A 4 DOFs variable stiffness soft module

The new generation of bio-inspired robots is constructed from soft materials. The low mechanical stiffness of the material enables continuous deformation by the robot, which can thus negotiate narrow and tortuous spaces. Additionally, soft robots are intrinsically atraumatic, with safe interaction with the surrounding environment without the need of an active compliant control. Several research centres are exploring solutions to vary the stiffness of the component material through use of either on-off or continuous mechanisms. The latter can be implemented by using a combination of pneumatic chambers and tendons to vary the air pressure, in a single compact design. To enhance the robot dexterity required to perform complex tasks, each module of the soft robot needs to exhibit several degrees of freedom (DOFs). In the present study, we describe a novel soft pneumatic cylindrical module made of fibre-reinforced polymer, that uses 2 pair disposed antagonistic tendons for variable stiffness control and capable of 4 DOFs. By tensioning twin pairs of antagonistic tendons, the module can rotate around X, Y and Z axes, and by inflating air in the 2 chambers, it can extend along Z but also rotate around the X axis. The wide cross-sectional area of the chamber reduces the maximal actuating internal air pressure. We report the design and construction of the soft module, and experimental data on the range of motion, forces generated, and the range of stiffness.