# Prognostics and Health Management Based on Next-Generation Technologies: A Literature Review
## 5 Application of AR to PHM
### Integration of AR and DTs  
After researching the literature on AR and DT integration, drawing on the 3R technology designed and combining with the technical framework of DTs in the field of manufacturing, we have compiled a technical framework for the AR and DT integration used in industrial PHM, as shown in Figure 7.  
The application framework for AR-DT technology consists of three parts: the physical layer, the virtual layer, and the application layer. The physical and application layers interact through the virtual layer, and the virtual space is the link between the physical space and the AR space. The physical layer constructs digital twins for devices through sensor monitoring and other data collection means. The virtual layer includes twin data and models for real-time interaction, design, manufacturing, and other necessary data. The virtual 3D model is projected into the physical space through virtual reality peripherals (movable displays, data gloves, position trackers and helmets, etc.) in the application layer. The operator can perform fault disputes and repairs on the digital twin of the target device through the AR device. After organizing and summarizing more than 200 literature works retrieved from China Knowledge Network and the Web of Science database, we found that the research on AR technology mainly focuses on vision and gesture in human-computer interaction to provide a technical basis for AR. After organizing them, we found that more and  
Figure 7: PHM based on AR-DT. more scholars are considering the application of digital twins to the process of PHM and intelligent maintenance and propose using AR technology in digital twin processes to enhance the connection between digital twins and the real world. For example, [PERSON] et al. propose an augmented reality-based multi-view interaction method for digital twin processing systems in response to the problems of heterogeneous and diverse contextual data and the need for more efficient visualization of the human-computer interface in digital twin processing systems [38]. [PERSON] et al. propose a digital twin-based AR multi-person collaborative assembly architecture, which decomposes the assembly sequence according to the complexity of collaborative assembly [39]. [PERSON] et al. proposed a framework that combines augmented reality and digital twin technology (AR-DT) to facilitate the development of digital capabilities [40]. [PERSON] et al. explored a construction machine's operation and job tracking by applying CPSs based on AR and DT [41]. [PERSON] et al. proposed a digital twin architecture that integrates the latest Mixed Reality (MR) devices and realizes real-time informatics integration of Mixed Reality and artificial intelligence, validated through simulation experiments of warehouse management scenarios [42]. [PERSON] et al. designed an interactive augmentation framework that combines virtual reality, augmented reality, and digital twins [43]. [PERSON] et al. used an augmented reality-based digital twin system to monitor the machining process [44]. [PERSON] et al. developed a tool that allows operators to match digital twins with their physical twins (PTs) in an augmented reality environment and update their positions when reconfiguring the system [45]. [PERSON] et al. developed a device-driven, large-scale, spatially unfoldable human-robot collaborative assembly system using digital twins and wearable augmented reality devices [46]. The realization of accurate human-computer interaction in AR requires multimodal data coupling, fast bidirectional data feedback, and accurate modeling, which requires the support of hardware with powerful algorithms. We investigated several studies that can be used to demonstrate the integration of AR into other industrial applications, such as healthcare, aerospace maintenance, and construction. On the other hand, by integrating the use of DT-AR, the recognition of object types and sizes can be further increased and diversified.