This paper proposes a novel sensorless force control algorithm for the belt-driven DD (Direct Drive) motor servomechanism. The proposed control is obtained by using the disturbance observer combined with modal space design. The two same motors are required and connected by belt-driven mechanism. Each motor has the same characteristics of the friction model and the vibration phenomenon. Therefore, it is easy for the proposed system to compensate these nonlinear effects. The sensorless interaction force control is designed by dual disturbance observer structure. A robust control system is obtained by applying the inner disturbance observer loop in order to cancel out any disturbance torque on the motor side. The outer disturbance observer combined with modal space design is applied to control both belt tension and interaction force of human. The common mode is utilized for control of vibration suppression and belt tension. The other mode is differential mode, which is a friction-free coordinates and useful for control interaction force of human. By using disturbance observer based on modal space design, it is possible to obtain the purity of external force with wide bandwidth. A validation experiment has been performed on a training system. The experimental results point out that the proposed system has a good realization of interaction force between human and the training system.