Abstract: This paper introduces fieldbus technology and focuses on its ten key features, comparing the wiring methods of fieldbus control systems with those of traditional control systems.
1. Fieldbus technology
Fieldbus-based control systems break away from the traditional structure of connecting devices one-to-one according to control loop requirements. By placing the control modules and input/output modules that were originally located in the control room of the DCS system into field devices, and with the communication capabilities of these field devices, the control system functions can be completed directly on-site without relying on the computer or control instruments in the control room, achieving completely distributed control.
A fieldbus control system is both an open communication network and a fully distributed control system. It connects intelligent devices as network nodes into an automated network system, realizing comprehensive automation functions such as basic control, compensation calculation, parameter modification, alarm, display, monitoring, and optimization. Fieldbus technology is a comprehensive technology primarily based on intelligent sensors, control, computers, digital communication, and networks.
Fieldbus technology is an intersection and integration of control, computer, and communication technologies, covering almost all continuous and discrete industrial fields, such as process automation, manufacturing automation, building automation, and home automation. Its emergence and rapid development reflect the control field's demands for reduced costs, improved reliability, enhanced maintainability, and increased intelligence in data acquisition. The development of fieldbus technology is reflected in two aspects: the continuous development and improvement of low-speed fieldbus technology and the development of high-speed fieldbus technology. Currently, fieldbus products are mainly low-speed bus products, applied in fields with lower operating speeds and less stringent network performance requirements. From a practical application perspective, most fieldbuses can effectively achieve process control with lower speed requirements.
2. Characteristics of Fieldbus
Fieldbus technology essentially replaces traditional parallel signal transmission and connection methods with serial data transmission and connection. It sequentially realizes data transmission between the control layer and the fieldbus device layer, while ensuring real-time transmission and maintaining information reliability and openness. Compared to traditional PLC point-to-point control methods, fieldbus control systems have unparalleled advantages. Typical fieldbus systems have the following characteristics:
1) High performance-price ratio. Simple wiring is a common characteristic of most fieldbuses. The biggest revolution in fieldbuses is the revolution in wiring methods; minimal wiring and maximized network topology significantly reduce system wiring and maintenance costs. Because they use a serial approach, most fieldbuses use twisted-pair cables, and some even have power applied directly to the two signal lines. This makes fieldbus-based devices and systems feel simple and intuitive. The significant reduction in wires and connectors reduces the number of control cables (from hundreds or even thousands) to a single bus cable, thus greatly reducing the need for terminals, cable trays, and other accessories. This significantly reduces design, installation, commissioning, and maintenance costs. Previously cumbersome schematic and wiring diagram designs become simple and easy; standard connectors allow for quick and easy installation, greatly reducing manpower and material costs; and powerful fault diagnosis capabilities significantly reduce the workload of system commissioning and maintenance.
2) System performance is significantly improved. Reliable data transmission, rapid data response, and strong anti-interference capabilities elevate the control system to a new level. Many buses have strict regulations regarding communication media, information verification, information error correction, and duplicate address detection, thereby ensuring fast and completely reliable bus communication.
3) Generally, buses possess a certain degree of anti-interference capability. Furthermore, when a system fault occurs, they have a certain diagnostic capability to maximize network protection. The extent of a bus's fault diagnosis capability is determined by the physical transmission medium and software protocol used, therefore different buses have different diagnostic and processing capabilities. They also possess powerful automatic diagnostic and fault display functions. Diagnosis includes communication faults and power supply faults of bus nodes, as well as open-circuit and short-circuit faults in field devices and connectors, thereby quickly identifying the location and status of various system faults and enabling rapid replacement of faulty nodes.
4) Employing digital signal communication effectively improves the measurement and control accuracy of the system. Various switching and analog signals are converted into digital signals at the nearest point, avoiding signal attenuation and distortion. The real-time requirements of the bus are designed to adapt to the characteristics of field control and data acquisition. Generally, fieldbuses require high transmission rates and efficiency while ensuring data reliability and integrity. The faster the bus transmission speed, the better, as a faster speed means a shorter system response time. However, transmission speed cannot be improved solely by increasing the transmission rate; transmission efficiency is also crucial. Transmission efficiency mainly refers to the ratio of valid user data in the transmitted frames and the ratio of successfully transmitted frames to all transmitted frames.
5) The bus nodes have an IP67 protection rating, providing waterproof, dustproof, and vibration-resistant properties. They can be directly installed on industrial equipment, significantly reducing the need for field junction boxes and improving system reliability.
