On the stiffness of three/four degree-of-freedom parallel pick-and-place robots with four identical limbs

This paper presents stiffness analysis of a family of three/four degree-of-freedom pick-and-place robots with four identical limbs. The Cartesian stiffness matrix of the robots is modeled with the virtual spring approach. By means of a nondimensionalization of the stiffness matrix, the 6 × 6 inhomogeneous stiffness matrix is decomposed into two homogeneous sub-matrices, of which two performance indices, corresponding to the translational and rotational stiffnesses, are defined to evaluate the stiffness behaviors of the manipulators. A comparative study among the robot counterparts is carried out with respect to the elasto-static performance.