## 6 Conclusions and Discussion  
In this paper, the control system for the developed AIRGAIT lower limb robotic orthosis is continued to be improved. First, the mathematical model of the robotic orthosis manipulated by additional bi-articular muscles is built. Based on the mathematical model, a modified computed torque controller, which utilizes the fractional order derivative is proposed for trajectory tracking mode. By adjusting the additional parameter, i.e., the fractional order of the derivative, the tracking performance is significantly improved in both transient and steady-state. Then, a new compliance controller based on new defined human-robot interactive torque and human active torque is proposed. As a result, AAN strategy is successfully implemented, i.e., the robotic orthosis dominates the movement of subjects in passive mode and reduces the support when the subjects become more active. Finally, the effectiveness of the proposed strategy is confirmed by various experiments with the participation of eight healthy subjects. Particularly, all the subjects report that they feel comfortable during the experiments.  
The MTE of hip and knee joint are less than 7\({}^{\circ}\) during the experiments. This control performance is in accordance with the result achieved in [19; 20; 21; 22] in which the 6-DOFs gait rehabilitation orthosis powered by PAMs is used, i.e., MTE < 15\({}^{\circ}\). However, the walking speed of the proposed system, i.e., 2.2 km/h, is lower than of its 6-DOF robot orthosis system counter part, i.e., 2.5 km/h. The trajectory tracking control strategy of 6-DOF system based on the boundary layer augmented sliding control (BASMC) law [19; 20] and the chattering free robust variable structure controller (CRVC) [21; 22], whereas the proposed system employed the modified computed torque control scheme with one more fractional order derivative for tracking purposed. Although, the nominal pressure is also used for AAN, the proposed strategy required less force sensors, i.e., 2 bar-shaped load cells attached at COM of thigh and shank part, in comparison with 4 load cells connected in series with each PAM in the 6-DOF robot system. Moreover, the use of actuator configuration based on the human musculoskeletal system also provides more power and redundant for the system.  
In future work, the improvement of the AAN controller which is able to adapt the robotic orthosis compliance according to the muscle responses of the patient is considered. Due to the fact that the patient disability is caused by different reasons, i.e., stroke or spinal cord injury, etc. which results in different responses of the patient muscles, the use of electromyography (EMG) sensor is going to be exploited. In that case, the AAN controller may evaluate the active levels of the muscles and provide the assistance accordingly. This is expected to enhance the patient's volition during the gait training process.  
[PERSON] conceived the methodology and developed the experiment, he also performed the experiment, analyzed data and wrote the paper. [PERSON] led the research efforts and led the preparation of this paper.  
The authors declare no conflict of interest.