PLCs and DCSs play a crucial role in industrial automation control, which is a core component of national industrial development strategies. Through continuous upgrades and improvements across various aspects of industrial control, PLCs and DCSs have become indispensable tools in modern industrial manufacturing.
1. Definitions of DCS and PLC
DCS control systems, also known as distributed control systems in the domestic automation industry, are a new type of computer control system that is developed and evolved from centralized control systems.
As a comprehensive computer system integrating process control and monitoring, DCS has evolved into a complete system integrating computer, communication, display, and control technologies (4C technologies) driven by continuous advancements in communication networks. Its main characteristics include distributed control, centralized operation, hierarchical management, flexible configuration, and convenient setup.
Today's DCS systems can be widely used for production control and operation management of industrial plants, and their application in process automation fields such as chemical, power, and metallurgy is already very common.
A PLC, or Programmable Logic Controller, is a digital electronic system designed for industrial applications. It uses a programmable memory to store programs, execute user-oriented instructions such as logic operations, sequential control, timing, counting, and arithmetic operations, and control various types of machinery or production processes through digital or analog inputs/outputs. It is a core component of industrial control.
2. Differences between DCS and PLC controllers
The main difference between DCS and PLC controllers lies in their handling of digital and analog signal processing. Even with some overlap in their approaches, distinctions remain. Since the 1980s, PLCs have incorporated control loop algorithms in addition to logic operations, but performing complex calculations remains challenging. PLCs use ladder logic programming, which makes analog signal processing less intuitive and more cumbersome. However, they offer a significant advantage in rapid logic decomposition. DCS, on the other hand, encapsulates analog and logic operations using function blocks. This results in a very clear expression for both logic and complex analog operations, although its logic expression efficiency is lower compared to PLCs.
3. Applications of DCS and PLC in thermal power plants
In the field of thermal power plant automation, DCS and PLC are two completely different yet intricately related concepts. Both DCS and PLC are products of the integration of computer technology and industrial control technology. The main control system of a thermal power plant uses DCS, while PLC is mainly used in auxiliary workshops of the power plant. Both DCS and PLC have operator stations to provide human-machine interaction, rely on computer-based controllers to complete control calculations, exchange data with primary components and actuators through I/O cards, and have a communication system called a network.
With the continuous expansion of installed power capacity in China and the advancement of power system reform, the requirements for auxiliary workshop control are also constantly increasing. In this context, the integration of DCS systems into auxiliary workshop control has become a trend. Due to its comprehensive technical and economic advantages, the NT6000DCS has been and will continue to play an increasingly important role in auxiliary workshop control.
PLCs, widely used in auxiliary workshops, will not disappear from the history of thermal automation. Unprecedented competitive pressure will force PLC manufacturers to align their technology with DCS standards and make greater efforts in pricing. The outcome of competition in the DCS and PLC markets will bring greater benefits to users.
4. Control and processing capabilities of DCS and PLC
A PLC controller can typically handle several thousand I/O points (up to 8,000 or more). In contrast, a DCS controller can generally only handle a few hundred I/O points (no more than 500).
From the perspective of distributed control systems, centralized control is not allowed. Too many controllers are useless in practical applications. DCS developers don't need to develop drivers for controllers with many I/O points; their main focus is on providing system reliability and flexibility. PLCs are different. As independent, flexible control devices, stronger I/O capabilities naturally indicate higher technical levels. The overall application level of the control system is primarily a concern for engineers and users, not the core objective of PLC manufacturers. Another indicator of control processing capability is processing speed, and PLCs are generally perceived to be much faster than DCSs.
The new DCS controller has adopted the design principles of large PLCs, resulting in a significant improvement in control cycle performance. Take the T2550 controller of the NT6000DCS as an example. The controller can be configured with four tasks of different priorities, with a minimum operation cycle of 10ms. Combined with high-speed I/O cards, the control cycle can reach 15-20ms. Analog signal operations are then assigned to other tasks with longer cycles.
5. Market situation and development trend of DCS and PLC
In the field of thermal power automation, the main plant control system almost invariably uses a DCS (Distributed Control System). PLCs are used only in auxiliary workshops. The main reason for this is that early DCS systems were very expensive. It was believed that auxiliary workshop operations could be intermittent, reliability requirements were not very high, and analog control requirements were minimal. From a cost-reduction perspective, PLCs were often chosen to build the control system. However, the control systems for boilers, turbines, and generators require long-term stable and reliable operation, and the signals contain a significant proportion of analog signals. From a system performance perspective, the expensive DCS had to be chosen.
Furthermore, analyzing the market competition for main plant DCS and auxiliary workshop control systems reveals an interesting phenomenon. Competition for main plant DCS often occurs between suppliers or agents of different brands, leading to fierce competition and continuous price reductions. In contrast, competition for auxiliary workshop control systems is typically among contractors using the same brand of PLCs. The entry barrier is lower, resulting in even fiercer competition, but PLC price reductions are not as significant as those for DCS. This is primarily because DCS manufacturers directly participate in the competition, continuously lowering equipment manufacturing and implementation costs under immense market pressure. PLC manufacturers, however, do not directly compete, limiting the scope for contractors to lower their own limited engineering costs. Currently, the price difference between DCS and high-end PLCs is no longer significant, and the continued widespread use of PLCs in auxiliary workshops is a result of market inertia.
