Compliant Beaded-String Jamming For Variable Stiffness Anthropomorphic Fingers

TL;DR

The paper addresses the challenge of achieving human-like dexterity with robust manipulation by introducing Compliant Joint Jamming (CJJ), a variable-stiffness mechanism that embeds passive residual compliance into anthropomorphic fingers. By integrating beaded-string jamming with compliant elements and a notch-enhanced bead geometry, the authors achieve a stiffness range of $0.48$ to $1.95$ Nm/rad ($4\times$ increase) while maintaining a human-like interphalangeal ROM of $72^{\circ}$. Experimental results include a non-linear hold-torque behavior due to bead deformation, significant stiffness changes with jamming tension, and a peg-in-hole task showing a $60\%$ higher success rate for the compliant gripper versus a rigid BSJ baseline. The work demonstrates that passive residual compliance can enhance manipulation robustness while reducing sensing and control requirements, with avenues for material/texturing improvements and modeling to predict stiffness as a function of geometry.

Abstract

Achieving human-like dexterity in robotic grippers remains an open challenge, particularly in ensuring robust manipulation in uncertain environments. Soft robotic hands try to address this by leveraging passive compliance, a characteristic that is crucial to the adaptability of the human hand, to achieve more robust manipulation while reducing reliance on high-resolution sensing and complex control. Further improvements in terms of precision and postural stability in manipulation tasks are achieved through the integration of variable stiffness mechanisms, but these tend to lack residual compliance, be bulky and have slow response times. To address these limitations, this work introduces a Compliant Joint Jamming mechanism for anthropomorphic fingers that exhibits passive residual compliance and adjustable stiffness, while achieving a range of motion in line with that of human interphalangeal joints. The stiffness range provided by the mechanism is controllable from 0.48 Nm/rad to 1.95 Nm/rad (a 4x increase). Repeatability, hysteresis and stiffness were also characterized as a function of the jamming force. To demonstrate the importance of the passive residual compliance afforded by the proposed system, a peg-in-hole task was conducted, which showed a 60% higher success rate for a gripper integrating our joint design when compared to a rigid one.

Figures (9)

• Figure 1: Synoptic view of the proposed Compliant Joint Jamming design. (a) Close-up view of the variable stiffness finger and its internal structure, alongside a three-fingered gripper. (b) Example highlighting the benefits of the passive residual compliance afforded by the Compliant Joint Jamming design. The the fingers passively adapt and allow the peg to orient itself properly without losing their grasp on it, enabling successful completion of the task despite the initial misalignment.
• Figure 2: Structure of the variable stiffness finger highlighting the main design parameters. The radius R affects the holding torque of the bead, while the length of the TPU bead elements determines the bending behavior of the joint. The effective angle determines the amount of rotation the phalanges can achieve when not jammed.
• Figure 3: Diagram illustrating how adding a notch to the bead design increases the overall maximum range of motion of the passive finger.
• Figure 4: Diagram showing the working principle of the proposed variable stiffness mechanism. When no tension is applied, the two halves of the bead are able to freely rotate relative to each other. The tension applied to the tendon translates into compression between the mating surfaces of the two halves, activating the bulk state: the static friction causes the bead to act in unit as a beam undergoing bending. The jamming tension also has the effect of altering the bead geometry, causing the it to contract axially and expand sideways, leading to an increase in its bending stiffness.
• Figure 5: Overview of the experimental setup. (a) Each test sample consists of a single VS joint with markers for visual tracking. Jamming tension is applied through a steel tendon actuated by a Dynamixel MX-106 controlled in torque, while calibrated weights are used to provide external torque on the joint being tested. (b) Free body diagram of the test sample highlighting the rotational displacement $\Delta \theta$ used in the calculations throughout the study.