What is the difference between PLC, DCS, and FCS control systems?

1 PLC

(1) From the development of switching quantity control to sequential control and arithmetic processing, it is from bottom to top;

(2) Logic control, timing control, counting control, step (sequence) control, continuous PID control, data control - PLC has multi-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 are slave stations;

(4) It is also possible to use one PLC as the main station and multiple PLCs of the same type as slave stations to form a PLC network. This is more convenient than using a PC as the main station: when there is user programming, you do not need to know the communication protocol, just write it according to the format of the 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, and the new PLC also has closed-loop control functions.

2 DCS

(1) Distributed Control System DCS and Distributed Control System TDCS is a fourth-generation process control system that integrates 4C (Communication, Computer, Control, CRT) technology. The computer control system has the advantages of advanced control formula, high precision and fast response speed, and the requirements of safe and reliable instrument control system and convenient maintenance;

(2) Tree top topology from top to bottom, where communication is the key;

(3) is a tree topology and a parallel continuous link structure, and there are also a large number of cables from the relay station in parallel to the field instrumentation;

(4) Analog signal, A/D-D/A, mixed with microprocessor. It consists of several computers and some smart meter smart components, and gradually replaces the analog signals with digital signals.

(5) One instrument is connected to the I/O by a pair of wires, and is connected to the LAN by the control station;

(6) DCS is a three-level structure of control (engineering station), operation (operator station), and field instrument (site monitoring station). The disadvantage is that the cost is high, the products of each company cannot be interchanged, and they cannot interoperate. The large DCS system is different from each other;

(7) For large-scale continuous process control, such as petrochemical, centralized control of large power plant units.

3 FCS

(1) FCS is the fifth-generation process control system, which is the direction of the 21st century automation control system. It is a fusion of 3C technology (Communication, Computer, Control). The basic tasks are: essential (intrinsic) safety, dangerous areas, volatile processes, and very difficult environments to deal with;

(2) Fully digital, intelligent and multifunctional replacement of analog single-function instruments, instruments and control devices;

(3) Use two wires to connect the scattered field instruments and control devices to replace the two wires of each instrument. “Field Control” replaces “Distributed Control”; data transmission adopts “bus” mode;

(4) The two-way digital communication bus from the control room to the field device is an interconnected, two-way, serial multi-node, open digital communication system that replaces the one-way, single-point, parallel, closed analog system;

(5) replacing the centralized control station with a decentralized virtual control station;

(6) Transfer the microcomputer processor into the field automatic control device, so that the device has digital computing and digital communication capabilities, high signal transmission accuracy and remote transmission. The signal transmission is fully digitized, the control functions are dispersed, and the standards are unified and open;

(7) can be connected to the local area network, and then communicate with the internet. Both a communication network and a control network;

(8) Typical applications of Class 3 FCS: 1) Continuous process automation such as petrochemical, where “intrinsically safe explosion-proof” technology is absolutely important; 2) discrete process motion automatic control such as automobile manufacturing robots, automobiles; ) Multi-point control such as building automation.

These three control systems, especially DCS and PLC, have been widely used in power stations, and the effect is very good.

Differences between the three control systems

The structure of the PLC system and the DCS system are not much different, but the focus of the function is different. The DCS focuses on closed-loop control and data processing. The PLC focuses on the control of logic control and switching quantity, and can also realize analog quantity control.

The key to a DCS or PLC system is communication. It can also be said that the data highway is the spine of the distributed control system DCS and PLC. Because its mission 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 pair of twisted wires, coaxial cables, or fiber optic cables.

The characteristics of DCS are: (1) The control function is strong. Complex control laws such as cascade, feedforward, decoupling, adaptive, optimal, and nonlinear control can be implemented. Sequential control is also possible. (2) High system reliability. (3) The CRT operator station has a good man-machine interface. (4) The hardware and software adopt modular modular structure. (5) The system is easy to develop. (6) With the configuration software, the programming is simple and the operation is convenient. (7) has a good price/performance ratio.

Through the design parameters of the data highway, you can basically understand the relative strengths and weaknesses of a particular DCS or PLC system.

