What is Fieldbus?
Fieldbus is an industrial data bus that has developed rapidly in recent years. It mainly solves the problems of digital communication between intelligent instruments, controllers, actuators and other field devices in industrial settings, as well as the information transmission between these field control devices and higher-level control systems. Due to its simplicity, reliability, cost-effectiveness and other outstanding advantages, fieldbus has received high attention from many standards organizations and computer manufacturers.
It is an industrial data bus, serving as the underlying data communication network in the field of automation . Simply put, fieldbus replaces the traditional 4-20mA analog signal and ordinary switch signal transmission with digital communication. It is a fully digital, bidirectional, multi-station communication system that connects intelligent field devices and automation systems.
Key features of fieldbus
1. System openness
Traditional control systems are self-contained systems, typically communicating with the outside world only through serial or parallel ports on workstations. In fieldbus technology, users can assemble systems of any size from different vendors according to their needs and target applications.
2. Operability and Reliability
When using the same communication protocol, fieldbus can achieve information transmission and communication between interconnected devices and systems by simply selecting appropriate bus network cards, ports and adapters, which greatly reduces the workload of wiring and troubleshooting and effectively improves the reliability of control.
3. Intelligence and functional autonomy of field equipment
Traditional CNC machine tools transmit signals in a one-way analog manner, resulting in significant errors during transmission. This makes it difficult for the system to quickly diagnose faults, leading to continued operation with malfunctions. In contrast, fieldbus systems employ bidirectional digital communication, distributing functions such as sensing and measurement, compensation calculation, engineering quantity processing, and control to field devices, enabling real-time diagnosis of equipment operating status.
4. Adaptability to the on-site environment
Fieldbus is designed to work in field environments and can support twisted pair, coaxial cable, optical fiber, radio frequency, infrared and power lines. It has strong anti-interference capabilities, can use two-wire system to achieve power transmission and communication, and can meet safety and explosion-proof requirements.
Advantages and disadvantages of fieldbus
advantage:
(1) Save on hardware quantity and investment
Since the intelligent devices distributed on-site can directly perform a variety of sensing, measurement, control, alarm and calculation functions, the number of transmitters can be reduced. There is no need for separate regulators, calculation units, etc., nor for the signal conditioning, conversion, isolation and other functional units of the DCS system and their complex wiring. An industrial control PC can also be used as an operator station, thus saving a lot of hardware investment and reducing the floor space of the control room.
(2) Save on installation costs
Wiring in a fieldbus system is very simple; multiple devices can typically be connected to a single twisted pair or cable. This significantly reduces the amount of cables, terminals, cable trays, and wiring harnesses needed, as well as the workload of wiring design and connector calibration. When adding field control devices, no new cables are required; they can be connected to existing cables, saving both investment and design and installation work. Calculations from typical pilot projects show that installation costs can be reduced by more than 60%.
(3) Save on maintenance costs
The field control equipment has the ability to self-diagnose and handle simple faults. It also sends relevant diagnostic and maintenance information to the control room through digital communication. Users can query the operation, diagnostic and maintenance information of all equipment to analyze the cause of faults early and quickly eliminate them, thus shortening maintenance downtime. At the same time, the simplified system structure and simple wiring reduce the workload of maintenance.
(4) Users have a high degree of initiative in system integration.
Users can freely choose equipment from different manufacturers to integrate their systems. This avoids limiting the range of equipment options due to choosing a particular brand, and prevents them from being at a loss due to incompatible protocols and interfaces during system integration. It ensures that the user maintains control over the system integration process.
(5) Improved the accuracy and reliability of the system
The intelligent and digital transformation of field equipment fundamentally improves the accuracy of measurement and control and reduces transmission errors compared to analog signals. The simplified system structure, with fewer devices and wiring, enhances the internal functions of field equipment, reduces signal round-trip transmission, and improves system reliability.
In addition, due to its standardized equipment and modular functions, it also has the advantages of simple design and easy reconfiguration.
shortcoming:
In network communication, data packet transmission delays, transient errors and data packet loss in communication systems, and inconsistencies in the order of sending and arriving data can all disrupt the determinism inherent in traditional control systems, making the analysis and synthesis of control systems more complex and negatively impacting the performance of control systems.
Composition of a fieldbus control system
1. Fieldbus Control System
Its software is a crucial component of the system. Control system software includes configuration software, maintenance software, simulation software, equipment software, and monitoring software. The first step is to select and develop configuration software and a human-machine interface (MMI) for control operation. Through the configuration software, connections between function blocks are established, function block parameters are selected, and network configuration is performed. During network operation, real-time data is collected from the system, and data processing and calculations are performed. This optimizes control and logic control, alarms, monitoring, display, and reporting.
