Robotic Modules for a Continuum Manipulator With Variable Stiffness Joints

This study presents a novel robotic module that combines rigid, rigid but compliant, and soft components to enable actuation, proprioceptive sensing, and variable stiffness. Three spring-reinforced soft actuators are employed for positioning the module in the 3D space by exploiting a ball joint as rotation center. Moreover, soft pads that are placed around the joint are used to tune the module stiffness regardless of the module position. Both actuation and stiffening are controlled by pressure, independently. Finally, the spring elements are used not only as reinforcement structures, but also as inductive sensors. Design, experimental characterization, and closed-loop control of the module are reported here. 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.