Development of modular multi-degree-of-freedom hybrid joints and robotic flexible legs via fluidic elastomer actuators

Abstract Soft actuators with high safety, adaptivity, and energy-to-weight ratio have the potential to be used in developing more adaptive legged robots. In this work, we incorporate soft actuators into rigid parallel mechanisms and develop multi-degree-of-freedom (multi-DOF) soft-rigid hybrid joints that can actively achieve 1, 2, and 3 DOFs actuated by 2, 4, and 8 bellows-type fluidic elastomer actuators (FEAs), respectively. The FEAs exhibit large axial strain ( ϵ e max = 176%, ϵ c max = 25%), small radial expansion ( ϵ r max = 12%) at 70 kPa, and are light weight, and the rigid parallel mechanisms constrain motions of the joints to the desired DOFs. We characterize the proposed joints’ kinematic and static performances by measuring their range of motion and blocked torque upon actuation. Results show that these joints successfully achieve all desired DOFs and are of high torque to weight ratio (4.07 N·m·kg −1 ). A bucking prediction model is established to evaluate the critical buckling pressure. As a demonstration for legged robots, we use the proposed joints and develop two types of multi-DOF legs based on inspirations from the DOF configuration of legged mammals’ musculoskeletal systems. Preliminary results demonstrate that FEAs-based multi-DOF legs can perform fundamental biomimetic movements (e.g. leg swing) through pressure adjustment, and high-speed tasks (e.g. ball kicking and jumping) through high-pressure and short-pulse actuation.