Development and Improvement of Parallel Yield Sensors for Measuring Individual Weights of Onion Bulbs

Abstract Measuring the weights of individual tubers, bulbs, and fruits on the harvester may bring progress to Precision Agriculture and enhance their market value. In order to measure the individual weights on an onion picker in situ, we developed two impact-based parallel yield sensors and carried out the experiments in different cropping years. Both sensors (the indirect sensor and the direct sensor) consisted of plural load cells and sponge rubber cushions. The indirect sensor was set near the end of the conveyor of the picker and a bounce plate consisting of an acrylic plate and polyurethane cushion was set between the indirect sensor and the conveyor. On the other hand, the direct sensor was set directly under the end of the conveyor. Calibration were first carried out off the field to relate the impulses received by the sensor to the individual weights of the bulbs, resulted in the standard error of 26.3g and 16.1g, the root-mean squared relative error of 8.8% and 12.2% for the indirect sensor and the direct sensor, respectively. As the validation, field measurements were carried out. The bulbs were harvested in 12 and 26 containers (20kg each) for the indirect sensor and the direct sensor, respectively. After the harvest, we weighed all the bulbs in each container manually and performed the Kolmogorov-Smirnov tests of the distributions of the actual and the estimated weights at the significance level of 5 %. Using the indirect sensor, the similarity between the actual and estimated weight distributions was accepted for 9 of 12 containers. The difference in the estimated number of bulbs was about ±3 per container, caused mainly by the same bulb colliding twice or by no bulb contacting the sensor at all. On the other hand, using the direct sensor, the similarity of the distributions was accepted for 23 of 26 containers. The overestimation of the number of bulbs was a maximum of 40 bulbs per container containing about 100 bulbs.