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Recognizing and targeting weeds
The entire system is mounted on a trailer that can be towed over the field.
During the first stage, the weed must be detected. To this end, two Marlin F-201 digital cameras by Allied Vision Technologies are engaged. Both cameras are equipped with 2 megapixel sensors; one of the cameras is a color version, while the other is an infrared sensitive monochrome variety with a 780 nm IR pass filter. The cameras serve two functions: first, they localize the plants against the earth background; then, they identify the plants either as weed or agricultural crop.
In a captured ground area of 150 x 18 cm, resolution of just over 1 pixel/mm is achieved (1.08 Pixel/mm). For optimal, comparable images, the cameras are installed in a bin that is open from below, allowing five Xenon lamps to provide controlled illumination. Furthermore, a distance measurement device is mounted on one of the trailer wheels, documenting the exact position of the image on the field.
To control the image capture and analysis, Dutch developers decided on National Instruments hardware (NI PXI system with Virtex-5 FPGA) and software (NI LabView).
Intelligent image analysis accomodates naturally occurring
Image analysis must first and foremost recognize the weeds. Supporting this task, the system is adaptive. "On a mechanically planted field, the path of the furrows is a clearly defined constant: anything growing between the furrows can only be weed," explained Dr. Ard Nieuwenhuizen. Still, weeds can also grow in the furrows themselves between the cultivated plants. Therefore, this system concentrates first on plants beyond the furrow and analyzes their color and infrared properties. Having learned the characteristic features of the wild potato, the software can differentiate between plant types on the furrow as well, even when located between sugar beet plants.
"One additional challenge for an imaging system in agriculture is that the ground properties, such as water or nitrogen content, aren't homogenous in nature, which leads to variations in the color properties of plants," according to Nieuwenhuizen. To circumvent this problem, the system recalibrates anew every ten meters so that only adjacent plants are compared to each other.