1. Introduction
Before the 1970s, analog systems were used to transmit and control signals through analog quantities. These systems had low accuracy and were highly susceptible to interference, resulting in poor overall system stability and control effectiveness. By the late 1970s, with the development of large-scale integrated circuits and microprocessor technology, microprocessors became more powerful, compact, and reliable. They were used to control systems via interface circuits, improving control performance. However, these systems still relied on centralized control. As process control, automation instrumentation, and computer networking technologies matured, a new technological shift occurred in the control field. This shift significantly advanced traditional control systems, such as distributed control systems (DCS), both in structure and performance. The foundation of this change was the emergence of fieldbus technology.
Fieldbus is a two-way serial, digital, multi-node communication network that connects intelligent field devices with automation control systems. It is also known as the Field Underlying Device Control Network (INFRANET). Since the 1980s, various fieldbus technologies have emerged. There has been growing demand to replace traditional analog instrumentation and control systems, making fieldbus a global technological trend. In 1984, the American Instrumentation Association (ISA) began developing fieldbus standards. In Europe, Germany introduced PROFIBUS, and France developed FIP, leading to the formation of multiple fieldbus standards. Key examples include Foundation Fieldbus (FF), Controller Area Network (CAN), Local Operating Network (LonWorks), Process Field Bus PROFIBUS, and HART Protocol. Despite their differences, the development of a single, open international fieldbus standard is inevitable.
2. Fieldbus Technology and Its Characteristics
Fieldbus is one of the most prominent developments in automation technology today. Often referred to as the "LAN of automation," it enables digital communication between field devices. Most existing field devices now use microprocessors and digital components internally, requiring digital communication between them. Fieldbus is a fully digital, two-way, multi-station communication system that links intelligent field devices with automation systems. It addresses the need for digital communication between field instruments, controllers, and actuators, integrating technologies like digital communication, computing, control, networking, and smart instrumentation. It breaks away from the limitations of traditional point-to-point analog or digital signal control, creating a fully distributed, digital, intelligent, bidirectional, and interconnected system.
Fieldbus is a digital, two-way transmission, multi-branch communication network that connects intelligent field devices with automation systems, based on smart meters. These smart meters, located across industrial sites, are connected via a digital fieldbus and combined with controllers and monitors in the control room to form a Fieldbus Control System (FCS). Following international standards, fieldbus products from different manufacturers can be integrated into the same FCS, ensuring interoperability and interchangeability. FCS decentralizes the control functions of traditional DCS to on-site smart meters, allowing them to perform data acquisition, processing, control operations, and output. Field instrument data, including collected and diagnostic information, is transmitted to the control room. The control room monitors each field device, saves data from smart meters, and executes advanced control features that would otherwise require many field instruments. Additionally, FCS can connect to an enterprise’s management network through a gateway, enabling managers to access real-time data and support decision-making.
Fieldbus offers several outstanding features:
2.1 Open Communication: FCS uses open communication protocols, allowing devices from different manufacturers to interconnect and exchange information. Users can choose products from various vendors to build an optimal control system.
2.2 Interoperability: Devices from different manufacturers can not only communicate but also be configured uniformly, achieving the same control strategy and “plug-and-play†functionality. Devices with similar performance from different manufacturers can be swapped.
2.3 Flexible Network Topology: FCS can adapt to complex field conditions by forming different network topologies, such as tree, star, bus, and hierarchical structures.
2.4 Highly Decentralized Structure: Field devices are intelligent, capable of independent control, fundamentally changing the centralized-decentralized DCS model. Control functions are fully distributed, enhancing reliability and simplifying system architecture. Even if the fieldbus is disconnected from the upper network, field devices can operate independently, greatly increasing their intelligence.
2.5 Intelligent Field Devices: Unlike traditional DCS, which relies on centralized control stations, FCS distributes control functions to field devices. These devices can handle tasks like data acquisition, PID control, self-diagnosis, and remote monitoring, enabling operators to adjust settings and diagnose faults from the control room.
2.6 Environmental Adaptability: Designed for industrial environments, fieldbus supports twisted pair, coaxial cable, fiber optics, and wireless communication. It is highly resistant to interference. Commonly used twisted pairs are cost-effective, and they can power field devices while meeting intrinsic safety requirements.
3. Overview of Major Fieldbuses
3.1 Foundation Fieldbus (FF): Developed by the Fieldbus Foundation, FF is an open protocol not tied to any company or country. It follows the ISO OSI model and includes a user layer. FF is widely supported by global instrument manufacturers and has strong potential as an international standard.
FF provides two physical layers: H1 (low-speed, 31.25 kbps) and H2 (high-speed, 1 Mbps or 2.5 Mbps). H1 supports bus-powered and intrinsically safe devices, while H2 is suitable for high-speed data transfer. Both can be interconnected using bridges.
3.2 PROFIBUS: Proposed by Siemens, PROFIBUS is a German national and European standard. It includes three series: PA (Process Automation), DP (Decentralized Periphery), and FMS (Fieldbus Message Specification). PROFIBUS-PA is suitable for low-speed process automation, while DP supports high-speed communication. FMS is ideal for medium-speed general automation.
3.3 LonWorks: A network control technology based on the ISO OSI seven-layer model, using Neuron Chip technology. It allows object-oriented programming and communication via network variables or explicit messages.
3.4 CAN (Controller Area Network): Developed by Bosch in 1983 for automotive applications, CAN supports high bit rates and strong immunity to electromagnetic interference. It is widely used in automotive and aerospace industries.
3.5 HART: Introduced by Rosemount in 1986, HART is a communication protocol for smart field devices. It operates over analog lines and is a transitional solution during the shift from analog to digital systems. It remains popular due to its compatibility and market share.
4. FCS Control System
Traditional DCS systems rely on centralized control, requiring extensive wiring and complex configurations. This leads to high costs, signal degradation, and maintenance challenges. In contrast, FCS (Fieldbus Control System) places all I/O modules in the field, converting signals to digital format. This reduces cabling, improves efficiency, and enhances signal integrity. FCS is open, allowing interoperability between hardware and software, and supports IEC 1131-3 programming languages. It also integrates PC resources for improved performance and reduced costs.
The basic structure of FCS includes an industrial computer or commercial PC, fieldbus master interface card, fieldbus I/O modules, PLC/NC/CNC control software, configuration tools, and application software. The host computer handles system configuration, data display, alarm management, and human-machine interaction, achieving control concentration, data sharing, and open control requirements.
5. Conclusion
Currently, various fieldbus protocols coexist in the control industry. LonWorks and CAN have advantages in building automation, while HART, FF, and PROFIBUS dominate process automation. HART will remain a key protocol for smart instrumentation in the near future, while FF shows great promise in process automation. Due to its openness, LonWorks has attracted many developers in China. While the transition to a unified fieldbus standard is gradual, fieldbus technology continues to evolve, promising widespread adoption in the future.
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