The procedure for decompositional synthesis of the robot manipulator control system with electric actuators has been developed. The procedure ensures the exponential convergence of generalized coordinates to given trajectories under the action of external unmatched disturbances and the uncertainty of the parameters of the mechanical subsystem. It is supposed to use discontinuous controls and organization of sliding modes in the virtual space of mixed variables (functions of state variables, external influences and their derivatives). The original method for the synthesis of a reduced order observer with high gains is presented. The observer makes it possible to estimate the mixed variables from measurements of only tracking errors under conditions of uncertainty of the input channels.