# Extended Reality Application Framework for a Digital-Twin-Based Smart Crane
## 3 Results
### Use Case
#### 3.2.1 VR Training Application  
The use of VR in manufacturing training is an upcoming and ongoing research process. VR enables the creation of a realistic-looking world, providing an immersive and interactive experience for the user. To highlight the interactivity of the development framework, the first case is a VR training application of the \"Ilmatar\" crane, the development contents are based on the design document (see Table 6). Taking lessons learned from previous research [44], the complex operation of the controller would hamper the users' experience; the users would be confused with excessive text description; physical VR hands were essential to improve the users' immersion; a gradual and adaptive learning process shall be designed for the users. Table 6 illustrates the design document of the VR training application. To enhance immersion and operability, the application was featured by:  
* **Physical VR hand:** The two learning contents (the operation of the device controller, and the operation of the virtual remote controller of the crane) in the VR environment increased the operation complexity. The development of the physical VR hands eased the sense of unfamiliarity and improved the immersion. * **Multi-Interactor:** Implemented the different Interactors to improve the user experience. Interactors are used for interacting with interactable. It contained three kinds of Interactors: Ray Interactor, used for interacting with interactable at a distance; Direct Interactor, used for directly interacting with interactables that are touching; and Socket Interactor, that would hold an interactable and raise an event when an interactable was placed into, or removed from, the Socket. * **Multi-modal interaction capabilities:** video and sound streams were imported, video stream was used for the tutorial of the crane features instead of the text description, while audio functions were set up to give feedback on the operation. * **Multi-scene:** Four scenes were organized to give a progressive learning process, including the introduction of the VR environment, getting familiar with the interaction features in the VR environment, learning about the functional features of the \"Ilmatar\" crane, and completing a virtual training (see Figure 9). * **Design parameter synchronization:** The design parameters (see Table 3) were stored in an independent script-based component in Unity, enabling the request of updated \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Network Layer** & **OPC UA-MQTT** & **OPC UA-GraphQL** & **OPC UA-Unity** \\
**Wrapper** & **Wrapper** & **Client** \\ \hline Coding Language & Python and C\# & C\# & C\# \\ Communication & MQTT & HTTP & OPC UA \\ Protocol & Medium & Low & High \\ Learning Cost & 0.273 s & 0.505 s & 0.531 s \\ \multirow{2}{*}{Applied Scenario} & Continuous data flow & Discrete data & \multirow{2}{*}{Complex information} \\  & remote monitor & request/response & \\ \multicolumn{3}{c}{interaction} & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Communication middleware comparison: coding language is the programming language needed for the development of the application in the service layer; communication protocol is applied between the network and service layer; learning cost concentrates on the service layer. data from the external resource (DT document) through HTTP. In other words, the VR training application is a dynamic data-driven XR environment.