## 2 System Flexibility Framework Driven by Digital Twin  
As an effective means to realize the digital transformation and intelligent upgrading of traditional industries, digital twin technology can realize effective management and analysis of the entire product life cycle. Digital twin exists in the entire life cycle of the product, and uses virtual environments and simulation computing technologies to improve the performance of the system [20]. In the manufacturing industry, the ISO/DIS 23247-1 [21] standard defines that the digital twin can realize the digital expression of all aspects of observable objects, among which the observable objects can be personnel, equipment, materials, environment, supporting documents, etc. The supporting documents defined in this standard are any form of artefact (requirement, plan, model, specification, and configuration) that assists manufacturing. Based on the above foundation, with the support of digital twin technology, this paper proposes a flexibility research framework for production system in the design phase as shown in Figure 1. This framework is used to realize the data extraction, modelling and analysis of flexibility features in the production system design scheme, helping to make fast and accurate flexibility evaluation and optimization decision making for each design scheme. The system flexibility research framework consists of three main subjects. The first subject is the physical system, which stores all design documents including the design scheme of the production system, and the design documents include the design requirements for new products production, design documents, calculation documents, design models, etc. These documents record the whole process of the production system from the initial requirement analysis to the generation of the demand-oriented design scheme. These are the original datasets to realize the flexible digital expression of the system. The second subject is the digital world, which is the digital expression of system flexibility. Flexibility is the observable and research object defined in ISO/DIS 23247-1 standard. All aspects of it can be digital expressed with the support of digital twin technology, and can keep the same update iteration frequency with the physical object. [PERSON] [22] adopted digital shadow to assist decision making in the machining process through data management and analysis. The constructed process-oriented digital shadow includes the data model and data analysis of the process data. The ISO/DIS 23247-3 standard proposes that the information contained in the observable object includes not only static but also dynamic information. For this reason, the digital model constructed in this paper adopts data models and data analysis methods in the process of digital expression oriented to system flexibility, and the system flexibility features concerned include static and dynamic dimensions. The data model is used to extract the characteristic parameters related to  
Figure 1: Production system flexibility research framework driven by digital twin. the system flexibility performance analysis in the original data set, and form a flexibility evaluation model based on the analysis of these characteristic parameters. Data analysis put forward an analysis method on the data model, and realized the quantitative evaluation of the flexibility evaluation model from dynamic and static dimensions to help analyse system flexibility. The third subject is the information channel, which is to realize data interaction between the physical world and the digital world. On the one hand, it helps to realize the data extraction and collection related to the analysis of system flexibility characteristics from the physical world to the digital world, and on the other hand, it feeds back the analysis and evaluation results of system flexibility realized in the digital world to help the evaluation and development of the demand-oriented production system in the physical world.