Development and test of an autonomous air-assisted sprayer based on single hanging track for solar greenhouse

Highlights

• A novel autonomous air-assisted sprayer based on single hanging track was developed and tested.

• Movement direction of the boom strongly affectedthe axial spray distribution.

• The pattern of spray during the boom rising obtained more uniform distribution.

• Arrangement of nozzles and structure of airflow also affected spray distribution.

Abstract

Manual spray operations of conventional pesticides in the greenhouse are highly dependent on labor and have been associated with potential health risks. To overcome the constraints the narrow crop space and the complexity of auxiliary facilities impose on ground autonomous sprayers, this study developed an autonomous air-assisted sprayer based on a single hanging track for solar greenhouses. The spray distribution from the boom and spray patterns were evaluated. Laboratory tests were conducted to investigate the airflow distribution and static spray distribution of the boom. The sprayer was tested on cucumber crops in a solar greenhouse to evaluate the performances of two spray patterns: spraying during boom rising-traveling (P1) and during traveling-falling (P2). The coefficient of variation of the static axial spray distribution was 33% based on liquid collection. The coefficient of variation of axial and radial spray distribution of P1 and P2 (based on coverage rate) were 37.2% and 58.3%, as well as 46.3% and 69.4%, respectively. This confirmed that the motion direction of the boom strongly influenced the spray distribution. The structure of the airflow also affected the radial spray distribution. Moreover, the axial spray distributions of both spray patterns had high correlation coefficients of 0.921 and 0.889 with a static distribution detected in the laboratory. Optimizing the configuration of the nozzles and airflow of the boom is expected to further improve the uniformity of the radial and axial spray distributions. This autonomous air-assisted sprayer based on single hanging track provides a predictable solution for plant protection in solar greenhouses.

Introduction

According to statistical data from the Agricultural Mechanization Management Department of the Ministry of Agriculture and Rural Affairs (MOARA), the planted greenhouse area for vegetables in China reached about 4 million hectares in 2018. However, the application of mechanization in the greenhouse is still at a low level, especially in the field of plant protection (Zhang, 2019).

Diseases and pests are occur extremely easily in the microclimatic environment of a solar greenhouse with its high temperature and humidity, which severely threatens the yield and quality of cultivated vegetables. Cho (1999) suggested that the effective spraying of pesticides can control pests and diseases, thus saving 30–35% of the total yield losses. Because of the structural complexity and the conditions of auxiliary facilities of a solar greenhouse, the development and promotion of automated spray equipment is extremely slow. Backpack sprayers are still the most utilized plant protection equipment for greenhouses in China, which are characterized by their simple structure, flexible application, and low cost. Similar applications of backpack sprayers are also common in Europe (Cerruto et al., 2009; Goossens et al., 2004).

Many technologies such as electrostatic spraying (Mamidi et al., 2013) and variable spraying (Liu et al., 2012) are commonly utilized to improve the performances of backpack sprayers. Several researchers focused on the improvement of spraying performances by optimizing operation parameters (Rincon et al., 2017; Sánchez-Hermosilla et al., 2013). Nevertheless, an inevitable problem is that the operation efficiency is insufficient, and the stability of the operation quality mainly depends on the level of technical expertise of the operator (Derksen et al., 2008; Derksen, 2010).

Compared to a handheld spray gun, a vertical boom sprayer with hand-push type performs satisfactorily with regard to deposition quality and operation efficiency (Sanchez-Hermosilla et al., 2012). This advantage may benefit from an optimized configuration of multiple nozzles in the boom (Nuyttens et al., 2004a). Foqué (2012a) studied the performance of the vertical spray boom and recommended the addition of an air-assistance function for satisfactory penetration.

Air assistance is considered to be the most effective way to improve penetrability and uniformity of the sprayed compounds (Li et al., 2018; Llop et al., 2015, 2016). Furthermore, Llop et al. (2015) suggested that lower wind speed (14 m s⁻¹) can achieve higher deposition quality, especially on the upper and lower surfaces of leaves. The possible reason is that the high airflow velocity increases the swing amplitude of the leaf, thereby reducing the retention of droplets on the leaf surface. This conclusion is consistent with results reported by Lee et al. (2000) and Pascuzzi et al. (2017).

