Theoretical and Experimental Study on Active Stiffness Control of a Two-Degrees-of-Freedom Rope-Driven Parallel Mechanism

Rope-driven mechanisms with the characteristics of high speed, low inertia, and high precision are widely utilized in numerous fields. Stiffness is an important indicator to illustrate the precision and compliance of the mechanism. However, realizing active stiffness control is difficult for the mechanisms due to the coupling of rope tension and controller stiffness. To solve the problem, a verification prototype, 2-DOF rope-driven parallel mechanism (RDPM), is designed and manufactured, and its mechanical model is established. And then the general stiffness model of the RDPM is derived. Meanwhile, the rope-hole friction is calculated based on the Stribeck model. An active stiffness control scheme considering the pose retention, vibration suppression, and friction compensation is proposed. According to the stiffness model and active stiffness control law, the linear motion stiffness of the RDPM is analyzed in detail. The conclusion shows the motion stiffness is linear with the controller stiffness and initial rope tension. Finally, the theoretical stiffness, simulation stiffness and experimental stiffness are calculated and compared by the co-simulation technique and physical prototype experiment. The error between experimental data and simulation data is within 10%, which verifies the stiffness model and active stiffness control scheme.