Dynamic Finger Gaits via Pivoting and Adapting Contact Forces

For over three decades, finger gaiting has remained largely a subject for theoretical inquiries. Successful execution of a sequence of finger gaits does not simply reduce to planning collision-free paths for the involved fingers. A major issue is how to move the gaiting finger without losing the finger contacts with the object, which will most likely undergo a motion as the contact forces need to be adapted during the gait. This paper focuses on a single finger gait executed on a tool by an anthropomorphic hand driven by an arm. To improve stability, the tool's tip is leveraged as a pivot on the supporting plane. The gait consists of three stages: removal, during which the contact force on the gaiting finger gradually decreases to zero; relocation, during which the finger follows a pre-planned path (relative to the moving object) to establish a new contact; and addition, during which the contact force on the relocated finger increases to some desired level. Hybrid position/impedance control employs reference finger forces that satisfy the friction cone constraints and are dynamically consistent with the object's motion, which in turn provides reference poses for the fingertips to maintain their contacts during the gait. Finger gaits have been demonstrated on a kitchen knife and a screwdriver with an Adept SCARA robot and a Shadow Dexterous Hand.

I. Introduction

Until today, no algorithms existed for a robotic hand to achieve power grasps on naturally resting tools (or objects for that matter). Several issues surrounding successful grasp execution have often been overlooked in research. First, part of a tool's handle may not be accessible because the tool often lies on a supporting surface instead of standing on it or being hanged in the air. Consequently, some pre-grasp operations are needed. Second, finger gaits need to be carried out to achieve the desired relative pose of the hand to the handle. This requires planning of the motions of the tool and hand along with the finger contact forces under dynamics and contact constraints. The resulting position and force trajectories can then be used as profiles for control and execution. Third, primitive operations, such as gaits and pivots, are vital for preventing loss of finger contact.