When it comes to control systems, many people seem to speak eloquently. Continuing to delve deeper, what are controllers, PLCs, DCS, and FCS? After introducing the basic concepts in a few sentences, I suddenly became speechless and couldn’t get to the point no matter how much we talked.
1. The evolutionary history of DCS
People generally believe that to analyze the characteristics and differences of anything, two philosophical questions must be answered: 1) Where does it come from? Where to go?
Similarly, if we want to know what DCS is and what distributed is, we cannot do without the evolutionary history and development direction of DCS.
Next, let’s talk about the evolution and development of DCS from the impact of the four industrial revolutions on process industry control systems. Help everyone gain a deeper understanding of DCS.
In the first industrial revolution, the steam age, automatic control could only perform very simple functions. According to the physical point calculation, only the current DI and DO can achieve the functions of opening and closing, that is, the start and stop of a locomotive. At most, forward and reverse rotation and simple speed control can be achieved with the participation of some peripheral mechanical devices. The well-known Watt is widely recognized as the inventor of the steam engine, but in fact, the appearance of the steam engine was much earlier than Watt’s birth, and the earliest DCS was closely related to the steam engine.
During the Second Industrial Revolution and the era of electrification, electronic devices such as relays became popular, and people began to control large equipment on-site. The improvement compared to before is that, for example, if a device is particularly large, I don’t have to climb up the device to perform start stop operations by using local control. Later on, people still found it troublesome because it was also troublesome to run from one device to another for operation. So, all these buttons that need to be controlled are placed in one room, and simple indicator lights are installed. The operator stares at a pile of indicator lights on the display screen, ready to press the start stop buttons on the control panel at any time.
The third industrial revolution, the information age, with the development of electronic computer technology, control systems have been constantly evolving, forming mature centralized control systems. Today, CCR (Central Control Room) can also be called a centralized control center. Why did they disperse again later? Because people have found that centralized control can lead to many problems. When the points are too centralized and the control scale is too large, if a critical path (module) has a problem, it may cause a large area of control to be paralyzed. This is a common problem of centralized systems, where everything falls into place. So, people are considering dispersing the hardware and placing it near the control area, increasing the number of operator stations, and retaining the CCR. The operator station handles specific on-site affairs, while the CCR supervises the connection and coordination of the entire control area. People are still in the control room, but the entire system is running more stably.
In addition, in order to improve communication efficiency, a new race called FCS (Fildebus Control System) has evolved through improvement. This means that originally, you had to equip each control unit with a role similar to a communicator for communication, but the organization was too bulky and the team was too large, resulting in low efficiency. Therefore, one-on-one communication was cancelled, and everyone completed all communication work through a designated organization, which is the Fildebus.
In the era of the Fourth Industrial Revolution and the Internet of Things, people have done a lot of improvement work to make control systems look more sophisticated, with more decision-making assistance, and more data statistics and analysis. So many functions were branched out or added, such as OEE, MES, ERP
With the advent of the Internet of Things era, a new control system has recently emerged: ICS. The general idea is system functionalization, hardware modularization, and organizational flattening
2. System composition of DCS
The evolution history of DCS mentioned above is a bit vague. Friends may ask, what is the basic form and composition of DCS now? I believe the following image can intuitively explain what DCS is and what it is used for?For example, due to the requirements of a certain process, the liquid level of the water tank needs to be maintained within a certain range. When the liquid level of the water tank fluctuates significantly and exceeds this range, the valve that controls the water level can be manually adjusted on site, or instructions can be issued through DCS to adjust it.As shown in the figure below, DCS is equivalent to the brain, which responds to what the eyes see (i.e. signals transmitted by on-site detection instruments, such as a decrease in water level) by issuing instructions, such as increasing the opening of the inlet valve. The on-site execution equipment receives the instructions and takes corresponding actions, such as increasing the opening of the inlet valve to maintain the water level within a certain range.
The examples given seem to belong to the process industry category, but what about discrete manufacturing industries such as automobiles? There is another star product PLC there.PLC and DCS play a crucial role in industrial automation control, which is the core of the national industrial development strategy. PLC and DCS are constantly upgraded and improved in various aspects of industrial control, and have become indispensable tools in modern industrial production and manufacturing.
