Fieldbus Technology and Control Systems (Full Text Reproduction)

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　　 　　　　1 Introduction 　　　　Before the 1970s, analog signals were used in control systems to convert and transmit control signals; however, their accuracy was poor and they were highly susceptible to interference, which resulted in poor control performance and system stability overall. At the end of the 1970s, with the emergence of large-scale integrated circuits, microprocessor technology made great progress. Microprocessors feature strong performance, small size, and high reliability; when used in control systems through appropriate interface circuits, they enhance the control efficiency ； Nevertheless, it still belongs to a centralized control system. With the maturity and development of process control technology, automated instrumentation technology, and computer network technology, another technological revolution has taken place in the field of control. This transformation brought about a tremendous leap in both structure and performance for traditional control systems such as distributed control systems, and the foundation of this transformation was the emergence of fieldbus technology. 　　　　A field bus is a two-way, serial, digital, multi-node communication network that connects field intelligent devices with automated control devices; it is also known as the Infrastructure Network for Field Devices (INFRANET). Since the 1980s, various fieldbus technologies have emerged, and there has been an increasing demand for changes to traditional analog instruments and control systems, which has turned fieldbuses into a global wave of technological transformation. The Instrumentation Society of America (ISA) began developing fieldbus standards in 1984; in Europe, standards such as PROFIBUS in Germany and FIP in France emerged, and various fieldbus standards were gradually created. The main ones include: Foundation Fieldbus, Controller Area Network, Local Operating Network, Process Field Bus, and HART protocol (Highway Addressable Remote Transducer), among others. However, the development of bus standards has not been smooth; due to historical factors such as industry and regional development, as well as the pursuit of self-interest by various companies and corporate groups, progress toward international standards for field buses has been slow. But in any case, the development of a single open international fieldbus standard is an inevitable trend. 　　　　2　Several currently popular types of fieldbuses　　　　2.1　Foundation Fieldbus FF　　　　Foundation Fieldbus FF is a technology that enjoys wide support in the field of process automation and has good prospects for development. Its predecessors were the ISP protocol developed by American company Fisher-Rosemount in collaboration with 80 other companies including Foxboro, Yokogawa, ABB, and Siemens, and the World FIP protocol developed by Honeywell in partnership with 150 companies from Europe and other regions. These two major groups merged in September 1994 to establish the Fieldbus Foundation, which is dedicated to developing a unified international fieldbus protocol. 　　　　The Foundation fieldbus comes in two communication speeds: H1 and high-speed H2. H1 has a transmission rate of 31.25 Kbps, a communication range of up to 1.9 km, and supports bus power supply as well as intrinsically safe use in explosive environments. The transmission rate of H2 can be 1Mbps or 2.5Mbps, with communication distances of 750m and 500m respectively. Physical transmission media can be twisted pair, optical fiber, and wireless, with Manchester encoding used for signal transmission. The Foundation Fieldbus is based on the ISO/OSI open system interconnection model, adopting its physical layer, data link layer, and application layer as the corresponding layers of the FF communication model, with an additional user layer added at the application layer. The user layer is designed to meet the needs of automated measurement and control applications; it defines unified rules for information access and specifies a set of common functional blocks using a device description language. The FF bus includes the FF communication protocol, a stack of communication protocols for layers 2 to 7 in the ISO model, the DDL device description language used to describe device characteristics and operation interfaces, device description dictionaries, application function blocks for performing measurement, control, and unit conversion tasks, system software technologies for managing system configuration, as well as system integration technologies for building integrated automation systems and network systems. 　　　　2.2 CAN Bus The CAN bus was first introduced by the German company Bosch as a data communication protocol for use between various measurement and control components within vehicles. Its bus specification has been established as an international standard by the ISO International Organization, and it is widely used in the field of discrete control. It is also based on the OSI model, but has been optimized by using its physical layer, data link layer, and application layer to improve real-time performance. Its nodes have priority settings, and it supports point-to-point, one-to-many, and broadcast communication modes. Each node can send messages at any time. The transmission medium is twisted pair, and the communication rate depends on the bus length. The CAN bus uses short message frames, with 8 valid bytes per frame ； It can shut down automatically when a node fails, boasts strong interference resistance, and high reliability. 　　　　