## 6 Maintenance Improvement Brought about by DT with XR Devices
### Maintenance Improvements  
When the operator is performing maintenance procedures on-site, it might be necessary to locate or identify specific equipment. XR devices have often been used to improve this _locating_ task, using specific equipment data issued from a centralised DT or a BIM to detect its position [25; 47; 55; 72]. Once the equipment location is known, XR devices can then be used to highlight it with, for example, a colour-coded 3D model [43; 47; 67; 69; 70; 71; 72]. For example, [PERSON] et al. show the use of a Hololens headset combined with detection algorithms to facilitate the localisation and identification of malfunctioning elements [43]. Furthermore, the augmentations are not limited to the real-world obstacles. Even if the equipment is hidden or hard to see (e.g., behind a wall), a 3D model can be used to display it (see Figure 9). This simulated X-ray vision can also be used to highlight a specific part of a \begin{table}
\begin{tabular}{c c c} \hline \hline
**Supported Activity** & **Features** & **Reference** \\ \hline \multirow{4}{*}{Maintenance} & Visual annotation & [25; 43; 47; 55; 64; 67; 68; 69; 72; 73] \\  & Track location & [47; 55; 72] \\  & Ease inspection & [22; 43; 53; 55; 66; 72; 79] \\  & Asset identification & [43; 53; 68; 71] \\ \hline \multirow{4}{*}{Collaboration during maintenance procedures} & Shared reports & [22; 72] \\  & Connected systems & [43; 64; 70] \\  & Distant expert & [43; 69; 72] \\  & Telepresence & [70; 72; 78] \\ \hline \multirow{4}{*}{Visualisation} & Real-time, historical and & [22; 25; 43; 47; 53; 55; 68; 70; 71; 72] \\  & documentation data & [30; 43; 47; 67; 69; 72; 79] \\ \cline{1-1}  & 3D model & [43; 47; 68; 72; 43; 77; 69; 73; 74; 75; 76; 77; 78; 79] \\ \cline{1-1}  & Filtered data & [43; 47; 68; 72] \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Categorical review of maintenance-related papers using DT and XR technologies with/without BIM. Figure 8: Image captured by the Hololens. A bounding box is drawn to delimit the object to label [71]. piece of equipment, as shown by [PERSON] et al. in [68], which can be helpful to inspect specific subparts of complex equipment. It can also be used to display specific documentation, such as a 3D representation of an ultrasound inspection [72]. Some authors have observed that the combined use of centralised DT and XR devices can provide on-site operators with easy and intuitive access to the necessary data for maintenance procedures, such as equipment documentation or maintenance instructions [22; 25; 43; 47; 53; 55; 68; 70; 71; 72]. Through the use of specific filters, both upstream and by the on-site operator, these data can also be specified to display some of them only for specific roles [47], but also through the use of the interaction methods offered by the XR devices (e.g., filter buttons, operator's location and operator's orientation) (see Section 5.3). Once the specification is done, the on-site operator will see only the needed information for the current procedure, preventing a mental load overflow due to too much information being displayed [68]. These augmentations can also help to display warning messages if needed to indicate either a failure or a security alert [64]. Once the information is chosen, various displaying methods have been proposed by the authors using XR devices. Most of them are based on the use of an AR or an MR device, thanks to its ability to overlay information onto the physical environment of the operator using augmentations (see Section 5.1). It has been observed in the literature that these augmentations can bring benefits to maintenance procedures. For example, during inspection of a fault [22; 43; 55; 66; 72], some authors have observed that the use of augmentations to display specific information over or near the equipment can help the operator to better identify the faulty equipment. This benefit also seemed to be reinforced when the operator's position and orientation, relative to that of the equipment, are considered to hide or display the information.