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Development and Evaluation of an Optical Sensing System for Detection of Herbicide Spray Droplets

Real time monitoring of herbicide spray droplet drift is important for crop production management and environmental protection. Existing spray droplet drift detection methods, such as water-sensitive paper and tracers of fluorescence and Rubidium chloride, are time-consuming and laborious, and the accuracies are not high in general. Also, the tracer methods indirectly quantify the spray deposition from the concentration of the tracer and may change the drift characteristics of the sprayed herbicides. In this study, a new optical sensor system was developed to directly detect the spray droplets without the need to add any tracer in the spray liquid. The system was prototyped using a single broadband programmable LED light source and a near infrared sensor containing 6 broadband spectral detectors at 610, 680, 730, 760, 810, and 860 nm to build a detection system for monitoring and analysis of herbicide spray droplet drift. A rotatory structure driven by a stepper motor in the system was created to shift the droplet capture line going under the optical sensor to measure and collect the spectral signals that reflect spray drift droplets along the line. The system prototype was tested for detection of small (Very Fine and Fine), medium (Medium), and large (Coarse) droplets within the droplet classifications of the American Society of Agricultural and Biological Engineers. Laboratory testing results indicated that the system could detect the droplets of different sizes and determine the droplet positions on the droplet capture line with 100% accuracy at the wavelength of 610 nm selected from the 6 bands to detect the droplets.


Introduction
Spray application of herbicides is an important means to control weeds in crop fields. In general, the purpose of using pesticides in crop production is to help ensure the yields of crops and to preserve the environment surrounding the crop fields at the same time. Specifically, the impact of herbicides on crop production is huge and in the United States, more than 95% of crop lands for maize, cotton, soybean and sugarbeet are treated with herbicides for weed control [1]. However, because of some uncontrollable factors, especially weather factors, herbicides can protect targeted crops but their off-target drift may also be negative to susceptible crops and the environment [2] [3]. Crops can suffer unpredictable damage when applied herbicides drift off-target close to sensitive crops [4] [5] [6]. Although the spray drift is usually very small relatively to the total amount of spray, it is still of great concern to producers. Smith et al. [7] indicated that even ultra-low herbicide drift will still have a clear negative response to the yield of cotton. The assessment of the risk of herbicide drift has been widely studied [8]. In the United States, aerial and ground applications of herbicides have been widely adopted, and problems of off-target drift are serious concerns. The spray drift is influenced by many dynamic factors, such as spray droplets generated by different system configurations and under different weather conditions, during the spray operation [6] [9] [10].
To assess the impact of spray drift, an effective method of spray droplet detection is critical [11]. Existing spray droplet detection methods include using mylar sheet and water-sensitive paper along with imaging techniques for tracer intensity, such as fluorescence [12], Rubidium chloride (RbCl) [13], and droplet sizes [14]. Spray tracers are typically mixed in the spray tank with the herbicide to measure the concentration of the tracer and indirectly obtain the amount of herbicide spray deposition and drift. However, the added tracer may change the true herbicide spray characteristics, so the insight of the herbicide spray and drift under certain conditions may be misleading. Water-sensitive paper provides a rough estimate of spray droplet sizes relying on discoloration of water on the specially made paper. These methods are laborious and time consuming.
This study was undertaken to develop an optical sensing system to directly measure spray distribution with droplet sizes without need of adding any tracer in the spray liquid, which has not done before by anybody else. The system is built with inexpensive, open-source electronic hardware and software and nearinfrared (NIR) spectroscopy.

System Prototype
The spray droplet detection system consists of a microcontroller-based circuit designed to detect and measure the distribution of herbicide (chemical) droplets deposited on a capture line string. The system includes a programmable microcontroller, multiband NIR sensor, stepper motor, and droplet capture line string, and is configured as a portable, battery-powered monitoring system. The main components are described in the following section, with a schematic diagram of the system prototype shown in Figure 1 and a listing of the components described in Table 1.

Programmable Microcontroller
The

Near Infrared Sensor
The

Droplet Capture Line
The droplet capture line string consists of commonly available nylon fishing line.
To evaluate the potential impact of the type and color of the fishing lines used, lines with four different colors (red, moss green, green, and clear) and three different diameters (0.25, 0.38, and 0.46 mm) were purchased for testing.

Assembled System Prototype
The prototype system depicted in Figure 1 was assembled for testing and evaluation. A circuit board was fabricated for mounting the electronic components and consisted of a protoboard and male and female headers. Female headers were arranged and soldered to the protoboard to mount the microcontroller and motor driver boards, which mated via male headers soldered to the two boards.
Male headers were soldered to the protoboard to connect the NIR sensor via a short cable soldered to the sensor board. A schematic of the electronic circuit is shown in Figure 2.
The microcontroller was programmed to manage all sensor, stepper motor, and data storage functions. When electrical power was supplied to the system, the microcontroller program began execution by first running a setup routine, which configured input/output pins, initialized the NIR sensor and motor driver,

System Testing
The

Results
For system testing, the LED light was set to 99 for the Color Rendering Index

Effect of Droplet Capture Line Colors
LED light is a mixture of many colors for white light. The LED light is reflected by the droplet deposited and adhered on the different colors of the droplet capture fishing line after spraying, so the light intensity of specific wavelengths will be affected. In the study, a test was conducted to investigate whether the effect of line color on the spectrum was significant. For this test, fishing lines of 4 different colors and 3 different cross-sectional diameters as described above were tested. The test indicated that the red fishing line is the most sensitive in 610 nm (R), then 680 nm (S), wavebands, and others insignificant regardless of diameters ( Figure 3). Following testing, red fishing line (ZS1412, Red Lightnin, Zebco, Inc., OK, USA) was used in the system.

Droplet Detection
For calibration of droplet size, 9 droplets of three different particle sizes were placed on the fishing line and the position coordinates of the droplets on the  Figure 4 shows the detection of the 9 droplets at the 6 wavelengths of the NIR sensor, especially at 610 nm and 680 nm with significant responses.
Each curve in Figure 4 is the average of 11 replicated tests. With repeated testing, multiple depositions of the herbicide would contaminate the fishing line,

Conclusion
In this study, an optical spray droplet detection system prototype was developed without the need of adding any tracer in the herbicide spray tank mix. The system prototype was developed with inexpensive open-source electronics and software and tested with droplets of different sizes within ASABE categories. The system reliably detected varied sized droplets, especially M and C droplets with large enough areas of the droplet reflection surface on the droplet capture fishing line. With the results of this study, the next step will be to build and test a multiple droplet capture line system prototype to detect herbicide drift in the field.