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The choice between PLC and DCS-how to choose the right control system?

Last Updated on 2021-08-04 Hits:234

The choice between PLC and DCS-how to choose the right control system?

If an automation project is to be successful, the automation engineer and design engineer must first evaluate the application requirements together, and then select the most effective control system platform. These decisions will have a long-term impact on the factory’s operating performance. In some cases, the impact can last as long as 25 years or more. Most control system decisions can be boiled down to choosing a programmable logic controller (PLC) or a distributed control system (DCS). Sometimes, a certain option is very suitable for a factory, and in another case, it may not be suitable. The more factors considered when choosing a control system, the more helpful it is to achieve short- and long-term goals.

The control system platform will have a certain impact on the way the automation system meets the needs of optimizing production, maintaining availability, and obtaining data. The lack of foresight in choosing a control system may also affect future expansion, process optimization, user satisfaction, and company profits.

In addition to some basic guidelines (such as how to control the process), the design team must also consider various factors such as installation, scalability, maintenance, and maintenance.


At present, although the PLC system may be the most cost-effective for small equipment, the DCS system provides more economical scalability and is more likely to obtain a higher initial return on investment.

PLC is an industrial computer used to control manufacturing processes such as robots, high-speed packaging, bottling, and motion control. In the past 20 years, PLC has added more functions, creating more benefits for small factories and installations. PLC usually runs as a stand-alone system, but it can also be integrated with other systems to realize the connection between each other through communication. Since each PLC has its own database, integration requires some degree of mapping between controllers. This makes PLC particularly suitable for small applications that do not have much need for expansion.

The DCS system disperses the controllers in the automation system, and provides universal interfaces, advanced control, system-level databases, and easy-to-share information. Traditionally, DCS is mainly used in process technology and larger factories. In the whole life cycle of the factory, large-scale system applications are easier to maintain.

PLC and DCS systems are generally suitable for discrete and process manufacturing, respectively. Discrete manufacturing facilities using PLC systems are generally composed of separate production devices, which are mainly used to complete the assembly of components, such as labeling, filling, or grinding. Process manufacturing facilities usually use automated systems to produce in a continuous and batch manner according to recipes rather than pieces. Large-scale continuous processing equipment, such as oil refineries and chemical plants, all use DCS automation systems. Hybrid applications usually use both the PLC system and the DCS system. To select a controller for an application, you need to consider many factors such as the scale of the process, scalability and future update plans, integration requirements, functions, high availability, and return on investment for the entire life cycle of plant facilities.

Process scale: How many input/output (I/O) points are needed? Small systems (<300 I/O points) may have a smaller budget, so PLC systems are more suitable. It is not easy to apply the DCS system to smaller projects. On the contrary, it can perform its functions better in large factory applications. With a global database, the DCS system is easier to manage and upgrade, and any changes are global.

Upgrade plan: PLC systems can be applied to smaller-scale industrial processes, but if the process needs to be expanded or upgraded, more PLC hardware and databases need to be added, and separate maintenance is required. This is a time-consuming, laborious process, and error-prone. The DCS system is easier to upgrade, for example, the user trust can be managed from the central hub, so it is easier to maintain and maintain
Integration requirements: For stand-alone devices, the PLC system is an ideal choice. When multiple PLC systems are configured in a factory, interconnection requirements will arise. This is generally difficult to achieve, because it is usually necessary to use a communication protocol to map the data. The integration is of course no problem, but when there is a change requirement, the user's troubles come: once a PLC system is changed, it may cause the two PLCs to fail to communicate normally. This is because the data mapping is affected. For the DCS system, there is no need for mapping at all, and configuration changes are just a simple process; the controller comes with the system.
High availability: For processes with high availability requirements, the DCS system can provide redundant configuration
Life cycle return on investment: The demand for facilities varies from industry to industry. For small-scale process engineering, there is no need for expansion and integration with other process areas, so the PLC system has a better return on investment. The DCS system may have a higher installation cost, but from the perspective of the whole life cycle, the increase in output and safety benefits brought by the DCS system will offset part of the cost.

Balancing short-term needs and long-term vision is critical to operational certainty and improving plant operation and maintenance.

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