2. Fieldbus Measurement System
Its features include multivariable and high-performance measurement, giving the measuring instrument more functions such as computing capabilities. Due to the use of digital signals, it has high resolution, high accuracy, strong anti-interference and anti-distortion capabilities, and also has the status information of the instrument equipment, which can be used to adjust the processing.
3. Equipment Management System
It can provide diagnostic information, management information, equipment operating status information (including smart meters), and equipment manufacturing information provided by the manufacturer, both for the equipment itself and the process. For example, Fisher-Rosemoune offers the AMS management system, which is installed on the main computer and performs management functions, forming a comprehensive management system database for field equipment. Based on this, it enables equipment reliability analysis and predictive maintenance, transforming a passive management model into a predictive management and maintenance model. The AMS software uses a T-shaped structure with a field server as the platform, supporting modular, feature-rich application software and providing users with a graphical interface.
4. Bus system computer service mode
The client/server model is a popular network computer service model. The server represents the data source (provider), while the application client represents the data user, retrieving data from the data source and processing it further. The client machine runs on a PC or workstation. The server runs on a minicomputer or mainframe, utilizing the intelligence, resources, and data of both to complete the task.
5. Database
It can organize and dynamically store large amounts of relevant data and applications, enabling full data sharing and cross-access, while maintaining a high degree of independence. Industrial equipment experiences continuous parameter changes and generates large amounts of data during operation, requiring high real-time performance in operation and control. Therefore, a distributed, real-time database system capable of cross-access and operation has been developed. Mature and readily available options include relational databases such as Oracle, Sybass, Informix, and SQL Server; and real-time databases such as Infoplus, PI, and ONSPEC.
6. Hardware and software of the network system
Network system hardware includes: system management host, servers, gateways, protocol converters, hubs, user computers, and underlying intelligent instruments. Network system software includes network operating software such as NetWarc, LANMangger, and Vines; server operating software such as Lenix, OS/2, and Windows NT; and application software such as databases, communication protocols, and network management protocols.
Nine types of fieldbus technology
1. Foundation Fieldbus
This is the merging of the ISP protocol, developed by Fisher-Rousemount (USA) in conjunction with 80 companies including Yokogawa, ABB, Siemens, and Invensys, and the WorldFIP protocol, developed by Honeywell (USA) in conjunction with over 150 companies in Europe and other regions, in September 1994. This bus has been widely used in process automation and has promising development prospects. The Foundation Fieldbus adopts a simplified OSI model (layers 1, 2, and 7) from the International Organization for Standardization (ISO), namely the physical layer, data link layer, and application layer, with the addition of a user layer. FF offers two communication rates: low-speed H1 and high-speed H2. The former has a transmission rate of 31.25 Kbit/s and a communication distance of up to 1900m, supporting bus power supply and intrinsically safe explosion-proof environments. The latter has transmission rates of 1 Mbit/s and 2.5 Mbit/s, communication distances of 750m and 500m respectively, supports twisted-pair cabling, fiber optic cable, and wireless transmission, and conforms to the IEC 1158-2 standard. The physical medium of FF uses Manchester encoding for signal transmission.
2. CAN
Originally introduced by the German company Bosch, CAN is widely used in discrete control applications. Its bus specification has been standardized by the ISO international standards organization and is supported by companies such as Intel, Motorola, and NEC. The CAN protocol consists of two layers: the physical layer and the data link layer. CAN signal transmission uses a short frame structure, resulting in short transmission times, automatic shutdown, and strong anti-interference capabilities. CAN supports multi-master operation and employs non-destructive bus arbitration technology, avoiding conflicts by setting priorities. The maximum communication distance is 10 km/5 Kbps/s, the maximum communication rate is 40 Mbps/1 Mbps/s, and the actual number of network nodes can reach 110. Currently, several companies have developed communication chips compliant with the CAN protocol.
3. Lonworks
Introduced by Echelon Technologies AG and co-sponsored by Motorola and Toshiba, Lonworks technology utilizes all seven layers of the ISO/OSI model's communication protocol. Employing an object-oriented design approach, it simplifies network communication design to parameter settings through network variables. Supporting various communication media such as twisted-pair cable, coaxial cable, fiber optic cable, and infrared, with communication speeds ranging from 300 bit/s to 1.5 Mbps and direct communication distances up to 2700 meters (78 Kbit/s), it is hailed as a universal control network. The LonTalk protocol used in Lonworks technology is encapsulated within the Neuron chip and implemented. Products employing Lonworks technology and Neuron chips are widely used in building automation , home automation, security systems, office equipment, transportation, and industrial process control.
