FCS can be considered the fifth generation of process control systems, developed from PLC (Programmable Logic Controller) or DCS (Distributed Control System). FCS has intricate connections with both PLC and DCS, but also has essential differences. This article analyzes the characteristics, performance, and differences of the three major control systems: PLC, DCS, and FCS.
Basic Characteristics of PLC, DCS, and FCS Three Major Control Systems
Currently, in continuous process production industrial control, there are three major control systems: PLC, DCS, and FCS. Their basic characteristics are as follows:
1 PLC
(1) Evolved from switch control to sequential control and computational processing, moving from bottom to top;
(2) Logic control, timing control, counting control, step (sequential) control, continuous PID control, data control—PLC has data processing capabilities, communication, and networking functions;
(3) A single PC can serve as the master station, with multiple identical PLCs as slaves;
(4) A single PLC can also serve as the master station with multiple identical PLCs as slaves, forming a PLC network. This is more convenient than using a PC as the master station: when user programming, there is no need to know the communication protocol, just write according to the manual format;
(5) The PLC network can serve as an independent DCS/TDCS or as a subsystem of DCS/TDCS;
(6) Mainly used for sequential control in industrial processes, with new PLCs also featuring closed-loop control functions.
2 DCS
(1) The Distributed Control System (DCS) and the Centralized Control System (TDCS) integrate 4C (Communication, Computer, Control, CRT) technologies, representing the fourth generation of process control systems. It combines the advantages of computer control systems with advanced calculations, high precision, and fast response, along with the safety and maintenance requirements of instrument control systems;
(2) A top-down tree topology system, where communication (Communication) is key;
(3) A tree topology and parallel continuous link structure, with numerous cables running from relay stations to field instruments;
(4) Analog signals, A/D—D/A, mixed with microprocessor components. It consists of several computers and intelligent instrument components, gradually replacing analog signals with digital signals;
(5) Each instrument connects to I/O via a pair of wires, linked to the local area network (LAN) by the control station;
(6) DCS has a three-level structure: control (engineer station), operation (operator station), and field instruments (field measurement and control station). Its drawbacks include high costs, incompatibility among different manufacturers’ products, and lack of interoperability—large DCS systems are often unique to each manufacturer;
(7) Used for large-scale continuous process control, such as centralized control in petrochemical and large power plant units.
3 FCS
(1) FCS is the fifth generation of process control systems and represents the direction of automation control systems in the 21st century. It integrates 3C technologies (Communication, Computer, Control). Its essential tasks include intrinsic safety, hazardous areas, variable processes, and challenging environments;
(2) Fully digital, intelligent, and multifunctional, replacing analog single-function instruments and control devices;
(3) Uses two wires to connect dispersed field instruments and control devices, replacing the two wires for each instrument. “Field control” replaces “distributed control”; data transmission uses a “bus” method;
(4) A bidirectional digital communication bus from the control room to field equipment, which is interconnected, bidirectional, serial multi-node, and an open digital communication system, replacing unidirectional, single-point, parallel, closed analog systems;
(5) Uses decentralized virtual control stations to replace centralized control stations;
(6) Incorporates microprocessors into field self-control devices, enabling them to have digital computation and communication capabilities, with high signal transmission accuracy and remote transmission. Achieves fully digital signal transmission, decentralized control functions, and standardized open systems;
(7) Can connect to a local area network and communicate with the internet. It serves as both a communication network and a control network;
(8) Three typical applications of FCS: 1) Continuous process automation control such as petrochemicals, where “intrinsically safe explosion-proof” technology is critically important; 2) Discrete process automation control such as automotive manufacturing robots; 3) Multi-point control such as building automation.
These three major control systems, especially DCS and PLC, have been widely used in power plants with very good results.
Differences Between the Three Control Systems
The structural differences between PLC and DCS systems are not significant; the difference lies in the emphasis on functionality, with DCS focusing on closed-loop control and data processing. PLC emphasizes logic control and switch quantity control, and can also achieve analog quantity control.
The key to DCS or PLC systems is communication. One could say that the data highway is the backbone of the distributed control system DCS and PLC. Since its task is to provide a communication network for all components of the system, the design of the data highway itself determines the overall flexibility and safety. The media for the data highway can be: a pair of twisted wires, coaxial cables, or fiber optic cables.
The characteristics of DCS include: (1) Strong control functions. It can implement complex control laws such as cascade, feedforward, decoupling, adaptive, optimal, and nonlinear control. It can also achieve sequential control. (2) High system reliability. (3) Good human-machine interface with CRT operator stations. (4) Modular block structure for hardware and software. (5) The system is easy to develop. (6) Simple programming and convenient operation with configuration software. (7) Good cost-performance ratio.
