Detection of 3-D objects with a 2-D laser scanning sensor for greenhouse spray applications

Precise measurement of plant dimensions can provide essential information for variable-rate spray application technologies to discharge the amounts of chemicals actually needed by the plants. The accuracy of a 270° radial laser scanning sensor combined with a specially-designed plant surface mapping algorithm was evaluated in detection of complex-shaped object surfaces and sizes. Data acquisition and three-dimensional (3-D) image construction were supported with the specially-designed algorithm. The objects in the test were toy balls with a pink smooth surface, light brown rectangular cardboard boxes, black and red texture surfaced basketballs, white smooth cylinders, and two different sized artificial plants. Test variables included four different detection heights (0.25, 0.5, 0.75 and 1.0 m) above each object, five sensor travel speeds (1.6, 2.4, 3.2, 4.0 and 4.8 km h−1), and 8-15 horizontal distances to the sensor ranging from 0 to 3.5 m. Horizontal distances of the objects to the laser sensor influenced the accuracy significantly, and the influence became weaker when the objects were closer to each other. The color and finish of object surfaces did not apparently affect the sensor detection accuracy. The average root mean square error (RMSE) and coefficient of variation (CV) in the laser sensor travel direction and object height direction varied slightly with detection heights, travel speeds, and object positions. The highest RMSE and CV were 83 mm and 50.9% in the horizontal direction, 41 mm and 15.2% in the travel direction, and 16 mm and 14.0% in the height direction, respectively. These tests demonstrated the potential adaptation of the laser scanning sensor and the specially-designed algorithm to measure complex-shaped targets for future development of automatic spray systems to achieve variable-rate functions in greenhouse applications.