Which is better, a DCS system or a PLC? Comparison of the advantages and disadvantages of DCS and PLC

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PLCs and DCS play a crucial role in industrial automation control, which is **the core of industrial development strategies**. PLC and DCS continue to be upgraded and improved in various aspects of industrial control, and have become indispensable tools in modern industrial manufacturing. 　　1. Definitions of DCS and PLC A DCS control system is also referred to as a distributed control system in China’s automation industry. The so-called distributed control system is a new type of computer control system as opposed to centralized control systems; it has developed and evolved from centralized control systems. 　　As a comprehensive computer system that integrates process control and process monitoring, DCS has, driven by communication networks, evolved into a complete system encompassing the 4C technologies of computing, communication, display, and control. Its main features are decentralized control, centralized operation, hierarchical management, flexible configuration, and easy setup. 　　Today’s DCS systems can be widely used for the production control and operational management of industrial facilities, and their use in fields such as process automation in the chemical, power, and metallurgy industries is already very widespread. 　　A PLC, or Logic Programmable Controller, is an electronic system that performs digital calculations, designed specifically for use in industrial environments. It uses a type of programmable memory for storing programs internally, to carry out user-oriented commands such as logical operations, sequential control, timing, counting, and arithmetic operations; it also controls various types of machinery or production processes through digital or analog input/output, and constitutes the core component of industrial control. 　　2. Differences between DCS and PLC controllers The main difference between DCS and PLC controllers lies in the processing of digital and analog signals; although there has been some overlap between the two over time, differences still exist. After the 1980s, in addition to logical operations, PLCs also incorporated some control loop algorithms; however, performing certain complex calculations remained difficult. PLCs are programmed using ladder diagrams, and performing arithmetic operations on analog values is not very intuitive in this programming approach, making it rather cumbersome. However, it shows advantages in terms of calculation logic speed. DCS uses function blocks to encapsulate analog and logical operations; the representation of both logical and complex analog operations is very clear, yet its efficiency in handling logical operations is lower compared to PLCs. 　　3. Applications of DCS and PLC in thermal power plants In the field of thermal automation in thermal power plants, DCS and PLC are two distinct concepts that are yet closely interconnected. Both DCS and PLC are products of the combination of computer technology and industrial control technology. DCS is used in the main control systems of thermal power plants, while PLCs are primarily applied in the auxiliary workshops of power plants. Both DCS and PLC have operator stations that provide a means for human-computer interaction; they both rely on computer-based controllers to carry out control calculations; they both use I/O cards to exchange data with primary components and actuating devices; and they both possess communication systems referred to as networks. 　　With the continuous expansion of the installed capacity of power plants in China and the advancement of power system reforms, the requirements for control in auxiliary workshops are also increasing. In this context, it has become a trend for DCS systems to be used for controlling auxiliary workshops. Due to its comprehensive technical and economic advantages, the NT6000DCS is playing, and will continue to play, an increasingly important role in the control of auxiliary workshops. 　　PLCs, which are widely used in auxiliary workshops, will not disappear from the history of thermal process automation. Unprecedented competitive pressures will drive PLC manufacturers to align their technologies with DCS standards and to make greater efforts in terms of price. The outcome of the competition between DCS and PLC markets will bring greater benefits to users. 　　4. Control and processing capabilities of DCS and PLCs A PLC controller is typically capable of handling thousands of I/O points (up to over 8,000 I/Os). The controllers of DCS can generally handle only a few hundred I/O points (no more than 500 I/Os). 　　From the perspective of the requirements of distribution systems, centralized control is not allowed; controllers with too many I/O points are useless in practical applications. DCS developers do not need to create drivers for controllers with a large number of I/O points, as their main focus is on ensuring the reliability and flexibility of the system. PLC is different; as an independent flexible control device, the stronger its processing capabilities, the higher its technical level. As for the level of application of the entire control system, that is primarily the responsibility of engineers and users, rather than the core goal of PLC manufacturers. Another indicator of control processing capacity is processing speed, and it is generally believed that PLCs are much faster than DCSs. 　　