VARISA - A VARIable Stiffness soft robotics Arm based on inverse pneumatic actuators and differential drive fiber jamming

Variable stiffness technologies are promising to fill the existing gap between the capabilities of robots based on soft materials and real-case applications, which may require high stiffness in specific working phases or conditions. Among these technologies, jamming transition emerged as a suitable option for devices that are intended to experience large deformations. Building upon the first version of the already introduced variable stiffness linear actuator (based on the combination of inverse pneumatic artificial muscles, fiber jamming, and positive pressure jamming), here we present the design of the VARISA, a novel multidirectional modular soft arm with tuneable stiffness. A tailored fabrication process, considered also in the design choices, is reported. Both the single module, made of three actuators, and the arm, which consists of two modules connected in series, were tested to assess deformability and variable stiffness capabilities. VARISA is 45 mm in diameter and 285 mm in length and it reached 100 mm of elongation and 82 degrees of maximum bending angle, covering a 300 mm wide workspace. Moreover, it achieved a stiffness variation close to one order of magnitude (a maximum stiffness ratio of 9.57) and, in particular, the possibility to tune the absolute stiffness between 0.06 and 0.52 N/mm in bent configuration.