Probe-to-Grasp Manipulation Using Self-Sensing Pneumatic Variable-Stiffness Joints

2026-03-29

AI Summary

This study addresses the challenge of unstable or damaging grasps on deformable objects caused by spatially varying stiffness. To this end, the authors propose an integrated exploration-and-grasping framework employing a soft-rigid hybrid two-finger gripper equipped with self-sensing pneumatic variable-stiffness joints. By monitoring internal chamber pressure in real time, the system estimates contact normal forces and, combined with finger closure displacement, infers the local relative stiffness of the object to autonomously select optimal grasp locations. A key innovation lies in achieving highly integrated, low-cost stiffness-aware grasping without requiring additional sensors. Experimental results validate the self-sensing model across varying bending angles and pneumatic pressures, and demonstrate successful stable, non-damaging manipulation of real fruits based on their stiffness distribution.

Abstract

Grasping deformable objects with varying stiffness remains a significant challenge in robotics. Estimating the local stiffness of a target object is important for determining an optimal grasp pose that enables stable pickup without damaging the object. This paper presents a probe-to-grasp manipulation framework for estimating the relative stiffness of objects using a passive soft-rigid two-finger hybrid gripper equipped with self-sensing pneumatic variable-stiffness joints. Each finger of the gripper consists of two rigid links connected by a soft pneumatic ring placed at the joint, enabling both compliant interaction and controllable joint stiffness via internal pressurization. By measuring the pressure inside the pneumatic ring, we can estimate the interaction force during contact. Building on this, we propose a practical probing strategy to infer relative object stiffness by correlating the estimated normal force with known gripper closing displacement. We validate the self-sensing model through stiffness characterization experiments across bending angles and pressure ranges, and demonstrate stiffness-aware probing-and-grasping in real-life applications: selecting grasp locations on fruits with spatially varying stiffness. The proposed system offers a minimal, low-cost sensing approach for stiffness-aware soft manipulation while retaining probing and grasping capability.

Problem

• deformable objects
• variable stiffness
• grasp planning
• stiffness estimation
• soft robotics

Innovation

• self-sensing
• pneumatic variable-stiffness joints
• stiffness estimation
• soft-rigid hybrid gripper
• probe-to-grasp manipulation