Differences between DCS, PLC, and PID

DCS is distributed control, offering good redundancy and is suitable for large and medium-sized companies. Investment is also high. PLC program logic control, with no redundancy. Suitable for small and medium-sized companies. Investments are also small. APC, or Advanced Process Control, is a method that uses various models for calculation and analysis, through programs in DCS and PLCs, to achieve optimization of the process control. It is a completely independent software with no connection to hardware. For different brands of DCS and PLC, the software and hardware are integrated together. PID is simply proportional-integral-differential control; it is not the same concept as DCS, PLC, or APC. It’s hard to compare.

DCS features risk dispersion, centralized control system, good redundancy, and strong functional computing capabilities. Controlled by PLC program logic, with no redundancy – the optimal choice for timing control, suitable for small-scale control systems. PID is simply proportional-integral-derivative control; it is just one type of control method, and it is not the same concept as DCS or PLC.

DCS is a distributed control system that is good at handling analog signals. PLC is a logic programming system that is adept at dealing with digital signals. PID is used for adjusting control loops, not as part of the overall system

A few years ago, there were still some differences between DCS and PLCs. For systems with a small number of points but many digital inputs/outputs, PLCs were generally used for construction. PLCs have a simple structure, are inexpensive, and easy to use; common brands include Siemens, AB, ABB, Honeywell, etc. For some more complex systems that require extensive tuning and control, DCS is used in systems with many analog signals; however, it is relatively expensive and its programming is complex. However, over the years, the established PLC manufacturers have reduced the price of analog modules and added many more functions. As a result, for many medium-sized systems that previously used small DCS systems, PLCs are now being employed instead. The various PLC manufacturers also try their best to blur the distinction between DCS and PLCs. However, for truly large-scale systems, it can be said that DCS still dominates, such as in power plants and the petrochemical industry; whereas in metallurgy and steel rolling, PLCs are more commonly used

The rapid development of computer and network technologies has led to changes in the structure of automated control systems. One of the most advanced control systems in the world is the Fieldbus Control System (FCS), which became practical in the 1990s and continues to develop at a rapid pace. Fieldbus control systems are a hot topic in automation technology today, attracting increasing attention from automation equipment manufacturers and users both at home and abroad. The emergence of fieldbus control systems will bring about another revolution in the field of automation, particularly in process control systems; its impact will be greater in terms of depth and scope than any previous revolution, thus ushering in a new era for automation. FCS can be considered the fifth generation of process control systems, having evolved from PLCs (Programmable Controllers) or DCSs (Distributed Control Systems). FCS is closely connected to PLC and DCS, yet there are fundamental differences between them. This article analyzes the characteristics, performance, and differences among the three major control systems: PLC, DCS, and FCS. 1 Basic characteristics of the three major control systems: PLC, DCS, and FCS. Currently, in the industrial process control of continuous flow processes, there are three major control systems, namely PLC, DCS, and FCS. Their respective basic characteristics are as follows: 1.1 PLC (1) Evolved from digital control to sequential control and arithmetic processing, following a bottom-up approach. (2) Logical control, timing control, counting control, step (sequential) control, continuous PID control, data control – PLCs possess multiple functions such as data processing, communication, and networking. (3) A single PC can be used as the master station, with multiple identical PLCs serving as slave stations. (4) A single PLC can also serve as the master station, with multiple identical PLCs acting as 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 need not know the communication protocol; they just need to write according to the instructions provided. (5) The PLC network can function as an independent DCS/TDCS or as a subsystem of a DCS/TDCS. (6) It is mainly used for sequential control in industrial processes, and modern PLCs also possess closed-loop control capabilities. 1.2 DCS (1) The Distributed Control System, or DCS, and the Distributed Control and Data Acquisition System, or TDCS, are monitoring technologies that integrate 4C technologies (Communication, Computer, Control, CRT); they represent the fourth generation of process control systems. Computer control systems offer the advantages of advanced control algorithms, high precision, and fast response speeds, while instrument control systems meet the requirements of being safe and reliable as well as easy to maintain. (2) A top-down tree-like topological large system, in which communication is key. (3) It features a tree topology and a parallel, continuous link structure, with a large number of cables running from the relay stations to the field instruments. (4) Analog signals, A/D–D/A, microprocessor-based hybrids. It consists of several computers and some intelligent instruments and components, and gradually replaces analog signals with digital signals. (5) One instrument is connected to I/O via a pair of wires, and the control station connects it to the local area network LAN. (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 costs, products from different companies cannot be interchanged or made interoperable, and large DCS systems vary from one company to another. (7) Used for large-scale continuous process control, such as the centralized control of petrochemical plants and large power plant units. 