Currently, there isn't a strict boundary between DCS and PLC. Most people consider large systems to be DCS and small systems to be PLC. While this isn't entirely wrong, it's not accurate either. Let's re-establish this concept.
First, what are the differences between DCS and PLC?
1. From a developmental perspective:
DCS evolved from traditional instrument panel monitoring systems. Therefore, DCS inherently focuses more on instrument control. For example, the YOKOGAWACS3000DCS system we use does not even have a limit on the number of PIDs (PID, proportional-derivative-integral algorithm, is the standard algorithm for closed-loop control of control valves and frequency converters, and the number of PIDs usually determines the number of control valves that can be used).
PLCs evolved from traditional relay circuits. Early PLCs didn't even have analog signal processing capabilities. Therefore, from the beginning, PLCs emphasized logic operation capabilities.
2. From the perspective of system scalability and compatibility:
The market offers a wide variety of control products, with numerous manufacturers producing and selling both DCS and PLC systems. PLC systems generally have little or no need for expansion, as they are typically designed for specific devices. Compatibility requirements for PLCs are also rare; for example, resource sharing between two or more systems is very difficult for a PLC. Furthermore, PLCs typically use dedicated network architectures, such as Siemens' MPI bus network, making even adding an operator station difficult or costly.
During the development of DCS (Distributed Control Systems), each manufacturer developed its own system. However, most DCS systems, such as Yokogawa, Honeywell, and ABB, while using different communication protocols at the process level, unanimously chose Ethernet for their operational network platforms, employing standard or modified TCP/IP protocols. This provides convenient scalability. In this network, controllers and computers exist as nodes; the number and location of nodes can be freely added or removed wherever the network reaches. Furthermore, based on open protocols such as OPC and DDE used in Windows systems, different systems can communicate easily to achieve resource sharing.
3. From a database perspective:
DCS systems typically provide a unified database. In other words, once data exists in the database of a DCS system, it can be referenced in any situation, such as in configuration software, monitoring software, trend charts, reports, etc. However, PLC systems usually do not have a unified database; configuration software, monitoring software, and even archiving software each have their own databases. Why is it often said that Siemens' S7400 series only qualifies as a DCS at the S7414 level or higher? Because Siemens' PCS7 system uses a unified database, and PCS7 requires controllers to be at least S7414-3 or higher.
4. From a time scheduling perspective:
PLC programs generally cannot run according to a pre-set cycle. A PLC program executes from beginning to end once and then starts again. (Some newer PLCs have improved this, but there are still limitations on the number of task cycles.) DCS, on the other hand, allows setting task cycles, such as fast tasks. For example, when sampling sensors, pressure sensors have very short change times, so we can use a 200ms task cycle for sampling, while temperature sensors have much longer hysteresis, so we can use a 2s task cycle. In this way, DCS can rationally allocate controller resources.
5. From the perspective of network structure:
Generally, DCS systems typically use a two-layer network structure. The first layer is the process-level network, where most DCS systems use their own bus protocols, such as Yokogawa's Modbus, Siemens and ABB's Profibus, and ABB's CANbus. These protocols are all based on standard serial port transmission protocols RS232 or RS485. Field I/O modules, especially those handling analog sampling data (machine code, 2^13/scan cycle), generate a large amount of data. Furthermore, there are many interference factors in the field, so a network standard with high data throughput and strong anti-interference capabilities should be adopted. A bus structure based on RS485 asynchronous serial communication meets the requirements of field communication. The I/O sampling data is converted into integer or real data by the CPU and transmitted on the operator-level network (the second-layer network). (http://www.diangon.com/ Copyright) Therefore, the operator-level network can adopt a network standard with moderate data throughput, high transmission speed, and convenient connection. Since the operator-level network is generally located in the control room, the requirements for anti-interference are relatively low. Therefore, using standard Ethernet is the best choice. TCP/IP is a standard Ethernet protocol, and we typically use a communication speed of 100 Mbit/s.
PLC systems typically have relatively simple tasks, so the amount of data they need to transmit is generally not large. Therefore, common PLC systems use a single-layer network structure. The process-level network and the operator-level network are either combined, or the process-level network is simplified into internal connections between modules. PLCs rarely or never use Ethernet.
6. In terms of the scale of the application objects:
PLCs are generally used in small-scale automation applications, such as equipment control or control and interlocking of a small number of analog signals, while large-scale applications typically use DCSs. While this concept isn't entirely accurate, it's intuitive; conventionally, we refer to systems with more than 600 points as DCSs, and those smaller as PLCs. Our heat pumps, QCSs, and the control systems配套的控制制 for our horizontal products are generally referred to as PLCs.
Having discussed so many differences between PLC and DCS, we should recognize that as PLC and DCS have evolved to this point, they are actually converging. Strictly speaking, PLC and DCS can no longer be clearly distinguished, and the concepts between them have become blurred in many cases.
Now, let's discuss the similarities between them.
1. In terms of function:
PLCs already possess analog signal control capabilities, and some PLC systems even have quite powerful analog signal processing capabilities, such as Yokogawa FA-MA3, Siemens S7400, ABB ControlLogix, and Schneider Electric's Quantum system. DCS also has quite strong logic processing capabilities; for example, we have implemented all possible process interlocks and equipment linkage start-stop on the CS3000.
2. From the perspective of system structure:
PLCs and DCSs share the same basic structure. Today, PLCs have been fully integrated into computer system control, rendering traditional programmers obsolete. Small-scale PLC applications typically use touchscreens, while large-scale applications rely entirely on computer systems. Like DCSs, the controller and I/O stations use a fieldbus (usually based on RS485 or RS232 asynchronous serial communication protocols). This bus is also used for communication between the controller and the computer if there are no expansion requirements, meaning only one computer is used. However, if more than one computer is used, the system architecture becomes similar to DCS, with the host computer platform using an Ethernet structure. This is one reason why the distinction between PLCs and DCS has become blurred as PLCs have become larger.
3. Development trends of PLC and DCS:
Miniaturized PLCs will evolve towards more specialized applications, such as more targeted functions and more specific application environments. The boundaries between large PLCs and DCSs will gradually blur until they are completely integrated.
DCS will continue to evolve towards FCS. Besides a more decentralized control system, the core of FCS is, in particular, instrumentation. FCS applications abroad have already reached the instrumentation level. The control system only needs to handle signal acquisition, provide a human-machine interface, and handle logic control; the control of analog quantities is distributed to field instruments. Traditional cable connections are no longer needed between the instruments and the control system; instead, fieldbuses connect the entire instrumentation system. (Currently, Yokogawa has used FCS in its CNOOC-Shell petrochemical project, employing intelligent instruments such as EJX at the instrumentation level, achieving the world's most advanced control standards.)
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