The new DCS controller incorporates the design principles of large-scale PLCs, resulting in a significant improvement in its performance in terms of control cycle time. Taking the T2550 controller of the NT6000DCS as an example. The controller can set four tasks with different priorities; the minimum operation cycle can be set to 10 ms, and with high-speed I/O cards, the control cycle can reach 15–20 ms. Analog operations are scheduled in other tasks with longer cycles. 　　5. Market situation and development trends of DCS and PLC In the field of thermal process automation, DCS is used in control systems for main plant facilities without exception. PLC is only used in the auxiliary workshop. The main reason is that early DCS systems were extremely expensive; it was believed that the operation of auxiliary workshops could be intermittent, with lower requirements for reliability, and fewer demands for analog control. To reduce costs, PLCs were often chosen to build the control systems. The control systems for boilers, turbines, and generators require long-term stable and reliable operation, and the signals contain a significant proportion of analog values; considering the performance requirements of such systems, people were forced to opt for expensive DCS systems. 　　Furthermore, by analyzing the market competition in the DCS systems of the main plant and the control systems of the auxiliary workshops, we find an interesting phenomenon. Competition for the DCS in the main plant often takes place among suppliers or agents of different brands; it is fierce, and the prices of DCS continue to drop. Competition in auxiliary workshop control systems often takes place among various manufacturers of PLCs from the same brand; the entry barrier is low, so competition is fierce. However, the decline in PLC prices is not as significant as that of DCS. The main reason is that DCS manufacturers are directly involved in competition, and under fierce market pressure, they continuously reduce the costs of equipment manufacturing and project implementation. The PLC manufacturers do not participate in this competition directly, and engineering firms can only reduce their limited project costs, leaving little room for further reduction. Judging from the current situation, the price gap between DCS and high-end PLCs is no longer significant; auxiliary workshops still rely more on PLCs, due to market inertia. 　　The PLC and DCS product markets are extremely dynamic and highly competitive. The PLC market is home to a multitude of outstanding players: over 200 companies around the world produce more than 400 different models of PLCs, which are used in various industries such as power generation, oil and petrochemicals, metallurgy, materials processing, packaging, paper manufacturing, the automotive industry, and municipal services. From an industry perspective, foreign manufacturers dominate the market and have their own spheres of influence. 　　The DCS market is similar to the PLC market, with it being dominated primarily by foreign industry leaders. Fortunately, a number of domestic manufacturers such as Hollysys, Supcon, and Xinhua are gradually growing stronger. Due to the high technical complexity of DCS, many demands for these products arise from project-based initiatives; as a result, the demand for DCS will also follow a cyclical pattern over extended periods of time. Therefore, it is unlikely that there will be significant changes in the DCS market landscape in the short term. Of course, due to the irregular pace of industry development, companies that focus on different sectors within this industry may experience some changes as a result. 　　In the future development of control systems, we will see a gradual integration of DCS and PLC technologies, which will facilitate the growth of both of these technologies as well as that of various industries. Differences between PLCs and DCS: 1. In terms of development history: DCS evolved from traditional panel-based monitoring systems. Therefore, DCS systems are inherently focused on the control of instruments; for example, the YOKOGAWA CS3000 DCS system we use does not even have any limit on the number of PIDs (PID, or proportional-integral-differential algorithm, is the standard algorithm for closed-loop control of control valves and frequency converters, and typically the number of PIDs determines the number of control valves that can be used). PLCs evolved from traditional relay circuits; the earliest PLCs did not even have the capability to process analog signals. Therefore, logic processing capabilities were emphasized in PLCs from the very beginning. 　　