The PLC and DCS product market is booming and fiercely competitive. The PLC market is a constellation of players: over 200 companies worldwide produce more than 400 different PLC models, used in various industries including power, petrochemicals, metallurgy, materials, packaging, papermaking, automotive, and municipal engineering. From an industry perspective, foreign manufacturers dominate, each with their own sphere of influence.
The DCS market is similar to the PLC market, largely dominated by foreign industry leaders. Encouragingly, a number of domestic manufacturers, such as Hollysys, Supcon, and Xinhua, are gradually growing stronger. Due to the high technological content of DCS, many product demands are project-based, meaning the demand for DCS will be a long-term, cyclical process. Therefore, the DCS market structure is unlikely to change significantly in the short term. Of course, due to the irregular pace of industry development, companies focusing on different areas may experience some changes.
In the future development of control systems, we will see the gradual integration of DCS and PLC technologies, which will promote their respective development as well as the development of various industries.
The difference between PLC and DCS
1. From a developmental perspective:
DCS evolved from traditional instrument panel monitoring systems. Therefore, DCS inherently emphasizes instrument control; for example, the YOKOGAWACS3000DCS system we use doesn't even have a limit on the number of PID controllers (PID, Proportional-Derivative-Integral algorithm, is the standard algorithm for closed-loop control of control valves and frequency converters; the number of PID controllers usually determines the number of control valves that can be used). PLC evolved from traditional relay circuits; early PLCs didn't even have analog signal processing capabilities. Therefore, PLCs emphasized logic operation capabilities from the beginning.
2. From the perspective of system scalability and compatibility:
The market offers a wide variety of control products, with numerous manufacturers producing and selling both DCS and PLC systems. PLC systems generally have little or no need for expansion, as they are typically designed for specific devices. Compatibility requirements for PLCs are also rare; for example, resource sharing between two or more systems is very difficult for a PLC. Furthermore, PLCs typically use dedicated network architectures, such as Siemens' MPI bus network, making even adding an operator station difficult or costly.
During the development of DCS (Distributed Control Systems), each manufacturer developed its own system. However, most DCS systems, such as Yokogawa, Honeywell, and ABB , while using different communication protocols at the process level, unanimously chose Ethernet for their operational network platforms, employing standard or modified TCP/IP protocols. This provides convenient scalability. In this network, controllers and computers exist as nodes; the number and location of nodes can be freely added or removed wherever the network reaches. Furthermore, based on open protocols such as OPC and DDE used in Windows systems, different systems can communicate easily to achieve resource sharing.
3. From a database perspective:
DCS systems typically provide a unified database. In other words, once data exists in the database of a DCS system, it can be referenced in any situation, such as in configuration software, monitoring software, trend charts, reports, etc. However, PLC systems usually do not have a unified database; configuration software, monitoring software, and even archiving software each have their own databases. Why is it often said that Siemens' S7400 series only qualifies as a DCS at the S7414 level or higher? Because Siemens' PCS7 system uses a unified database, and PCS7 requires controllers to be at least S7414-3 or higher.
4. From a time scheduling perspective:
PLC programs generally cannot run according to a pre-set cycle. A PLC program executes from beginning to end once and then starts again. (Some newer PLCs have improved this, but there are still limitations on the number of task cycles.) DCS, on the other hand, allows setting task cycles, such as fast tasks. For example, when sampling sensors, pressure sensors have very short change times, so we can use a 200ms task cycle for sampling, while temperature sensors have much longer hysteresis, so we can use a 2s task cycle. This allows the DCS to rationally allocate controller resources.
5. From the perspective of network structure:
Generally, DCS systems typically use a two-layer network structure. The first layer is the process-level network, and most DCS systems use their own bus protocols, such as Yokogawa's Modbus, Siemens and ABB's Profibus, and ABB's CANbus. These protocols are all based on the standard serial port transmission protocols RS232 or RS485. Field I/O modules, especially those handling analog sampling data (machine code, 2^13/scan cycle), generate a large amount of data, and there are many interference factors in the field. Therefore, a network standard with high data throughput and strong anti-interference capabilities should be adopted. A bus structure based on RS485 asynchronous serial communication meets the requirements of field communication.
The sampled I/O data is converted into integer or real data by the CPU and transmitted over the operator-level network (Layer 2 network). Therefore, the operator-level network can adopt a network standard with moderate data throughput, high transmission speed, and convenient connection. Furthermore, since the operator-level network is generally located in a control room, the requirements for interference resistance are relatively low. Therefore, using standard Ethernet is the best choice. TCP/IP is a standard Ethernet protocol, and we generally use a communication speed of 100 Mbit/s.
PLC systems typically have relatively simple tasks, so the amount of data they need to transmit is generally not large. Therefore, common PLC systems use a single-layer network structure. The process-level network and the operator-level network are either combined, or the process-level network is simplified into internal connections between modules. PLCs rarely or never use Ethernet.
6. In terms of the scale of the application objects:
PLCs are generally used in small-scale automation applications, such as equipment control or control and interlocking of a small number of analog signals, while large-scale applications typically use DCSs. While this concept isn't entirely accurate, it's intuitive; conventionally, we refer to systems with more than 600 points as DCSs, and those smaller as PLCs. Our heat pumps, QCSs, and the control systems配套的控制制 for our horizontal products are generally referred to as PLCs.
