In some industries, FCS has developed from PLC; in others, FCS has developed from DCS. Therefore, there are intricate connections between FCS, PLC, and DCS, along with essential differences.This article analyzes the characteristics and differences of the three major control systems: PLC, DCS, and FCS, pointing out their origins and development directions.
PLC, DCS, FCS
Basic Characteristics of the Three Control Systems
Currently, in continuous process production automation control (PA), commonly referred to as industrial process control, there are three major control systems: PLC, DCS, and FCS, each with the following basic characteristics:
PLC
(1) Developed from switch control to sequential control and transport processing, it is bottom-up.
(2) Multi-functional features such as continuous PID control, with PID at the interruption station.
(3) A PC can be used as the master station, with multiple identical PLCs as slave stations.
(4) Alternatively, one PLC can serve as the master station, with multiple identical PLCs as slave stations to form a PLC network. This is more convenient than using a PC as the master station because users do not need to know the communication protocol; they just need to write according to the manual format.
(5) The PLC network can act as an independent DCS/TDCS or as a subsystem of DCS/TDCS.
(6) Large systems can be similar to DCS/TDCS, such as TDC3000, CENTUMCS, WDPFI, MOD300.
(7) Examples of PLC networks include Siemens’ SINEC—L1, SINEC—H1, S4, S5, S6, S7, GE’s GENET, and Mitsubishi’s MELSEC—NET, MELSEC—NET/MINI.
(8) Primarily used for sequential control in industrial processes, new PLCs also have closed-loop control functions.
(9) Manufacturers: GOULD (USA), AB (USA), GE (USA), OMRON (Japan), MITSUBISHI (Japan), Siemens (Germany), etc.
DCS or TDCS
(1) The Distributed Control System (DCS) and the Total Distributed Control System (TDCS) integrate 4C (Communication, Computer, Control, CRT) technologies into a monitoring technology.
(2) A top-down tree topology large system, where communication is key.
(3) PID at the interruption station connects computers with field instruments and control devices.
(4) It has a tree topology and parallel continuous link structure, with numerous cables running parallel from relay stations to field instruments.
(5) Analog signals, A/D—D/A, and hybrid with microprocessors.
(6) Each instrument connects to I/O with a pair of wires, linked to the local area network (LAN) from the control station.
(7) DCS consists of three levels: control (engineer station), operation (operator station), and field instruments (field measurement and control station).
(8) The disadvantage is high cost; products from different companies are not interchangeable and cannot interoperate; DCS systems vary by manufacturer.
(9) Used in large-scale continuous process control, such as petrochemicals.
(10) Manufacturers: Bailey (USA), Westinghouse (USA), HITACHI (Japan), LEEDS & NORTHRMP (USA), SIEMENS (Germany), Foxboro (USA), ABB (Switzerland), Hartmann & Braun (Germany), Yokogawa (Japan), Honeywell (USA), Taylor (USA), etc.
FCS
(1) The primary task is: intrinsic safety, hazardous areas, variable processes, and challenging environments.
(2) Fully digital, intelligent, multifunctional, replacing analog single-function instruments and control devices.
(3) Uses two wires to connect distributed field instruments, control devices, PID, and the control center, replacing two wires for each instrument.
(4) On the bus, PID, instruments, and control devices are all equal.
(5) Multivariable, multi-node, serial, digital communication systems replace single-variable, single-point, parallel, analog systems.
(6) It is interconnected, bidirectional, and open, replacing unidirectional and closed systems.
(7) Uses decentralized virtual control stations instead of centralized control stations.
(8) Controlled by field computers; can also connect to upper-level computers on the same bus.
(9) Local area network can also connect to the internet.
(10) Changes traditional signal standards, communication standards, and system standards in enterprise management networks.
(11) Manufacturers: Honeywell (USA), Smar, Fisher—Rosemount, AB/Rockwell, Elsag—Bailey, Foxboro, Yamatake, Yokogawa (Japan), Siemens (Europe), GEC—Alsthom, Schneider, proces—Data, ABB, etc.
