Prismatic-Bending Transformable (PBT) Joint for a Modular, Foldable Manipulator with Enhanced Reachability and Dexterity

TL;DR

The Prismatic-Bending Transformable (PBT) Joint addresses the dexterity and modularity gap in traditional rigid manipulators by integrating prismatic and bending capabilities within a single, foldable module. Through a scissors-inspired design with a 3D direction-maintenance mechanism, it enables reconfigurable joint chains that can operate in four transformable modes and scale across large, medium, and small sizes. The work provides detailed design, kinematic/dynamic modeling, IK for multiple modes, and experimental validation of single and dual-PBT configurations, showing expanded reach and obstacle-navigation capabilities in confined spaces. The PBT approach offers a scalable, standardized SKU-based path to versatile, human-centered manipulation in dynamic environments, with future work focused on control, planning, and multi-joint integration.

Abstract

Robotic manipulators, traditionally designed with classical joint-link articulated structures, excel in industrial applications but face challenges in human-centered and general-purpose tasks requiring greater dexterity and adaptability. To address these challenges, we propose the Prismatic-Bending Transformable (PBT) Joint, a novel, scissors-inspired mechanism with directional maintenance capability that provides bending, rotation, and elongation/contraction within a single module. This design enables transformable kinematic chains that are modular, reconfigurable, and scalable for diverse tasks. We detail the mechanical design, optimization, kinematic and dynamic modeling, and experimental validation of the PBT joint, demonstrating its integration into foldable, modular robotic manipulators. The PBT joint functions as a single stock keeping unit (SKU), enabling manipulators to be constructed entirely from standardized PBT joints. It also serves as a modular extension for existing systems, such as wrist modules, streamlining design, deployment, transportation, and maintenance. Three joint sizes have been developed and tested, showcasing enhanced dexterity, reachability, and adaptability, particularly in confined and cluttered spaces. This work presents a promising approach to robotic manipulator development, providing a compact and versatile solution for operation in dynamic and constrained environments.

Figures (6)

• Figure 1: (A) The modular manipulator composed of two PBT joints and a dexterous end-effector. (B) and (C) show the PBT wrist. The PBT wrist is a small-sized PBT joint with a suction cup at its tip. (B) is performing the revolute motion. (C) demonstrates the prismatic motion.
• Figure 2: Concept of the Prismatic-Bending transformable (PBT) joint. (A) The motion illustration of the PBT joint. The first three stages are linear motion, while the third and fourth stages are revolute motion. The desired joint angles in the moving process are shown by the coordinates $S_1, S_2, and S_3$. (B) Serial combination of two PBT joints. The 'P' represents prismatic, while 'B' represents bending.
• Figure 3: 3D Direction Maintenance Mechanism. (A) Illustrates the Direction Maintenance Challenge in the PBT Joint. Using an open-loop link connection to implement the PBT joint causes the direction change at the PBT joint's end. (B) PBT Joint with 3D Direction Maintenance Mechanism: Each end of the base links is equipped with a direction maintenance structure. (C) 3D Direction Maintenance Structure: The mechanism effectively withstands overturning torque across all three axes.
• Figure 4: Structure and Assembly of the Two-Module PBT Joint Modular Manipulator (A) Partial sectional front view of the modular manipulator: 1 represents the timing belt, 2 is the servo motor driving the linear motion of the PBT joint, and 3 represents the driving gear of the PBT joint directly connected to the servo motor. (B) Partial exploded sectional view of the modular manipulator: From bottom to top are the large revolute module, large PBT module, medium revolute module, and medium PBT module. Two potential structures for folding are shown. Force singularity occurs in the structure in the second row. (C) Full view of the modular manipulator: 4 represents the servo motors driving the revolute motion of the PBT joint, 5 represents the servo motor driving the revolute motion of the medium PBT joint, and 6 represents the servo motor driving the linear motion of the medium PBT joint.
• Figure 5: Reachability and Manipulability Analysis of PBT Joints (A) Reachability set of a single PBT joint: (i) and (ii) correspond to the prismatic and bending modes, respectively. (B) Translational Manipulability Distribution in Task Space of dual-unit PBT: (i)-(iv) correspond to the PP, PB, BP, and BB modes defined in Fig. \ref{['fig:Concept']}. Comparing different motion modes with a conventional UR5 arm in an compact (C) and a clustered (D) environments. (i) Environment visualization, the dual-unit PBT and UR5 are scaled to a normalized stretched length. (ii)-(iV) show the BP mode, BB mode and UR5 workspaces. The scatter plots are generated from 30,000 randomly sampled joint configurations, excluding collisions. Extended conventional robot's workspace with PBT units (E) The reachability set in a compact environment is extended with a PBT joint mounted on a UR5. (i) visualizes the environment, while (ii) and (iii) show workspaces with and without the mounted PBT joint. In (ii), sampling density is increased in the cavity area to better capture reachability.