The rapid development of computer and network technologies has led to a transformation in the structure of automated control systems. A new type of control system, the Fieldbus Control System (FCS), entered practical application in the 1990s and is developing rapidly. Fieldbus control systems are currently a hot topic in automation technology, attracting increasing attention from automation equipment manufacturers and users both domestically and internationally. The emergence of fieldbus control systems will bring another revolution to process control systems in the field of automation, surpassing any previous revolution in its depth and breadth, thus ushering in a new era of automation.
FCS can be considered the fifth generation of process control systems, evolving from PLC (Programmable Controller) or DCS (Distributed Control System). While FCS is closely related to PLC and DCS, it also has fundamental differences. This article analyzes the characteristics, performance, and differences of these three major control systems: PLC, DCS, and FCS.
1. Basic characteristics of the three major control systems: PLC, DCS, and FCS
Currently, there are three main control systems in continuous process manufacturing industrial processes: PLC, DCS, and FCS. Their basic characteristics are as follows:
1.1 PLC
(1) The development from switch control to sequential control and arithmetic processing is a bottom-up process.
(2) Logic control, timing control, counting control, step (sequence) control, continuous PID control, data control - PLC has multiple functions such as data processing capability, communication and networking.
(3) One PC can be used as the master station and multiple PLCs of the same type can be used as slave stations.
(4) Alternatively, one PLC can be the master station and multiple PLCs of the same type can be the slave stations to form a PLC network. The advantage of this over using a PC as the master station is that when users are programming, they do not need to know the communication protocol, they only need to write it according to the format in the instruction manual.
(5) The PLC network can be used as an independent DCS/TDCS or as a subsystem of DCS/TDCS.
(6) It is mainly used for sequential control in industrial processes. The new PLC also has closed-loop control function.
1.2DCS
(1) Distributed Control System (DCS) and Distributed Control System (TDCS) are monitoring technologies that integrate 4C (Communication, Computer, Control, CRT) technologies and are the fourth generation of process control systems. They have the advantages of advanced control algorithms, high precision, and fast response speed of computer control systems, as well as the requirements of safety, reliability, and convenient maintenance of instrument control systems.
(2) A large tree-like topology system from top to bottom, in which communication is the key.
(3) is a tree topology and a parallel continuous link structure, with a large number of cables running in parallel from the relay station to the field instruments.
(4) Analog signals, A/D-D/A, and microprocessor-based hybrid signals. These consist of several computers and some intelligent instruments and components, and are gradually being replaced by digital signals.
(5) One instrument is connected to the I/O with one pair of wires, and then connected to the local area network (LAN) by the control station.
(6) DCS is a three-level structure consisting of control (engineer station), operation (operator station), and field instruments (field measurement and control station). The disadvantages are high cost, incompatibility and non-interoperability between products from different companies, and different large DCS systems from different manufacturers.
(7) Used for large-scale continuous process control, such as centralized control of petrochemical and large power plant units.
1.3FCS
(1) FCS is the fifth generation of process control system, which is the direction of automation control system in the 21st century. It is a fusion of 3C technologies (Communication, Computer, Control). Its basic tasks are: inherent safety, hazardous areas, volatile processes, and difficult-to-handle extreme environments.
(2) Fully digital, intelligent, and multifunctional instruments, meters, and control devices replace analog single-function instruments, meters, and control devices.
(3) Use two wires to connect the distributed field instruments and control devices, replacing the two wires of each instrument. "Field control" replaces "distributed control"; data transmission adopts the "bus" method.
(4) The bidirectional digital communication bus from the control room to the field equipment is an interconnected, bidirectional, serial multi-node, open digital communication system that replaces the unidirectional, single-point, parallel, closed analog system.
(5) Replace centralized control stations with decentralized virtual control stations.
(6) Transfer the microcomputer processor to the field control equipment to enable the equipment to have digital computing and digital communication capabilities, high signal transmission accuracy, and remote transmission. Realize fully digital signal transmission, decentralized control functions, and unified and fully open standards.
(7) It can access a local area network and then connect to the Internet. It is both a communication network and a control network.
(8) Typical applications of three types of FCS: 1) Continuous process automatic control such as petrochemicals, in which "intrinsically safe explosion protection" technology is absolutely important; 2) Separate process automatic control such as automobile manufacturing robots and automobiles; 3) Multi-point control such as building automation.
These three control systems, especially DCS and PLC, have been widely used in power plants and have achieved excellent results.
2. Differences between the three major control systems
2.1 Differences
2.1.1 DCS or PLC
The structures of PLC systems and DCS systems are not significantly different; the main difference lies in their functional focus. DCS emphasizes closed-loop control and data processing, while PLC focuses on logic control and control of switching quantities, and can also implement analog quantity control.
Communication is crucial for DCS or PLC systems. The data highway can be considered the backbone of a distributed control system (DCS) or PLC. Since its task is to provide a communication network between all components of the system, the design of the data highway itself determines the overall flexibility and security. The media for the data highway can be: a twisted pair of wires, coaxial cable, or fiber optic cable.
