Dynamic modeling of hybrid robots through stiffness directionally releasing and multiple-node dynamics assembling

More and more hybrid robots are being applied in various fields, but researches on their dynamic modeling have not kept pace. The key to modeling hybrid robots lies in assembling of parallel and serial mechanisms, which gives rise to two unresolved problems: 1) modeling of parallel mechanisms with free boundary conditions; and 2) dynamic assembling at multiple nodes between the two mechanisms. To solve them, this paper proposes a dual-node joints modeling method and a multiple-node receptance coupling method respectively. The first method achieves stiffness releasing for any joint by constructing constraint wrench spaces, allowing extraction and elimination of stiffness in the directions of DOF. Then, the dynamics of parallel mechanisms with free boundaries is assembled by the dual-node joints. The second method achieves multiple-node dynamic assembling through partition of receptance matrices and boundary condition analysis. Subsequently, dynamic models of hybrid robots are built. To verify the methods, a hybrid machining robot is presented as an example and modeled. The modal shapes and frequency responses are predicted and compared to the experimental ones, which show high modeling accuracy.