This study investigated a pregrasp strategy for hooking and caging ring-shaped objects. Through-hole features enable the robot hand to hook an object with holes by inserting its finger into one of the holes. Compared to directly grasping the ring, an inserting motion is more convenient to allow the uncertainty of positioning errors and avoid collisions between the hand and the object. Instead of recognizing the exact shape of the object, we only detected its ring-shaped feature as a through-hole to be inserted and estimated its approximate center position and orientation from the point cloud of the object. The estimated geometric properties enabled the approaching motion of the robotic gripper to complete insertion. The proposed perception and motion-planning method was demonstrated for rigid and deformable objects with holes.

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