Development of a Shoulder Joint for Humanoid Robotics Application

In this paper we present the design and the kinematic model of a low-inertia, high-stiffness, tendon-driven shoulder joint for humanoid robotics application. The current system includes three limbs connected in a parallel architecture with 2 DOFs of mobility that allow rotations about the pitch and roll axes. In addition a third DOF can be easily included to implement the yaw rotation. Motion is possible thanks to three tendons displaced at 120 deg one from another and moved by pulleys connected to motors integrated in the base of the joint. The forces applied by the three tendons to the moving platform are measured with precise load cells sensors integrated in the joint structure. A customized motherboard was developed to allow integrating the micro-controller unit, the motor drivers and the instrumentation amplifiers. The forward kinematic model of a single limb of the parallel shoulder joint was obtained using screw theory and the inverse kinematics calculated from the orientation matrix. Preliminary tests of the joint were conducted using a customized graphical user interface that facilitate monitoring and controlling the actuators' status and all the system variables.