Robotic Modules for a Continuum Manipulator with Variable Stiffness Joints

This study introduces a novel robotic module that integrates three spring-reinforced soft actuators for positioning the module in 3D space. This is achieved by utilizing a ball joint as the rotation center and leveraging the spring elements not only as reinforcement structures but also as inductive sensors. Additionally, soft pads are strategically placed around the ball joint to adjust the module stiffness irrespective of its position. Both actuation and stiffening mechanisms are independently controlled by pressure. Design, experimental characterization, and closed-loop control of the module are reported. In addition, a multifunctional manipulator that is built by integrating three modules in a series is demonstrated. A specific architecture has been pursued to reduce the overall number of fluidic tubes required when adding a new module. It resulted in a manipulator with continuum soft actuators, but independent variable stiffness joints, which are the key feature for guaranteeing different bending angles of each segment. Results show that a single module can bend up to 30° omnidirectionally, its stiffness can increase up to 95% in a controllable way, and the output voltage change of the springs can be employed for position sensing. This design offers a highly compact, lightweight, and low-cost solution exploitable in a wide range of applications, from medical to rescue missions, where actions behind obstacles in highly confined areas are needed.