Labor dependency is a common challenge for all manual sprayers. Coupled with poor air circulation and insufficient protection awareness, operators may be subject to potential health risks from pesticides (Cang et al., 2018; Cerruto et al., 2008; Nuyttens et al., 2004b; Rincon et al., 2018).

A navigation-based autonomous greenhouse sprayer (NAGS) was developed to replace human labor (i.e., hand spraying). Most NAGSs are composed of a vertical boom system and an inter-row mobile platform. The techniques of autonomous detection and navigation are the keys to developing a mobile platform. Detecting preset guidance signals is the easiest way to navigate. Several NAGS systems have been successfully developed based on detecting electromagnetic signals (Liu et al., 2011), ultrasonic signals (Lee et al., 2015), and attitude signals overhead (Gat et al., 2016). Autonomous mapping and navigation was applied for the development of NAGSs, which was based on various autonomous detection technologies, such as ultrasonic sensing (Mandow et al., 1996), LIDAR ranging (Singh et al., 2017), monocular or binocular machine vision (Li et al., 2009; Singh et al., 2005), laser-based guiding (Sanchez-Hermosilla et al., 2013), and sonar detection (González et al., 2009). These studies have verified the feasibility of using autonomous sprayers for greenhouses based on navigation technology. However, their high cost is an important reason that NAGSs are not widely accepted and applied (Sánchez-Hermosilla et al., 2013). In addition, Chinese growers prefer a dense planting pattern, which caused the inter-row space of crops to be too small to satisfy cruising NAGSs.

A track-based autonomous greenhouse sprayer (TAGS) is a compromise solution. This sprayer travels autonomously on a track laid on the sidewalk near the wall and sprays perpendicular to the direction of travel. Qi et al. designed an air-assisted variable rate mist sprayer and evaluated it with spray-on artificial crops in the laboratory (Gao et al., 2017; Musiu et al., 2019; Qi et al., 2016). Li (2019) reported a two-degree-of-freedom air-assisted TAGS, for which a genetic algorithm-based control strategy was developed that plans the motion (both swaying and pitching) of the air cylinder to adjust spray direction. Field trials showed that this control strategy improves the uniformity of spray distribution on the top canopy of the crop. However, TAGSs spraying from the sidewalk of a greenhouse inherits the native disadvantage of spraying from above the crop, which often suffers from insufficient penetration of the bottom of the crop canopy, especially at the boundary of the spray range (Derksen, 2010; Foqué et al., 2012a). Therefore, the pursuit of a uniform spray distribution across the entire range is the goal for the design and optimization of long-range air-assisted sprayers for greenhouses.

Water Sensitive Paper (WSP) is commonly used to evaluate spray deposition and coverage. Several researchers suggested that because of droplet stacking, WSP has an insufficient ability to characterize the deposition and coverage for operation at high-volume rates while performing very well at low-rate treatments (Salyani et al., 2013; Salyani and Fox, 1999; Zhu et al., 2011). In addition, Patel (2017) confirmed that WSP analysis identified the same spray distribution trend as the traditional liquid collecting method. In the present study, the sprayer operated at a low application rate, and WSP was used to evaluate spray coverage.

The objective of this study was to validate the performances of a newly developed autonomous air-assisted sprayer based on single hanging track. A short two-way spray boom with two centrifugal fans and joint stacking nozzles is equipped on the sprayer, which can spray over and drop into the inter-row of the crops through the auxiliary grid. Both the air velocity distribution and static axial spray distribution were tested in the laboratory. Two different spray patterns of the sprayer were tested in the solar greenhouse on cucumber crops. The specific objectives of this study were: (a) to evaluate the operation quality of the sprayer; (b) to evaluate the effects of spray patterns on spray distribution; (c) to guide the optimization of the sprayer to further improve operation quality.