Modeling, design, and experimental evaluation of rotational elastic joints for underactuated robotic fingers

In this paper, a novel 3D printed Rotational Joint (RJ) embedding an integrated elastic element is presented. The RJ, produced as a single piece by means of an FDM printer, comprises a traditional pin hinge coupled with a pair of spiral torsion springs, providing the desired compliance for the application at hand. Benefits of the proposed design include monolithic manufacturing and possibility to be successfully employed in robotic articulated devices requiring joint elasticity for their functioning. On the other hand, the sub-optimal RJ behavior, mainly caused by the unavoidable friction between 3D printed mating surfaces, must be accurately taken into account for design purposes. In this context, preliminary reliability tests have been performed showing promising results in terms of lifetime and negligible fatigue effects. Then, a mathematical model of the system is derived, which comprises the spring elasticity along with any frictional effects that may be due to either the pin hinge itself or the tendon transmission (frequently employed in underactuated robotic devices). The model parameters have been empirically evaluated by comparing simulated and experimental data. In addition, the last part of the paper describes how the proposed RJ can be effectively employed for the design of modular, underactuated fingers, providing three degrees of freedom and a single tendon transmission. To this end the model of the joint module proposed in this work will be the starting point for the geometry dimensioning of a finger with a desired free closure motion.