How to Choose between PLC and DCS Systems
【 Introduction 】 How to choose between programmable logic controllers (PLCs) and distributed control systems (DCS) depends on specific situations and analysis. Due to different application scenarios, the requirements for control systems also vary.
【1】 PLC and DCS
PLC
From switch quantity control to sequential control, transportation processing, it is a multifunctional system with continuous PID control from bottom to top. PID is used in interrupt stations.
A single PC can be used as the master station, and multiple PLCs of the same type can be used as slaves.
It is also possible to have one PLC as the master station and multiple PLCs of the same type as the slave stations, forming a PLC network. The convenience of using a PC as the main station is that when users program, they do not need to know the communication protocol, as long as they write according to the manual format.
The PLC grid can serve as both an independent DCS and a subsystem of DCS.
PLC is mainly used for sequential control in industrial processes, and the new PLC also has closed-loop control function.
DCS
Distributed control system DCS is a monitoring technology that integrates 4C (Communication, Computer, Control, CRT) technology.
A tree topology system from top to bottom, where communication is crucial.
In the interrupt station, PID connects the computer with the on-site 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 on-site instruments and control devices.
Analog signal, A/D-D/A, mixed with microprocessor.
A pair of instruments are connected to I/O and connected from the control station to the local area network LAN.
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.
【2】 How to Choose between PLC and DCS Systems
The choice between programmable logic controllers (PLCs) and distributed control systems (DCS) needs to be analyzed on a case by case basis, as different application scenarios have varying requirements for control systems.
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 cost-effective scalability and are more likely to achieve higher initial investment returns.
PLC is an industrial computer used to control production and 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 devices.
PLC is usually operated as a standalone system, but it 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 controllers within the automation system and provides universal interfaces, advanced controls, system level databases, and easily shared information. Traditionally, DCS has been mainly applied in process technology and relatively large factories, and large 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 item by item during runtime to complete the operations required by the process flow.
【3】 Comparison of Engineering Analysis between PLC and DCS
There is a program counter in the CPU of the PLC that indicates the storage address of the program step. During the program operation, the counter automatically increases by 1 when a step is executed. The program is executed sequentially from the starting step (step number is zero) to the final step (usually the end instruction), and then returns to the starting step for cyclic operation.
The time required for a PLC to complete each cycle operation is called a scanning cycle. Different models of PLCs have a cyclic scanning cycle between 1 microsecond and several tens of microseconds. The loop operation of program counters is not available in DCS. This is also the reason why the redundancy of PLC is not as good as DCS.
DCS was developed on the basis of operational amplifiers. Make all the relationships between functions and process variables into functional blocks (some DCS systems refer to them as expansion blocks). The main difference in performance between DCS and PLC lies in the logical calculation of switch quantities and the operation of analog quantities. Even though there is some infiltration between the two later on, there are still differences.
After the 1980s, in addition to logical operations, the algorithmic functions used in the control circuit of PLC have been greatly enhanced. However, PLC programming using ladder diagrams makes analog operations less intuitive and programming more cumbersome. But in terms of solving logic, it shows the advantage of being fast, and at the microsecond level, solving 1k logic programs takes less than 1 millisecond. It processes all inputs as switch variables, with 16 bits (or 32 bits) being an analog variable.
And DCS treats all inputs as analog quantities, with one bit being the switch quantity. Solving a logic takes several hundred microseconds to a few milliseconds. For PLC, solving a PID operation takes several tens of milliseconds, which is comparable to the operation time of DCS.
In terms of grounding resistance, PLC may not have high requirements, but DCS must be below a few ohms (usually below 4 ohms). Analog isolation is also very important.
For systems with the same number of I/O points, using PLC is more cost-effective than using DCS (about 40% can be saved). PLC does not have a dedicated operating station, and the software and hardware used are universal, so the maintenance cost is much lower than DCS. If the controlled object is mainly equipment interlocking with relatively few circuits, using PLC is more suitable.
If it is mainly analog control and there are many function operations, it is best to use DCS. DCS is much better than PLC in terms of redundancy in controllers, I/O boards, communication networks, and other advanced computing and industry specific requirements. Due to the use of universal monitoring software, PLC is easier to design in the management information system of enterprises.
PLC and DCS systems are generally suitable for discrete and process production manufacturing, respectively. Discrete production manufacturing facilities using PLC systems typically consist of separate production units, primarily used to complete component assembly, such as labeling, filling, or grinding. Process manufacturing facilities typically use automated systems to produce according to formulas rather than parts in a continuous and batch processing manner. Large scale continuous processing equipment, such as oil refineries and chemical plants, all use DCS automation systems. Mixed applications typically use both PLC and DCS systems simultaneously. Choosing a controller for a certain application requires consideration of factors such as the scale of the process, scalability, future update plans, integration requirements, functionality, high availability, and the return on investment for the entire lifecycle of the factory facility.
【4】 The relevant factors that affect how to make decisions
Process scale: How many input/output (I/O) points are needed? A small system (<300 I/O points) may have a smaller budget, so using a PLC system is more suitable. It is not easy to apply the DCS system to smaller projects, on the contrary, it can better fulfill its 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 will need to be added, and separate maintenance will be required. This is a time-consuming, laborious process, and prone to errors. DCS systems are easier to upgrade, such as managing user trust from a central hub, making maintenance and upkeep easier (see Figure 1).
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Figure 1: DCS system architecture with a single database, allowing users to maintain and operate the system from a central control station
Integration requirement: For standalone devices, PLC systems are an ideal choice. When multiple PLC systems are configured in the factory, there is a requirement for interconnection. This is generally difficult to implement because communication protocols are often used to map data. Integration is certainly not a problem, but when there is a need for changes, the user’s trouble arises: once a PLC system undergoes a change, it may cause the two PLCs to be unable to communicate properly, due to the impact on 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 (see Figure 2). Efficiency and ease of redundancy are crucial for maintaining costs within budget. Functional requirements: Some industries and facilities require a historical database, streamlined alarm management, and a central control room with a universal user interface configuration. Some require 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 to automation engineering applications without the need for independent servers or increased integration costs. In this regard, DCS systems are more cost-effective and can improve productivity and reduce risks.
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Figure 3: Each system platform has unique database requirements
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 higher 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.