A dynamic system consisting of a servomotor, elastic shaft and inertial lend, which may represent a disk or a robot arm, is chosen as an elastic control system. Equations of the torsional vibratory system are coupled with the servomotor control system equations and the transfer function of the elastic control system relating output angle to the desired input angle is obtained including not only control parameters but also the structural natural frequency and the structural damping of the torsional vibratory system. The ratio of the structural natural frequency to the substructural natural frequency and the ratio of substructural natural frequency to the control system frequency are defined as nondimensional parameters. System behavior is analyzed with respect to these parameters. The fourth order elastic control system transfer function is also reduced to the second order one and formulas for elastic control system frequency and damping are developed with respect to these parameters. [S0739-3717(00)70104-8].