(12) Three types of typical FCS:
1) Continuous process automation control such as petrochemicals, where “intrinsic explosion-proof” technology is critically important; typical products are FF, World FIP, Profibus—PA;
2) Discrete process automation control such as automotive manufacturing robots and cars, typical products are Profibus—DP, CANbus;
3) Multipoint control such as building automation, typical products are LON Work, Profibus—FMS.
From the above basic points, have we noticed thatnone of the three major systems for process control were developed specifically for power plants, or that in their early development, they were not primarily aimed at power plants as control objects. Moreover, the usage instructions of these systems do not list power plants as their primary applicable range, and some do not mention power plants at all.
It is strange that these three major control systems, especially DCS and PLC, are widely used in power plants today, and the results are very good.
2Differences Between the Three Control Systems
As we already know,FCS has developed from both DCS and PLC, FCS not only possesses the characteristics of DCS and PLC but also takes a revolutionary step. Currently, both new DCS and new PLC show a trend of convergence.
The new DCS has strong sequential control capabilities; the new PLC is also competent in closed-loop control, and both can form large networks, with a significant overlap in the applicable range of DCS and PLC.The next section will compare DCS and FCS. The differences between DCS and FCS have been touched upon in previous sections, and the following will discuss aspects such as architecture, investment, design, and use.
Key Differences
The key to DCS systems iscommunication. It can also be said that the data highway is the backbone 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 highway itself determines the overallflexibility and safety. The media for the data highway can be: a pair of twisted wires, coaxial cables, or fiber optic cables.
By examining the design parameters of the data highway, one can basically understand the relative advantages and weaknesses of a specific DCS system.
(1) How much I/O information can the system handle? (2) How much control loop information related to control can the system handle? (3) How many users and devices (CRT, control stations, etc.) can it accommodate? (4) How thoroughly is the integrity of the transmitted data checked? (5) What is the maximum allowable length of the data highway? (6) How many branches can the data highway support? (7) Does the data highway support hardware produced by other manufacturers (programmable controllers, computers, data recording devices, etc.)?
To ensure complete communication, most DCS manufacturers provide redundant data highways.
To ensure the safety of the system, complex communication protocols and error-checking technologies are employed. The so-called communication protocol is a set of rules to ensure that the transmitted data is received and understood as the same as the data sent.
Currently, DCS systems generally usetwo types of communication means, namely 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…
Key Points of FCS
(1) The core of the FCS system
The core of the FCS system is the bus protocol; once its bus protocol is determined, the related key technologies and devices are also determined. In terms of the basic principles of its bus protocol, all types of buses are the same, based on solving bidirectional serial digital communication transmission. However, for various reasons, there are significant differences in the bus protocols of various types of buses.
To meet interoperability requirements, the field bus must become a truly open system. In the IEC international standard, the user layer of the field bus communication protocol model explicitly states that the user layer has device description functionality.
To achieve interoperability, each field bus device is described using a Device Description (DD). DD can be considered a driver for the device, containing all necessary parameter descriptions and operational steps required by the master station. Since DD includes all the information needed for device communication and is independent of the master station, it allows field devices to achieve true interoperability.
There are eight types, while the original IEC international standard is just one of the eight types, with the other seven types of buses being equal in status. Each bus protocol, regardless of market share, has its own set of software and hardware support. They can form systems and products, while the original IEC field bus international standard is an empty framework without software or hardware support.
Therefore, achieving mutual compatibility and interoperability among these buses is nearly impossible given the current state.
From the above, can we conclude that: the interoperability of open field bus control systems, for a specific type of field bus, only requires compliance with the bus protocol of that type of field bus to ensure openness and interoperability of its products.
In other words, regardless of the manufacturer, as long as the products comply with the bus protocol, they can form a bus network.
(2) The foundation of the FCS system is digital intelligent field devices
Digital intelligent field devices are the hardware support of the FCS system and its foundation. The reason is simple; the FCS system executes bidirectional digital communication field bus signals between automatic control devices and field devices.
If the field devices do not follow a unified bus protocol, i.e., relevant communication protocols, and lack digital communication capabilities, then the so-called bidirectional digital communication is just an empty phrase, and it cannot be called a field bus control system.
