# Generic Digital Twin Architecture for Industrial Energy Systems
## 5 Discussion
The proposed reference framework and the identified structural properties of the DT are very generic, but some additional aspects can be found in our GDTA. So we introduced a Service Management component and defined groups of common service types inside a DT, which help to structure and implement certain services. We also see the HMI interaction as an important part of our GDTA which almost every service has to implement. Thus, we include a HMI component in our GDTA. The conceptual architecture of the Cognitive Twin Toolbox presented in [15] also introduces a \"Knowledge Repository\" in combination with \"Cognitive Services\". This is only applied for a \"cognitive Twin\". The basic DT concept implements only a so-called \"Metadata Repository\". A separation into two distinguishable components, as presented in [15] seems neither beneficial nor has practical advantages. Therefore, we introduced the Shared Knowledge base, which acts as a central point of information in our GDTA. Also a \"Model Management Layer\" is not explicitly stated in our architecture as resource management has to be done by the service implementations themselves. The information about the resources has to be made available through the Shared Knowledge base. The functions described for the \"Twin Management Layer\" can be located in a Reconfiguration Service in the GDTA. The COGNITIVE Twin toolbox is a solution with various components that seems to be specific for their definition of the so-called \"Hybrid\" or \"Cognitive Twin\". Thus, we tried to reduce the components of the proposed GDTA to be as simple and generic as possible. The conceptual model of a DT in the context of IoT, as presented in [16] introduces five layers which are quite similar to the layers of RAMI 4.0. The \"Physical Space\" corresponds to the Asset and Integration Layer; the \"Virtual Layer\", to the Information Layer; the \"Data Analytics and Visualization\" and \"Application\", to the Functional Layer. In our architecture, we chose a more service-oriented view of the various applications of the DT. Providing semantics of the data and a shared knowledge base is not explicitly handled in this conceptual model, in contrast to the suggested GDTA. The conceptual DT model emphasizes the importance of security by defining a vertical module overall specified layers. In our generic architecture, such a module is not explicitly depicted, as it applies to all levels of RAMI 4.0 implicitly and must be considered for the asset as a whole [40]. The AAS is a promising way of standardization as shown in [17] but still ongoing work. In its current state, the concept of the AAS meta-model can be applied and implemented in line with our proposed architecture. It was already shown, that the defined data model of the AAS can be semantically lifted to a knowledge representation based on RDF [41]. In the context of our proposed architecture, this enables the representation of the AAS inside the Shared Knowledge base. The presented proof-of-concept implementation is the basis for further development of other services inside the DT. Additionally, the knowledge representation of the simulation models can be extended with application-specific domain ontologies. This would facilitate the integration of other semantic models of the DT as described in [6]. The hierarchical and modular ontology design, using a top-level ontology as well as various domain ontologies, facilitates the integration of new concepts and ontologies. These hierarchies can be further increased by introducing additional levels, like application or even task ontologies. The alignment with already existing or emerging standards, represented in OWL, can be performed by having similar concepts in the upper layers of the ontology hierarchy and using the ow:equivalentClass property defined in OWL 2. The presented use case was chosen in the domain of industrial energy system, but the proposed GDTA can also be applied for a broader spectrum of applications.