Preliminary experimental study on variable stiffness structures based on fiber jamming for soft robots

Soft robotics opened a new set of technological challenges in using soft materials to build robots. The ability of change stiffness is among the most important, because it enables the possibility to tune forces exchanged with the environment. Granular and layer jamming transition have already demonstrated to be a promising approach for developing variable stiffness structures, but no studies have been dedicated to specifically fiber jamming so far. In this paper, we report a preliminary comparative study on fiber jamming applied to cylindrical structures. Fibers of different materials have been used as filler and tested under bending conditions. The results show that up to 380% of stiffness increase can be achieved, in line with data collected on similar devices that use granular jamming. The role of the main mechanical and geometrical parameters have been discussed: elastic properties are fundamental, but optimal performance can be achieved only with ordered arrangements of the fibers; geometrical features seem secondary, but surface roughness has an important role in preventing sliding. This is a preliminary study, but it already defines a first set of guidelines that can help and promote future works on the development and integration of the fiber jamming in soft systems.

I. Introduction

The robotic disciplines recently experienced a paradigmatic revolution. The traditional approach that counts on well-established conventions for designing and developing robots based on hard materials has been revised by the observation that natural agents (humans, animals and even plants) heavily rely on the presence of soft and flexible structures to cope with everyday tasks (1, 2). In this context, soft robotics and soft mechatronics devices can represent a game changer in the development of a new generation of robots (3). The complexity in the use of soft and flexible materials is no more an issue to solve, but a resource to exploit. However, this approach can be effective only if softness is considered together with variable stiffness possibility to be obtained when and where necessary (4). This topic is extremely important in soft robotics as witnessed by the effort spent by several research groups on tunable stiffness actuators and structures (5–9). Among the technologies and physical principles that are recently gaining researchers' interest, the jamming transition has a prominent role. In this context, jamming refers to the capability of some materials to stiffen when their density is increased.