Location: Application Technology Research Unit
2013 Annual Report
Objective 2: Develop computer control systems for laser-guided precision sprayers to accommodate various tree canopy characteristics.
After deposition and evaporation, residue patterns of 500 µm sessile droplets that incorporated four classes of adjuvants on five different waxy plants were investigated. Droplets were generated with a single-droplet generator and deposited on target leaves placed in an environmentally-controlled chamber at 60% relative humidity and 25 ºC ambient temperature. Adjuvants tested were two types of oil-based crop oil concentrate and modified vegetable oil, a nonionic surfactant, and a type of mixture oil surfactant blend . Water-only droplets were also tested for comparative purposes. The five waxy plants were difficult-to-wet and had a water contact angle greater than 90º. The water-only droplets did not spread at all and formed extremely small wetted areas on the leaf surface. The addition of an adjuvant to the spray solution significantly reduced the contact angle and increased the wetted area, but the change/improvements varied with the plant species and the adjuvant class. In general, the adjuvants enhanced the droplet spread and maintained the droplet evaporation time on the waxy leaf surfaces. After evaporation, the residues formed patterns of “coffee rings”. Droplets with oil-based adjuvants had more uniform residual distribution in the deposition patterns than droplets with the surfactant adjuvant. Results of this study demonstrated that selection of the appropriate class of adjuvants significantly improved deposit formation on waxy leaves, leading to more effectiveness of pesticides.
A high speed laser scanner was investigated to detect gaps between trees, and measure tree characteristics. Interfaced program between laser and computer was developed to determine the tree size and shape. Droplet size distributions from spray nozzles were measured. Laboratory and field tests were conducted to verify the accuracy of spray controller timing and modulation. A precision air-assisted sprayer implementing an automatic variable rate control system is in the process of development for ornamental nurseries and fruit trees.
An intelligent air-assisted sprayer implementing a high speed laser scanning sensor was developed to vary spray output of each individual nozzle to match target tree needs in real time. Each nozzle was coupled with a pulse width modulation solenoid valve to achieve variable rates based on the occurrence and canopy characteristics of the target, such as height, width and foliage density. A unique density algorithm was developed to calculate foliage density by mapping the surface roughness of the canopy during the spray application. A back pressure control unit was integrated into the system to minimize the pressure fluctuation due to frequent changes in nozzle flow rates. Delay time between the sensor detection of the canopy and the nozzle activation was determined with a high-speed video camera. Laboratory tests demonstrated that the design criteria of the experimental sprayer were acceptable for performing variable rate functions.
The intelligent air-assisted sprayer with variable flow rate of individual nozzles was tested for ornamental nurseries and fruit trees. The sprayer was developed with a conventional air-assisted orchard sprayer by implementing a laser scanner to detect canopy characteristics, five-port air-assisted nozzles coupled with pulse width modulation solenoid valves, and an automatic flow rate controller to minimize pressure fluctuation. Spray performances were compared for the new sprayer with the same sprayer without the intelligent control and a conventional air blast sprayer in an orchard at three different growing stages. Measurements were made for spray deposition and coverage inside canopies, losses on the ground and beyond target trees, and airborne drift downwind from the target trees. Compared to conventional sprayers, the variable-rate sprayer produced relatively uniform spray coverage and deposition inside canopies, and reduced spray volume by 47% to 73% with significantly less off-target losses on the ground, through gaps between trees, and in the air.
An electronic flow rate control system with microprocessors and pulse width modulation controlled solenoid valves was designed to manipulate the output of spray nozzles independently to match tree structures which would be detected with a wide-range laser scanning sensor. The signals to control nozzle flow rates were generated by the microprocessors based on laser sensor detections. Multi-channel driver and protection circuits for activating solenoid valves were developed to modulate variable-rate outputs in real time. An embedded computer along with a touch screen was used to process control algorithms and to fulfill communications between the operator and the control system. Laboratory tests demonstrated that the flow rate control system was able to achieve linear nozzle outputs with the duty cycle of the solenoid valves.
A variable-rate air-assisted sprayer implementing laser scanning technology was evaluated in an apple orchard by quantifying spray deposition at three different growing stages with three sprayer treatments: the new variable-rate sprayer, the same sprayer without the variable-rate function and a conventional air blast sprayer. Their spray coverage and deposits inside canopies were measured and compared with water sensitive papers and nylon screens. Compared to conventional constant-rate sprayers, the new variable-rate sprayer only consumed 27% to 53% of the spray mixture while it still achieved adequate spray coverage inside canopies. Also, the spray deposition from the new sprayer was very consistent regardless of the canopy growth stages. The new sprayer was able to apply appropriate amount of pesticides based on tree canopy characteristics such as tree height, width, volume, foliage density and occurrence of trees, and thus increased spray efficiency and improved spray accuracy, resulting in reduced spray costs and potential environmental pollutions.
System delay times due to the laser-sensor data buffer, software operation, and hydraulic-mechanical component response were determined for a control system used for a LiDAR-guided air-assisted variable-rate sprayer. The delay times were used to determine how far the laser sensor should be mounted ahead of spray nozzles to ensure sufficient time for the sprayer to discharge desired amounts of sprays to the target in real time. A photoelectric detection unit was designed to measure the lag times between when the target was detected by the laser sensor and when the liquid was discharged from the nozzle at various sprayer travel speeds. An algorithm was also developed to compensate the system delay times for the spray outputs to match the detected targets at different travel speeds in real time.
A 270° radial range laser scanning sensor was tested for its scanning accuracy to detect tree canopy profiles. Signals from the laser sensor and a ground speed sensor were processed with an embedded computer along with a touch screen mounted on a tractor. An algorithm for data acquisition and 3-dimentioal (3-D) canopy image reconstruction were designed with C++ language and MATLAB software. The system accuracy was tested under indoor laboratory conditions with four regular-shape objects and two artificial trees and outdoor conditions with three field trees. Statistical analyses demonstrated the sensor measurements of the objects were not significantly different from those of the actual measurements. The mean RMS errors were not significant for scanning distance of 2 to 5 m and sensor travel speeds of 2 to 5 mph. Both indoor and outdoor tests verified that the wide-range laser sensor had the capability to accurately measure different sizes and shapes of objects. This confirmation offers the potential for the sensor to be integrated into spraying systems and provide variable-rate functions for tree crop applications.
This research addressed critical elements in ARS parent project Objective #1 "Develop precision sprayers that can continuously match canopy characteristics to deliver agrichemicals and bio-products accurately to nursery and fruit crops".