CFRP Spherical Joint for Robotic Limbs

Robotic limbs, especially lower limbs, have been used for decades both for humanoid robots and to restore the motor functions of patients subjected to amputation of one or more limbs. Joints used for those prosthetic devices are mostly cylindrical, therefore with a single rotational degree of freedom, or made with multi-bar linkage mechanism, which allows a polycentric motion, nevertheless included in the sagittal plane. Observing for example the motions to which the human foot is subjected, the degrees of freedom are not limited to the dorsiflexion/plantarflexion of the foot, but it is also subjected to eversion/inversion motions on frontal plane and to abduction/adduction motions on transversal plane. The same is for the knee joint, since the motions are not limited to the simple flexion/extension of the leg. The main aim of this project is the implementation of a CFRP spherical joint able to guarantee the three rotational degrees of freedom around the three local axes of the articulation; the use of composite materials allows a reduction of the overall weight of the prosthesis, in order to limit the consumption of electrical and/or metabolic energy, depending on whether it is a limb applied to humanoid robot or a prosthesis. The project focused in particular on the application of the spherical joint to the ankle-foot joint. Since the range of motion of dorsiflexion/plantarflexion of the foot is wider than the range of motion of eversion/inversion and adduction/abduction, the spherical seat has been properly functionalized limiting the motions in the space, in order to respect both single rotation on the three reference planes and the combinations of the three rotations. The optimization of the operating ranges of motion is based on the reference values given by studies about lower limb prosthesis and robotic lower limbs applied to humanoid robots. To guarantee the automatic realignment of the spherical joint to the default position, multiple solutions have been developed; some of those provide for the adoption of elastic elements, which in part are made of composite material. Furthermore, possible mode of integration of the spherical joint to other components of the robotic limb, which are in part in composite material too, and to actuation system like electric motors, elastic elements, hydraulic or pneumatic systems have been studied and are briefly presented. Structural analyses with finite element method and experiments of the spherical joints have been performed, simulating the operating conditions.