Development of a yield sensor for measuring individual weights of onion bulbs

Site-specific measurement of the weight of individual tubers, bulbs, and fruits on the harvester may enhance their market value and represent a significant advance for precision agriculture. An impact-based yield sensor was developed, consisting of two load cells, an acrylic plate and a polyurethane cushion, to determine the weight of individual onion bulbs through the impulse received by the sensor. A simple proportional relation was used to calibrate the sensitivity of the sensor, the precision of which was evaluated by standard and relative errors at calibration and validation. The experimental variables investigated under static conditions were the arrangement of the bulbs on the conveyor, the thickness of the cushion on the plate, and the clearance between the output end of the conveyor and the impact plate. The arrangement of the bulbs affected neither the precision nor the sensitivity of the sensor. With or without a 10 mm thick cushion on the plate, the precision decreased at the higher clearance. Practical precision was obtained (a relative error at validation of less than 2.0%) with a 30 mm thick cushion regardless of the clearance.