Dynamic Evaluation of a Suction Based Gripper for Fruit Picking Using a Physical Twin

We present and evaluate a novel suction-based gripper designed for fruit picking. This work is motivated by common problems observed in field trials of robotic harvest- ing: Calibration/perception errors, workspace obstacles, fruit swinging/moving when contacted, and varying stem and branch stiffnesses. The gripper consists of three suction-cups located on the palm, along with in-hand perception. To evaluate the gripper, we developed a physical proxy that approximates a realistic apple-stem-branch dynamic system. We performed 756 apple picks on the proxy with varying branch stiffness, stem strength and gripper pose (yaw, roll and offset w.r.t. the apple). Our results show that grasping performance improves when the gripper yaw w.r.t. the apple has two suction cups on the bottom of the apple and one suction cup on top. Even with ±15mm offset, at least two suction cups engaged with the apple 80% of the time, regardless of branch stiffness. Moreover, the gripper withstands ±20mm offset when it approaches the apple near its equator.