The torque required to cut branches is an important parameter for designing a robotic end-effector for pruning apple (Malus xdomestica) trees. In this study, the branch cutting torque was measured because it is important for the future development of a robotic pruning end-effector. To measure the branch cutting torque, a force-measuring sensor was integrated with a manual shear pruner. An inertial measurement unit sensor was also used to monitor the angle between the shear blades and the branch. Field tests were conducted for ‘Fuji’, ‘Gala’, ‘Honeycrisp’, and ‘Golden Delicious’ trees, and the cutting torque was calculated for different branch diameters. The results indicated that the branch diameter is one of the most important factors influencing the pruning torque requirements for all tested cultivars. The statistical tests (0.05 significance) revealed that the pruning torque varies significantly for different branch diameters ranging from 6 to 20 mm. It was found that the cutting torque required for the ‘Honeycrisp’ branches was significantly lower than that for ‘Gala’, ‘Fuji’, and ‘Golden Delicious’ branches. ‘Gala’ branches had the highest torque requirements. To cut branches of ‘Fuji’ trees, the required cutting torque for branches placed at the cutter center was higher compared with the cutter pivot. The statistical tests indicated that the difference in required cutting torques for both branch-blade contact points was significant (0.05 level of significance). The cutting torque requirement for a 30° angle (bevel) cut was higher compared with a 0° (straight) cut for ‘Fuji’ apple trees, but the statistical analysis suggested that the difference was insignificant at a level of significance of 0.05. Comparing all test results (four cultivars and cutting settings), the highest cutting torque of 6.98 N m was observed for ‘Fuji’ branches with a diameter of 20 mm for a straight cut with the branch placed at the shear cutter center. Therefore, it is suggested that the robotic pruner should provide a comparable torque for successful cutting. The outcomes of this study are important for the selection of appropriate cutting mechanisms for the future development of a robotic pruning system.
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