There are many names for control products, and each has different names. The commonly used control products include DCS and PLC. We have extended the concept of DCS to FCS.
DCS (Distributed Contorl System), also known as distributed control system, is a kind of computer control system relative to the centralized control system, which is developed and evolved on the basis of the centralized control system.
FCS (FieldBus Contorl Syestem), a fieldbus control system. It uses the open and interoperable network of fieldbus to interconnect various controllers and instruments and instrumentation equipment in the field to form a fieldbus control system, and at the same time, the control function is completely decentralized to the field, reducing installation costs and maintenance costs.
PLC（Program Logic Control ）,可编程逻辑控制器。
Detailed comparison of FCS and DCS
FCS is developed from DCS and PLC, which not only has the characteristics of DCS and PLC, but also takes a revolutionary step. At present, the new DCS and the new PLC have a tendency to move closer to each other. The new DCS already has a strong sequential control function; The new PLC is not bad in dealing with closed-loop control, and both can form a large network, and the scope of application of DCS and PLC has been greatly crossed.
The key to a DCS system is communication. It can also be said that the data bus is the spine of the distributed control system DCS. Since its task is to provide a communication network between all components of the system, the design of the data bus itself determines the overall flexibility and security. The media of the data bus can be: a pair of stranded cables, coaxial cables, or fiber optic cables. The design parameters of the data bus basically give an idea of the relative strengths and weaknesses of a particular DCS system.
In order to ensure the integrity of communication, most DCS manufacturers can provide redundant data buses. In order to ensure the security of the system, complex communication protocols and error detection techniques are used. A communication protocol is a set of rules that guarantee that the transmitted data is received and understood in the same way as the data sent. At present, two types of communication means are generally used in the DCS system, namely synchronous and asynchronous, synchronous communication relies on a clock signal to regulate the transmission and reception of data, and asynchronous network adopts a reporting system without a clock.
Figure Traditional: One-to-one I/O connection is used for the connection between field-level devices and controllers
Figure Fieldbus technology: All field devices are connected with one cable
| performance | FCS | DCS |
| structure | One-to-many: One pair of transmission lines is connected to multiple instruments, and multiple signals are transmitted in both directions. | One-to-one: A pair of transmission lines is connected to an instrument, and a signal is transmitted in one direction. |
| reliability | Good reliability: digital signal transmission has strong anti-interference ability and high precision. | Poor reliability: Analog signal transmission is not only inaccurate, but also susceptible to interference. |
| Out of control | In the control room, the operator can not only understand the working condition of the field equipment through the field instrument, but also adjust the parameters of the equipment, and also predict or find faults, so that the equipment is always in the process monitoring and controllable state of the operator. | In the control room, the operator does not know how the analog meter is working, cannot adjust its parameters, and cannot predict failures, resulting in the operator being in a state of "runaway" of the instrument. |
| control | The control functions are scattered across the various smart instruments. | All control functions are centralized in the control station. |
| Interchangeability | Users are free to choose the best performance-to-price field devices and meters from different manufacturers, and interconnect different brands of instruments to achieve "plug and play". | Although the analog instrument has unified the signal standard (4~20mA DC), most of the technical parameters are still set by the manufacturer, resulting in poor interchangeability of different brands of instruments. |
| appearance | In addition to the detection, conversion, compensation and other functions of analog instruments, smart meters also have digital communication capabilities, and have control and computing capabilities. | The analog instrument only has the functions of detection, conversion, compensation and so on. |
| Communication method | It adopts the communication mode of dual digital and two-way transmission. Fieldbus networks are used from the lowest level of sensors, transmitters and actuators, and communication networks are interconnected from the bottom up to the top. Multiple branch communication lines extend to the production site to connect the field digital instruments, using a pair of N connections. | Adopting a hierarchical architecture, the communication network is distributed in each layer and adopts digital communication mode, only the conventional analog instrument at the production site layer is still a one-to-one analog signal (such as 4~20mA DC) transmission mode, which is a "semi-digital signal" system. |
| Decentralized control | The input/output unit of the DCS is discarded and replaced by field instruments, that is, the functions of the DCS control station are reduced to zero, and the function blocks are distributed to the digital instruments on the fieldbus to achieve complete decentralized control. | The analog instruments at the production site are centrally connected to the input/output unit, and the input, output, control, calculation and other functional blocks related to control are concentrated in the control station of the DCS. DCS is just a "semi-decentralized" system. |
| Interoperability | As long as the field devices adopt the same bus standard, the products of different manufacturers can be interconnected or interchangeable, and can be configured in a unified manner, thus completely changing the closure and specificity of the traditional DCS control layer, and has good integration. | Field-level equipment is developed by each manufacturer, and the products of different manufacturers are difficult to interconnect and interoperate with each other due to the proprietary and incompatible communication protocols. |
PLC and DCS
PLC：
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From switching control to sequence control and delivery processing, it is from the bottom up
Continuous PID control and other multi-functions, PID in the interrupt station.