4. DeviceNet
DeviceNet is a low-cost communication connection and a simple networking solution with an open network standard. Its direct interconnectivity not only improves communication between devices but also provides crucial device-level infrastructure functionality. Based on CAN technology, DeviceNet offers transmission rates from 125 Kbit/s to 500 Kbit/s, with a maximum of 64 nodes per network. Its communication mode is Producer/Consumer, employing multi-channel broadcast messaging. Devices on the DeviceNet network can be freely connected or disconnected without affecting other devices on the network, and its installation and cabling costs are relatively low. The DeviceNet bus is organized by the Open DeviceNet Vendor Association (ODVA).
5. PROFIBUS
PROFIBUS is a fieldbus standard conforming to German (DIN 19245) and European (EN 50170) standards. It consists of the PROFIBUS-DP, PROFIBUS-FMS, and PROFIBUS-PA series. DP is used for high-speed data transmission between distributed peripherals and is suitable for manufacturing automation . FMS is suitable for textiles, building automation, programmable logic controllers (PLCs), low-voltage switches, etc. PA is a bus type used for process automation and conforms to the IEC 1158-2 standard. PROFIBUS supports several transmission modes, including master-slave systems, pure master systems, and multi-master/multi-slave hybrid systems. PROFIBUS transmission rates range from 9.6 Kbit/s to 12 Mbit/s, with a maximum transmission distance of 1200 m at 9.6 Kbit/s and 200 m at 12 Mbit/s. Repeaters can be used to extend the distance to 10 km. The transmission medium is twisted-pair cable or optical fiber, and up to 127 stations can be connected.
6. HART
HART stands for Highway Addressable Remote Transducer, originally developed by Rosemount. Its key feature is the implementation of digital signal communication over existing analog signal transmission lines, making it a transitional product in the shift from analog to digital systems. Its communication model employs three layers: physical layer, data link layer, and application layer, supporting point-to-point master-slave response and multi-point broadcast modes. Because it uses a mix of analog and digital signals, it is difficult to develop universal communication interface chips. HART can utilize bus power supply, meeting intrinsic safety and explosion-proof requirements, and can be used in dual-master systems where a handheld programmer and a management system host are the primary devices.
7. CC-Link
CC-Link is an abbreviation for Control & Communication Link. Launched in November 1996 by a group of companies led by Mitsubishi Electric, it experienced rapid growth and holds a significant market share in Asia. This system allows control and information data to be transmitted simultaneously to the field network at a high speed of 10 Mbit/s, offering advantages such as superior performance, ease of use, wide applicability, and cost savings. It not only solves the problem of complex wiring in industrial fields but also boasts excellent noise immunity and compatibility. CC-Link is a device-layer-based network that can also cover higher-level control layers and lower-level sensing layers. In July 2005, CC-Link was approved by the China National Standardization Management Committee as a guiding technical document for Chinese national standards.
8. WorldFIP
After the North American portion of WorkdFIP merged with ISPs to form FF, the European portion of WorldFIP remained independent, headquartered in France. It holds a significant position in the European market, particularly in France where it holds approximately 60% market share. WorldFIP is characterized by a single bus architecture to accommodate the needs of various application areas, and it uses software to handle high-speed and low-speed connections without any gateways or bridges. WorldFIP can achieve "transparent connectivity" with FFHSE and has extended the technology of FF's H1 standard, such as in terms of speed. WorldFIP excels in connectivity with IEC 61158 Type I standards, leading the world in this area.
In addition, other influential fieldbuses include P-Net proposed by the Danish company Process-Data A/S, which is mainly used in agriculture, forestry, water conservancy, food and other industries; SwiftNet fieldbus is mainly used in aerospace and other fields; and there are some other fieldbuses, which will not be discussed here.
9. INTERBUS Fieldbus
INTERBUS, introduced by the German company Phoenix, is one of the earliest fieldbuses and became the international standard IEC 61158 in February 2000. INTERBUS adopts a simplified OSI model (layers 1, 2, and 7) from the International Organization for Standardization (ISO), namely the physical layer, data link layer, and application layer, offering strong reliability, diagnostics, and maintainability. It uses lumped-frame data ring communication, characterized by low speed and high efficiency, while strictly guaranteeing the synchronization and periodicity of data transmission; the bus also boasts excellent real-time performance, anti-interference capabilities, and maintainability. INTERBUS is widely used in industries such as automotive, tobacco, warehousing, papermaking, packaging, and food, making it a leading international fieldbus provider.
In addition, other influential fieldbuses include P-Net, proposed by the Danish company Process-Data A/S, which is mainly used in industries such as agriculture, forestry, water conservancy, and food; and SwiftNet fieldbus, which is mainly used in aerospace and other fields.
