SPM-D11 Woodward 8440-1706B Power Station Manager

Description

PLC control and DCS control systems are not concepts at the same logical level. From the name, it can be seen that PLC is named after its function, while DCS is named after its architecture. In principle, PLC can form a DCS. Of course, there are still differences in performance between the two, depending on the product and needs. From an application perspective, simply distinguishing between PLC and DCS often leads to misunderstandings.

The difference between DCS control system and PLC control: DCS is a “decentralized control system”, while PLC (programmable controller) is only a control “device”, and the two are the difference between “system” and “device”. The system can achieve the functions and coordination of any device, while the PLC device only implements the functions that this unit has.

The DCS network is the central nervous system of the entire system, and the international standard protocol TCP/IP is commonly used in DCS systems. It is a safe and reliable dual redundant high-speed communication network, with better scalability and openness of the system. However, PLC, because it basically works as a single small system, uses a single network structure when communicating with other PLCs or upper computers, and the network protocol often does not comply with international standards. PLC does not have good protection measures in network security.

DCS considers the overall plan, and the operator station has the function of an engineer station. After the installation of the operation plan program, there is a close joint relationship between stations. Any station, any function, and any controlled device are interlocked and controlled in a coordinated manner; The system composed solely of PLC interconnections has a loose connection between stations (PLC and PLC), which cannot achieve coordinated control functions.

DCS has a large number of expandable interfaces in its entire design, making it very convenient to connect or expand external systems. After the entire system connected by PLC is completed, it is difficult to arbitrarily add or reduce operator stations.
To ensure the safety and reliability of the equipment controlled by DCS, DCS adopts dual redundant control units. When important control units fail, relevant redundant units will switch to working units in real-time without disturbance, ensuring the safety and reliability of the entire system. The system connected to the PLC needs to be configured with dual PLCs to achieve redundancy.

Updating various process control schemes is the most basic function of DCS. When a certain scheme changes, engineers only need to compile the changed scheme by the engineer station master and execute the download command. The download process is completed by the system without affecting the operation of the original control scheme. Various control software and algorithms in the system can improve the control accuracy of process objects.

For a system composed of PLCs, the workload is extremely large. Firstly, it is necessary to determine which PLC to edit and update, then use the corresponding compiler for program compilation, and finally use a dedicated machine (reader/writer) to one-on-one transfer the program to this PLC. During system debugging, it greatly increases debugging time and costs, and is extremely detrimental to future maintenance.
There is a significant difference in control accuracy. This determines why in large and medium-sized control projects (above 500 points), systems connected entirely by PLC are rarely used. The reason for this is that all I/O modules in the DCS system have CPUs, which can achieve quality judgment and scalar transformation of collected and output signals, fault electrification, and random replacement. The PLC module is just a simple electrical conversion element, without an intelligent chip, and all corresponding units will be paralyzed after a malfunction.

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