1.3 FCS (1) FCS is the fifth-generation process control system, and it represents the direction for automation control systems in the 21st century. It is the integration of 3C technologies (Communication, Computer, Control). The basic tasks are: intrinsic safety, hazardous areas, variable processes, and challenging extreme environments. (2) Fully digital, intelligent, and multi-functional devices replace analog single-functional instruments, meters, and control units. (3) Connect the dispersed field instruments and control devices with two wires, replacing the two wires required for each instrument. “On-site control” replaces “distributed control” ； Data transmission is carried out via a “bus” system. (4) The two-way digital communication bus from the control room to the field devices represents a interconnected, two-way, serial multi-node, open digital communication system that replaces the one-way, single-point, parallel, closed analog system. (5) Replace centralized control stations with distributed virtual control stations. (6) Integrate microcomputer processors into field automatic control devices, enabling them to perform digital calculations and digital communications, with high precision in signal transmission and the capability for remote transmission. Achieve fully digital signal transmission, decentralized control functions, and unified standards with full openness. (7) It can connect to a local network, and then to the Internet. It is both a communication network and a control network. (8) Typical applications of Category 3 FCS: 1) Automatic control of continuous industrial processes such as those in the petrochemical industry, where \"intrinsic safety\" technology is absolutely essential ； 2) Automatic control of discrete process actions, such as robotics in automobile manufacturing, automobiles ； 3) Multi-point control such as building automation. These three control systems, especially DCS and PLC, are widely used in power plants, with very good results. 2 Differences among the three major control systems 2.1 Differences 2.1.1 DCS or PLC The structural differences between PLC systems and DCS systems are not significant; the difference lies rather in the focus of their functions. DCS systems emphasize closed-loop control and data processing. PLCs focus on logical control and digital control, and can also perform analog control. The key to DCS or PLC systems is communication. It can also be said that the data highway is the backbone of distributed control systems DCS and PLCs. Since its task is to provide a communication network among all components of the system, the design of the data highway itself determines the overall flexibility and security. The media for data highways can be: twisted pair, coaxial cable, or fiber optic cable. The characteristics of DCS are: (1) strong control capabilities. Complex control laws can be implemented, such as cascade, feedforward, decoupling, adaptive, optimal, and nonlinear control. Sequential control can also be achieved. (2) The system has high reliability. (3) The CRT operation station features a good human-machine interface. (4) The software and hardware adopt a modular building-block structure. (5) The system is easy to develop. (6) Using configuration software, programming is simple and operation is convenient. (7) It offers good value for money. Through the design parameters of the data highway, it is possible to understand the relative advantages and disadvantages of a specific DCS or PLC system. (1) How much I/O information can the system handle. (2) How much information related to control loops can the system process? (3) How many users and devices (CRTs, control stations, etc.) can it support? (4) How is the integrity of the transmitted data thoroughly checked? (5) What is the maximum allowable length of a data highway? (6) How many branches can a data highway support? (7) Can the data highway 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 links. To ensure the security of the system, complex communication protocols and error detection techniques are used. A communication protocol is a set of rules used to ensure the proper 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 the transmission and reception of data, while asynchronous networks use a clock-free reporting system. 2.1.2 FCS FCS features (1) excellent openness, interoperability, and interchangeability. (2) All-digital communication. (3) Intelligence and functional autonomy. (4) High dispersity. (5) High applicability. There are three key points of FCS: (1) The core of the FCS system is the bus protocol, that is, the bus standard. Digital signals are transmitted via twisted pair, optical fiber, or radio, reducing the need for a large number of wires and enhancing reliability and resistance to interference. FCS is a digital signal from the sensor and transmitter to the regulator, which allows us to handle more complex and precise signals easily; moreover, the error detection capabilities of digital communication enable the identification of errors that occur during transmission. FCS can completely decentralize PID control to field devices. FCS based on field buses is a new generation of production process automation system that is fully decentralized, fully digital, fully open, and interoperable. It will replace the one-to-one 4–20mA analog signal lines in use today, bringing about revolutionary changes to the architecture of traditional industrial automation control systems. According to the definition of IEC61158, a field bus is a digital, bidirectional, multi-branch communication network that connects field devices installed in manufacturing or process areas with automatic control devices located in the control room. Fieldbuses endow measurement and control devices with digital computing and digital communication capabilities, improving the accuracy of signal measurement, transmission, and control, as well as the functionality and performance of the systems and devices. The SC65C/WG6 working group of IEC/TC65 began working in 1984 on developing a single fieldbus standard for the world; after 16 years of difficult efforts, IEC61158-2 was released in 1993, and standard development became chaotic thereafter. There are eight subsets of the international IEC61158 fieldbus standard published in early 2000, namely: ① Type 1 IEC technical report (FFH1) ； ②Type 2 Control-NET (supported by Rockwell in the United States) ； ③Type 3 Profibus (supported by German Siemens) ； ④Type 4 P-NET (supported by Danish Process Data) ； ⑤Type 