Abstract : This paper discusses the application and development history of Manufacturing Execution System (MES) based on its integration model, external environment model, functional model, and REPAC model. It also elaborates on the current research, application, and development status of MES in China, arguing that MES is a fundamental technical means to achieve agile workshop production management. Based on this, the paper looks forward to the future development trends of MES from three perspectives: international, domestic, and technological. It points out that agility, intelligence, and integration are important development trends for MES technology and systems, providing a reference for the manufacturing industry to formulate overall solutions for MES implementation. Keywords: Manufacturing Execution System, Enterprise Resource Planning, Process Control System. The three essential elements for the survival of today's manufacturing industry are: Information Technology (IT), Supply Chain Management (SCM), and Batch Manufacturing Technology. Using IT transforms manual labor into faster and more efficient operations, significantly reducing human intervention and lowering production and operating costs. Supply Chain Management optimizes the flow and allocation of logistics resources throughout the entire production process from raw material supply to product delivery; this differs from partial optimization in that it represents comprehensive optimization. Batch Manufacturing Technology optimizes production planning and scheduling for the appropriate quantity of products at the right time, and with the deepening of manufacturing technology, it improves equipment management. Here, MES technology plays the most important role in enterprise informatization. Therefore, the promotion and application of MES has become one of the important tasks in manufacturing informatization projects. Researching the application and development of MES will provide necessary reference for the manufacturing industry to formulate overall solutions for MES implementation. 1. The Emergence, Development, and Models of MES 1) Single-Function MES Systems In the latter half of the 1970s, single-function MES systems emerged, such as equipment status monitoring systems, quality management systems, and production management systems including production progress tracking and production statistics, used to solve individual problems. That is, before the implementation of overall solutions or information systems, enterprises only introduced single-function software products and individual systems. At that time, the ERP layer (called MRP) and DCS layer were also operated separately, thus creating two problems: first, information silos between horizontal systems; and second, missing rings or links between the MRP, MRPII, and DCS layers. 2) T-MES Prototype In the mid-1980s, in order to solve the above two problems, the information system of the production site began to develop. The production progress tracking information system, quality information system, performance information system, equipment information system and their integration have reached a consensus. At the same time, the original bottom process control system and the upper production planning system have also been developed, and the MES prototype, namely the traditional MES (TMES), mainly POP (point of production) and SFC (shop floor control). 3) MES in the middle layer of informatization In the early 1990s, the industry began to realize the need for a middle layer that can integrate business systems and control systems. Manufacturing Execution System (MES) has always been a general term for a specific set, used to represent a set of specific functions and the products that realize these specific functions. In 1992, the American consulting and research company AMR (advanced manufacturing research) advocated the use of the three-layer model (3rd layer model) [1] to represent informatization in the manufacturing industry (see Figure 1), and called the execution layer located in the middle of the planning layer and the control layer MES, and explained the functions and importance of each layer. In China, the terms "production" and "manufacturing" are sometimes used interchangeably or equated, but in reality, the scope of "production" is broader than that of "manufacturing." MES (Manufacturing Execution System) occupies an intermediate position between Enterprise Information Systems (ERP/SCM) and Process Control Systems (DCS/PLC). ERP serves as a business management system, DCS/PLC as a control system, while MES serves as a production execution system. Together with upper-level business systems like ERP and lower-level production equipment control systems like DCS, MES constitutes the nervous system of the enterprise. Its functions are twofold: first, to transmit business plan instructions to the production floor; and second, to collect, upload, and process information from the production floor in a timely manner. MES is not merely a system oriented towards the production floor, but rather a system for transmitting information between the upper and lower levels, a cutting-edge system connecting the production floor and the operational level, and improving production and operational efficiency. MES is not a concept specific to any particular industry, but rather an important information system applied to various manufacturing industries. Because IT applications at the ERP and DCS levels started earlier and are largely implemented, it is necessary to integrate the two through the functional implementation of the MES layer and fill the gaps or narrow areas between them. Initially, MES did not have a very clear definition; it encompassed almost all applications or products that could not be accurately assigned to other layers. Most of these products evolved from customized applications, which were developed by system integrators for specific users and were usually for specific fields (such as scheduling, laboratory, quality, product tracking, etc.). 