A Brief Discussion on the Characteristics and Differences of the Three Control Systems PLC, DCS, and FCS

Firstly, let’s briefly talk about the Chinese meanings of the three: DCS, Distributed Control System, also known as Distributed Control System; FCS, Fieldbus Control System; PLC, programmable logic controller. Below, we will specifically discuss the characteristics and differences of FCS, DCS, and PLC.

In some industries, FCS has evolved from PLC; In other industries, FCS has evolved from DCS, so there are intricate connections and essential differences between FCS, PLC, and DCS. This article analyzes the characteristics and differences of the three major control systems, PLC, DCS, and FCS, pointing out their origins and development directions.

1. Basic characteristics of PLC, DCS, and FCS control systems

At present, in continuous process production automatic control (PA) or commonly referred to as industrial process control, there are three major control systems, namely PLC, DCS, and FCS. Their basic characteristics are as follows:

1.1 PLC

 

Figure 1

(1) The development from switch quantity control to sequential control and transportation processing is from bottom to top.
(2) Continuous PID control and other multifunctional functions, PID in the interrupt station.
(3) A single PC can be used as the master station, and multiple PLCs of the same type can be used as slaves.
(4) It is also possible to have one PLC as the master station and multiple PLCs of the same type as the slave stations, forming a PLC network. The convenience of using a PC as the main station is that when users program, they do not need to know the communication protocol, as long as they write according to the manual format.
(5) The PLC grid can serve as both an independent DCS/TDCS and a subsystem of DCS/TDCS.
(6) Large systems are similar to DCS/TDCS, such as TDC3000, CENTUMCS, WDPFI, MOD300.

(7) PLC networks such as Siemens SINEC-L1, SINEC-H1, S4, S5, S6, S7, etc., GE’s GENET, Mitsubishi’s MELSEC-NET, MELSEC-NET/MINI.

(8) Mainly used for sequential control in industrial processes, the new PLC also has closed-loop control function.
(9) Manufacturers: GOULD (USA), AB (USA), GE (USA), OMRON (Japan), MITSUBISHI (Japan), Siemens (Germany), etc.

1.2 DCS or TDCS

(1) Distributed Control System: DCS and Distributed Control System (TDCS) are monitoring technologies that integrate 4C (Communication, Computer, Control, CRT) technology.
(2) A tree topology large system from top to bottom, where communication is crucial.
(3) In the interrupt station, PID connects the computer with on-site instruments and control devices.
(4) It is a tree topology and parallel continuous link structure, with a large number of cables running in parallel from relay stations to on-site instruments and meters.
(5) Analog signal, A/D-D/A, mixed with microprocessor.
(6) A pair of instruments are connected to I/O and connected from the control station to the local area network LAN.

(7) DCS is a three-level structure consisting of control (engineer station), operation (operator station), and on-site instruments (on-site measurement and control station).
(8) The disadvantage is high cost, products from different companies cannot be interchanged or interoperable, and DCS systems are different from each other.
(9) Used for large-scale continuous process control, such as petrochemicals.
(10) Manufacturers: Bailey (USA), Westinghouse (USA), HITACH (Japan), LEEDS&NORTHRMP (USA), SIEMENS (Germany), Foxboro (USA), ABB (Switzerland), Hartmann&Braun (Germany), Yokogawa (Japan), Honeywell (USA), Taylor (USA), etc.

1.3 FCS

(1) The basic tasks are: essential (intrinsic) safety, hazardous areas, volatile processes, and difficult to deal with extraordinary environments.
(2) Fully digital, intelligent, and multifunctional replacing analog single function instruments, meters, and control devices.
(3) Use two wires to connect scattered field instruments, control devices, PID, and control centers, replacing each instrument with two wires.
(4) On the bus, PID is equal to instruments, meters, and control devices.

