# Digital Twin and Reinforcement Learning-Based Resilient Production Control for Micro Smart Factory
## 2 Research Background  
### Cyber-Physical Production System and Digital Twin  
A cyber-physical system (CPS) advances processes in the physical world by approaching, processing, analyzing, and utilizing data through the internet-based connection between the physical world and virtual components [25; 26; 27; 28]. Thus, a CPS can be defined as \"a physical and engineering system that monitors, controls, coordinates and integrates physical elements by utilizing computing and communication technologies\" [26]. Furthermore, a CPPS is a CPS that enhances efficiency of production process of a manufacturing system. A CPPS is defined as:  
\"a physical and engineered system, which aggregates resources, equipment and products by interacting between the physical and the cyber world. This system utilizes knowledge about the overall product lifecycle to improve the efficiency of the production process. Here, the interface between the physical and cyber world is used to monitor, control, coordinate, and integrate resources, equipment, and products. Knowledge about the product lifecycle is applied for the operation of CPPS in an appropriate way for a specified time scale. In addition, heterogeneous advanced engineering applications can improve the value added by the operation of CPPS\" [26; 29; 30]. The above definition indicates that any study on CPPS must focus on the composition and interoperation of a complex system, and that the modularization and interoperability of technology and applications with various levels, layers, and scopes are core issues [29; 31; 32]. This SoS perspective is related to many issues in architectural design and can follow modular architectural design [29; 33]. A modular architecture consists of modules with one or several distinct functions that are connected through a simple interface. The overall system behavior is implemented based on the interactions through this interface, which can be loosely or tightly coupled [29; 34; 35]. DT is an advanced virtual factory that represents a heterogeneous configuration, reflects the functional units, and synchronizes information objects. The advanced attributes of a DT can improve management accuracy and decision-making efficiency. As a core technology of CPPS, DT can be used to achieve cyber-physical integration for work-center-level design and operation. A DT has the following advanced characteristics in comparison with the traditional simulation model [23; 36; 37; 38; 39; 40; 41; 42]:  
* automatic creation of DT with predefined configurations and functional units,
* transmission or reception of information from physical assets through vertical integration,
* advanced process that applies horizontal coordination to advanced engineering applications, and
* repeated derivation of performance indicators for prediction and diagnosis. A DT application is a software component for creating, synchronizing, and utilizing a DT. A virtual representation of a DT application (VREDI) is an asset description that supports vertical integration and horizontal coordination. VREDI considers four core advanced characteristics for applying a DT to a work-center-level asset administration shell (AAS), which includes DT-based technical functionality and the concepts of type and instance. The DT virtual representation is an asset description that abstracts the input to the DT application, thereby realizing an object through component-manager-enabled aggregation. The operation module runs the actual DT application; it runs with the DT engine and uses virtual representation-based objects as the input. The DT engine runs according to the creation, synchronization, and utilization procedures of the operation module; therefore, it must be appropriately designed or selected to achieve the required technical functionality. The configuration data library (CDL) stores the composition of the resources for an accurate and quick site simulation; the composition of the resources is divided into base model, metadata, and logic.