4) IMES and MES-II In the 1990s, MES evolved into IMES (integratedMES) and MES-II (manufacturing execution solutions). In 1993, AMR proposed an integrated model of MES [1] (see Figure 2), so MES appeared as an integrated system for the entire factory production site, hence it was also called IntegratedMES (IMES). This model includes four main functions and is supported by a real-time database. Under the guidance of this model, the focus of MES in the early 1990s was the integration of production site information. In 1997, the Manufacturing Execution System Association International (MESA) proposed the MES external environment model shown in Figure 3 [2]. As can be seen from Figure 3, Enterprise Resource Planning (ERP) system, Supply Chain Management (SCM) system, Sales and Service Management (SSM) system, Production Process (P&PE) system, Process Control (controls) system and Manufacturing Execution System (MES) are the six mainstream information systems for manufacturing enterprises. MES is closely connected with the other types of information systems, which makes it an important part of the overall information foundation of the enterprise. From the perspective of information integration, MES plays a role in vertical information integration between the enterprise-wide SCM, SSM, ERP and other systems and the control system for the factory's bottom equipment. At the same time, MES connects the SCM, SSM, ERP, P&PE and other systems, playing a role in horizontal information integration. In the mid-1990s, the idea of ​​MES standardization and functional componentization and modularization was proposed. At this time, many MES software implemented componentization, which also facilitated integration and consolidation, so that users could flexibly and quickly build their own MES according to their needs. In 1997, MESA proposed the functional model of MES [3] (see Figure 4). The model represents 11 functional modules arranged like a tortoise shell. MESA stipulates that even possessing only one or a few of these 11 modules constitutes a single-function product within the MES series. The AMR organization further refers to the overall solution implemented according to these 11 functions as MES-II. During this period, many groups, government agencies, and organizations participated in the standardization of MES and the research and establishment of standards and models. This involved distributed object technology, integration technology, platform technology, interoperability technology, and plug-and-play technologies. All organizations and scholars advocated for the standardization of MES functions and interfaces, as well as the importance of integration and interoperability. The use of ORB (object request broker) as the MES information access interface protocol was promoted between systems and between functional modules to achieve plug-and-play functionality of MES. Furthermore, the distributed object technology standard CORBA and related standard platforms are the foundation for developing MES software; however, factors such as high development costs, poor system performance, and unclear boundaries of the three-layer model have consistently hindered the componentization of MES functions. In 1998, AMR Corporation published the REPAC (ready, execute, process, analyze, coordinate) model [4] (see Figure 5). It is a process cycle consisting of five stages: preparation, implementation, processing, analysis, and adjustment. This model integrates and connects the five information systems corresponding to planning, technical solutions, scheduling, execution, and control. The REPAC model is not only for the middle layer of the three-layer MES model, but is connected with the corresponding system in the closed loop of the business system. In May 1994, AMR made the following statement about the concept of MES: MES provides the company with the means to achieve its execution goals when the company's entire resources are managed according to its business objectives. It connects the theoretical data of the basic information system and the actual data of the factory through a real-time database, and provides communication functions between the business planning system and the manufacturing control system. Therefore, MES is not just a single information system of the factory, but a system that integrates horizontally, vertically, and between systems, that is, the so-called business system. For various enterprise information systems that have received attention in recent years, such as SCP, ERP, CRM, and data warehouse, as long as they contain the factory as an object, they cannot do without MES. In summary, MES can be defined by one principle: how manufacturing is executed; two core databases: a real-time database and a relational database; two communication interfaces: one with the control layer and one with the business planning layer; four key functions: production management, process management, process management, and quality management; and eleven basic functions (including key and auxiliary functions). Over the past decade, the emergence of new business types has significantly driven technological innovation. This has led to considerable attention being paid to B2B and supply chain solutions. Although B2B and supply chain solutions are business-level solutions, their full implementation requires strong support from a Manufacturing Execution System (MES). Consequently, MES cannot simply be an interface layer between business and process; it also needs to establish numerous functions capable of fulfilling the company's critical business objectives. These functions cannot be independent of each other, nor can they be simply connected through a data exchange layer; rather, they must collaborate based on business and production strategies. This is clearly stated in the ISA-95 committee's documents. This not only describes the MES process but also the interactions between various functions, such as resource management and allocation, scheduling, data acquisition, quality assurance management, maintenance management, performance analysis, scheduling, document control, time management, and material and production tracking. 2. Current Status of MES Application and Development in China In the 1970s and 80s, China's industrial informatization focused primarily on factory automation, while in the 1980s and 90s it focused on management informatization. In the early 21st century, the focus remained on improving factory automation levels, popularizing DCS, PLC, and FCS, and enhancing management informatization, shifting from developing MIS to promoting ERP. MES, playing a bridging role in enterprise informatization, has primarily remained at the level of research on MES concepts, connotations, and system architecture; application system development has generally been limited to single MES functions. During the Ninth Five-Year Plan period, Beijing University of Aeronautics and Astronautics, Southeast University, Nanjing University of Science and Technology and other institutions have done some work in the development of MES theory and application systems with the support of the National 863 Program. For example, Xia Jinghua, Chen Jie and others [5,6] proposed the Advanced Management and Control System for Agile Manufacturing (AMCS) and attempted to build a workshop-level agile manufacturing system. Zhang Shuting, Zhou Hua, Yang Jianjun and others [7,8] started from the establishment of workshop production management system for agile manufacturing enterprises, analyzed and discussed the functions and application characteristics of manufacturing execution system, and proposed to use advanced MES ideas and software assistance to build agile workshop