JAMMit! Monolithic 3D-Printing of a Bead Jamming Soft Pneumatic Arm

TL;DR

The paper addresses achieving high stiffness in a monolithically fabricated soft robotic arm by embedding a bead-jamming mechanism along a central tendon. It introduces a tendon-driven bead column integrated into a 3D-printed bellow arm and uses COMSOL to bound the jam tension, followed by extensive ROM and stiffness experiments that reveal a clear trade-off between mobility and load-bearing. A practical switch-toggling task demonstrates that jammed configurations enable successful operation, illustrating potential for real-world manipulation. The work offers a scalable manufacturing approach for variable-stiffness soft robots with demonstrated gains in stiffness and a clear path toward closed-loop control.

Abstract

3D-printed bellow soft pneumatic arms are widely adopted for their flexible design, ease of fabrication, and large deformation capabilities. However, their low stiffness limits their real-world applications. Although several methods exist to enhance the stiffness of soft actuators, many involve complex manufacturing processes not in line with modern goals of monolithic and automated additive manufacturing. With its simplicity, bead-jamming represents a simple and effective solution to these challenges. This work introduces a method for monolithic printing of a bellow soft pneumatic arm, integrating a tendon-driven central spine of bowl-shaped beads. We experimentally characterized the arm's range of motion in both unjammed and jammed states, as well as its stiffness under various actuation and jamming conditions. As a result, we provide an optimal jamming policy as a trade-off between preserving the range of motion and maximizing stiffness. The proposed design was further demonstrated in a switch-toggling task, showing its potential for practical applications.

Figures (8)

• Figure 1: The variable stiffness soft arm. (a) The design of the soft arm enables monolithic fabrication and variable stiffness through beaded string jamming. (b) Performance of the soft arm while inflated, but without jamming enabled. (c) By engaging the integrated bead jamming mechanism, the load bearing capability of the arm drastically improves.
• Figure 2: Materials and geometry of the variable stiffness soft arm. Soft regions are printed with 40A Shore hardness, while rigid ones reach 83-86D Shore hardness. A clearance $g$ of 0.2 mm allows for the printing of the beads without them fusing together, while openings on the side walls between the bellow actuators enable easier removal of the support material during post-processing of the print.
• Figure 3: The FEM results for determining the maximum tension force: (left) the maximum volumetric von Mises stress as a function of the applied force; (right) the volumetric maximum von Mises stress distribution of an intermediate bead, shown from a cross-sectional view through the center for T=25 N which is used as the upper limit for the tension applied to the real system.
• Figure 4: The setup includes a tracking system with four cameras mounted on a $780\ \mathrm{mm} \times 700\ \mathrm{mm} \times \ 563\ \mathrm{mm}$ aluminum frame, reflective markers on the soft arm’s top and bottom plates for angle measurement, a servo for tendon tensioning.
• Figure 5: Range of motion and uniform deformation test: rotation angle results for each direction, with two pneumatic chambers actuated simultaneously at 20 kPa.