## 2 Materials and Methods
### Development of the Structure and Connection of the Dynamic Model and the Augmented Reality System
However, it did affect the result at a particular level where the dynamic model was applied. In this study, a framework has been developed to create a special real virtual environment for pumping equipment maintenance. The functionality of the developed framework is the same as that listed in Section 2.1. An essential feature is its modularity and ability to integrate easily with any digital solution and technology, such as digital twin and analytical modules based on artificial intelligence systems. The first step was to define the communication protocols between the dynamic model and the AR system. Aveva Dynamic Simulation as the main integration protocol supports OPC protocol (OPC UA). The communication channel between the dynamic model and the OPC server was configured by the developer of the dynamic simulation software environment and was not an object of this study. The subject of this study was essentially finding and testing a communication channel between the augmented reality system and the OPC server (OPC UA). Implementing and configuring the OPC UA in the augmented reality application development environment was challenging for the authors (it was possible to connect the values once without updating in real time). Since the augmented reality application functions like the WEB application, obtaining values from the OPC server required the development of a special polling block (i.e., the program code) which pulled the main code upon obtaining values from the server. The authors found this method unacceptable because of the potentially large number of  
Figure 1: The dynamic model connected to the augmented reality system. errors due to delays in obtaining the signal. Therefore, a structure was proposed which is shown in Figure 2.  
This structure was based on the application of the modern MQTT protocol for message transfer. In order to use it, the MQTT broker (Mosquito MQTT) was added to the framework to coordinate message transmission between the dynamic model and the augmented reality system. This protocol is a basic protocol in IoT systems, and the effectiveness of its use was repeatedly proven [34; 35]. Thus, according to the authors, the optimal structure for the development of a software application for the collaborative use of dynamic simulation and an augmented reality system should correspond to the scheme shown in the figure and include the following functionality and technological links:  
1. The system contains three levels: communication, application, and physical;
2. On the physical level, there is an object (laboratory equipment was used in this work), a physical object, and a dynamic model. The power system and the control system are distinguished separately on the physical level. Allocating these systems into a separate  
Figure 2: The structure of the augmented reality system. class is necessary to expand the system maintenance functionality. In the separate version, it is possible to transfer these parts to the real or virtual execution environment at any time as the production need arises. For instance, in the normal mode of operation, all systems are real. When the task of introducing new control algorithms arrives, a dynamic model is used instead of the object, and the control system remains real or is virtualized as needed. This approach will, among other benefits, significantly reduce costs during commissioning new systems and the maintenance of old systems;  
3. The main protocols of the communication module are MQTT and OPC (OPC UA). OPC allows communication between the dynamic model, automation system, and power supply system. Then, the data from the OPC server are published on the MQTT broker, which interacts with the augmented reality application on HoloLens 2 glasses. The SQL server in this structure is a tool to keep a history of all changes at all levels. All values are recorded with corresponding timestamps. For the system to work correctly, all components have to be time-synchronized;  
4.