real-time and precise adjustment of liquid flow rate.

3.3.3Three-dimensional remote uniform air supply. Three-dimensional reversing mechanism provides a delivery carrier for uniform distribution of second atomization of liquid droplets along the spraying range, realizing three-dimensional adjustment of spraying direction and air-assisted supply system, to satisfy spraying demands of different greenhouse widths, different crops, and crop growth periods, and effectively improving the uniform distribution of droplets on the target crops.

3.3.4Automatic control of operating status and parameters. With the aid of mechanical and electrical integrated automatic control technology such as single-chip technology and electronic remote control technology, this sprayer realizes automatic control of operating status or parameters of centrifugal atomization spraying component, variable liquid supply system, and three-dimensional air-assisted reversing system. The application of electronic remote control technology realizes the remote control of pesticide spraying operation and man-machine separation, greatly reduces the labor intensity, and provides more safe guarantee for operators through preventing them getting in touch with pesticides and inhaling pesticide fog for a long time.

4 Recommendations for the development of controlled droplet pesticide sprayer

The plant protection equipment in facility agriculture of China is various and popularization and utilization are in varying degrees. At present, main problems include unstable utilization effect, and the general use, adaptability and economy are poor, and ordinary farmers generally consider the cost and do not use the newly developed high technology pesticide application equipment[5-6,8]. For the front researchers, it is also necessary to strengthen researches of controlled droplet pesticide application technology in the facility agriculture, seek breakthroughs and innovations from the ideas and principles, and also stresses the researches of general use, adaptability, and economy of the equipment, to obtain ideal pesticide application performance. In addition, it is recommended to grasp the development trend both at home and abroad, learn related high and new technologies, and take actual situation of domestic plant protection as the basis, to develop efficient pesticide application equipment.

4.1 Strengthening basic researches

Combined with the BODS theory, it is recommended to undertake researches of controlled droplet pesticide application technology, study the droplet secondary breaking mechanism and factors influencing droplet size, obtain the fitting correlation of optimized atomization atomizer design, and study the secondary atomization mechanism of atomization teeth collision between drug liquid droplets and fixed tooth plate. Basic researches include the effects of existence, shape, quantity, and distribution of atomization teeth on the quality of secondary atomization, namely, effects on distribution of droplet spectrum (droplet size and droplet spectrum width) of collision type low speed centrifugal atomizer, to provide necessary references for obtaining the optimal spraying effect of centrifugal atomizer.

4.2 Main points urgently to be solved

4.2.1Improving the general use and adaptability of the equipment. The prevention and control of insect pests and diseases are varied in the south and north of China. Due to the difference in crop planting modes and climate, many pesticide application equipments are only suitable for crops of certain region, and the pesticide application equipment is greatly influenced by crop planting scale, and many new pesticide application equipments are less general[15-16]. Take the centrifugal atomizer, the core component of controlled droplet pesticide application equipment as an example, it is recommended to improve the adaptability of centrifugal atomizer, make it suit many kinds of crops. This plays an important role in promoting whole-process mechanization of prevention and control of crop pests and diseases.

4.2.2Increasing the government subsidies for purchase of efficient, low-pollution agricultural plant protection equipment. Facility agriculture is a hallmark of the development of modern agriculture. To promote the development of facility agriculture is an important task for the realization of agricultural modernization. In recent years, both the central and local governments show great concern, and China’s facility agriculture has entered a stage of rapid development. Since the period of the12th Five-Year Plan, the fiscal subsidy policy on agricultural machinery has greatly promoted scientific research and development, production, and utilization of agricultural machinery in China[12]. The period of the 13th Five-Year Plan is a crucial period for the modernization of agriculture. It is required to eliminate as soon as possible old plant protection machinery that is inferior in quality and inefficient and seriously pollutes the environment, and it is recommended to encourage farmers to purchase and use new and high-quality plant protection machinery. Besides, it is required to include the energy conservation and emission reduction into the financial subsidy system, and increase the subsidy for machinery with higher utilization efficiency, better prevention and control effect, and better conforming to safety and environmental protection requirements.

4.2.3Improving regulations about pesticide application technology. In accordance with the technical requirements of energy conservation and emission reduction, it is required to constantly improve the standard system of agricultural plant protection equipment; plant protection and agricultural machinery extension department should take the energy conservation and emission reduction as key economic and technical indicators, and include them into the experimental assessment and demonstration and extension of plant protection machinery[10,20].

4.2.4Strengthening training of new and large agricultural machinery technicians. Relevant departments should vigorously carry out the training on the application of pesticide application equipment and technology in the facility agriculture, improve the safe pesticide application level of agricultural technicians and farmers, guide them to change the concept of outdated pesticide application, and help farmers to grasp pesticide utilization skills and methods, so as to really bring into play the functions of cost saving, efficiency increasing, energy conservation and emission reduction.

4.3 Combining new technologies

With the advance of science and technology, and continuous development of integrated mechanical and electrical technology, some old pesticide application technology and equipment will be gradually replaced by new technology and equipment. In the simplification of pesticide application equipment, design objectives should be placed in changing the complex mechanical drive structure, power transmission mode, ordinary materials and backward production process, and extending the service life and reducing labor costs. With the further development of related technologies both at home and abroad, it is also necessary to solve the problem of how to apply new technologies into actual production as soon as possible[14,18].

4.4 Future research trend

With the continuous growth of the demands for the prevention and control of insect pests, diseases and weeds in crops in China, as well as the contradiction between the pesticides and the environment, ecology, quality of agricultural products and the national health, the market demands for high efficiency, low volume, low pollution and precise application equipment will also become larger and larger, and its industrialization prospects are very broad[14].

5 Conclusions

In the process of the development of facility agriculture in China, the whole society including levels of government cares more about the construction of infrastructure such as greenhouse system, but due to little attention to the supporting machinery and equipment in the greenhouse, the overall development level of China’s pesticide application technical equipment for facility agriculture is not high, and the mechanization level is relatively low. Research and development of the controlled droplet pesticide application equipment will provide the facility agriculture of China with new advanced, safe, and reliable pesticide application equipment suitable for requirements of prevention and control of insect pests and diseases of greenhouse crops. However, the mechanized development of pesticide application equipment for the facility agriculture is an arduous work. It needs pulling of actual demands of the facility agriculture, promotion of related policies, and constant improvement of machinery design and manufacturing process level. With these, China’s pesticide application equipment for the facility agriculture will get onto the road of rapid and sound development.