How to choose between a programmable logic controller (PLC) and a distributed control system (DCS) depends on the specific situation, as the requirements for the control system vary depending on the application scenario. When communicating with clients, we can start from the following aspects separately!
PLC
1. From switch control to sequential control and transportation processing, it is a multi-functional continuous PID control from bottom to top, with PID in the interrupt station.
2. One PC can be used as the master station, and multiple PLCs of the same type can be used as slave stations.
3. It is also possible to use one PLC as the master station and multiple PLCs of the same type as slave stations to form a PLC network. The convenience of using a PC as the main station is that when programming, users do not need to know the communication protocol, just write it in the format of the instruction manual.
4. PLC grid can serve as both an independent DCS and a subsystem of DCS.
5. PLC is mainly used for sequential control in industrial processes, and new PLCs also have closed-loop control functions.
DCS
1. Distributed Control System (DCS)
A monitoring technology that integrates 4C (Communication, Computer, Control, CRT) technology.
2. A tree topology system from top to bottom, where communication is crucial.
3. In the interrupt station, PID connects the computer with the field instruments and control devices in a tree topology and parallel continuous link structure. There are also a large number of cables running in parallel from the interrupt station to the field instruments and control devices.
4. Analog signal, A/D-D/A, mixed with microprocessor.
5. One instrument, one pair of wires connected to I/O, hung from the control station to the local area network LAN
6. DCS is a three-level structure consisting of control (engineer station), operation (operator station), and on-site instruments (on-site measurement and control station). Used for large-scale continuous process control, such as petrochemicals.
How to choose between PLC and DCS systems
How to choose between a programmable logic controller (PLC) and a distributed control system (DCS) depends on the specific situation, as the requirements for the control system vary depending on the application scenario.
The control system platform will have a certain impact on the way automation systems meet the needs of optimizing production, maintaining availability, and obtaining data. Lack of foresight in selecting control systems may also affect future expansion, process optimization, user satisfaction, and company profits.
In addition to some basic principles (such as how to control the process), the design team must also consider various factors such as installation, scalability, maintenance, and upkeep.
At present, although PLC systems may be the most cost-effective for small devices, DCS systems provide more economical scalability and are more likely to achieve higher initial investment returns.
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, PLCs have added more functions and created more benefits for small factories and devices. PLCs typically operate as standalone systems, but can also be integrated with other systems and connected to each other through communication. Due to each PLC having its own database, integration requires some degree of mapping between controllers. This makes PLC particularly suitable for small applications that do not require significant expansion.
The DCS system disperses the controllers in the automation system and provides universal interfaces, advanced controls, system level databases, and easily shareable information. Traditionally, DCS has been mainly used in process technology and larger factories, where large-scale system applications are easier to maintain throughout the entire lifecycle of the factory.
PLC is developed from the principle of relay control, which stores instructions for executing logical operations, sequential control, timing, counting, and arithmetic operations; And control various types of machinery or production processes through digital input and output operations. The control program developed by the user expresses the process requirements of the production process and is pre stored in the user program memory of the PLC. Execute the stored program content one by one during runtime to complete the operations required by the process flow.
Related factors that affect how to make decisions
Process scale: How many input/output (I/O) points are required? Small systems (<300 I/O points) may have a lower budget, so using a PLC system is more suitable. It is not easy to apply DCS systems to smaller projects. On the contrary, they can better perform their functions in large factory applications. Due to having a global database, DCS systems are easier to manage and upgrade, and any changes are global in nature.
Upgrade plan: Smaller industrial processes can be adapted to PLC systems, 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 prone to errors. DCS systems are easier to upgrade, such as being able to manage user trust from a central hub, making them easier to maintain and upkeep.
Integration requirements: For standalone devices, PLC systems are the ideal choice. When multiple PLC systems are configured in a factory, there will be a requirement for interconnection. This is generally difficult to achieve because it usually requires mapping data using communication protocols. Integration is certainly not a problem, but when there is a need for changes, the user's trouble arises: once a PLC system is changed, it may cause two PLCs to not communicate properly due to the impact of data mapping. For DCS systems, there is no need for mapping at all, and configuration changes are just a simple process; The controller is built-in to the system.
High availability: For processes that require high availability, DCS systems can provide redundant configurations.
Efficiency and ease of implementing redundancy are crucial for maintaining costs within the budget.
Functional requirements: Certain industries and facilities require historical databases, streamlined alarm management, and a central control room with a universal user interface. Some require the integration of Manufacturing Execution Systems (MES), advanced control, and asset management. The DCS system is equipped with these applications (see Figure 3), making it easy to add them to automation engineering applications without the need to add independent servers or increase integration costs. In this regard, DCS systems are more economical and can improve productivity and reduce risks.
Life cycle investment return rate: The demand for facilities varies by industry. For small-scale process engineering, there is no need for expansion or integration with other process areas, so PLC systems have a good return on investment. DCS systems may have high installation costs, but from a full lifecycle perspective, the increased production and safety benefits brought by DCS systems will offset some of the costs. Balancing short-term demand with long-term vision is crucial for operational certainty and improving factory operation and maintenance.