The characteristics of DCS are: (1) Strong control function. It can realize complex control laws, such as cascade, feedforward, decoupling, adaptive, optimal and nonlinear control. It can also realize sequential control. (2) High system reliability. (3) It adopts CRT operator station with good human-machine interface. (4) The hardware and software adopt modular building block structure. (5) The system is easy to develop. (6) It uses configuration software, which is simple to program and easy to operate. (7) It has good cost performance.
By analyzing the design parameters of a data highway, one can essentially understand the relative advantages and disadvantages of a specific DCS or PLC system.
(1) How much I/O information can the system process?
(2) How much information about control loops related to control can the system process?
(3) How many users and devices (CRT, control station, etc.) can it accommodate?
(4) How is the integrity of the transmitted data thoroughly checked?
(5) What is the maximum allowable length of the data highway?
(6) How many branch roads can the data highway support?
(7) Whether the data highway can support hardware (programmable logic controllers, computers, data recording devices, etc.) manufactured by other manufacturers. To ensure the integrity of communication, most DCS or PLC manufacturers can provide redundant data highways.
To ensure system security, complex communication protocols and error detection techniques are used. A communication protocol is a set of rules used to ensure the reception and transmission of data.
Currently, two types of communication methods are generally used in DCS and PLC systems: synchronous and asynchronous. Synchronous communication relies on a clock signal to regulate data transmission and reception, while asynchronous networks use a reporting system without a clock.
2.1.2 FCS
FCS has the following characteristics: (1) excellent openness, interoperability and interchangeability. (2) all-digital communication. (3) intelligence and functional autonomy. (4) high degree of decentralization. (5) strong applicability.
There are three key points about FCS:
(1) The core of the FCS system is the bus protocol, i.e. the bus standard.
Transmitting digital signals via twisted-pair cables, fiber optic cables, or radio reduces the number of wires, improving reliability and interference immunity. FCS uses digital signals throughout the sensor, transmitter, and controller, making it easy to handle more complex and precise signals, while the error detection capabilities of digital communication can detect errors during transmission.
FCS can completely distribute PID control to field devices. Based on fieldbus, FCS is a fully distributed, fully digital, fully open, and interoperable next-generation production process automation system. It will replace the one-to-one 4-20mA analog signal lines in the field, bringing revolutionary changes to the traditional industrial automation control system architecture.
According to the definition of IEC 61158, a fieldbus is a digital, bidirectional, multi-branch communication network installed between field devices in manufacturing or process areas and automatic control devices in control rooms. Fieldbus enables measurement and control equipment to possess digital computing and communication capabilities, improving the accuracy of signal measurement, transmission, and control, and enhancing the functionality and performance of systems and equipment. The IEC/TC65 SC65C/WG6 working group began its work in 1984 to develop the world's single fieldbus standard. After a difficult 16-year process, IEC 61158-2 was released in 1993. Subsequent standard development became chaotic. The IEC 61158 fieldbus international standard subset published in early 2000 contained eight sub-standards:
① Type 1 IEC Technical Report (FFH1); ② Type 2 Control-NET (supported by Rockwell Automation, USA); ③ Type 3 Profibus (supported by Siemens, Germany); ④ Type 4 P-NET (supported by ProcessData, Denmark); ⑤ Type 5 FFHSE (formerly FFH2) High-Speed Ethernet (supported by Fisher Rosemount, USA); ⑥ Type 6 Swift-Net (supported by Boeing, USA); ⑦ Type 7 WorldFIP (supported by Alsto, France); ⑧ Type 8 Interbus (supported by PhoenixContact, USA).
In addition to the eight fieldbuses specified in IEC 61158, IEC TC 17B has approved three bus standards: SDS (Smart Distributed System); ASI (Actuator Sensor Interface); and DeviceNET. Furthermore, ISO has published the ISO 11898 CAN standard. DeviceNET was approved as a national standard in China on October 8, 2002, and came into effect on April 1, 2003.
Therefore, achieving mutual compatibility and interoperability among these bus types is currently virtually impossible. The interoperability of open fieldbus control systems, for a specific type of fieldbus, means that as long as the products adhere to the same type of fieldbus's bus protocol, they are open and interoperable. In other words, regardless of the manufacturer or the specific fieldbus company, as long as the products follow the same type of bus protocol, are open, and interoperable, they can form a bus network.
In addition, FCS can connect to the enterprise's upper-level management network via a gateway, enabling managers to obtain first-hand information and provide a basis for decision-making. Therefore, fieldbus has many outstanding characteristics such as openness, interoperability, highly distributed system architecture, flexible network topology, highly intelligent field devices, and high adaptability to the environment.
(2) The foundation of the FCS system is digital intelligent field devices.
Control functions are delegated to field instruments, while the instruments in the control room mainly perform functions such as data processing, monitoring and control, optimization control, coordination control, and management automation.
Digital intelligent field devices are the hardware support and foundation of the FCS system. The reason is simple: the FCS system executes a two-way digital communication fieldbus signaling system between automatic control devices and field devices. Field devices must adhere to a unified bus protocol, i.e., the relevant communication protocol, and possess digital communication capabilities to achieve two-way digital communication. Furthermore, a key characteristic of fieldbus is the addition of field-level control functions.
(3) The essence of the FCS system is the on-site processing of information.
For a control system, whether it uses a DCS or a fieldbus, the amount of information the system needs to process is at least the same. In fact, with a fieldbus, the information can be obtained from the field.
