Finite element prediction and experimental verification for damage on citrus fruit from robot clamping

Objective To study internal stress variations of citrus fruits clamped by working robot under different clamping conditions, and to predict and analyze clamping damages on citrus fruits.

Method The mechanical parameters of citrus peel and pulp were measured. Finite element model of each tissue structure of citrus fruit was established. The processes of clamping citrus fruits by robot with different finger planes were simulated. The verification experiments of citrus fruit clamping were performed using a custom terminal clamping platform.

Result The elastic modulus of citrus peel and pulp under vertical stress were 11.408 and 0.277 MPa respectively, and the limiting stress of citrus peel and pulp were 1.250 and 0.048 MPa respectively. The equivalent stress distribution diagrams of citrus peel and pulp were obtained in the simulation tests. The stress of peel was lower than that of pulp. Because the limiting stress of pulp was lower than that of peel, the pulp started to damage before the peel did. The stress on both peel and pulp caused by arc fingers was lower than that caused by plane fingers under the same clamping force. The peel started to damage at 23 N clamping force for the plane fingers, while the damage initiated at 45 N clamping force for the arc fingers. At the clamping force of 12, 23, 34, 45, 56 and 67 N, the pulp damage rates for the plane fingers were 0, 10%, 30%, 40%, 20% and 20% respectively higher than those for the arc fingers. The results of verification experiments using the clamping platform were consistent with the simulation outcomes.

Conclusion The clamping damage on citrus fruit can be predicted and evaluated. This study provides a basis for designing damage reducing structures in citrus picking robot.