# A Method for Dynamic Insertion Order Scheduling in Flexible Job Shops Based on Digital Twins
## 2 Flexible Job Shop Dynamic Scheduling Framework Based on Digital Twins  
Arranging the production of workshop material can improve production efficiency and reasonable allocation of resources. With the advent of the intelligent era, workshop scheduling has become complicated. Under the background of intelligent manufacturing, effective information data analysis can solve many problems. The key to solving dynamic disturbance is real-time interaction between physical space and information space, and data analysis in the information space. Finally, the intelligent system dynamically decides the next production scheduling plan.  
Based on digital twins, this paper constructs a flexible job shop dynamic scheduling framework, as shown in Figure 1. The model consists of four layers.  
(1) Physical layer. The physical layer mainly refers to the set of objective entities such as the personnel, equipment, material and environment in the workshop, which is responsible for the production and processing activities in the workshop and functions to collect and transmit data in physical space, such as equipment data, material information, personnel information, and environmental data.  
(2) Model layer. The model layer is a virtual digital twin workshop, which is the real mapping of the physical workshop. Based on real-time data, through the corresponding models, rules, and knowledge of the model layer, the physical space production operations are analyzed, and the corresponding optimization schemes and decision-making instructions are formed. The model layer has three attributes: computation, interaction,and control. Computational performance truly reflects the state of physical products with the help of simulation tools. The interaction includes two aspects. One is to improve the accuracy of the digital twin model through continuous interaction with physical products. The second is the interaction with other digital twin models to complete the product production process simulation. Controllability refers to the control of the behavior and state of products in the physical space through data analysis.  
(3) Information layer. This layer is the data analysis and processing platform of the workshop. All kinds of data in the physical layer and model layer are transferred to the information layer and stored in the corresponding database, model base, rule base, and knowledge base. The rules and knowledge of the information layer can be directly used by the application system layer as a decision-making reference, and the encapsulated model can be directly called for production simulation and optimization. The information layer data is analyzed and collated by the workshop data analysis and processing platform, which serves as the decision-making basis for the application system layer to regulate production activities. The information layer realizes the data interaction and fusion between the physical layer and the model layer and ensures smooth communication between the information systems of the application system layer.  
(4) Application system layer. The information systems of the digital twin workshop are no longer independent of each other, but interconnected and cooperative to realize the digital management of the product lifecycle. By analyzing the actual needs of the production workshop and relying on the support of the data, models, rules, and knowledge of the information layer, the system layer regulates the operations of the physical layer and the model layer. The application systems include ERP (Enterprise Resource Planning), PDM (Product Data Management), PLM (Product Lifecycle Management), MES (Manufacturing Execution System), and PCS (Process Control System). The functions include workshop production process optimization, process control, intelligent production scheduling, equipment efficiency analysis, and product processing progress monitoring.  
Figure 1: Flexible job shop dynamic scheduling framework based on digital twins.