PLC and DCS are similar in some aspects, but the differences are quite obvious, as reflected in the following several aspects.
1. DCS is a “Distributed Control System”, while PLC (Programmable Logic Controller) is merely a control “device”; the distinction lies between “system” and “device”. A system can achieve the functions and coordination of any device, while a PLC device only realizes the functions inherent to its unit.
2. In terms of networking, the DCS network serves as the central nervous system of the entire system, featuring a safe, reliable, dual-redundant high-speed communication network, which offers better scalability and openness. In contrast, PLCs generally operate individually, and when communicating with other PLCs or host computers, they primarily use a single network structure, with network protocols often not aligning with international standards. Regarding network security, PLCs lack adequate protective measures. We employ dual redundancy for power supply, CPU, and network.
3. DCS considers the overall solution, where operator stations have engineer station functionalities, and the relationship between stations becomes a tightly integrated one under operational program execution, with any station, any function, and any controlled device being interlocked for control and coordinated operation; whereas a system composed solely of interconnected PLCs has a loose connection between stations (PLC to PLC), making coordinated control impossible.
4. DCS design incorporates numerous scalable interfaces, making it very convenient to connect external or expanded systems, whereas once a PLC-based system is completed, it becomes very challenging to freely add or remove operator stations.
5. DCS safety: To ensure the safety and reliability of devices controlled by DCS, it uses dual redundancy for control units. When a critical control unit fails, a corresponding redundant unit seamlessly switches to work as the operational unit, ensuring the entire system’s safety and reliability. PLC systems generally lack the concept of redundancy, let alone a redundancy control strategy. Particularly when a PLC unit fails, the entire system must be halted for replacement and maintenance, requiring reprogramming. Therefore, DCS systems are inherently more reliable and safe.
6. System software: Updating various process control schemes is a fundamental function of DCS. When a scheme changes, engineers only need to compile the modified scheme at the engineer station and execute the download command; the download process is automatically completed by the system without affecting the original control scheme’s operation. Various control software and algorithms in the system can enhance the precision of process control objects. In contrast, for PLC-based systems, the workload is enormous. First, it is necessary to identify which PLC needs to be edited or updated, then compile the program using the corresponding compiler, and finally use a dedicated machine (programmer) to transfer the program to each PLC one-on-one. This significantly increases debugging time and cost during system commissioning and is extremely unfavorable for future maintenance. The difference in control precision is substantial. This explains why in large and medium-sized control projects (over 500 points), systems composed entirely of PLCs are generally not adopted.
7. Modules: All I/O modules in DCS systems come with a CPU, allowing for quality judgment and scalar transformation of collected and output signals, hot-swapping of faulty components, and random replacement. In contrast, PLC modules are simply electrical conversion units without intelligent chips; when they fail, the corresponding unit becomes entirely inoperative.
8. Currently, high-end PLCs and DCS functionalities are quite similar. DCS has robust capabilities for network and distributed database scanning, as well as strong abilities in processing calculations and analog quantities.
9. PLCs are categorized into large, medium, small, and micro PLCs, with micro PLCs costing only a few hundred to 2000 yuan and having very few points, while large PLCs can support thousands of points and have computation capabilities similar to DCS, but weaker in multi-machine networking. Currently, both technical platforms are quite similar, just with different focuses.
Source: Today’s Chemical Topic Ranking
China Chemical Safety Association
Editor: An An