By analyzing the design parameters of the data highway, one can generally understand the relative advantages and disadvantages of a specific DCS or PLC system.
(1) How much I/O information the system can handle.
(2) How many control loops related to control the system can handle.
(3) How many users and devices (CRT, control stations, etc.) it can accommodate.
(4) How thoroughly the integrity of data transmission is checked.
(5) What is the maximum allowable length of the data highway.
(6) How many branches the data highway can support.
(7) Whether the data highway can support hardware produced by other manufacturers (programmable controllers, computers, data recording devices, etc.). To ensure the integrity of communication, most DCS or PLC manufacturers can provide redundant data highways.
To ensure the safety of the system, complex communication protocols and error detection technologies are used. Communication protocols are a set of rules to ensure the transmission and reception of data.
Currently, two types of communication methods are generally used in DCS and PLC systems, namely synchronous and asynchronous. Synchronous communication relies on a clock signal to regulate data transmission and reception, while asynchronous networks use a report system without a clock.
FCS has (1) good openness, interoperability, and interchangeability. (2) Fully digital communication. (3) Intelligence and functional autonomy. (4) High decentralization. (5) Strong applicability.
The key points of FCS are three:
(1) The core of the FCS system is the bus protocol, i.e., the bus standard.
Using twisted pairs, optical cables, or radio methods to transmit digital signals reduces the amount of wiring and improves reliability and anti-interference capability. FCS maintains digital signals from sensors, transmitters to regulators, making it easy to handle more complex and precise signals. Additionally, the error detection functionality of digital communication can identify transmission errors.
FCS can completely decentralize PID control to field devices (Field Device). Based on field bus, FCS is a new generation of fully decentralized, fully digital, fully open, and interoperable production process automation system, replacing the one-to-one 4-20mA analog signal lines, bringing revolutionary changes to the traditional industrial automation control system architecture.
According to IEC61158, the field bus is a digital, bidirectional transmission, multi-branch structured communication network installed between field devices in the manufacturing or process area and the automatic control devices in the control room. The field bus enables measurement and control devices to possess digital computation and communication capabilities, improving the precision of signal measurement, transmission, and control, enhancing the functionality and performance of systems and devices. The IEC/TC65 SC65C/WG6 working group has been dedicated to developing a single field bus standard since 1984, and after 16 years of difficult progress, it launched IEC61158-2 in 1993, but subsequent standardization efforts were chaotic. The IEC61158 field bus international standard subset published in early 2000 includes eight types:
① Type 1 IEC Technical Report (FFH1); ② Type 2 Control-NET (supported by Rockwell, USA); ③ Type 3 Profibus (supported by Siemens, Germany); ④ Type 4 P-NET (supported by Process Data, 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 Phoenix Contact, USA).
In addition to the eight types of field buses in IEC61158, IEC TC17B has approved three bus standards: SDS (Smart Distributed System); ASI (Actuator Sensor Interface); Device NET. Additionally, ISO has published the ISO 11898 CAN standard. Device NET was approved as a national standard by China on October 8, 2002, and was implemented on April 1, 2003.
Thus, achieving compatibility and interoperability among these bus types is currently almost impossible. The interoperability of open field bus control systems, for a specific type of field bus, is only possible if the bus protocol of the same type of field bus is followed, and the products are open and interoperable. In other words, regardless of the manufacturer, as long as the products adhere to the bus protocol of the same type of field bus, they can form a bus network.
Furthermore, FCS can also connect through gateways to the upper-level management network of the enterprise, allowing managers to access first-hand information to inform decision-making. Therefore, the field bus features many outstanding characteristics, including openness, interoperability, a highly decentralized system structure, flexible network topology, high intelligence of field devices, and strong adaptability to environments.
(2) The foundation of the FCS system is digital intelligent field devices.
Control functions are delegated to field instruments, while instruments in the control room mainly handle data processing, supervisory control, optimal control, coordinated control, and management automation.
Digital intelligent field devices are the hardware support of the FCS system and its foundation; this is straightforward, as the FCS system executes bidirectional digital communication field bus signals between automatic control devices and field devices. Field devices must adhere to a unified bus protocol, meaning relevant communication protocols, and possess digital communication capabilities to achieve bidirectional digital communication. Additionally, a significant characteristic of the field bus is to enhance the control functions at the field level.
(3) The essence of the FCS system is localized information processing.
For a control system, whether using DCS or field bus, the amount of information the system needs to process is at least the same. In fact, after adopting the field bus, it can be obtained from the field.
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