## 3 Design of UI System for the WalkON Suit
### Output Interface 2 of the WalkON Suit: Head-Llp Display with See-Through Display Glasses  
A head-up display (HUD) refers to any transparent display that provides necessary information without requiring users to look away from their usual viewpoints [33; 34]. As the name of HUD describes, it is a UI system that allows a user to obtain information for operating machines with the head positioned up. It is also a great advantage of an HUD that the users do not have to repetitively change the eye focus between distant objects and near displays. HUDs have been proposed for many applications. For example, an HUD is being utilized for military purposes to overlay tactical information.  
In this paper, an HUD is proposed for the user of the WalkON Suit to walk with the head positioned up, i.e., to minimize \(\theta_{t}\) and \(\theta_{n}\) in Figure 4. This is very important, because the larger \(\theta_{t}\) and \(\theta_{n}\), the more increased burden to the users' arms, as in (2). Excessive \(\theta_{t}\) and \(\theta_{n}\) may result  
Figure 8: UI images for the OLED display in the mode selection process: (**a**) initialization process, (**b**) stand mode action, (**c**) right foot forward action in walk mode, (**d**) mode selection procedure in a sitting state, (**e**) sit mode and (**f**) sit mode action.  
Figure 7: State transition rules in the physical layer (in Figure 6) given the selected mode.  
in falling forward, which is fatal for a complete paraplegic wearing a robot. In order for the user to be able to walk with his/her head straight up, an HUD may be a good solution to provide the necessary information to operate the WalkON Suit without looking at the toe or the display of the crutch. However, unlike the HUDs in the other applications, such as vehicles and airplanes, the powered exoskeletons do not have a windshield in front of the user. Therefore, to realize the HUD system in the WalkON Suit, smart glasses that have a see-through display (STD) are utilized, as shown in Figure 9. The transparent display of the smart glasses allows the user to view information while looking forward.  
The STD glasses system receives the operation status of the WalkON Suit (e.g., the motion modes) via Bluetooth serial communication. In addition to the motion modes, the remaining battery level, the total number of steps and the system error status are displayed on the STD glasses. As the WalkON Suit must operate safely even if the Bluetooth connection is lost, the data are transmitted only from the robot to the glasses.  
The STD glasses include a transparent screen, which shows a progress indicator, the foot positions, a warning indicator and an analysis result after completion of walking. The transparent screen of STD glasses shows an image overlapped with the environment. In the display that the user sees, a donut chart shows how long the user has walked with respect to the maximum step counts, which is preset (see Figure 9a). On the center of the donut chart, the number of steps and the current motion mode are displayed; e.g., walk mode is shown in Figure 9c. The foot positions are also shown, as in Figure 9d, which allows the user looking forward while walking. In addition to the necessary information related to motion modes, the STD glasses also show warning messages if any potential hazard is detected, as in Figure 9e. When the operation is terminated, the total operation time and the total number of steps are displayed as in Figure 9f.