PLC and DCS in industrial automation control occupies a pivotal position, and industrial automation control is the core of the national industrial development strategy. PLC as well as DCS in the various aspects of industrial control in the continuous upgrading, improvement, has become an indispensable tool in modern industrial production and manufacturing.
1, the definition of DCS and PLC
DCS control system, in the domestic automation industry is also known as the centralized control system. That is, the so-called distributed control system, is relative to the centralized control system, a new type of computer control system, which is based on the development of centralized control systems, evolution.
DCS as a set of process control and process monitoring as one of the computer integrated system, in the communication network continues to drive the DCS system has become a comprehensive computer, communication, display and control of the 4C technology of the complete system. Its main features are decentralized control, centralized operation, hierarchical management, flexible configuration and convenient configuration.
Today's DCS system can be widely used in the production control and management of industrial installations, in the chemical industry, electric power, metallurgy and other process automation applications have been very popular.
PLC, i.e. Logic Programmable Controller, is an electronic system of digital arithmetic operation designed for application in industrial environments. It uses a class of programmable memory for its internal storage program, the implementation of logical operations, sequence control, timing, counting and arithmetic operations, such as user-oriented instructions, and through the digital or analog input/output control of various types of machinery or production processes, is the core part of industrial control.
2, the difference between DCS and PLC controller
DCS and PLC controller is the main difference between the switching and analog operations, even if the two later some mutual penetration, but there is still a difference. 80's, PLC in addition to logical operations, but also increased some control loop algorithms, but to complete some complex operations or more difficult, PLC programming ladder diagrams, analog operations in the programming of less intuitive programming, programming is more troublesome. But in terms of solving the logic, it shows the advantage of fast. DCS use function blocks to encapsulate analog and logical operations, both logical operations or complex analog operations are very clear expression form, but relative to the PLC for the expression of logical operations is less efficient.
3, DCS and PLC in thermal power plant applications
In the thermal power plant thermal automation field, DCS and PLC are two completely different and inextricably linked concepts. DCS and PLC are the combination of computer technology and industrial control technology, thermal power plant host control system with DCS, and PLC is mainly used in the auxiliary workshop of the plant. DCS and PLC have operator stations to provide the means of human-computer interaction, rely on computer technology-based controller to complete the control operation. Both DCS and PLC have operator station to provide a means of human-computer interaction, both rely on computer-based controller to complete the control operation, both through the I/O card to complete the data exchange with the primary components and actuators, both have a communication system called network.
With the continuous expansion of the domestic power plant installed capacity and power system reform, the auxiliary workshop control requirements also continue to improve, in this environment, DCS system into the auxiliary workshop control has become a trend. NT6000DCS because of its comprehensive technical and economic advantages, has been and will continue to play an increasingly large role in the auxiliary workshop control.
Widely used in auxiliary workshop PLC will not be out of the historical stage of thermal automation, unprecedented competitive pressure, will prompt PLC manufacturers in the technology to DCS standards closer to the price to make greater efforts. DCS and PLC market competition will result in greater benefits to the user.
4, DCS and PLC control processing capacity
A PLC controller, often able to handle thousands of I / O points (up to more than 8000 I / O). The controller of the DCS, generally can only handle a few hundred I / O points (not more than 500 I / O).
From the centralized system requirements, do not allow a centralized control of the situation, too many points of the controller in the actual application is useless, DCS developers simply do not have to develop a lot of I / O points with the need to drive the controller, their main focus is to provide the reliability and flexibility of the system. PLC is not the same, as an independent flexible control device, with the ability to point the stronger of course, also represents the higher the level of its technology, as for the application level of the entire control system, which is mainly engineers and users of the matter, rather than the PLC manufacturer's core objectives. Another indicator of control processing power, computing speed, in people's impression of PLC is also much faster than DCS.
New DCS controllers have learned from the design of large PLCs and have gained substantial improvements in control cycle performance. Take the T2550 controller of NT6000DCS as an example. Controller can be set to four different priority tasks, the minimum operation cycle can be set to 10ms, with high-speed I / O card, the control cycle can reach 15-20ms. and analog operations are set in other tasks with a longer cycle.
5, DCS and PLC market situation and development direction
In the field of thermal automation, the main plant control system basically without exception, the use of DCS, while in the auxiliary workshop only use PLC, the main reason is that the early DCS system is very expensive, people think that the auxiliary workshop operation can be intermittent, the reliability of the requirements are not very high, and the analog control requirements of the less, from the perspective of cost reduction, often choose PLC to build the control system. While the control system of boiler, steam engine and generator, requires long-term stable and reliable operation, the signal contains a considerable proportion of analog, from the performance of the system, people have to choose the expensive DCS.
In addition, analyzing the market competition between main plant DCS and auxiliary plant control system, we will find an interesting phenomenon. The competition of main plant DCS tends to be among suppliers or agents of different brands, and the competition is so fierce that the price of DCS keeps going down. While the competition of auxiliary plant control system is often carried out between various engineers of the same brand of PLC, the threshold is lower, the competition is more intense, but the price of PLC downward adjustment is not as obvious as DCS. The main reason is that the manufacturer of DCS directly involved in the competition, in the huge market pressure, constantly downward adjustment of equipment manufacturing costs and engineering implementation costs. PLC manufacturers are not directly involved in the competition, each engineering business can only adjust their own limited engineering costs, space is limited. From the present situation, DCS and high-grade PLC price gap is no longer obvious, auxiliary workshops are still more often used PLC, is the market inertia.
PLC and DCS product market can be described as windy and competitive. Throughout the PLC market stars: more than 200 companies in the world to produce more than 400 varieties of series of PLC, used in electric power, petroleum and petrochemical, metallurgy, materials, packaging, paper, automotive and municipal industries. From the industry's point of view, foreign manufacturers occupy the mountain as the king, with their respective spheres of influence.
DCS market and PLC is not much different, basically by the foreign industry strong hold, it is encouraging that a number of local manufacturers such as Hollysys, Supcon, Xinhua and other enterprises to gradually grow and develop. Due to the high technical content of DCS, a lot of demand for products are built on the basis of the project, so that the demand for DCS will also be a cyclical process, so the DCS market pattern is difficult to have a large change in a short period of time. Of course, due to the irregularity of the development speed of the industry, companies focusing on different industries may undergo some changes as a result.
In the future development of control systems, we will see the gradual integration of DCS and PLC technology, which is their respective development and the development of various industries have a role in promoting.