Though many research efforts have been devoted to developing robot controllers with uncertainties in the robot dynamics, most research so far has assumed that the model of actuator is known exactly. In implementing the robot controllers, calibration is necessary to identify the exact parameters of the actuator model. When there is an error in calibrating or modeling, it is unknown whether the stability of the system could still be guaranteed. In addition, the actuator model could change as the temperature varies due to overheating of the motor or changes in the ambient temperature. To overcome this problem, we propose in this paper, a task-space controller for the regulation of the robot with an approximate actuator model. We consider second-order dynamics of electric actuators with negligible armature inductances. Sufficient conditions for choosing the feedback gains are presented to guarantee stability.