(1) How much I/O information the system can handle.

(2) How much control loop related information the system can handle.

(3) How many users and devices (CRT, control station, etc.) can be accommodated.

(4) How to completely check the integrity of the transmitted data.

(5) What is the maximum allowable length of the data highway?

(6) How many branches can be supported by the data highway.

(7) Whether the data highway can support hardware (programmable controllers, computers, data recording devices, etc.) produced by other manufacturers. To ensure the integrity of the communication, most DCS or PLC manufacturers can provide redundant data highways.

In order to ensure the security of the system, complex communication protocols and error detection techniques are used. The so-called communication protocol is a set of rules to ensure that the transmitted data is received and transmitted.

At present, 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, and asynchronous network uses a reporting system without clock.

FCS has (1) good openness, interoperability and interchangeability. (2) All digital communication. (3) Intelligent and functional autonomy. (4) Highly dispersible. (5) Strong applicability.

There are three key points of FCS:

(1) The core of the FCS system is the bus protocol, the bus standard.

Transmission of digital signals over twisted pair, fiber optic cable or radio reduces the number of wires and improves reliability and immunity. The FCS is always a digital signal from the sensor, transmitter to regulator, which makes it easy to handle more complex and accurate signals, while the error function of digital communication can detect errors in transmission.

FCS can completely spread the PID control into the Field Device. The fieldbus-based FCS is a fully-distributed, fully digital, fully open and interoperable next-generation production process automation system that will replace the on-site one-to-one 4-20mA analog signal line to the traditional industrial automation control system. The structure brings revolutionary changes.

According to the definition of IEC61158, a fieldbus is a digital, bidirectional, multi-branch communication network installed between a field device in a manufacturing or process area and an automatic control device in a control room. The fieldbus enables the measurement and control equipment to have digital computing and digital communication capabilities, which improves the measurement, transmission and control accuracy of the signal and improves the function and performance of the system and equipment. The SC65C/WG6 working group of IEC/TC65 began to work on the introduction of the world's single fieldbus standard in 1984. After 16 years of hard work, IEC61158-2 was introduced in 1993, and the standard setting was later confusing. . There are eight sub-sets of IEC61158 fieldbus international standards announced in early 2000, namely:

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 fieldbuses of IEC61158, IEC TC17B has adopted three bus standards: SDS (Smart Distributed System); ASI (Actuator Sensor Interface); Device NET. In addition, ISO has published the ISO 11898 CAN standard. Among them, Device NET was approved as a national standard by China on October 8, 2002, and was implemented on April 1, 2003.

Therefore, to achieve mutual compatibility and interoperability of these bus types, it is almost impossible in the current state. The interoperability of open fieldbus control systems, for a particular type of fieldbus, is open and interoperable for products that follow the same type of fieldbus bus protocol. In other words, no matter what manufacturer's products, nor one is the product of the fieldbus company, as long as the bus protocol of the same type of bus is followed, the products are open and interoperable, and the bus network can be formed.

In addition, the FCS can also be connected to the enterprise's superior management network through the gateway, so that the manager can grasp the first-hand information and provide the basis for decision-making. Therefore, the field bus has many outstanding features such as openness, interoperability, highly decentralized system structure, flexible network topology, highly intelligent field devices, and high adaptability to the environment.

(2) The basis of the FCS system is the digital intelligent field device

The control function is placed in the field instrument, and the indoor instrument device is controlled to complete functions such as data processing, supervisory control, optimization control, coordinated control and management automation.

The digital intelligent field device is the hardware support of the FCS system, which is the basis; the reason is very simple, the FCS system performs the two-way digital communication field bus signal system between the automatic control device and the field device. The field device must follow a unified bus protocol, that is, the relevant communication protocol, with digital communication function, can realize two-way digital communication. Another point, a major feature of the fieldbus is to increase the level of control at the site.

(3) The essence of FCS system is information processing on-site

For a control system, whether it is DCS or a fieldbus, the system needs to process at least as much information. In fact, after using the field bus, you can get it from the field.