## 3 Digital Twin, from Concept to Architecture Model
### Classification of Digital Twins  
In ref. [11], [PERSON] et al., realized a hierarchization of DTs, considering the moment in which it was created, the level of integration, the applications, the hierarchy, and the level of maturity. They classified DTs according to several parameters to provide a complex view \begin{table}
\begin{tabular}{l c l} \hline \hline
**Authors** & **Year** & **Definition** \\ \hline \multirow{3}{*}{[PERSON] et al. [2]} & \multirow{3}{*}{2016} & “The Digital Twin theoretical model [...] contains three main components: (a) real environment physical products (b) virtual products in virtual space, and (c) the information and data links which connects real and virtual products.” \\ \hline \multirow{3}{*}{[PERSON] et al. [6]} & \multirow{3}{*}{2017} & “The Digital Twin is a production system virtual representation which can be executed on various simulation environments, through the synchronization between the real and virtual system, using mathematical models, appropriate information. connected intelligent devices and mathematical models.” \\ \hline \multirow{2}{*}{[PERSON] et al. [7]} & \multirow{2}{*}{2018} & “The DT is the cyber part of a cyber-physical system (CPS).” \\ \hline \multirow{3}{*}{[PERSON] et al. [8]} & \multirow{3}{*}{2018} & “The Digital Twin vision consists of a complex description at the functional and physical layer of a system, product or component, that integrates useful data, which can be of interest for all phases of the on-going and and subsequent life cycles.” \\ \hline \multirow{2}{*}{[PERSON] et al. [9]} & \multirow{2}{*}{2018} & “Full DT should integrate five components: services, data, connections, virtual component and physical component.” \\ \hline \multirow{2}{*}{[PERSON] et al. [10]} & \multirow{2}{*}{2018} & “DT represents an integrated system which is able to monitor, simulate, regulate, compute and control the system’s process and status.” \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Digital Twin definitions. of how information is stored, managed, and interpreted. Following the classification by [PERSON] et al., it is desirable to create an ontology--Figure 7.  
Depending on the time of the creation of the DTs, [PERSON] and [PERSON] [12] proposed two categories of DTs, which were developed at different stages of the life cycle: before the creation of the prototype, that is, in the design stage, and in the production phase. These DTs were integrated and used in a Digital Twin Environment (DTE). * _Digital Twin Prototype (DTP):_ The DTP is the DT that contains the essential information for creating a physical copy of a virtual version. The product cycle will be initiated upon achievement of the DTP, which undergoes several tests before the physical twin is created. Once the DTP is realized and validated, its physical counterpart can be produced in the real environment. The simulation accuracy determines the physical twin quality. * _Digital Twin Instance (DTI):_ The DTI originates in the production phase and is a DT that is strongly correlated, throughout its life cycle. After building the physical system, real space data are transmitted to digital space and, reversely, to predict and monitor the system behavior. From the application point of view, [PERSON] and [PERSON] [12] classify DTs according to their roles, implicit prediction and query:  
* _Predictive DT:_ DT predicts the behavior and performance of its physical counterpart. * _Interrogative DT:_ The DT is used to query the status of its counterpart. Depending on the integration level, [PERSON] et al. [13] structured DTs into four categories, as follows:  
* _Digital Model:_ The data between the physical and virtual object are manually changed by the user. Any change to the state of the physical object is not reflected in the virtual object, and any change to the state of the virtual object is not reflected in the physical object. * _Digital Shadow:_ The data of the physical object is automatically transmitted to the virtual object, so there is one-way communication between the two objects.