- One PC can be used as the main station, and multiple PLCs of the same type can be used as slaves.
- It can also be a PLC as the master 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 a master station: when there is a user programming, you don't need to know the communication protocol, just write it in the format of the manual.
- The PLC grid can be used either as a stand-alone DCS or as a subsystem of the DCS.
- PLCs are mainly used for sequential control in industrial processes, and the new PLCs also have closed-loop control functions.
DCS：
- The distributed control system (DCS) is a monitoring technology that integrates 4C (Communication, Computer, Control, CRT) technology.
- A large top-to-bottom tree topology system, in which communication is the key.
- PID In the interrupt station, the interrupt station connects the computer with the field instrumentation and control device is a tree topology and parallel continuous link structure, and there are also a large number of cables from the relay station to the field instrumentation in parallel.
- Analog signals, A/D—D/A, hybrid with microprocessor.
- A pair of lines of an instrument is connected to I/O, and the control station is attached to the local area network LAN.
- DCS is a three-level structure of control (engineer station), operation (operator station), and field instrument (field measurement and control station). It is used for large-scale continuous process control, such as petrochemical, etc.
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Industry insiders elaborate on the difference between PLC and DCS
PLC cannot be compared with DCS, PLC is a controller, is an isolated product, and DCS is a system. However, PLC can be compared with the control station of DCS, the cycle cycle of PLC is about 10 milliseconds, while the DCS control station is about 500 milliseconds, the openness of PLC is better, and its ability to work independently as a product is stronger.
- OMRON Corporation
DCS is a system that includes upper-level software, network and controller, while PLC is only a controller, and the upper-level SCADA system and the network connected to it are also required to form a system.
For PID loop control, Mitsubishi's process controller can now realize the same FBD programming as SAMA configuration, and the DCS system is larger, the number of control loops is more, and there are more controls and algorithms, which can complete the control between more complex loops. The hardware reliability is about the same. DCS can achieve I/O redundancy, but PLC cannot. Comparatively speaking, the cost of the system made up of PLC is lower.
—— Manager of Mitsubishi Electric Automation Precision Control Department
DCS is a kind of "decentralized control system", the hardware includes the field controller, the operator station computer, the engineer station computer, and the network system that connects them; the DCS software is an overall solution, which solves all the technical problems of a system, and the various parts of the system are tightly integrated.
The PLC is a device, which is equivalent to the field controller in the DCS in terms of hardware, and a local solution in software, and the stations are loosely organized from station to station.
—— B&R's technical manager responsible for the application development of APROL products
Analyzing the difference between DCS and PLC, the most important points are two points, one is that DCS is distributed control and has a global database, and the other is that PLC is a sequential scanning mechanism, and DCS is time-based control. Our system complies with the first point, such as the modification of an I/O tag, which can also be reflected simultaneously in the HMI.
- Product Manager, Automated Process Market, Rockwell
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