2.3 LonWorks Bus The LonWorks technology was developed by the American company ECHELON, and it is a fieldbus technology promoted in collaboration with Motorola and Toshiba. It adopts the full seven-layer protocol structure of the OSI reference model. The core of LonWorks technology is the Neuron chip, which possesses communication and control functions. The Neuron chip implements the full LonTalk communication protocol of LonWorks. It integrates three 8-bit CPUs. A CPU that performs the functions of the first and second layers of the OSI model is called a media access processor. A CPU is an application processor that runs the operating system and user code. There is also a CPU that serves as a network processor; acting as an intermediary between the first two, it handles tasks such as network variable addressing, updating, path selection, and network communication management. Nodes composed of neural chips can communicate peer-to-peer. LonWorks supports a variety of physical media and multiple topologies, offering flexible networking options; its IS-78 intrinsically safe physical channel enables its use in hazardous areas. The main application areas of LonWorks include building automation and industrial control, and it offers excellent performance in establishing distributed monitoring networks. 　　　　2.4 PROFIBUS Bus 　　　　PROFIBUS is a fieldbus that complies with the German standard DIN19245 and the European standard EN50179; it consists of three variants: PROFIBUS‑DP, PROFIBUS‑FMS, and PROFIBUS‑PA. It also only uses the physical layer, data link layer, and application layer of the OSI model. PROFIBUS supports master-slave mode, pure master mode, and multi-master multi-slave communication mode. The master station has control over the bus, and control over the bus is transferred between master stations by passing tokens. The master station that has gained control can send and receive information from the slave stations. PROFIBUS‑DP is used for high-speed data transmission between distributed peripherals, and is suitable for the field of process automation. The FMS type is suitable for textiles, building automation, programmable controllers, low-voltage switches, etc. The PA type is a bus type used for process automation. 　　　　2.5 HART Bus The HART protocol is a communication protocol developed by Rosemount in 1986. It is a protocol used for communication between field smart instruments and control room equipment. It includes the physical layer, data link layer, and application layer of the ISO/OSI model. HART communication can operate in either point-to-point or multi-point connection modes. This protocol is an open communication protocol for high-speed channels of addressable remote sensors; it features the transmission of digital signals over existing analog signal lines. Being a transitional product in the shift from analog systems to digital systems, it possesses strong market competitiveness during this current transition period and holds a significant share in the smart meter market. 　　　　3 Structure and Characteristics of Field Bus Control Systems (FCS) 　　　3.1 Structure 　　　With the emergence and maturity of field bus technology, control systems have transitioned from Distributed Control Systems (DCS) to Field Bus Control Systems (FCS). In a typical FCS system, field instruments that comply with certain field bus protocols can form control loops, allowing some of the control functions of the control station to be decentralized and distributed among these field instruments. This reduces the burden on the control station, allowing it to focus solely on executing complex, high-level control algorithms. For simple control applications, it is even possible to eliminate the control station; in its place, bridges and hubs that serve as connectors for the field bus are used, with the operation stations being connected directly to the field instruments, thus forming a distributed control system. 　　　　3.2 Characteristics 　　　 The distributed FCS system better embodies the concept of \"centralized information, decentralized control\" compared to the DCS system. Compared to traditional DCS, FCS has its own characteristics. The FCS system is highly decentralized; it can consist of field devices that form autonomous control loops. On-site instruments or equipment possess a high degree of intelligence and functional autonomy; they can carry out the basic control functions and diagnose the operating condition of the equipment at any time. Furthermore, the structure of FCS is simpler than that of DCS. Some FCS systems omit the control station layer in DCS, with the operation stations being connected directly to the field instruments. These improve the reliability of FCS. 　　　　Fieldbus systems are open. The system ensures consistency and transparency regarding relevant standards, emphasizing consensus and compliance with them. Communication protocols are publicly standardized, enabling information exchange between devices from different manufacturers; open and interconnected systems in the field of automation can be established through field buses. The openness of the system determines its interoperability and reusability. Interoperability refers to the transmission of information and communication between interconnected devices and systems ； Interoperability, on the other hand, means that devices with similar performance from different manufacturers can be substituted for one another. As the fieldbus at the foundation of factory networks, it also possesses strong adaptability to the field environment. It supports twisted pair, coaxial cable, optical fiber, wireless, and power lines, etc., and has strong interference resistance. 　　　　