2. In terms of the system’s scalability and compatibility: There are a large number of control products available on the market; many manufacturers produce and sell both DCS and PLC systems. For PLC systems, there is generally no need for expansion or such needs are rare, as PLC systems are typically used for equipment. Generally speaking, PLCs also have few compatibility requirements; for example, it is very difficult for PLCs to enable two or more systems to share resources. Moreover, PLCs generally use dedicated network architectures, such as Siemens’ MPI linear network; it is even difficult or costly to add an operator station. 　　Throughout their development, different manufacturers have created their own systems for DCS. However, for most DCS systems, such as those from YOKOGAWA, Honeywell, ABB, etc., although the communication protocols at the process level within these systems vary, the network platforms at the operational level have all opted for Ethernet, using standard or modified TCP/IP protocols. In this way, diangon.com provides very convenient scalability. In such a network, controllers and computers exist as nodes, and the number of nodes as well as their locations can be adjusted freely wherever the network extends. Furthermore, thanks to open protocols such as OPC and DDE based on the Windows system, various systems can communicate with each other easily to achieve resource sharing. 　　3. In terms of databases: DCS systems generally provide a unified database. In other words, in a DCS system, once data is stored in the database, it can be referenced under any circumstances – such as in configuration software, monitoring software, trend charts, reports, etc. Meanwhile, the databases in PLC systems are usually not unified; configuration software, monitoring software, and even archiving software all have their own databases. Why is it often said that Siemens’ S7 400 becomes a DCS only when it moves to version 414 or higher? This is because only Siemens’ PCS7 system uses a unified database, and PCS7 requires controllers of model S7 414-3 or higher. 　4. In terms of timing scheduling: PLC programs generally cannot operate at a pre-set cyclic interval. The PLC program is executed from start to finish once, and then starts executing again from the beginning. (Some new types of PLCs have seen improvements, but there are still limits on the number of task cycles), whereas DCS allows for the setting of task cycles. For example, quick tasks, etc. Regarding sensor sampling as well, pressure sensors have a very short response time, so we can use a sampling interval of 200 ms; whereas temperature sensors have a large lag time, allowing us to use a sampling interval of 2 seconds. In this way, DCS can reasonably schedule the resources of the controllers. 　　5. In terms of network structure: Generally speaking, DCS systems typically employ a two-layer network structure. One layer is the process-level network. Most DCS systems utilize their own bus protocols, such as Yokogawa’s Modbus, Siemens and ABB’s Profibus, and ABB’s CAN bus. All these protocols are based on the standard serial communication protocols RS232 or RS485. Field IO modules, particularly the sampling data for analog signals (machine code, 213/scanning cycle), generate a large volume of data; moreover, there are many interference factors in the field. Therefore, network standards with high data throughput and strong resistance to interference should be employed. The bus structure is based on the asynchronous communication mode of RS485 serial ports, meeting the requirements for field communication. 　　The sampled data from the IO is converted by the CPU into integer or real data, which is then transmitted over the operational level network (layer 2 network). Therefore, operational-level networks can adopt network standards that offer moderate data throughput, fast transmission speeds, and easy connectivity; moreover, since such networks are usually located in control rooms, the requirements for interference resistance are relatively low. Therefore, using standard Ethernet is the best choice. The TCP/IP protocol is a standard Ethernet protocol, and we generally use a communication speed of 100 Mbit/s. 　　The tasks performed by PLC systems are relatively simple; therefore, the amount of data that needs to be transmitted is generally not large. As a result, common PLC systems adopt a single-layer network structure. The process-level network and the operation-level network are either combined, or the process-level network is simplified to internal connections between modules. PLCs do not or rarely use Ethernet. 　　6. In terms of the scale of the application: PLCs are generally used in small automated control systems, such as for controlling equipment or managing a small number of analog signals as well as implementing interlocks, while large-scale applications typically make use of DCS. Of course, this concept isn’t entirely accurate, but it is intuitive; *we usually refer to systems with more than 600 points as DCS, while those with fewer points are called PLCs. Our heat pumps, QCS systems, and the control systems for related horizontal products are generally referred to as PLCs.