In this paper, we first propose a nonlinear static friction model for robot joints, which is then extended to an improved GMS model. The nonlinear characteristics of viscous friction and the influence of different parameters on Coulomb friction and the Stribeck effect are considered. Additionally, a four-quadrant model is employed to characterize the friction torque induced by asymmetric loads. Experiments demonstrate that the proposed friction model exhibits significant advantages compared to other models. And by combining the traditional GMS model with the proposed static friction model, a better description of hysteresis phenomena at low speeds can be achieved by the proposed dynamic model. The results indicate that the dynamic friction model was more accurate in characterizing friction behavior at low speeds compared to the static friction model. By combining both models to achieve an accurate estimation of friction behavior in robot joints. In future work, we will be considering additional factors influencing the friction of robot joint and applying the model in robot control to enhance friction compensation for improved control performance.
