This paper presents a novel sensor-less force control approach for the robot-assisted motion of human arm movements. The twin direct-drive motor system with wire rope mechanism has been developed in order to provide precise force sensation for human-robot interaction. The proposed control is obtained by using two disturbance observers combined with modal space design. In the common mode of modal space design, each motor has almost the same characteristics of the friction effect and other nonlinearity. Therefore, it is easy for the proposed system to compensate these nonlinear effects. Moreover, the bandwidth and the stiffness of mechanism can be enlarged by increasing the wire rope tension command. In the differential mode, the purity of human external force with compensation of friction force is obtained based on modal space design. This mode is useful for control interaction force of human arm movements. Variable powers assist control method based on a real-time estimation of the stiffness of the human arms is also introduced. By considering the stiffness of human arm movements, this method increases the efficiency of force control system and realizes comfortable force for human-robot interaction. The effectiveness of the method is verified by experimental results.