A Compliant Robotic Leg Based on Fibre Jamming

Abstract

Humans possess a remarkable ability to react to unpredictable perturbations through immediate mechanical responses, which harness the visco-elastic properties of muscles to maintain balance. Inspired by this behaviour, we propose a novel design of a robotic leg utilising fibre jammed structures as passive compliant mechanisms to achieve variable joint stiffness and damping. We developed multi-material fibre jammed tendons with tunable mechanical properties, which can be 3D printed in one-go without need for assembly. Through extensive numerical simulations and experimentation, we demonstrate the usefulness of these tendons for shock absorbance and maintaining joint stability. We investigate how they could be used effectively in a multi-joint robotic leg by evaluating the relative contribution of each tendon to the overall stiffness of the leg. Further, we showcase the potential of these jammed structures for legged locomotion, highlighting how morphological properties of the tendons can be used to enhance stability in robotic legs.

Figures (15)

• Figure 1: Our Jamming lEG, JEG, is a belt-driven robotic leg with antagonistic variable stiffness multi-material fibre jammed tendons.
• Figure 2: (a) Left: Fibre jammed tendon with concertina membrane. Right: Fibre jammed tendon with fibre layout shown for different sections along the length of the tendon. Multi-material fibres consists of shorter sections of soft shore A 30 (clear segment) and longer sections of stiff shore A 85 (blue segment) fibres. (b) Experimental setup to perform tensile tests, where tendons are are stretched up to 20 mm in their unjammed and jammed states.
• Figure 3: (A-J) Plots showing tensile force vs displacement for fibre jammed tendons of 1.5, 2.0, 2.5, and 3.0 mm fibre diameters in hexagonal 3, 4, and 5 layer configurations, while unjammed (0 kPA) and jammed (50 kPA).
• Figure 4: (a) FEM simulation of unjammed vs jammed (50 kPa) tendon deformation with extension of up to 20 mm. When unjammed, deformation is localised around the Shore A 30 (soft) sections of the individual fibres. However, upon jamming, the bundle of fibres deform as a single column and elongation is more distributed between the Shore A 30 (soft) and Shore A 85 (stiffer) materials. (b)(c) Simulation model validation: Force-displacement curves of unjammed and jammed tendon upon elongation, comparing FEM simulation with real experimental results. Upon jamming, tensile stiffness and damping capacity of the tendon increases. Through simulation, the slip point, which is the onset at which frictional sliding occurs could be identified. Following the slip point, the fibres exhibit a stick-slip behaviour, as shown by the jagged region of the force-displacement curve. (d) Low-order polynomial fitting of simulation data to approximate the tendon's mechanical properties.
• Figure 5: Left: Experimental setup to perform drop tests. Right: Behaviour of the joint after collision when unjammed vs jammed.