Moreover, one major feature of the field bus is to enhance the control function at the field level. If the field devices are not multifunctional intelligent products, then the characteristics of the field bus control system do not exist, and the advantages of simplifying systems, facilitating design, and ease of maintenance are also illusory.
(3) The essence of the FCS system is localized information processing
For a control system, whether using DCS or field bus, the amount of information the system needs to process is at least the same. In fact, using a field bus allows for more information to be obtained from the field.
The amount of information in the field bus system has not decreased; it has even increased, while the cables for transmitting information have significantly decreased. This requires, on one hand, greatly to enhance the capacity of cables to transmit information, and on the other hand, to process a large amount of information on-site, reducing the information back and forth between the field and the control room. It can be said that the essence of the field bus is localized information processing.
Reducing information back and forth is an important principle of network design and system configuration. Reducing information back and forth can often lead to improvements in system response time. Therefore, when designing the network, nodes with high information exchange should be prioritized on the same branch.
Reducing information back and forth and reducing the number of cables can sometimes conflict. In such cases, the principle of saving investment should guide the choice. If the selected system’s response time allows, the solution that saves cables should be chosen. If the selected system has a tight response time, slightly reducing the amount of transmitted information may suffice, then the solution that reduces information transmission should be chosen.
Currently, some field instruments with field buses have many functional blocks built-in; although different products may have slight performance differences, it is a fact that many similar functional blocks exist on one network branch. Choosing which functional block to use on which field instrument is a problem to be solved in system configuration.
The principle for considering this issue is: minimize information back and forth on the bus. Generally, one can choose the functional block with the most output related to that function from the instrument.
Typical System Comparison
By using field buses, users can significantly reduce field wiring, achieving multivariable communication with a single field instrument, and devices from different manufacturers can fully interoperate, enhancing control functions at the field level, greatly simplifying system integration, and making maintenance very easy.
In traditional process control instrument systems, each field device requires a dedicated pair of twisted wires to transmit 4~20mA signals to the control room; in a field bus system, each field device can still use twisted wires to connect to the junction box, but only one twisted wire is needed from the junction box to the central control room for digital communication.
The exact amount of cable saved by adopting a field bus control system has not been calculated by the author. However, from the cable kilometers used in power plants adopting DCS systems related to automatic control systems, we can see the share of cables in infrastructure investment.
In a certain power plant, 2×300MW coal-fired units. The thermal system is unit-based. Each unit is equipped with a centralized control building, adopting unit centralized control for machine, furnace, and electricity. The elevation of the unit control room is 12.6 meters, consistent with the operating layer elevation. DCS uses WDPF—II, with a designed I/O point of4500 points.
Cable laying is done using EC software,8 people took 1.5 months to complete the cable laying design task; in the main plant, each 300MW unit has approximately 4038 cables for automation; the total cable length for each 300MW unit’s automation is approximately 350 kilometers.
These cable counts and lengths do not include the fire alarm cables and cables for various auxiliary production workshops in the entire plant; the cable trays’ columns, trays, and small troughs are all made of galvanized steel, with each unit weighing about 95 tons.
Other cable trays include straight-through, bends, tees, four-way junctions, covers, terminal end caps, widening pieces, direct pieces, etc., using aluminum alloy materials, with each 300MW unit weighing about 55 tons.
In another power plant, 4×MW oil-gas power station. The thermal system is unit-based. DCS uses TELEPERM-XP, with a designed I/O point count of5804 points.
Cable laying is done using EC software,12 people took 2.5 months to complete the cable laying design task; in the main plant, each 325MW unit has approximately 4413 cables for automation; the total cable length for each 235MW unit’s automation is approximately 360 kilometers.
All cables in each unit are made of galvanized steel cable trays, weighing about 200 tons.
The cables in power plants can be divided into six categories: high-voltage power cables, low-voltage power cables, control cables, thermal control cables, weak current cables (mainly referring to computer cables), and other cables.
If two 300MW units are simultaneously laid with cables, the number of automation-related cables is approximately 8500. Among them, thermal control cables and weak current cables will exceed 5000, accounting for about 60% (measured by count).
Design, Investment, and Use
The above comparison is focused on purely technical aspects; the following comparison intends to incorporate economic factors.
