PLC and DCS: What's the difference?

1. Definition of DCS and PLC

DCS control system, in the domestic automatic control industry is also called distributed control system. The so-called distributed control system is a new type of computer control system relative to the centralized control system, which is developed and evolved on the basis of the centralized control system.

DCS as a set of process control and process monitoring as one of the computer integrated system, under the continuous drive of the communication network, DCS system has become a comprehensive computer, communication, display and control and other 4C technology 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 equipment, in the chemical industry, electric power, metallurgy and other process automation applications have been very popular.

A PLC, or logic Programmable controller, is an electronic system that operates numerically and is designed for use in industrial environments. It uses a programmable memory for its internal storage programs, performs user-oriented instructions such as logical operations, sequence control, timing, counting and arithmetic operations, and controls various types of machinery or production processes through digital or analog input/output, which is the core part of industrial control.

2, the difference between DCS and PLC controller

The main difference between DCS and PLC controller is in the operation of switching quantity and analog quantity, even if the two later penetrate each other, but there is still a difference. After the 80's, PLC in addition to logic operations, also added some control loop algorithms, but to complete some complex operations is still more difficult, PLC with ladder diagram programming, analog operations in programming is not intuitive, programming is more troublesome. However, in terms of solving logic, it has the advantage of being fast. DCS uses functional blocks to encapsulate analog operations and logic operations, whether it is a logical operation or a complex analog operation, the expression form is very clear, but the expression efficiency of logic operations is low compared with PLC.

3, DCS and PLC in the thermal power plant application

In the field of thermal automation in thermal power plants, DCS and PLC are two completely different and inextricably linked concepts. DCS and PLC are the product of the combination of computer technology and industrial control technology, the control system of the thermal power plant is DCS, and PLC is mainly used in the auxiliary workshop of the power plant. DCS and PLC have the operator station to provide the means of human-computer interaction, rely on the controller based on computer technology to complete the control operation, through the I/O card to complete the data exchange with the primary component and the execution device, and have a communication system called the network.

With the continuous expansion of installed capacity of domestic power plants and the promotion of power system reform, the requirements for auxiliary workshop control are also increasing, in this environment, DCS system into auxiliary workshop control has become a trend. NT6000DCS, due to its comprehensive technical and economic advantages, has and will continue to play an increasing role in auxiliary workshop control.

The PLC widely used in the auxiliary workshop will not exit the historical stage of thermal automation, unprecedented competitive pressure will prompt PLC manufacturers to move closer to the DCS standard in technology and make greater efforts in price. DCS and PLC market competition results will enable users to obtain greater benefits.

4, DCS and PLC control processing capacity

A PLC controller is often able to handle thousands of I/O points (up to more than 8,000 I/O). The DCS controller can generally only handle a few hundred I/O points (no more than 500 I/O).

From the requirements of the distributed system, do not allow centralized control of the situation, too many points of the controller is useless in practical applications, DCS developers simply do not develop with a lot of I/O points of the controller needs to drive, their main energy is to provide system reliability and flexibility. The PLC is not the same, as an independent flexible control device, the stronger the ability, of course, the higher the technical level, as for the application level of the entire control system, which is mainly the matter of engineers and users, rather than the core goal of PLC manufacturers. Another indicator of control processing power, computing speed, in people's impression of PLC is also much faster than DCS.

The new DCS controller has learned from the design of large PLCS, and its performance in terms of control cycles has been greatly improved. The T2550 controller of NT6000DCS is used as an example. The controller can set four tasks with different priorities, and the minimum operation cycle can be set to 10ms. With high-speed I/O cards, the control cycle can reach 15-20ms. The analog computation is set in other tasks with longer cycles.