Stiffness Variability in Jamming of Compliant Granules and a Case Study Application in Climbing Vertical Shafts

Jamming of granular media has been shown to possess the property of stiffness variation, transitioning from a soft to a quasi-solid state depending on the packing density of the granules. Recently, a gradual stiffness change for bending has been reported by using compliant, cubic shaped granules. Here we demonstrate that the same method and material also exhibits a gradual stiffness change for compression. As a potential application of “compliant jamming”, a bio-inspired robotic platform with jamming membranes as end effectors is designed and tasked with climbing straight vertical shafts. First, the benefit of varying the bending stiffness is investigated in the climbing task by measuring the friction force that the end effectors are applying to the shaft walls, especially if the walls are irregularly shaped. Then the role of compressive stiffness variation is explored by analyzing the performance of the robot in the climbing task, showing multi-modal properties of jamming membranes: (i) enabling a pressure sensor to detect the shaft walls, acting (ii) as grippers that actively use the irregularities of the walls to climb up by state-switching the granular material and (iii) as force dissipators that can dissipate internal forces caused by closed kinematic chains.

I. Introduction

Controlling the stiffness of the joints in a robot has a major influence on its versatility, safety and capability to adapt [1]. Stiff robots are traditionally used in industrial pick-and-place applications with highly specialized control strategies and often large payloads, whereas soft robots are starting to be established in rehabilitation, prostheses, manipulation, wearable and bio-inspired robotics [2], often with lower payloads. If a robot is to perform tasks from both regimes, a strategy for varying its joint stiffness is required. On the one hand, stiffness variability can be achieved purely by the control of the robot. On the other hand, such potentially complex control can be offloaded to the mechanical design of the robot by introducing new materials or mechanisms that inherently possess compliance. Focusing on the latter option, many prototypes with mechanical designs of variable stiffness joints have been the subject of research, ranging from pretensioning of springs [3], Series Elastic Actuators (SEA) [4] [5], tightening tendons [6] to Shape Memory Polymers (SMP) [7] and magneto-rheology [8].