Development of a Trigger-On Indicator for a Weed Sensing Spray Unit

Abstract

Intermittent spray systems that automatically spray only when weeds are present have been commercially available since 1992 and can significantly reduce herbicide usage by 50 to 90% as compared to broadcast applications (2). The Weedseeker (NTech Industries, Ukiah, Calif.) is one such system that utilizes optics to detect the presence of weeds. The system works by exploiting the fact that chlorophyll selectively absorbs red wavebands of light and reflects near-infrared light (1). Each sensor unit has its own built-in light source and a sensor that detects the light reflected back to the unit over a 12-inch-wide field of view. When the sensor detects an increase in the ratio of near-infrared to red light above the base threshold level, a solenoid valve is activated to spray the weed. Although the sensor unit reliably detects the presence of green plants (2), a major drawback with the current design is that it doesn't provide any feedback to the user when one of the spray units is actively spraying. This is problematic, particularly on wide-boom, multi-sensor-unit sprayers since each sensing unit is calibrated independently in the field prior to spraying from the reflectance of the ground surface beneath it whenever the calibration button is pushed. If one of the sensing units is calibrated when the sensor is over a weed, the unit will not detect weeds of similar size or smaller when spraying. Also, because the sensing unit is sensitive to background reflectance, if one of the sensors is calibrated when it is positioned over a spot of ground that is not characteristic of the rest of the field, the unit will ghost fire and spray when weeds are not present. Either scenario is undesirable, but likely to occur since recalibration is necessary several times during the day due to changes in ambient light and field conditions. To overcome this problem, a trigger-on indicating device was developed to help the operator determine that the sensor units are operating correctly. The Weedseeker unit has a small LED on the back of the unit that illuminates whenever the unit is spraying. The device developed utilizes a phototransistor to detect when the LED is illuminated and activate a super-bright LED that is visible from the tractor cab (Fig. 1). The circuit designed includes a resistor and a NPN transistor that function to prevent low levels of light from activating the super-bright LED (Fig. 2). All electronic components are encased in a waterproof BUD box and attached to a Weedseeker sensor via Velcro and zip ties (Figs. 1 and 3). An opaque, nontranslucent flexible rubber washer is used to seal the phototransistor from ambient light and dust (Fig. 1). Power is supplied by two 1.5-V, AA batteries. Fig. 1. Trigger-on indicator assembly for Weedseeker sensor unit.