and Lisi plc - the difference between DCS and PLC

The choice between a programmable logic controller (PLC) and a distributed control system (DCS) is a case-by-case analysis, as the requirements for the control system vary depending on the application. You can start from the following aspects!

PLC and DCS

　　PLC

1. From switching control to sequence control, transportation processing, it is multi-function such as continuous PID control from bottom to top, and PID is 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. A PLC can also be the master station, and multiple PLCs of the same type can be used as slaves to form a PLC network. This is more convenient than using a PC as a master station: when there is a user programming, you don't need to know the communication protocol, just write it in the format of the manual.

4. The PLC grid can be used as an independent DCS or a subsystem of the DCS.

5. PLC is mainly used for sequence control in industrial processes, and the new PLC also has closed-loop control functions.

and Lisi plc - the difference between DCS and PLC

　　DCS

1. Distributed control system DCS set

4C (Communication, Computer, Control, CRT) technology in one monitoring technology.

2. A large tree-like topology system from top to bottom, in which communication is the key.

3. PID In the interrupt station, the interrupt station connects the computer with the field instrumentation and control device is a tree-like topology and parallel continuous link structure, and there are also a large number of cables from the interrupt station in parallel to the field instrumentation.

4. Analog signal, A/D---D/A, mixed with microprocessor.

5. A pair of wires of an instrument is connected to I/O, and the control station is connected to the LAN LAN

6. DCS is a three-level structure of control (engineer station), operation (operator station) and field instrument (field measurement and control station). It is used for large-scale continuous process control, such as petrochemical, etc.

How to choose between PLC and DCS systems

The choice between a programmable logic controller (PLC) and a distributed control system (DCS) is a case-by-case analysis, as the requirements for the control system vary depending on the application.

Control system platforms have an impact on how automation systems meet the needs of optimizing production, maintaining availability, and obtaining data. A lack of foresight in choosing a control system can 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 consider various factors such as installation, scalability, maintenance, upkeep, etc.

Currently, while a PLC system may be the most cost-effective for smaller equipment, a DCS system offers more cost-effective scalability and a higher initial return on investment.

A PLC is an industrial computer that is used to control manufacturing processes such as robotics, high-speed packaging, bottling, and motion control. Over the past 20 years, PLCs have added more functionality to create more benefits for smaller factories and installations. PLCs usually operate as stand-alone systems, but can also be integrated with other systems to communicate with each other. Since each PLC has its own database, integration requires some degree of mapping between controllers. This makes PLCs particularly suitable for smaller applications that don't have much need for expansion.

The DCS system decentralizes the controllers in the automation system and provides a common interface, advanced control, a system-level database, and information that is easy to share. Traditionally, DCS has been used primarily in process processes and relatively large plants, where large system applications are easier to maintain throughout the plant's lifecycle.

PLC is developed by the principle of relay control, which stores instructions for performing operations such as logic operation, sequence control, timing, counting and operation; And through digital input and output operation, all kinds of machinery or production processes can be controlled. The user-programmed control program expresses the process requirements of the production process and is stored in the PLC's user program memory in advance. The runtime executes the contents of the stored program one by one to complete the operations required by the process.

Comparison of PLC and DCS engineering analysis

The CPU of the PLC has a program counter that indicates the storage address of the program step, and in the process of program running, the counter automatically adds 1 when one step is executed, and the program is executed sequentially from the starting step (the step sequence number is zero) to the final step (usually the end instruction), and then returns to the starting step for cyclic operation. The time it takes for a PLC to complete a cycle operation is called a scan cycle. Depending on the type of PLC, the cyclic scanning cycle is between 1 microsecond and tens of microseconds. The program counter is a cyclic operation that the DCS does not have. This is also what makes the redundancy of the PLC inferior to the DCS.

DCS was developed on the basis of operational amplifiers. The relationships between all functions and process variables are made into function blocks (some DCS systems call them puffing blocks). The main difference between the performance of DCS and PLC is in the logical calculation of switching quantities and the calculation of analog quantities, even if the two penetrate each other later, but there are still differences.