6) Adaptability to the field environment. Operating at the front end of the production site as the underlying fieldbus of the factory network, it is specifically designed for the field environment. It supports twisted-pair cables, coaxial cables, optical fibers, radio frequency, infrared, power lines, etc., has strong anti-interference capabilities, can achieve power supply and communication using a two-wire system, and meets intrinsically safe explosion-proof requirements. It is more suitable for direct installation in hazardous explosion-proof locations such as petroleum and chemical plants, reducing the possibility of system hazards.
7) Intelligence and Autonomy of Field Devices. Fieldbus distributes measurement, compensation calculation, engineering quantity processing, and control functions to field devices, enabling them to perform basic automatic control functions and diagnose equipment operating status at any time. Because PID control can be integrated into transmitters or actuators, the control cycle is significantly shortened. Currently, the number of adjustments per second can be increased from 2-3 times per second in a DCS to 10-20 times per second, thus improving regulation performance.
8) Interoperability and Interoperability. Interoperability refers to the ability to transmit and communicate information between interconnected devices and systems; it enables point-to-point and point-to-multipoint digital communication. Interoperability means that devices with similar performance from different manufacturers can be interchanged. By eliminating bottlenecks at the host computer's entry point, system security and reliability can be improved, allowing the host computer more time to perform optimization and other tasks to increase efficiency.
9) System Openness. An open system refers to a system with publicly available communication protocols, allowing interconnection and information exchange between devices from different manufacturers. Fieldbus developers aim to establish a unified, open system for the factory's underlying network. Here, openness refers to consistency and transparency regarding relevant standards, emphasizing consensus and adherence to those standards. An open system can connect to any other device or system that complies with the same standards. A fieldbus network system with bus functionality must be open, giving users the right to integrate the system. Users can combine products from different vendors into systems of any size according to their needs and objectives. The openness of the bus has two aspects: firstly, the ability to connect to different control systems, i.e., application openness; secondly, the openness of the communication protocol, i.e., development openness. Only with openness can a fieldbus possess both the low cost of traditional buses and the network and system requirements of advanced control.
10) Highly distributed system architecture. Since field devices can already perform basic automatic control functions, the fieldbus has become a new fully distributed control system architecture. This fundamentally changes the existing DCS (Distributed Control System) architecture, which combines centralized and decentralized approaches, simplifying the system structure and improving reliability.
3. Wiring method for fieldbus system
The application of fieldbus technology requires that field devices (sensors, drivers, actuators, etc.) be intelligent (programmable or parameterizable) devices with serial communication interfaces. Therefore, fieldbus technology is based on the development of large-scale integrated circuits in computers. Fieldbus technology, which integrates remote control, parameterization, and fault diagnosis of field devices, uses computer digital communication technology to connect intelligent field devices. Therefore, the controller can obtain a wealth of information from the field devices, enabling the transmission of device status, fault, and parameter information, and completing remote control, parameterization, and fault diagnosis of the devices.
Fieldbus technology is a key technology for realizing the integration of information at the factory level. It is the information integration and support technology for implementing CIMS at all levels. Fieldbus is an extension of the factory computer network to field-level devices and is the technical foundation for supporting the integration of information at the field level and the workshop level.
Fieldbus systems break away from the one-to-one device connection mode used in traditional analog control systems, adopting a bus communication method. Therefore, control functions can be completed directly on-site without relying on a control room computer, achieving thoroughly distributed control. This simplifies system structure, saves hardware equipment, and reduces connection cables and various installation and maintenance costs. Figure 1 shows a comparison of the wiring between fieldbus control systems and traditional control systems.
The functions of fieldbus are: to construct a network-integrated, fully distributed system; and to serve as a low-level control network. Fieldbus enhances the information acquisition capabilities at the field level, enabling it to obtain a wealth of information from field devices, effectively meeting the information integration requirements of factory automation and even CIMS systems. As a digital communication network, fieldbus does not simply replace 4-20mA signals; it also enables the transmission of equipment status, fault, and parameter information. In addition to remote control, the system can also perform remote parameterization.
Products from different manufacturers are interoperable and interchangeable as long as they use the same bus standard, thus the equipment has excellent integrability. The system is open, allowing other manufacturers to integrate their proprietary control technologies, such as control algorithms, process methods, and formulas, into the general control system. Therefore, there will be many monitoring and control systems on the market tailored to specific industry needs.
