Improvement of mobile citrus fruit grading machine

Abstract

A mobile grading machine was earlier developed to establish precision citrus fruit production. This machine could travel in citrus orchards and measure appearance and internal qualities of fruit harvested by humans on-site. In addition, it could record harvesting date and fruit locations. It consisted of a mobile mechanism, a differential global positioning system, a fruit conveyor system, two machine vision systems (one for fruit inspection, the other for trees monitoring), a sugar content measurement system using a near-infrared (NIR) spectrometer, and a personal computer for control and database. The color, size, and sugar content of each fruit and leaf color of each tree were measured by the machine vision devices and the NIR inspection system. As fruit features were extracted based on image analysis by the machine vision system, it was observed that the result of size had a correlation with actual citrus size and the result of color was accurately compared with human eyes. However, it was considered that the lighting device positions should be changed for higher quality images, because the edges of the fruit were dark due to radiation angles. This paper reports on moves of the lighting device position and to obtain lower light radiation angles so that the estimate of fruit size could get higher correlation with the actual fruit size. Furthermore, the earlier results of sugar content measurement had a low correlation with the actual brix values because the sensing head of NIR inspection system was pressed to each fruit from the top. The pressing direction was changed to from the side to obtain higher performance of the system. The improvements caused higher correlations of the sensor outputs to the fruit size and sugar content.