Abstract: The stiffness requirements of robot wrists vary with processes during automatic assembling-clamping of robots. The precision of robots moving workpieces to operating positions in the process of rigid localization is achieved if robot wrists equip with a large stiffness. The pose errors of workpieces in the process of compliant assembling-clamping can easily be compensated if robot wrists with a low stiffness is utilized. A 5-DOF (degrees of freedom) robot wrist with a large stiffness variation is proposed, and the rotations of the workpieces around X axis, Y axis and Z axis and the translations along X axis, Y axis can be realized. The stiffness variation mechanisms of the wrist modulating pretension and the mechanisms of both rigid localization and compliant assembling-clamping are studied. The stiffness of wrist joints increase linearly with the increase of pretension of elastic elements. The influences of pretensions and geometrical parameters on the variable-stiffness characteristics and the stiffness distribution of a wrist are researched. By designing geometric parameters of wrist joints, the stiffness distribution of the wrist is obtained and remains constant in the process of modulating wrist stiffness. The pose models of wrists caused by deformations during assembling-clamping and the models of compensating posture errors of workpieces are established. The models of wrist stiffness varying with wrist positions and postures are obtained, the assembly operations and clamping operations can be obtained by the wrist overcoming resistances. The results show that the stiffness of the wrist can be modulated largely, the requirements of both rigid localization and compliant assembling-clamping can be satisfied.

Key words: stiffness variation, robot wrist, compliant assembling-clamping, rigid localization, stiffness distribution