(5) Multi variable, multi node, serial, digital communication systems replace single variable, single point, parallel, and analog systems.
(6) It is interconnected, bidirectional, and open, replacing unidirectional and closed.
(7) Replace centralized control stations with decentralized virtual control stations.
(8) Operated by on-site computers, it can also be connected to the upper computer and connected to the same bus’s upper level computer.
(9) Local area network can be connected to the internet.

(10) Change traditional signal standards, communication standards, and system standards into the enterprise management network.

(11) Manufacturers: Honeywell, Smar, Fisher Rosemount, AB/Rockwell, Elsag Bailey, Foxboro, Yamatake, Yokogawa, Siemens, GEC Alstom, Schneider, Process Data, ABB, etc.
(12) Typical examples of 3 types of FCS

A. Continuous process automatic control, such as in the petrochemical industry, where “intrinsic safety and explosion-proof” technology is absolutely important, typical products are FF, World FIP, and Profibus PA;

B. Discrete process action automatic control, such as automotive manufacturing robots and automobiles, typical products are Profibus DP and CANbus;

C. Multi point control, such as building automation, typical products are LON Work and Profibus FMS

Differences between the three major control systems

We already know that FCS has evolved from DCS and PLC, and it not only possesses the characteristics of DCS and PLC, but also takes a revolutionary step forward. At present, there is a trend for new DCS and new PLC to approach each other. The new DCS already has strong sequential control functions; The new type of PLC is not inferior in handling closed-loop control, and both can form a large network. The application range of DCS and PLC has a significant overlap.

2.1 Differences between FCS and DCS

2.1.1 Technical comparison between FCS and DCS

Traditional method: The connection between on-site devices and controllers adopts a one-to-one I/O connection method

2.1.2 Specific economic comparison between FCS and DCS:

(1) The DCS system is a large system, and its controller has strong functions and plays a very important role in the system. The data highway is the key to the system, so the overall investment must be made in one step, and it is difficult to expand afterwards. The thorough decentralization of FCS functions, on-site information processing, and widespread adoption of digital intelligent field devices have led to a relative reduction in the functionality and importance of controllers. Therefore, the investment starting point of the FCS system is low, and it can be used, expanded, and put into operation simultaneously.

(2) The DCS system is a closed system, and the products of various companies are basically incompatible. The FCS system is an open system, where users can choose various devices from different manufacturers and brands to connect to the fieldbus, achieving optimal system integration.

(3) The information of the DCS system is all formed by binary or analog signals, and there must be D/A and A/D conversion. The FCS system is fully digital, eliminating the need for D/A and A/D conversion, with high integration and performance, enabling accuracy to increase from ± 0.5% to ± 0.1%

(4) The FCS system can incorporate PID closed-loop control functions into transmitters or actuators, shortening the control cycle. Currently, it can increase from 2-5 times per second in DCS to 10-20 times per second in FCS, thereby improving regulation performance.

(5) DCS can control and monitor the entire process of the process, diagnose, maintain, and configure itself. However, due to its own fatal weakness, its I/O signals use traditional analog signals, making it impossible to remotely diagnose, maintain, and configure on-site instruments (including transmitters, actuators, etc.) on the DCS engineer station. FCS adopts fully digital technology, and digital intelligent field devices send multivariate information, not just single variable information, but also have the function of detecting information errors. FCS adopts a bidirectional digital communication fieldbus signaling system. Therefore, it can perform remote diagnosis, maintenance, and configuration of on-site devices (including transmitters, actuators, etc.). The superiority of FCS is incomparable to DCS.

(6) Due to the fieldization of information processing, FCS can save a considerable number of isolators, terminal cabinets, I/O terminals, I/O cards, I/O files, and I/O cabinets compared to DCS. At the same time, it also saves space and floor space for I/O devices and device rooms. Some experts believe that it can save 60%

(7) For the same reason as (6), FCS can reduce a large number of cables and cable trays used for cable laying, while also saving design, installation, and maintenance costs. Some experts believe that it can save 66%. Regarding points (6) and (7), it should be noted that there is no doubt about the investment saving effect of using the FCS system, but whether it can reach 60-66% as some experts say. These numbers appear in multiple articles, and the editor believes that they are the result of mutual borrowing. Currently, the original source of these numbers has not been found, so readers should be cautious when quoting these numbers.