5 FFHSE (formerly FFH2) High-Speed Ethernet (supported by Fisher Rosemount in the United States) ； ⑥Type 6 Swift-Net (supported by Boeing in the United States) ； ⑦Type 7 WorldFIP (supported by French company Alsto) ； ⑧Type 8 Interbus (supported by Phoenix Contact in the United States). In addition to the 8 fieldbus standards specified in IEC61158, IEC TC17B has approved three additional bus standards: SDS (Smart Distributed System) ； ASI (Actuator Sensor Interface) ； Device NET. Additionally, ISO published the ISO 11898 CAN standard. Among them, Device NET was approved as a **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 practically impossible at the current stage. The interoperability of open fieldbus control systems means that, for a specific type of fieldbus, as long as the bus protocols of that same type are followed, products using it are compatible with each other and exhibit interoperability. In other words, regardless of the manufacturer, and as long as none of the products are from that specific fieldbus company, as long as they follow the bus protocol of the same type of bus, the products can be interconnected to form a bus network, thanks to their interoperability. Additionally, FCS can also be connected to the enterprise’s higher-level management network via a gateway, so that managers can obtain first-hand information to support decision-making. Therefore, fieldbuses possess many outstanding features such as openness, interoperability, a highly decentralized system architecture, flexible network topologies, high levels of intelligence in field devices, and strong adaptability to the environment. (2) The foundation of the FCS system is digital intelligent field devices; control functions are delegated to the field instruments, while the instrument devices in the control room are primarily responsible for tasks such as data processing, supervision and control, optimization control, coordination control, and management automation. Digital intelligent field devices serve as the hardware foundation for FCS systems; they are the basis ； The principle is simple: the FCS system utilizes a two-way digital communication field bus signaling system between automatic control devices and field devices. On-site devices must adhere to a unified bus protocol, that is, the relevant communication standards, and possess digital communication capabilities to enable two-way digital communication. Furthermore, a key feature of fieldbuses is the ability to enhance control functions at the field level. (3) The essence of the FCS system is on-site information processing. For a control system, whether DCS or fieldbus is used, the amount of information that needs to be processed is at least the same. In fact, by using fieldbuses, more information can be obtained from the field. The amount of information in the fieldbus system has not decreased; in fact, it has increased, while the cables used for transmitting this information have **decreased**. This requires, on the one hand, **improving the capacity of cables to transmit information, and on the other hand, processing large amounts of information on-site to reduce the back-and-forth of information between the field site and the control room.** It can be said that the essence of fieldbuses is the localization of information processing. On-site smart instruments are responsible for functions such as data collection, data processing, control calculations, and data output. The data from the field instruments (including the data collected and diagnostic data) is transmitted via a field bus to the control devices in the control room. These control devices are used to monitor the operating status of each field instrument, store the data sent by the intelligent instruments, and carry out advanced control functions that some field instruments are not capable of performing. 2.2 Comparison of typical systems: By using fieldbuses, users can significantly reduce the number of on-site wiring connections; multiple variables can be communicated using a single field instrument. Devices produced by different manufacturers can be fully interoperable with each other, control functions at the field level are enhanced, system integration is simplified, and maintenance is greatly facilitated. A typical fieldbus system block diagram is shown in Figure 1. As can be seen from Figure 1, in traditional process control instrument systems, a pair of dedicated cables or twisted pairs is required for each field device to connect to the control room in order to transmit 4mA–20mA signals. In the field bus system shown in Figure 2, the twisted pair from each field device to the junction box can still be used, but digital communication from the field junction box to the central control room is accomplished using only one twisted pair. The editor will not conduct a detailed calculation of how much cable can be saved by using a fieldbus control system. 2.3 Application Differences The comparison mentioned above is primarily technical in nature; below, a comparison is made between DCS and FCS systems in terms of their specific applications. The premise is a comparison between the DCS system and a typical, ideal FCS system. Specific comparison: (1) The DCS system is a large-scale system; its controllers have powerful functions and play a crucial role within the system. The data highways are even more vital to the system. Therefore, it is necessary to make an upfront investment for the entire system, as it is difficult to expand it later on. With the thorough delegation of FCS functions, on-site information processing, and the widespread use of digital intelligent field devices, the functions and importance of controllers have relatively diminished. Therefore, the investment threshold for FCS systems is low, allowing for use, expansion, and commissioning simultaneously. (2) The DCS system is a closed system, and products from different companies are generally not compatible with each other. The FCS system is an open system; products from different manufacturers and brands can generally be connected to the same fieldbus simultaneously, achieving optimal system integration. (3) All information in the DCS system is in the form of binary or analog signals, and it must undergo D/A and A/D conversion. The FCS system performs the D/A and A/D conversions at the field