# Conceiving a Digital Twin for a Flexible Manufacturing System
## 1 Introduction  
The new generation of integrated manufacturing systems (IMS) consists of the collaborative integration of humans, cyber systems, and physical systems [1]. Humans are the creators and users of the physical system. The physical system is the main part, which is the provider of the manufacturing process. The cyber system is the core; it can analyze, calculate, and control the manufacturing process. The human-cyber-physical system (HCPS) represents an innovation archetype brought by the new generation of artificial intelligence technology in the intelligent manufacturingsector [2]. It demonstrates the deep integration and continuous upgrade of the cyber-physical system (CPS) and human-cyber system [3]. A basic component of Industry 4.0 is the flexible manufacturing system (FMS), an advanced production system that interconnects machines, workstations, and logistics equipment, the entire manufacturing process being coordinated with the computer [4]. In industrial applications, flexible manufacturing systems (FMS) are the most important, automated, and technologically sophisticated of the machine cell types used to implement cellular manufacturing with advanced technology. An FMS is essentially a computer-controlled production system, which brings together different tools, machines, and control equipment capable of processing a variety of part types or jobs. The FMS concept integrates many advanced technologies, including flexible automation, CNC (computer numerical control) machines, distributed computer control, and automated material handling and storage. Some advantages of FMS include: improved capital/equipment utilization, reduced work-in-progress and set up, substantially reduced throughput times/lead times, reduced inventory and smaller batches, and reduced manpower [5]. Thus, emerging technologies are bringing new approaches to manufacturing parts and products, employing distributed and collaborative production. In this context, simulation systems and software are also very much used. Through these tools, it is possible to simulate business systems and manufacturing processes by analyzing system input and output in real-time and obtaining a detailed report about the process under study [6]. The implementation and commissioning of a manufacturing system and, in particular, an FMS, involves high expenses, and the expected savings are difficult to specify. For this reason, both during the design of the structure of the flexible manufacturing system and in its operation, modeling and simulation techniques are used [4]. A virtual simulation model reflects the constraints of physical assets without errors and direct training on physical assets to extract appropriate solutions through virtual simulation [7]. As such, it is an efficient tool for extracting appropriate actions for physical assets, being more advantageous than performing the trial-and-error method, which is more expensive and time-consuming. Thus, it is possible to solve problems with physical assets in advance through 65 experiments [8; 9]. Nevertheless, because virtual simulation does not involve linking data between the physical and cyber worlds, it cannot actually generate a \"twin\". Therefore, virtual simulation cannot be regarded as digital twin [10]. Digital twin (DT) creates the virtual model of a physical entity in a digital way, promotes the interaction and integration of the physical and digital worlds, and builds a reliable bridge for industrial information integration. DT has been evolving at a fast pace, being widely used in smart manufacturing [11]. DT can link the physical and cyber worlds to enable the optimization of the entire manufacturing process [12]. According to a recent report by Grand View Research, the size of the global DT market was valued at USD 7.48 billion in 2021 and is expected to grow at a compound annual growth rate (CAGR) of 39.1% from 2022 to 2030.