Hybrid Soft-Rigid: An effective approach for enhancing the capabilities of soft robots

Despite its relatively young history, soft robotics has become increasingly popular due to its salient advantages, such as compliance and deformability, allowing for safe adaptation and interaction with unstructured environments. However, such an inherent softness brings along limited load capability and narrow stiffness range, combined with controllability and predictability challenges which hinder enabling their strong potential and widening their real-world application areas. In this context, one objective is developing solid structures able to tune their stiffness in a controllable way. To achieve this goal, this Thesis addresses the development of novel low-cost structures based on the hybrid soft-rigid (HSR) approach. It relies on combining materials with different mechanical properties to benefit from the main advantages of both soft (i.e., structural compliance and many degrees of freedom) and rigid (i.e., force generation and precision) structures, while overcoming some of their respective limitations. The study and development of a novel variable stiffness revolute joint (VSRJ) is firstly reported, as a fundamental component for many HSR designs. The VSRJ provided up to 8-fold controllable stiffness enhancement through air pressure, regardless of its position and with high repeatability. Secondly, the integration of a stiffness control structure (SCS) -built through the assembly of three downscaled VSRJs- into a soft manipulator is presented to demonstrate the effective potential of the proposed variable stiffness concept in terms of position-independent stiffness control and enhanced precision of the path followed by the manipulator end-effector. Thirdly, a module that combines actuation, proprioceptive sensing, and variable stiffness is presented. The module was proposed as a modular solution able to mitigate the tubing challenge -by reducing the number of the required tubes in the overall system- which is typical in fluidic systems. The design, experimental characterization, and a demonstration of the module were presented. Overall, the Thesis proposes a set of solutions to show the potential of the hybrid soft-rigid approach to enhance the capabilities of the soft robots in terms of force, stiffness range, controllability, precision, and modularity.