terminal, enabling fully digital communication and thus significantly improving accuracy to 0.1%. Furthermore, the FCS system can incorporate PID closed-loop control functions into field devices, reducing the control cycle and increasing processing speed, thereby improving regulatory performance. (4) DCS can control and monitor the entire process, as well as perform self-diagnosis, maintenance, and configuration. However, due to its fatal weakness of using traditional analog I/O signals, it is unable to perform remote diagnosis, maintenance, and configuration of field instruments (including transmitters, actuators, etc.) on the DCS engineer station. FCS utilizes fully digital technology; digital intelligent field devices transmit multi-variable information, rather than just single-variable information, and they also have the capability to detect information errors. FCS uses a bidirectional digital communication field bus signaling system. Therefore, it enables remote diagnosis, maintenance, and configuration of field devices (including transmitters, actuators, etc.). (5) Due to on-site information processing, FCS can eliminate a considerable number of isolators, terminal cabinets, I/O terminals, I/O cards, I/O files, and I/O cabinets compared to DCS, thereby saving space and floor area for I/O devices and device rooms as well. At the same time, FCS can reduce the need for a large number of cables and cable trays used for laying them, thereby saving on design, installation, and maintenance costs. (6) FCS has a simpler configuration compared to DCS; thanks to standardized structure and performance, it is easier to install, operate, and maintain. 3 The Future of PLCs and DCSs As we all know, FCS evolved from PLCs or DCSs, and FCS systems are now widely used. So what is the future holding for PLCs and DCSs? PLC first appeared in the United States at the end of the 1960s, with the aim of replacing relays to carry out sequential control functions such as logic, timing, and counting, thereby creating flexible program control systems. It was officially named and defined in 1976: PLC (Programmable Logic Controller) is a specialized digital computer for control purposes. It uses programmable memory to store instructions, enabling it to perform functions such as logic operations, sequencing, timing, counting, and arithmetic calculations. Through analog and digital input/output components, it can control various mechanical devices or operational processes. After more than 30 years of development, PLCs have become highly mature and sophisticated, possessing powerful computing, processing, and data transmission capabilities. and is defined as a Programmable Controller (PLC). The role of PLCs in FCS systems seems to have been established with little debate. Refer to Figure 3: The IEC-recommended architecture for fieldbus control systems. The PLC is connected to the high-speed bus as a station. Make full use of the advantages of PLCs in handling digital signals. Additionally, in the auxiliary workshops of power plants, such as water treatment workshops, circulating water workshops, ash and slag removal workshops, and coal transportation workshops, the process operations in these workshops are mainly based on sequential control. PLCs have unique advantages for sequential control. For the control system of auxiliary workshops, PLCs that comply with fieldbus communication protocols or those capable of communicating and exchanging information with the FCS are the preferred choices. The application of fieldbuses is one of the main trends in the development of industrial process control. It can be said that the development and application of FCS represent a revolution in the field of automation. By utilizing fieldbus technology to develop low-cost fieldbus control systems, it promotes the intelligence of field instruments, the decentralization of control functions, and the openness of control systems, in line with the technological development trends of industrial control systems. In summary, after going through base-type pneumatic instrument control systems, electric unit combination analog instrument control systems, centralized digital control systems, and Distributed Control Systems (DCS), the development of computer control systems will move in the direction of Fieldbus Control Systems (FCS). Although FCS based on fieldbuses has developed rapidly, there is still much work to be done in its advancement, such as standardization and the intelligentization of instruments. Furthermore, the maintenance and modification of traditional control systems still require DCS; therefore, it will take a long time for FCS to completely replace traditional DCS. At the same time, DCS itself is also continuously evolving and improving. It is certain that FCS, combined with new technologies such as DCS, industrial Ethernet, and advanced control, will possess strong viability. Industrial Ethernet and fieldbus technologies, as flexible, convenient, and reliable methods of data transmission, are being increasingly used in industrial settings, and they will play an even more important role in the field of control. 4 Conclusion In the future, within industrial process control systems, digital technology will evolve toward intelligence, openness, networking, and informatization. At the same time, industrial control software will also develop in terms of standardization, networking, intelligence, and openness. Therefore, with the emergence of fieldbus control systems such as FCS, digital distributed control systems DCS and PLCs will not disappear; instead, DCS and PLC systems will continue to evolve toward greater intelligence, openness, networking, and informatization. Or simply move the DCS, which used to be at the center of the control system, to a station on the field bus. With this, the situation in which DCS or PLCs hold a central position in control systems will be broken from now on. Future control systems will be of the type where the FCS occupies a central role; it represents a new type of standardized, intelligent, open, networked, and information-driven control system that combines the features of DCS and PLC systems. PID stands for proportional-integral-differential control, and it is an essential parameter in automatic control; different control loops employ various combinations of PID control.