(8) FCS has a simpler configuration compared to DCS, and due to standardized structure and performance, it is easy to install, operate, and maintain.

(9) Key points of FCS design and development for process control. This is not intended as a comparison with DCS, but rather to illustrate the key issues that should be considered in the design and development of FCS for process control or simulation of continuous processes.
1) The requirement for intrinsic safety and explosion-proof function of the bus is of utmost importance.
2) The basic monitoring of changes such as flow rate, material level, temperature, pressure, etc. is slow and has a lag effect. Therefore, node monitoring does not require fast electronic response time, but requires complex analog processing capabilities. This physical characteristic determines that the system generally adopts a centralized polling system between master and slave, which is technically reasonable and economically advantageous.
3) The physical principles of measuring parameters such as flow rate, material level, temperature, and pressure are classical, but sensors, transmitters, and controllers should develop towards digital intelligence.
4) As an FCS developed for continuous process and its instrumentation, it should focus on improving the design of the low-speed bus H1.

2.2 Differences between PLC and DCS

2.2.1 Definition of DCS and PLC

DCS control system, also known as distributed control system in the domestic automation industry. The so-called distributed control system is a new type of computer control system compared to centralized control systems. It is developed and evolved on the basis of centralized control systems.

DCS, as a computer integrated system that integrates process control and monitoring, has become a complete system that integrates computer, communication, display, and control technologies under the continuous drive of communication networks. Its main characteristics are decentralized control, centralized operation, hierarchical management, flexible configuration, and convenient configuration.

Nowadays, DCS systems can be widely used for production control and business management of industrial devices, and their applications in process automation fields such as chemical, power, and metallurgy have become very popular.

PLC, also known as logic programmable controller, is an electronic system designed for digital operation and designed specifically for industrial applications. It uses a type of programmable memory for storing programs internally, executing user oriented instructions such as logical operations, sequential control, timing, counting, and arithmetic operations, and controlling various types of machinery or production processes through digital or analog input/output. It is the core part of industrial control.

2.2.2 Differences between DCS and PLC controllers

The main difference between DCS and PLC controllers is in the calculation of switching and analog quantities, even though the two may have some infiltration into each other later on, there are still differences. After the 1980s, in addition to logical operations, PLC also added some control circuit algorithms, but it was still difficult to complete some complex operations. PLC used ladder diagram programming, and analog operations were not very intuitive during programming, making programming more cumbersome. But in terms of solving logic, it shows the advantage of being fast. DCS uses functional blocks to encapsulate analog and logical operations, and the expression of both logical and complex analog operations is very clear. However, compared to PLC, the expression efficiency of logical operations is relatively low.

2.2.3 Control and processing capabilities of DCS and PLC

A PLC controller can often handle thousands of I/O points (up to over 8000 I/Os). And DCS controllers can generally only handle hundreds of I/O points (not exceeding 500 I/O points).

From the requirements of a distributed system, it is not allowed to have centralized control. Controllers with too many I/O points are useless in practical applications. DCS developers do not need to drive controllers with many I/O points, and their main focus is on improving the reliability and flexibility of the system.

However, PLC is different. As an independent flexible control device, the stronger the point capability, the higher its technical level. As for the application level of the entire control system, it is mainly the responsibility of the engineering company and users, rather than the core goal of PLC manufacturers. Another indicator of control processing capability, computing speed, is that PLC is also perceived to be much faster than DCS.

The new DCS controller has learned the design of large PLCs and achieved a significant improvement in control cycle performance. Taking the T2550 controller of NT6000DCS as an example. The controller can set four tasks with different priorities, and the minimum operation cycle can be set to 10ms. Combined with high-speed I/O cards, the control cycle can reach 15-20ms. Analog operations are set in other tasks with longer cycles.

