Fruit detachment force of multiple varieties kiwifruit with different fruit-stem angles for designing universal robotic picking end-effector

Fruit detachment force (FDF) between fruit and stem, is one of the important physical parameters for designing fruit picking robot. Orchards planted with multiple varieties kiwifruit generally exist in China because of their rich variety resources. FDFs at different fruit-stem angles (FSA) for multiple varieties kiwifruit make it difficult for the fruit picking robot to maintain optimal working conditions for different varieties. Determining the FDF of a single variety is not suitable for developing a universal end-effector that enables fruit picking of multiple varieties kiwifruit. Therefore, it is necessary to measure the FDF of multiple varieties kiwifruit at different FSA. In this study, a special fixture was developed based on external characteristics of fruit, which was installed on the texture analyzer to form a platform for measuring the FDF with different FSA. Fruits of different variety with stems hand-picked, i.e. 'Hayward', 'Xuxiang', 'Huayou', 'Cuixiang' and 'Qinmei', were measured for FDF at FSAs of 180°, 160°, 140°, 120°, 100°, 80°, and 60°. Computer-controlled texture analyzer led screw drives the lifting bracket to rise for realizing the detachment of the stem and fruit. FSAs corresponding to the minimum FDF of both 'Qinmei' and 'Cuixiang' were 80°, while those of other three varieties were 60°. The FDF at the FSA of 180° was smaller compared of the FSA of 160° for 'Hayward' and 'Cuixiang'. Additionally, the stem breakage occurred when the FDF of 'Xuxiang' was measured at the FSAs of 180° or 160°. These results have potential to provide guidance for designing a universal robotic picking end-effector for multiple varieties kiwifruit.