After the 80s, in addition to logic operation, the algorithm function of PLC control loop has been greatly strengthened, but PLC is programmed with ladder diagram, and the operation of analog quantities is not intuitive when programming, and programming is more troublesome. However, in terms of solving logic, it shows the advantage of being fast, and in the order of microseconds, it is less than 1 millisecond to solve 1k logic programs. It treats all inputs as switching quantities, with 16 bits (and 32 bits as well) as an analogue quantity.

DCS, on the other hand, treats all inputs as analogue, and 1 bit is the switching quantity. Solving a logic is on the order of a few hundred microseconds to a few milliseconds. For PLC to solve a pid operation in tens of milliseconds, which is comparable to the operation time of DCS.

In terms of ground resistance, the requirements for PLC may not be high, but for DCS, it must be less than a few ohms (usually less than 4 ohms). Analog isolation is also very important.

For a system with the same number of I/O points, the cost of using PLC is lower than that of DCS (about 40% saving). PLC does not have a dedicated operating station, and the software and hardware it uses are universal, so the maintenance cost is much lower than that of DCS. If the controlled object is mainly interlocking equipment and there are relatively few circuits, it is more appropriate to use PLC.

If the main thing is analog control and there are a lot of function operations, it is better to use DCS. DCS is much better than PLC in terms of redundancy of controllers, I/O boards, communication networks, etc., some advanced computing, and special requirements of the industry. Due to the use of general monitoring software, PLC is easier in designing the management information system of the enterprise.

PLC and DCS systems are generally suitable for discrete and process manufacturing, respectively. Discrete manufacturing facilities using PLC systems, typically consisting of separate production units, are primarily used to complete the assembly of parts, such as labeling, filling, or grinding. Process manufacturing facilities, often using automated systems, produce on a recipe rather than by piece in a continuous and batch fashion. Large-scale continuous processing equipment, such as oil refineries and chemical plants, use DCS automation systems. Hybrid applications often use both a PLC system and a DCS system. Selecting a controller for an application requires consideration of the size of the process, scalability and future update schedules, integration needs, functionality, high availability, and return on investment over the life cycle of the plant facility.

Relevant factors influencing how to choose

Process size: How many input/output (I/O) points are needed? Small systems (< 300 I/O points) may have a smaller budget and are therefore more suitable with a PLC system. It is not easy to apply a DCS system to a smaller project, on the contrary, it is more effective in a large factory application. Thanks to having a global database, the DCS system is easier to manage and upgrade, and any changes are global.

Upgrade plan: A PLC system can be used for a smaller industrial process, but if the process needs to be expanded or upgraded, more PLC hardware and databases need to be added, and separate maintenance is required. It's a time-consuming, laborious process, and it's prone to errors. DCS systems are easier to upgrade, e.g. user trust can be managed from a central hub, making them easier to service and maintain.

Integration requirements: For stand-alone installations, PLC systems are ideal. When a plant is configured with multiple PLC systems, the need to be interconnected arises. This is often difficult to achieve, as the data needs to be mapped using communication protocols. Of course, there is no problem with integration, but when there is a need for a change, the problem comes for the user: once a PLC system is changed, it may cause the two PLCs to not communicate properly, because the data mapping is affected. For DCS systems, there is no need for mapping, and configuration changes are a simple process. The controller is built into the system.

High availability: For processes that require high availability, the DCS system can provide redundant configurations.

Efficiency and ease of redundancy are key to keeping costs within budget.

Functional requirements: Some industries and facilities require historians, streamlined alarm management, and a central control room with a common user interface. Others require manufacturing execution system (MES) integration, advanced control, and asset management. These applications are built into the DCS system (see Figure 3), making it easy to add to automation engineering applications without the need for separate servers and integration costs. In this respect, DCS systems are more economical, increase productivity and reduce risk.

Lifecycle ROI: The needs of a facility, which varies by industry. For smaller process projects, there is no need for expansion and no need to integrate with other process areas, so PLC systems have a good ROI. DCS systems may have higher installation costs, but they are partially offset by the increased production and safety benefits that come with DCS systems throughout their life cycle. Balancing short-term needs with long-term vision is critical to operational certainty and improving plant operations and maintenance.

and Lisi plc - the difference between DCS and PLC

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