2.2.4 System Scalability and Compatibility of PLC and DCS

There are many control products in the market, both DCS and PLC, which are produced and sold by many manufacturers. For PLC systems, there is generally no or very little demand for expansion, as PLC systems are generally designed for equipment use. Generally speaking, PLCs also rarely have compatibility requirements, such as the requirement for resource sharing between two or more systems, which is also very difficult for PLCs. Moreover, PLCs generally adopt dedicated network structures, such as Siemens’ MPI total linear network, and even adding an operator station is not easy or costly.

During the development process of DCS, various manufacturers have their own systems. However, most DCS systems, such as Yokogawa YOKOGAWA, Honeywell, ABB, and so on, although the communication protocols within the system (process level) are not the same, the network platforms at the operation level all choose Ethernet networks and use standard or modified TCP/IP protocols. This provides convenient scalability. In this type of network, both the controller and the computer exist as a node, and as long as the network reaches its destination, the number of nodes can be increased or decreased and the position of nodes can be arranged arbitrarily. In addition, based on open protocols such as OPC and DDE in Windows systems, various systems can also communicate conveniently to achieve resource sharing.

2.2.5 Database of PLC and DCS

DCS generally provides a unified database. In other words, once a data exists in the database in a DCS system, it can be referenced in any situation, such as in configuration software, monitoring software, trend charts, reports… However, the database of PLC systems is usually not unified, and configuration software, monitoring software, and even archiving software have their own databases. Why is it often said that Siemens S7 400 is called DCS only when it reaches 414 or above? Because Siemens PCS7 systems only use a unified database, and PCS7 requires controllers to be at least S7 414-3 or higher.

Time scheduling of PLC and DCS

PLC programs generally cannot run according to pre-set cycle times. PLC programs are executed from start to finish and then from scratch. Some new PLCs have been improved, but there are still limitations on the number of task cycles. DCS can also set task cycles. For example, fast tasks, etc. Similarly, for the sampling of sensors, the change time of pressure sensors is very short. We can use a task cycle of 200ms for sampling, while the lag time of temperature sensors is large. We can use a task cycle of 2s for sampling. In this way, DCS can schedule the resources of the controller reasonably.

Network structure of PLC and DCS

Generally speaking, DCS commonly uses a two-layer network structure, with one layer being a process level network. Most DCS uses their own bus protocols, such as Yokogawa’s Modbus, Siemens and ABB’s Profibus, ABB’s CAN bus, etc. These protocols are built on the basis of standard serial transmission protocols RS232 or RS485. On site IO modules, especially analog sampling data (machine code, 213/scan cycle), are very large, and there are many interference factors on site. Therefore, network standards with high data throughput and strong anti-interference ability should be adopted. The bus structure based on RS485 serial asynchronous communication method meets the requirements of on-site communication. The sampling data of IO is converted by the CPU into shaped or solid data and transmitted on the operation level network (second layer network). Therefore, the operation level network can adopt network standards with moderate data throughput, fast transmission speed, and convenient connection. At the same time, since the operation level network is generally arranged in the control room, the requirements for anti-interference are relatively low. Therefore, using standard Ethernet is the best choice. TCP/IP protocol is a standard Ethernet protocol that typically uses a communication speed of 100Mbit/s.

The working task of PLC system is relatively simple, so the amount of data that needs to be transmitted is generally not too large, so the common PLC system is a one layer network structure. The process level network and the operation level network can either be merged together, or the process level network can be simplified into internal connections between modules. PLC does not or rarely uses Ethernet.

2.2.8 Application Object Scale of PLC and DCS

PLC is generally used in small-scale self-control places, such as equipment control or control and interlocking of a small amount of analog signals, while large-scale applications are generally DCS. Of course, this concept is not very accurate, but it is very intuitive. Traditionally, we refer to systems larger than 600 points as DCS, and systems smaller than this scale as PLC. Our heat pump, QCS, and horizontal product control systems are generally referred to as PLCs.