A Variable Stiffness Spherical Joint Motor by Magnetic Energy Shaping

This article presents a robotic joint motor capable of providing bio-joint-like motion and controllable stiffness in omnidirection. Unlike existing compliant actuators relying on elastic elements or force/impedance control, the output shaft of the proposed actuator can be stabilized at any equilibrium in antagonistic sense by purely manipulating the motor currents in the electromagnets distributed on the spherical surface of the joint socket. The multi-DOF orientation and the joint stiffness can be simultaneously adjusted by dynamically shaping the magnetic energy of the motor in the vicinity of any specified equilibrium. The relationship between the motor currents and the motor torque as well as the torque gradients is established in closed-form for the spherical joint motor (SJM), which allows for real-time shaping of the magnetic energy and manipulation of the equilibrium and the stiffness. The concept of the proposed variable stiffness motor with the energy shaping method have been validated as a robotic wrist equipped on a robotic manipulator. The results demonstrate that the SJM is capable of providing dexterous motion and intrinsic joint compliance in omnidirection while also allowing simple joint design and efficient compliant manipulations for robots.