The difference between PLC, DCS and FCS - Xinjie

The fundamental characteristics of the three major control systems of PLC, DCS and FCS

At present, there are three major control systems in the operation of industrial processes in continuous process production, namely PLC, DCS and FCS. The fundamental characteristics of each of them are as follows:

　　1PLC

(1) From switching control to sequence control and operation processing, it is from bottom to top;

(2) Logic control, timing control, counting control, step (sequence) control, continuous PID control, data control - PLC has multiple functions such as data processing ability, communication and networking;

(3) One PC can be used as the master station, and multiple PLCs of the same type are slave stations;

(4) A PLC can also be the master station, and multiple PLCs of the same type can be 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 don't need to know the communication protocol, you just need to write it according to the layout 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 the function of closed-loop control.

　　2DCS

Distributed control system (DistributedControlSystem), also known as computer distributed control system, it is the mid-70s of the 20th century agile development, it is a multi-level computer system composed of process control level and process monitoring level with communication network as a link, it combines control skills, computer skills, picture display skills and communication skills to complete the supervision, control and management of the production process. It not only breaks the constraints of conventional manipulation and appearance, but also better solves the risk caused by the excessive concentration of information, management and control effects in the early computer system. It is mainly used in large-scale continuous process control systems, such as petrochemical, electric power, etc. At the center of this is communications, the data highway.

The key points are:

(1) A top-to-bottom tree-like system, in which communication is the key.

(2) The PID is in the stop station, and the stop station combines the appearance and control equipment of the computer and the field instrument.

(3) It is a tree-like topology and parallel link structure, and there are many cables running in parallel from the relay station to the surface of the field instrument.

(4) The signal system includes switching signals and analog signals.

(5) DCS is a three-level structure of control (engineer station), operation (operator station), and on-site appearance (on-site measurement and control station).

　　3FCS

FieldbusControlSystem (FCS) is the fieldbus control system, which uses the open and interoperable network of the fieldbus to interconnect the various controllers and the appearance and appearance equipment of the field to form a fieldbus control system, and the control function is completely delegated to the field, reducing the equipment cost and repair cost. Therefore, FCS is essentially an open, interoperable, and completely loose distributed control system, which is expected to become the mainstream product of the control system in the 21st century. The center of the fieldbus control system is the bus protocol, the foundation is the digital intelligent field equipment, and the essence is the on-site information processing.

The key points of FCS are:

FCS is a fusion of the 3C skills (CommunicaTIon, Computer, Contro1). It is suitable for physical (intrinsic) security, risk areas, volatile processes, and extraordinary environments that are difficult to deal with.

The field equipment is highly intelligent and provides all-digital signals; One bus connects all the devices.

The two-way digital communication bus from the control room to the field equipment is an interconnected, bidirectional, serial multi-node, and open digital communication system that replaces a one-way, single-point, parallel, and closed imitation system.

The control function is completely lost.

The difference between PLC, DCS and FCS - Xinjie

The difference between PLC, DCS and FCS - Xinjie

Second, the difference between the three major control systems

1PLC vs. DCS

The structure of the PLC system and the DCS system is not much different, only in the focus of the function, DCS focuses on closed-loop control and data processing. PLC focuses on logic control and switching control, and can also complete imitation 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 sloppy control system DCS and PLC. Because its mission is to provide a communication network between all components of the system, the planning of the data highway itself determines the overall flexibility and security. The media of the data highway can be: a pair of stranded cables, coaxial cables, or fiber optic cables.

The characteristics of DCS are: (1) Strong control function. Complex manipulation rules can be completed, such as cascade, feedforward, decoupling, adaptive, optimal, and non-linear manipulation. Sequential control can also be done. (2) High system reliability. (3) The CRT operation station has an excellent man-machine interface. (4) The software and hardware are modular building block structure. (5) Abbreviated development of the system. (6) With configuration software, the programming is simple and the operation is convenient. (7) Outstanding cost performance.

Through the planning parameters of the data highway, it is fundamentally possible to understand the relative strengths and weaknesses of a particular DCS or PLC system.

Now in the DCS and PLC system, two types of communication methods are generally used, namely synchronous and asynchronous, synchronous communication relies on a clock signal to regulate the transmission and reception of data, and asynchronous network uses a reporting system without a clock.

2. Differences between DCS and FCS

The DCS system is closed, and the products of various manufacturers are incompatible with each other; FCS is a fully open system, its skill standards are also fully open, FCS field devices are interoperable, equipment is compatible with each other, so users can choose different manufacturers, different brands of products together to connect to the same fieldbus, to achieve the best system integration.

The DCS system is a large system, the function of its controller is strong and the effect in the system is very important, and the data highway is the key point of the system, so it must be funded as a whole in one step, and it is difficult to expand the capacity afterwards; However, the decentralization of FCS function is relatively complete, the information processing is on-site, and the wide selection of digital intelligent field equipment makes the function and importance of the controller relatively weakened.

The signal transmission of FCS has completed the full digitalization, and its communication can provide strong support for the MES and ERP of the enterprise from the lowest level of sensors and actuators to the highest level, and more importantly, it can also carry out long-distance diagnosis, protection and configuration of field equipment; The communication function of DCS is greatly constrained, although it can also connect to the Internet, it cannot connect to the bottom layer, the amount of information it provides is also limited, and it cannot perform long-distance operation on field devices.

The structure of FCS is fully scattered, which abandons the I/O unit and control station in DCS, and delegates the control function to the field equipment, which completes the complete dispersion and makes the system expansion very simple. The sloppiness of the DCS is only at the controller level, which emphasizes the function of the controller, and the data highway is its key point.

The signal system of DCS is binary or imitation, and there must be A/D, D/A links, so its control accuracy is 0.5%; The FCS is fully digital, the control system has high accuracy, which can reach 0.1% of the soil, and the FCS system can load the PID closed-loop control function into the field equipment, shorten the control cycle, improve the calculation speed, and then improve the conditioning function.

FCS's ability to put the PID closed-loop function into a transmitter or actuator in the field, coupled with digital communication, has shortened the sampling and manipulation cycle, and can now be increased from 2-5 times per second to 10-20 times per second in the DCS, with improved conditioning capabilities.

FCS is related to DCS configuration, because the structure and function are standardized, because FCS eliminates a lot of hardware equipment, cables and cable equipment auxiliary equipment, and saves a lot of equipment and debugging costs, so its cost is much lower than DCS.

The difference between PLC, DCS and FCS - Xinjie

Sections viewed

Contact

working hours