## 5 Discussion and Conclusions
This may be a general case for people with complete paraplegia, as their body is vulnerable and their curability with respect to any further injury is very poor. Therefore, the most important requirement for the powered exoskeletons of complete paraplegics is reliability. This is the reason why the proposed system was developed based on buttons and an HUD to reduce their physical burden to check their leg positions visually frequently, while maintaining a high operation reliability. Complete paraplegics with the ASIA level of A or B, in particular those with complete loss of sensory functions, neither generate any EMG signal nor feel the damaged body parts. Therefore, it is clear that the EMG is not a proper solution for the user-interface for such users. A haptic device may be a good solution to deliver the interaction forces with an environment (e.g., the ground reaction force) to the body part that is not damaged (e.g., the trunk) by bypassing the cut spinal nervous system. For example, the human subject who participated in this research has a spinal injury at the level of T-10, which means he does not have any feeling below his navel. For this case, it is impossible to transfer all the necessary tactile information through the haptic system. Nevertheless, the authors fully admit that a haptic device is a good solution for people with incomplete paraplegia and with a low paraplegia level. In particular, for those with loss of sensory function only below the knee joints, the haptic device would be a perfect solution for regaining the gait stability. This will definitely be the future work of this paper. In this paper, a combination of buttons and an HUD was applied as a user interface for a complete paraplegic user wearing a powered exoskeleton. The proposed method was developed for the sake of 100% reliability while providing the minimal necessary information for realizing a stable gait. The finite state machine for operating the robot with buttons was optimized to minimize the number of clicks to minimize any potential malfunction by selecting an undesired motion mode. The proposed UI system was applied to the WalkON Suit, a powered exoskeleton for complete paraplegics. As the STD glasses provided the necessary information for operating a powered exoskeleton, such as a progress indicator, the foot positions, a warning indicator, etc., the user of the WalkON Suit did not have to look down continuously to check the leg movements and to watch the display on the crutch. Consequently, the proposed UI system resulted in a critical improvement in the gait stability, the gait speed, as well as the user experience. The WalkON Suit with the proposed UI was utilized for the torch relay of the 2018 Pyeongchang Paralymics. A complete paraplegic successfully walked about 20 m carrying a torch and delivered it to the next torch bearer. The proposed UI method was verified by only one subject in this paper. For the proof of the generality of the effectiveness of the proposed method, a statistical study with enough human subjects may be necessary. A practical problem was that the powered exoskeleton used in the experiment, called the WalkON Suit, was custom-fitted, which means that the robot could be worn only by the human subject who participated in this paperwork. Nevertheless, we believe that the improvement in the gait speed and the neck posture was scientifically meaningful even though it was a result verified by only a single subject. We assumed that the limited gait speed and the poor gait stability were due to the problem of complete paraplegics needing to check their leg positions visually frequently, and we proposed an engineering solution to solve this problem by adopting the see-through display technology. The experimental result showed that the proposed hypothesis was correct from the engineering viewpoint. A new version of the WalkON Suit, which can be worn by multiple users, is being developed by Angel Robotics.