Due to structural changes, FCS is more hardware-efficient than DCS. Using FCS can reduce the need for a large number of isolators, terminal cabinets, I/O cards, and I/O ports, thereby saving space for I/O devices and device rooms ； It also reduces the number of cables significantly, which can greatly save on installation costs. Meanwhile, FCS offers improved performance compared to DCS. By eliminating the D/A and A/D conversions, the accuracy of the instrument is greatly improved ； By integrating the PID function into the corresponding smart sensors, the control cycle is significantly shortened. Currently, FCS can be increased from 2–5 adjustments per second in DCS to 10–20 adjustments per second, improving the adjustment performance. A comparison of the structure between the FCS control system and the DCS control system is shown in Figure 1. 　　　　3.3 Advantages of Field Buses 　　　　Due to the above characteristics of field buses, especially the simplification of their system architecture, they exhibit advantages throughout the entire process, from design and installation and commissioning to normal operational performance as well as maintenance. It not only reduces the number of hardware units and associated investment as well as installation costs, but it also **significantly lowers** the maintenance expenses of the system. Fieldbus control systems not only offer high precision and reliability, but FCS also has advantages over DCS in terms of ease of use and maintenance. FCS uses a unified configuration method, making installation, operation, and maintenance simple ； Intelligent field instruments make predicted maintenance possible ； Thanks to the system’s interoperability and interusability, users can freely choose devices from different brands to achieve optimal system integration. In the event of a device failure, they can select replacement devices at will, ensuring them full control over system integration. 　　　　In addition, it also features a simple design and is easy to refactor. 　　　　4 Development Trends 　　　　Traditional Distributed Control System (DCS) systems feature centralized monitoring, decentralized control, and ease of operation. However, in practical applications, it has also been found that the structure of DCS has certain shortcomings; for example, control cannot be fully decentralized, and risks remain relatively concentrated ； Due to the lack of openness of the system, products from different manufacturers cannot be interchanged or interconnected, which limits the range of choices available to users. By utilizing fieldbus technology, the goal of developing the FCS system is to address certain shortcomings of existing DCS systems, improve the structure of the control system, and enhance its performance and versatility. 　　　　FCS aims to replace DCS in practice, needing to possess the functions of DCS while also being able to overcome its shortcomings. Due to the use of fieldbus technology, FCS is superior to traditional DCS in terms of openness and decentralized control. However, since it is a new technology, unified standards have not yet been established, which means that compared to mature DCS systems, FCS still has the following shortcomings. 　　　　(1) Since the fieldbus standards themselves are still in the process of development, this poses difficulties in the development and testing of products. To some extent, this results in a small number of product developers and manufacturers, leading to a limited variety of products at high prices. 　　　　(2) In some cases, FCS is also unable to provide the control functions already available in DCS. Due to limitations in software and hardware capabilities, the functions of its functional blocks are not very strong, and the range of available options is also limited ； Using on-site instruments, only basic control loops such as single-loop, cascade, and proportional control can be implemented; complex and advanced control algorithms cannot be realized with these instruments. There is also a lack of effective solutions for situations in which a single loop has multiple inputs and outputs. 　　　　(3) There are few successful application examples of FCS at present, making it difficult to evaluate its actual performance in practice. 　　　　For these reasons, the replacement of DCS with FCS will be a gradual process. During this process, some transitional system architectures emerge, such as the replacement of certain subsystems in DCS with FCS. The user connects the fieldbus devices to the fieldbus network server**, which is equipped with an upper-level network interface connected to the operation stations in the DCS, allowing direct communication with those operation stations. Appropriate communication and management software can be added to the DCS software system. This way, no major structural changes to the existing control system are required. 　　　　5 Conclusion 　　　　Currently, various forms of fieldbus protocols coexist in the field of control. In the field of building automation, Lonworks and CAN networks have certain advantages ； In the field of process automation, there are primarily the transitional HART protocol, the widely adopted FF fieldbus protocol, and the equally competitive PROFIBUS protocol. The HART protocol will be the main communication protocol for intelligent instruments in the coming years ； The Foundation Fieldbus is a promising type of field bus in the field of process automation, and it enjoys support from many manufacturers of automated instrumentation equipment ； Due to the openness of Lonworks technology, many developers in China have emerged to use it for developing control systems. Given the long-term nature and difficulty of establishing a unified open fieldbus protocol standard, the phasing out of traditional DCS systems will be a gradual process. For a period of time, there will be a situation in which several fieldbuses coexist, with multiple heterogeneous networks interconnected and communicating within the same production site. However, it is an inevitable trend to develop unified standard specifications that are commonly followed in order to truly create open interconnection systems.