HPA Gripper: An Underactuated Gripper for Parallel Pinching and Adaptive Enveloping

Abstract

This paper presents the Hart-based Parallelpinching Adaptive gripper, termed the HPA gripper, a compact underactuated gripper intended for robotic grasping tasks that require both controlled fingertip motion and passive adaptation to object geometry. Existing underactuated grippers provide simple and robust grasping solutions, but it remains challenging to integrate parallel pinching and adaptive enveloping in a single compact mechanism without additional actuators, sensors, or complex control logic. The proposed design addresses this challenge by coupling a modified Hart mechanism with a parallelogram constraint and a lost-motion transmission. During the approach stage, the modified Hart mechanism guides the distal endpoint along a near-horizontal path, while the parallelogram constraint maintains a suitable fingertip posture for parallel contact. After object contact, the lost-motion transmission redirects the remaining input motion and triggers inward rotation of the distal phalanx, enabling adaptive enveloping. Kinematic analysis was conducted to locate the exact straight-line point and evaluate the straightness of the practical fingertip endpoint. A prototype was fabricated to verify the mechanical feasibility of the concept through representative demonstrations, including small-range parallel pinching, wide-range parallel pinching, and adaptive enveloping. The demonstrations indicate that the HPA gripper can realize dual-mode grasping through passive mechanical coordination, offering a simple and compact solution for low-cost robotic manipulation tasks with limited sensing and control resources.