The premise of comparison is between DCS systems and typical, ideal FCS systems. Why make such assumptions? As the DCS system has developed to today, the technical requirements proposed during its initial development have been met and improved, and the current situation is further enhancement, so there is no typical, ideal statement.
On the other hand, FCS systems entered practical use in the 1990s, and the initial technical requirements for development: compatibility, open communication, digital intelligent field devices, high-speed bus etc., are still not ideal and need improvement. This state is not unrelated to the formulation of international standards for field buses. In the past decade, various bus organizations have been busy formulating standards, developing products, and capturing more markets, aiming to integrate into international standards and legally occupy a larger market.
Now that the battle over international standards has come to a close, major companies and organizations have realized that to truly capture the market, they must improve systems and related products. We can predict that in the near future, well-developed field bus systems and related products will become the mainstream of global field bus technology.
Specific Comparisons:
(1) DCS systems are large systems, whose controllers are powerful and play a crucial role in the system; the data highway is key to the system, so overall investment must be made in one go, and subsequent expansion is challenging. In contrast, FCS has thoroughly decentralized functions, localized information processing, and widespread use of digital intelligent field devices, making the importance of controller functions relatively diminished. Therefore, the investment threshold for FCS systems is low, allowing for gradual use, expansion, and commissioning.
(2) DCS systems are closed systems, and products from different companies are generally incompatible. In contrast, FCS systems are open systems, allowing users to select various devices from different manufacturers and brands to connect to the field bus for optimal system integration.
(3) DCS systems only use binary or analog signals, which require D/A and A/D conversion. In contrast, FCS systems are fully digital, eliminating the need for D/A and A/D conversions, achieving high integration and performance, thus improving accuracy from ±0.5% to ±0.1%.
(4) FCS systems can embed PID closed-loop control functions into transmitters or actuators, shortening control cycles from DCS’s 2-5 times per second to FCS’s 10-20 times per second, thereby improving regulation performance.
(5) DCS can control and monitor the entire process, perform diagnostics, maintenance, and configuration. However, due to its inherent weaknesses, its I/O signals use traditional analog signals, making it impossible to perform remote diagnostics, maintenance, and configuration on field instruments (including transmitters, actuators, etc.) from the DCS engineer station.
(6) Due to localized 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, while also saving space and area for I/O devices and device rooms. Some experts believe that it can save up to 60%.
(7) For the same reasons as (6), FCS can significantly reduce the number of cables and the cable trays used for laying cables, while also saving design, installation, and maintenance costs. Some experts estimate savings of 66%. It should be noted that while the investment savings effect of adopting FCS systems is indisputable, whether it reaches 60-66% as some experts claim is still debated. These figures have appeared in multiple articles, and the author believes they are results of mutual citation; currently, the original sources of these figures have not been found, so readers should exercise caution when citing these numbers.
(8) FCS is simpler to configure than DCS due to standardized structure and performance, facilitating installation, operation, and maintenance.
(9) The design and development points for FCS used in process control should not be compared with DCS; they simply indicate key issues to consider for FCS used in process control or continuous process simulations.
3Prospects for PLC and DCS
We already know that some FCS have developed from PLC, while others have developed from DCS. Now that FCS has become practical, what are the prospects for PLC and DCS?
PLC first appeared in the late 1960s in the United States, intended to replace relays and execute logical, timing, counting, and other sequential control functions to establish flexible program control systems.
In 1976, it was officially named and defined: PLC is a type of digital control dedicated electronic computer that uses programmable memory to store instructions, executes functions like logic, sequencing, timing, counting, and calculations, and controls various machines or work processes through analog and digital input/output components.After more than 30 years of development, PLC has matured and improved, also developing closed-loop control functions for analog signals.
The position of PLC in the FCS system seems to have been established with little debate.PLC functions as a station on a high-speed bus, fully leveraging its advantages in processing switch quantities.
Furthermore, in auxiliary workshops of thermal power plants, such as feedwater treatment workshops, circulating water workshops, ash and slag removal workshops, and coal transport workshops, the process control is primarily sequential. PLC has unique advantages for sequential control.
The control systems for auxiliary workshops should preferably use PLCs that follow field bus communication protocols or PLCs that can communicate and exchange information with FCS.