（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 that has developed and evolved from centralized control systems.DCS, as a comprehensive computer system that integrates process control and monitoring, has become a complete system of 4C technologies such as communication, display, and control, driven by the continuous development of communication networks. Its main features 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 equipment, and their application in process automation fields such as chemical, power, and metallurgical industries has become very popular.
PLC， A logic programmable controller is an electronic system designed specifically for industrial applications that involves digital operations. It adopts 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）The difference between DCS and PLC controllers
The main difference between DCS and PLC controllers lies in the calculation of switch and analog quantities, even though the two have some mutual infiltration later on, there are still differences. After the 1980s, in addition to logical operations, PLCs also added some control circuit algorithms, but it was still difficult to complete some complex operations. PLCs were programmed using ladder diagrams, and analog operations were not very intuitive during programming, making programming more complicated. 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 forms of both logical and complex analog operations are very clear. However, compared to PLC, the expression efficiency of logical operations is relatively low.
（3） Application of DCS and PLC in Thermal Power Plants
In the field of thermal automation in thermal power plants, DCS and PLC are two completely different yet intricately related concepts. DCS and PLC are both products of the combination of computer technology and industrial control technology. DCS is used for the main control system of thermal power plants, while PLC is mainly used in the auxiliary workshop of power plants. Both DCS and PLC have operator stations to provide means of human-machine interaction, rely on computer technology-based controllers to complete control operations, exchange data with primary components and execution devices through I/O cards, and have communication systems called networks.With the continuous expansion of installed capacity in domestic power plants and the promotion of power system reform, the requirements for auxiliary workshop control are also constantly increasing. In this environment, DCS system entering auxiliary workshop control has become a trend. Due to its comprehensive technological and economic advantages, NT6000DCS has played and will continue to play an increasingly important role in auxiliary workshop control.
The PLC widely used in auxiliary workshops will not withdraw from the historical stage of thermal automation. Unprecedented competitive pressure will prompt PLC manufacturers to approach DCS standards in technology and make greater efforts in price. The competition between DCS and PLC markets will result in greater benefits for users.
（4）Control processing capability of DCS and PLC
A PLC controller can often handle thousands of I/O points (up to over 8000 I/O points). And DCS controllers can generally only handle a few hundred I/O points (no more than 500 I/O).From the requirements of the distributed system, it is not allowed to have centralized control. Controllers with too many points are useless in practical applications. DCS developers have not developed drivers with many I/O point controllers, 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 its 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 the user, rather than the core goal of PLC manufacturers. Another indicator of control processing capability, computing speed, is often perceived as much faster by PLC compared to DCS.The new DCS controller has learned the design of large PLCs and achieved significant improvements 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 computation cycle can be set to 10ms. With high-speed I/O cards, the control cycle can reach 15-20 ms. Analog computation is set in other tasks with longer cycles.
（5） Scalability and compatibility of PLC and DCS systems
There are numerous control products in the market, with many manufacturers producing and selling both DCS and PLC. For PLC systems, there is generally no or very little need for expansion, as PLC systems are typically designed for equipment use. Generally speaking, PLCs rarely have compatibility requirements, such as resource sharing between two or more systems, which is also a difficult task 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.
In the development process of DCS, each manufacturer has its own system, but most DCS systems, such as Yokogawa YOKOGAWA, Honeywell, ABB, etc., although the communication protocols within the system (process level) are not the same, the network platform at the operation level has unanimously chosen Ethernet network, using standard or modified TCP/IP protocol. This provides convenient scalability. In this network, both the controller and the computer exist as nodes, and the number and placement of nodes can be freely increased or decreased wherever the network reaches. In addition, based on open protocols such as OPC and DDE in the Windows system, various systems can also communicate conveniently to achieve resource sharing.
（6） Database for PLC and DCS
DCS generally provides a unified database. In other words, once a 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… However, the database of a PLC system 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 system only uses a unified database, and PCS7 requires controllers of at least S7 414-3 or higher models.
