What is the difference between DCS and PLC?
DCS is a distributed control system. It is also called a distributed control system in the domestic self-control industry. It is mainly used to centrally monitor and automatically control geographically dispersed equipment. It relies on a central system for centralized processing, and the data update speed is faster than that of the PLC system. slow.
What is the difference between DCS and PLC?
DCS is a distributed control system. It is also called a distributed control system in the domestic self-control industry. It is mainly used to centrally monitor and automatically control geographically dispersed equipment. It is mainly used in large-scale process industries to centrally reflect current production conditions. It relies on a central system for centralized processing, multiple operating stations to monitor and control, and data update speed is slower than PLC system.
What is DCS?
DCS is a distributed control system. It is also called a distributed control system in the domestic self-control industry. It is mainly used to centrally monitor and automatically control geographically dispersed equipment. It is mainly used in large-scale process industries to centrally reflect current production conditions. It relies on a central system for centralized processing, multiple operating stations to monitor and control, and data update speed is slower than PLC system.
For example: because a certain process requires, the tank level needs to be kept within a certain range. When the tank level fluctuates greatly and exceeds this range, you can choose to manually adjust the valve to control the water level, or send it through DCS. Instructions to adjust. DCS is equivalent to the brain, responding to the situation seen by the eye (that is, the signal transmitted by the on-site instrumentation, such as the water level is reduced) (ie, issuing instructions, such as increasing the inlet of the inlet valve), and the on-site execution device receives the instruction. The corresponding action (such as increasing the inlet of the inlet valve and keeping the water level within a certain range).
Difference between DCS and PLC
Control processing capability
We know that a PLC controller can often handle thousands of I/O points. The DCS controller can only handle hundreds of I/O points. From the requirements of the distribution system, it is not allowed to have centralized control. Too many points of controllers are useless in practical applications. DCS developers simply do not need to develop many I/O point controllers. Drive, their main focus is to provide system reliability and flexibility. The PLC is different. As an independent flexible control device, it has a strong point capability.
In people's minds, the computing speed of PLC is also much faster than DCS. From a certain point of view, this is true. The efficiency of PLC execution logic operation is very high. The execution of 1K logic program is less than 1 millisecond, and its control period (for example, DI input directly sends DO output) can be controlled within 50ms; while DCS is The same way is used when processing logic operations and analog operations, and the control cycle is often more than 100ms. When we use the PID algorithm to compare, we can find that the PLC performs a PID operation in a few milliseconds, and the T2550 controller of the NETWORK6000+DCS also needs 1 millisecond to solve a PID. This shows that the PLC and DCS are equivalent to the actual computing power. A certain type of DCS controller is even stronger. The difference in control cycle is mainly related to the scheduling design of the controller. The new DCS controller has learned the design of large PLCs and has greatly improved its performance in terms of control cycles. Take the T2550 controller of NETWORK6000+DCS as an example. The controller can set up four tasks with different priorities. The minimum calculation period can be set to 10ms. With high-speed I/O cards, the control cycle can reach 15~20ms. The analog operation is set in other tasks with longer periods.
Data communication exchange
In this respect DCS has an innate advantage. From the early development of DCS, the network has become the core technology direction of DCS manufacturers. Redundant technology and narrow-band transmission technology are the first successful development or application of DCS manufacturers. PLC is mainly designed according to independent devices, and its "network" is actually serial communication.
The problem of PLC data communication exchange mainly stems from the fact that PLC has been developing as an independent device for a long time, and there is no system concept; and it is mainly used in small control systems, and the problem is not obvious, so the development is slow. There are also some large PLCs that have improved in this area, but it takes a long time to reach the level of DCS.
Configuration maintenance function
Configuration maintenance functions include logic configuration, download modification, operation debugging, remote diagnosis, and more.
In the early days, PLC was dominated by ladder diagrams, and DCS was based on module function diagrams. After years of development, the International Electrotechnical Commission has defined five programming languages through the IEC1131-3 standard. Currently mainstream DCS and PLCs have met this standard and support several or all of them. From the perspective of development efficiency and program readability, module function diagrams and sequential function diagrams are increasingly becoming the main programming methods. Ladder logic and structured text become the development tools for custom modules. Large PLCs are more and more like DCS in the configuration mode, and the gap is gradually narrowing, while the small PLC is still dominated by ladder diagrams.
After years of development, DCS has accumulated a large number of advanced algorithm modules. For example, the NETWORK6000+ has a device-level module that centrally completes the basic control and fault alarm function for the device. In the network communication, the module is also transmitted by the unit, which greatly improves the efficiency of software development. A device pole module is equivalent to a 0.5K ladder logic. It is much more cumbersome for the PLC to perform the same function.
Hardware package structure
PLC is generally a large-bottom version of the rack, closed I / O modules, closed structure is beneficial to improve the reliability of I / O modules, anti-RF, anti-static, anti-damage. The number of I/O points of the PLC module is 8 points, 16 points, and 32 points.
Most of the DCS are 19-inch standard chassis plus plug-in I/O modules, and the I/O modules are bare structures. The number of I/O points per module is 8 and 16 points, and the 32-point module is rarely used.
This structure of DCS stems from its use in large-scale control objects. The 19-inch standard chassis facilitates dense layout, and the fewer I/O points are due to the dispersion requirements. PLC's outsole layout rack, closed modular structure is more flexible in management and configuration, and the reliability of a single device is higher. Therefore, many DCS also absorb the advantages of PLC in the structure, using a similar package structure with PLC, such as I / A with a metal shell, NETWORK-6000 + with a conductive plastic shell.
Human-computer interaction device
In the early days, DCS was used as a system, and its human-computer interaction device was a dedicated device provided by DCS manufacturers. PLC manufacturers generally do not provide human-computer interaction devices, which are often completed by the engineering company using universal monitoring software (such as ifix, intouch, and configuration king). The human-computer interaction device integrated by DCS often has the characteristics of more professional functions and better stability, but its price is also very high. With the rapid development of PC technology, some general-purpose monitoring software has developed rapidly, and its functions and performance have gradually surpassed the special devices provided by DCS manufacturers. Therefore, many DCS manufacturers gradually gave up the dedicated human-computer interaction device, and instead used the general monitoring software like PLC. The general monitoring software used by DCS manufacturers is not simply assembled, but based on the general monitoring software, through the cooperative development, the network communication technology and system self-diagnosis technology accumulated over the years have been retained and inherited in the form of special software packages. .
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