X-Ray Based Stem Detection in an Automatic Tomato Weeding System

A real-time, non-contact stem detection system was developed for crop plant location sensing in automatic weed control in transplanted tomato fields. A tractor-mounted, portable x-ray source projected an x-ray beam perpendicular to the crop row and parallel to the soil surface. In operation, the plant's main stem absorbs x-ray energy, decreasing the voltage output (signal) from the detector, allowing robust main stem detection even in the case where the main stem is occluded by weed or crop foliage. This signal was used to automatically control the operation of a pair of in-row mechanical weed knives. Minimizing the source to detector distance allowed for differences in signal strength between stems and background between 180 and 300 mV (60% to 100% drop in signal strength) at low x-ray energy and current levels (25 keV, 7 mA), which is a significant advantage for safety reasons. Background noise levels were ±30 mV, corresponding to a range of 10% to 16.7% of signal strength. The detector utilized a linear array of photodiodes aligned perpendicular to the soil surface. This configuration helped differentiate branches, which are angled and block only some of the photodiodes, from the main stem which has a similar vertical alignment as the array and hence blocks most or all photodiodes. A field trial was conducted at the standard time of first cultivation in a 17.5-m section of row in an organic transplanted tomato field, containing 43 tomato transplants. At a travel speed of 1.6 km/h, the detection system correctly identified 90.7% of main stems of the tomato plants. Four tomato plants (9.3%) were not detected because they had fallen over and passed below the detector. There were no false positive detections.