system. They also attempted to build an MES framework structure based on the concept of omnipotent body to solve the agility of MES. They believed that the MES system based on omnipotent body has the characteristics of autonomy, virtuality, self-organization, adaptability, dynamism, and a combination of distributed and centralized control. Song Haisheng and others [9] studied the MES software architecture of Web-based distributed production unit in network alliance enterprises, while Yu Haibin and others [10] explored the integrable MES technology system. In general, domestic research on MES focuses primarily on software modeling and development, lacking a systematic and in-depth study of the information nature and operational laws of MES. Independent product development is still in its early stages. Although a few domestic IT companies are both imitating foreign models and summarizing and refining the experience of Chinese industrial enterprises in MES, most products remain laboratory-scale and far from commercialization. In terms of application, some industries such as metallurgy, petrochemicals, tobacco, and food have already implemented MES, but these are still pioneering efforts. Nevertheless, products like Hollysys' HOLLiAS-MES (manufacturing execution system) and Zhejiang University Control Technology's ESPSuite-Enterprise Integrated Automation Solution will develop into relatively mature, large-scale, influential, promising, and potentially large-scale domestic MES products. Hollysys' HOLLiAS-MES (manufacturing execution system) uses an enterprise-level real-time database and relational database as its core, providing functions such as a real-time information system, quality analysis system, equipment maintenance management system, energy consumption management system, batch management system, production cost accounting system, and production scheduling system. HOLLiASMES provides enterprise production managers with a flexible and powerful tool for process monitoring and management, ensuring normal production operation, and controlling product quality and production costs. It features rich communication interfaces with control devices such as DCS and PLCs, exchanging data with different control systems through the OPCClient module. I/OServers support multiple communication interface protocols such as OPC and DDE, and can connect to DCS, PLCs, RTUs, and other devices manufactured by industrial automation companies such as Hollysys, Yokogawa, Honeywell, Siemens, Rockwell, ABB, Fisher-Rosemount, Hitachi, Mitsubishi, and Omron. It also provides an ODBC interface for integration with enterprise management systems such as ERP, achieving integrated management and control of enterprise production. HOLLiAS-MES focuses on production process information, providing direct support for enterprise decision-making systems. Its rich and flexibly configurable functional modules can meet the application requirements of different industries. Although much work remains to be done in terms of commercialization, productization, and standardization, this software can be considered a practical MES tailored to Chinese enterprises. Zhejiang University Control Technology Co., Ltd.'s ESP-Suite Enterprise Integrated Automation Solution is a comprehensive automation solution for process industry enterprises, encompassing software products, engineering, and technical services across three levels: Enterprise Resource Planning (ERP), Manufacturing Execution System (MES), and Process Control System (PCS). The ESP-Suite solution includes a series of application software centered around a comprehensive information integration software platform (based on a relational database) and a real-time monitoring software platform (based on a real-time database). Only a small portion of the vast ERP vendors and development teams are moving towards MES. This is because MES is closely integrated with industrial control, and its development and implementation require a strong foundation in industrial automation and extensive industrial field engineering experience—a significant technical barrier that excludes many ERP-type IT vendors. However, MES software developers and system integrators are facing pressure from two directions: top-down penetration from ERP software developers and system integrators, extending their functionality to MES; and bottom-up impact from HMI and SCADA software developers and system integrators, increasing the functionality of MES. Furthermore, powerful automation system suppliers (such as Siemens and Rockwell Automation) are acquiring successful MES companies or developing MES software packages to seize the MES market. Promoting the widespread adoption of MES still has a long way to go. With the development of industries and economies, the development of CIMS, and the continuous improvement of workshop automation levels, the demand for integrated, intelligent, and agile workshop production management will promote mergers or close cooperation between a considerable number of IT vendors, automation vendors, and manufacturing enterprises. In the next few years, MES vendors may emerge rapidly, and their technologies and implementation methods will inevitably become more mature, greatly promoting the in-depth application of domestic MES products in China. 3 Conclusion From the perspective of international development trends, a number of MES software products and solutions have been developed, and a number of industrial enterprise management application software companies with MES as their core product have emerged. Moreover, the focus of enterprise information technology applications has shifted from ERP to MES. However, much work still needs to be done in the standardization of MES. From a domestic development perspective, concepts like CIMS, MES, and ERP entered China relatively early. However, only the adoption of lower-level automation represented by DCS and management systems represented by ERP has been rapid. Consequently, information silos and broken links are more pronounced in Chinese industrial enterprises compared to their foreign counterparts, and the understanding of the MES layer lags behind. Nevertheless, some companies are accelerating the development of Chinese-style MES products or applying foreign MES products, and the momentum is positive. Simultaneously, a shift in mindset from focusing on ERP to focusing on MES has begun. From a technical perspective, MES functions are penetrating into the control and planning layers, potentially blurring the lines of division among the three layers. The Planning, Scheduling, and Execution Scheduling (APS) module within MES is becoming increasingly prominent and widespread, and it should be considered the core of MES. Single-function MES products are evolving into integrated MES and overall solutions. 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