Design and evaluation of end effectors for a vacuum-based robotic apple harvester

Authors: Lu, Renfu; DICKINSON, NATHAN - Michigan State University; LAMMERS, KYLE - Michigan State University; ZHANG, KAIXIANG - Michigan State University; CHU, PENGYU - Michigan State University; LI, ZHAOJIAN - Michigan State University

Submitted to: Journal of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/10/2022
Publication Date: 6/17/2022
Citation: Lu, R., Dickinson, N., Lammers, K., Zhang, K., Chu, P., Li, Z. 2022. Design and evaluation of end effectors for a vacuum-based robotic apple harvester. Journal of the ASABE. 65(5):963-974. https://doi.org/10.13031/ja.14970.
DOI: https://doi.org/10.13031/ja.14970

Interpretive Summary: The end effector of a harvesting robot, like a human hand, is responsible for gripping and detaching fruit from trees. Conventional harvesting robots use active end effectors that require locomotive mechanisms and thus are complex and bulky in design. We recently developed a vacuum-based apple harvesting robot using a twist-and-pull picking method, which greatly simplifies the design of the end effector for gripping and detaching fruit. However, a preliminary orchard study showed that the original end effector has failed to achieve acceptable picking performance, because it could not conform to fruit of varying sizes/shapes and thus result in air leakage, reducing suction forces needed to detach fruit. To improve the robot’s picking performance, two common apple picking methods, i.e., straight pulling and twisting, were first evaluated, using a force gauge and a specially built torque measuring device, in orchard for ‘Gala’, ‘Golden Delicious’ and ‘Red Delicious’ apples to provide basic information and guidance for the design of new end effectors. Three new end effectors, made of soft silicone and with different geometries, denoted as ‘Straight’, ‘Bellow’, and ‘Curved’, were designed and fabricated. Laboratory and field experiments were performed to evaluate the performance of the three new end effectors compared to the original end effector (the control). The field straight pulling and twisting experiments showed that the critical pulling force needed to pick 95% of apples was determined to be 28.3 N, while the critical torque for the twist picking method was found to be 0.257 N-m, corresponding to the critical suction or pulling force of 21 N. This confirmed that the twisting method is a preferred mode of fruit picking, because it requires less force and would reduce potential damage to fruit and branches. The three new end effectors performed significantly better than the original non-conformable end effector based on multiple performance metrics including vacuum pressure, overall picking success rate, success rate by the robot’s twisting actions, and fruit attachment orientation to the end effector. The ‘Straight’ end effector overall had best performance among the three new end effectors; it achieved 87% picking success rate, 65% success rate by twisting actions alone, and 89% success rate when fruit attached the end effector by their cheeks. With further improvements, the new ‘Straight’ end effector should meet the picking requirement for the new vacuum-based robotic apple harvester.

Technical Abstract: The end effector plays a critical role in fruit picking by a robotic fruit harvesting system. A newly developed vacuum-based robotic apple harvesting system, using the twist-and-pull fruit picking method, has shown promise in picking fruit from clusters and navigating through the tree canopies. However, the original end effector has failed to achieve acceptable picking performance because it was unable to conform to fruit of different sizes and thus could not generate sufficient suction forces needed to detach fruit. This research was therefore aimed at developing new end effectors to greatly enhance the robot’s fruit picking performance. Field manual pulling and twisting experiments for three varieties of apples were conducted, and the critical pulling or suction force and twisting torque needed to detach 95% of apples were determined to be 28.3 N and 0.257 N-m (equivalent to a pulling force of 21.0 N for the current robot’s configuration), respectively. Three new silicone-based end effectors of different geometries (denoted as “Straight”, “Bellow” and “Curved”) were designed and fabricated, and they were evaluated in lab and field experiments. Results showed that the three new end effectors performed significantly better than the original, unconformable end effector based on multiple performance metrics, including vacuum pressure, overall picking success rate, picking rate by the rotation mechanism, and fruit attachment orientation. The “Straight” end effector performed consistently better than the other two new end effectors; it had 87% overall picking success rate, 65% success rate by the rotation mechanism alone, and 89% success rate when the middle section or cheek of fruit attached to the end effector. With further improvements, the “Straight” end effector should meet the apple picking performance requirement for the new vacuum-based robotic harvesting system.