（7）Time scheduling of PLC and DCS
The program of PLC generally cannot run according to the pre-set cycle. The PLC program is executed from start to finish and then restarted from scratch. Some new PLCs have been improved, but there are still limitations on the number of task cycles. DCS can set task cycles. For example, quick tasks, etc. Similarly, for the sampling of sensors, the change time of pressure sensors is very short, and we can sample with a task cycle of 200ms, while the lag time of temperature sensors is large, and we can sample with a task cycle of 2s. In this way, DCS can schedule the resources of the controller reasonably.
（8）Network structure of PLC and DCS
Generally speaking, DCS commonly uses a two-layer network structure, with one layer being the process level network. Most DCS use their own bus protocols, such as Yokogawa’s Modbus, Siemens and ABB’s Profibus, ABB’s CAN bus, etc. These protocols are based on the standard serial transmission protocols RS232 or RS485. The on-site IO module, especially the analog sampling data (machine code, 213/scan cycle), is very large, and there are many on-site interference factors. Therefore, a network standard with high data throughput and strong anti-interference ability should be adopted. The bus structure based on RS485 serial asynchronous communication meets the requirements of on-site communication.
The sampling data of IO is converted into shaped data or real data by the CPU and transmitted on the operational level network (second layer network). Therefore, operational level networks can adopt network standards with moderate data throughput, fast transmission speed, and convenient connection. At the same time, as operational level networks are generally located in control rooms, the requirements for anti-interference are relatively low. Therefore, using standard Ethernet is the best choice. TCP/IP protocol is a standard Ethernet protocol, and we usually use a communication speed of 100Mbit/s.The work tasks of PLC systems are relatively simple, so the amount of data that needs to be transmitted is generally not too large, so the common PLC system is a layer network structure. Process level networks and operation level networks are either merged together, or process level networks are simplified into internal connections between modules. PLCs do not or rarely use Ethernet.
（9）Scale of application objects for PLC and DCS
PLC is generally used in small self-control places, such as equipment control or small analog control and interlocking, while large-scale applications are usually 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 and QCS, as well as the control system that comes with our horizontal products, are generally referred to as PLCs. （10） Market situation and development direction of DCS and PLC
In the field of thermal automation, DCS is widely used in the control system of the main plant without exception. PLC is only used in the auxiliary workshop. The main reason is that early DCS systems were very expensive, and people believed that the operation of auxiliary workshops could be interrupted. The reliability requirements were not very high, and there were fewer requirements for analog control. From the perspective of reducing costs, PLCs were often chosen to build control systems. The control systems of boilers, steam turbines, and generators require long-term stable and reliable operation, and the signals contain a considerable proportion of analog signals. From the perspective of system performance, people have to choose expensive DCS.
Additionally, analyzing the market competition between the main plant DCS and auxiliary workshop control systems, we will discover an interesting phenomenon. The competition for DCS in the main factory building is often fierce among suppliers or agents of different brands, and the price of DCS is constantly decreasing. The competition for auxiliary workshop control systems often occurs among various engineering vendors of the same brand PLC, with lower barriers to entry and more intense competition. However, the price reduction of PLC is not as significant as that of DCS. The main reason is that DCS manufacturers directly participate in competition, constantly reducing equipment manufacturing costs and engineering implementation costs under enormous market pressure. However, PLC manufacturers do not directly participate in competition, and each engineering company can only reduce their own limited engineering costs, resulting in limited space. From the current situation, the price difference between DCS and high-end PLC is no longer significant, and PLC is still widely used in auxiliary workshops due to market inertia.
The market for PLC and DCS products is booming and fiercely competitive. Looking at the PLC market, there are many shining stars: more than 200 companies in the world produce more than 400 varieties and series of PLCs, which are applied in various industries such as power, petroleum and petrochemical, metallurgy, materials, packaging, papermaking, automobiles, and municipal engineering. From an industry perspective, foreign manufacturers dominate and have their own spheres of influence.
The DCS market is almost the same as the PLC market, mainly dominated by strong players in the foreign industry. Fortunately, a group of local manufacturers such as Hollysys, Supcon, Xinhua, etc. are gradually developing and growing. Due to the high technological content of DCS, many demands for products are based on projects, and this demand for DCS will also be a long-term cyclical process. Therefore, it is difficult for the DCS market pattern to undergo significant changes in a short period of time. Of course, due to the irregularity of industry development speed, companies that focus on different industries may undergo 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 have a promoting effect on their respective development and the development of various industries.
