Basic characteristics of PLC, DCS, and FCS three major control systems
Currently, there are three main control systems in continuous process manufacturing industrial processes: PLC, DCS, and FCS. Their basic characteristics are as follows:
1 PLC
(1) The development from switch control to sequential control and computational processing is a bottom-up process;
(2) Logic control, timing control, counting control, step (sequential) control, continuous PID control, data control—PLC has multiple functions such as data processing, communication, and networking;
(3) One PC can be used as the master station, and multiple PLCs of the same type can be used as slave stations;
(4) Alternatively, one PLC can be used as the master station, and multiple PLCs of the same type can be used as slave stations to form a PLC network. The advantage of this over using a PC as the master station is that users do not need to know the communication protocol when programming; they only need to write the code according to the instruction manual format.
(5) The PLC network can function as either an independent DCS/TDCS or a subsystem of the DCS/TDCS;
(6) It is mainly used for sequential control in industrial processes. New PLCs also have closed-loop control functions.
2DCS
Distributed Control Systems (DCS), also known as computer-distributed control systems, developed rapidly in the mid-1970s. They are multi-level computer systems comprised of process control and process monitoring levels linked by a communication network. Combining control technology, computer technology, image display technology, and communication technology, they enable the monitoring, control, and management of production processes. They overcome the limitations of conventional control instruments and effectively address the risks associated with the over-centralization of information, management, and control functions in early computer systems. They are primarily used in large-scale continuous process control systems, such as in petrochemical and power industries. Their core is communication, i.e., the data highway.
Its basic points are:
(1) A tree-like system from top to bottom, in which communication is key.
(2) In the interrupt station, the PID controller connects the computer to the field instruments and control equipment.
(3) is a tree topology and a parallel continuous link structure, with a large number of cables running in parallel from the relay station to the field instruments.
(4) The signal system includes digital signals and analog signals.
(5) DCS is a three-level structure consisting of control (engineer station), operation (operator station), and field instruments (field measurement and control station).
3FCS
Fieldbus Control System (FCS) is an open, interoperable network that interconnects various controllers, instruments, and equipment in the field, forming a fieldbus control system. It decentralizes control functions entirely to the field, reducing installation and maintenance costs. Therefore, FCS is essentially an open, interoperable, and thoroughly distributed control system, and it is expected to become a mainstream control system product in the 21st century. The core of a fieldbus control system is the bus protocol, its foundation is digital intelligent field devices, and its essence is the field-based processing of information.
The key points of FCS are:
FCS is a fusion of 3C technologies (Communication, Computer, Control). It is suitable for inherently safe, hazardous areas, volatile processes, and challenging environments.
The field equipment is highly intelligent, providing fully digital signals; a single bus connects all the equipment.
The bidirectional digital communication bus from the control room to the field devices is an interconnected, bidirectional, serial multi-node, open digital communication system that replaces the unidirectional, single-point, parallel, and closed analog system.
The control functions are completely decentralized.
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The differences between the three major control systems
1. PLC and DCS
The structures of PLC systems and DCS systems are not significantly different; the main difference lies in their functional focus. DCS emphasizes closed-loop control and data processing, while PLC focuses on logic control and control of switching quantities, and can also implement analog quantity control.
Communication is crucial for DCS or PLC systems. The data highway can be considered the backbone of a distributed control system (DCS) or PLC. Since its task 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 twisted pair of wires, coaxial cable, or fiber optic cable.
The characteristics of DCS are: (1) Strong control function. It can realize complex control laws, such as cascade, feedforward, decoupling, adaptive, optimal and nonlinear control. It can also realize sequential control. (2) High system reliability. (3) It adopts CRT operator station with good human-machine interface. (4) The hardware and software adopt modular building block structure. (5) The system is easy to develop. (6) It uses configuration software, which is simple to program and easy to operate. (7) It has good cost performance.
By analyzing the design parameters of a data highway, one can essentially understand the relative advantages and disadvantages of a specific DCS or PLC system.
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 data transmission and reception, while asynchronous networks use a reporting system without a clock.
2. Differences between DCS and FCS
DCS systems are closed systems, and products from different manufacturers are incompatible with each other; FCS is a fully open system, and its technical standards are also fully open. FCS field devices are interoperable and compatible with each other. Therefore, users can choose products from different manufacturers and brands to connect to the same fieldbus at the same time to achieve optimal system integration.
DCS systems are large systems with powerful controllers that play a crucial role, and data highways are key components. Therefore, a full investment must be made in one step, and subsequent expansion is difficult. In contrast, FCS systems have more decentralized functions, localized information processing, and widespread adoption of digital intelligent field devices, which reduces the importance of controllers. As a result, FCS systems have a lower initial investment and can be used, expanded, and put into operation simultaneously.
FCS achieves fully digital signal transmission, and its communication can reach from the lowest level of sensors and actuators all the way to the highest level, providing strong support for enterprise MES and ERP. More importantly, it can also perform remote diagnosis, maintenance and configuration of field devices. DCS's communication function is greatly limited. Although it can connect to the Internet, it cannot connect to the lower level, the amount of information it can provide is limited, and it cannot remotely operate field devices.
The FCS has a fully distributed structure, which eliminates the I/O units and control stations in the DCS and decentralizes the control functions to field devices, achieving complete decentralization and making system expansion very easy; the decentralization of the DCS only extends to the controller level, emphasizing the functions of the controller, and the data highway is its key feature.
The DCS signal system is binary or analog, and must have A/D and D/A links, so its control accuracy is ±0.5%. The FCS is fully digital, and the control system has high accuracy, which can reach ±0.1%. In addition, the FCS system can integrate PID closed-loop control function into field equipment, shorten the control cycle, improve the calculation speed, and thus improve the regulation performance.
FCS can integrate the PID closed-loop function into the field transmitter or actuator, and with the addition of digital communication, it shortens the sampling and control cycle, which can be increased from 2-5 times per second in DCS to 10-20 times per second, thereby improving regulation performance.
Compared to DCS, FCS has a simpler configuration. Due to its standardized structure and performance, and because FCS eliminates a large amount of hardware equipment, cables, and cable installation auxiliary equipment, it saves a significant amount of installation and commissioning costs, making its cost much lower than that of DCS.
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