PLC and DCS
PLC
- It has developed from switching control to sequential control, transportation processing, and multifunctional functions such as continuous PID control from the bottom to the top, and PID is in the interrupt station.
- One PC can be used as the master station and multiple PLCs of the same type as the slave station.
- A PLC can also be used as 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 the master is: there is user programming, do not have to know the communication protocol, as long as the format of the manual to write.
- PLC grid both as a stand-alone DCS, can also be used as a subsystem of the DCS.
- PLCs are mainly used for sequential control of industrial processes, and new PLCs also have closed-loop control functions.
DCS
- Decentralized Control System DCS is a monitoring and control technology that combines 4C (Communication, Computer, Control, CRT) technologies.
- From top to bottom of the tree topology of the large system, in which communication is the key.
- PID in the interrupt station, the interrupt station link computer and field instrumentation and control devices is a tree topology and parallel continuous link structure, but also a large number of cables from the interrupt station parallel to the field instrumentation.
- Analog signals, A/D - D/A, mixed with microprocessor.
- A pair of wires from one instrument to the I/O is hooked up to the LAN LAN from the control station.
- DCS is a 3-level structure of control (engineer station), operation (operator station), and field instrumentation (field measurement and control station). It is used for large-scale continuous process control, such as petrochemicals.
How to choose between PLC and DCS
In the programmable logic controller (PLC) and decentralized control system (DCS) between how to choose, to analyze the specific circumstances, because the application of different occasions, the requirements of the control system are also different.
Control system platforms can have an impact on the way automation systems meet the need to optimize production, maintain availability, and access data. A lack of vision in selecting a control system can also impact future expansion, process optimization, user satisfaction, and company profitability. In addition to some basic guidelines (e.g., how to control the process), the design team must also consider a variety of factors in terms of installation, scalability, maintenance, and upkeep.
Currently, while a PLC system may be the most cost-effective for a small facility, a DCS system offers more economical scalability and is more likely to yield a higher return on the initial investment.
A PLC is an industrial computer used to control manufacturing processes such as robotics, high-speed packaging, bottling and motion control. Over the past 20 years, PLCs have added more features to create more benefits for smaller plants and installations.
PLCs usually operate as stand-alone systems, but they can also be integrated with other systems and connected to each other via communications. Since each PLC has its own database, integration requires some level of mapping between controllers. This makes PLCs particularly suitable for smaller applications where there is little need for expansion.
DCS systems, on the other hand, decentralize controllers across the automation system and provide common interfaces, advanced controls, system-level databases, and easily shared information. Traditionally, DCSs have been used primarily in process processes and in relatively large plants, where larger system applications are easier to maintain throughout the plant's life cycle.
PLC is developed from the relay control principle to store instructions for performing operations such as logic operations, sequence control, timing, counting and arithmetic; and to control various types of machinery or production processes through digital input and output operations. The control program prepared by the user expresses the process requirements of the production process and is stored in advance in the user program memory of the PLC. During operation, the program is executed line by line according to the contents of the stored program to complete the operations required by the process.
Comparison of PLC and DCS engineering analysis
PLC's CPU has an indication of the program step storage address of the program counter, in the process of program operation, the execution of a step of the counter is automatically increased by 1, the program from the starting step (step number zero) from the sequential execution to the final step (usually end instruction), and then return to the starting step of the cyclic operation.
PLC every cycle to complete the operation of the time required is called a scan cycle. Different models of PLCs have cyclic scanning cycles ranging from 1 microsecond to tens of microseconds. The program counter such a cyclic operation, which is not available in DCS. This is what makes PLCs less redundant than DCSs.
DCS was developed on the basis of operational amplifiers. The relationship between all functions and process variables are made into function blocks (some DCS systems are called expansion blocks). the main difference between the performance of DCS and PLC is in the logic of the switching solver and analog operations, even if the two later penetrate into each other a little bit, but there is still a difference.
After the 80's, PLC in addition to logic operations, control loop with the algorithm function has been greatly enhanced, but PLC programming with ladder diagrams, analog operations in the programming is not very intuitive, programming is more troublesome. However, in terms of solving the logic, it shows the advantage of fast, in the microsecond scale, solving 1k logic program is less than 1 millisecond. It treats all inputs as switches, with 16 bits (or 32 bits) as an analog.
DCS treats all inputs as analog, and 1 bit is a switch. Solving a logic is on the order of hundreds of microseconds to milliseconds. For a PLC to solve a pid operation is in the order of tens of milliseconds, which is comparable to the DCS's computation time.
In terms of grounding resistance, the PLC may not require much, but the DCS must be under a few ohms (usually under 4 ohms). Analog isolation is also very important.
The same number of I / O points of the system, with PLC than with DCS, its cost is lower (about 40% of the savings). PLC does not have a dedicated operator station, it uses the software and hardware are common, so the maintenance cost is much lower than DCS. If the object to be controlled is mainly a chain of equipment, the circuit is relatively small, the use of PLC is more appropriate.
If the main analog control, and a lot of function operations, it is best to use DCS. DCS in the controller, I / O boards, communication networks, such as redundancy, some of the advanced computing, industry-specific requirements than the PLC is much better than the PLC. PLC due to the use of common monitoring software, in the design of the enterprise's management information system, it is easier to some.
PLC and DCS systems are generally applicable to discrete and process manufacturing respectively. Discrete manufacturing facilities that use PLC systems generally consist of individual production units used primarily to complete the assembly of components, such as labeling, filling, or grinding. Process manufacturing facilities, which typically use automated systems, produce in a continuous and batch mode according to recipes rather than on a piece-by-piece basis. Large continuous process facilities, such as refineries and chemical plants, use DCS automation systems. Hybrid applications often use both PLC systems and DCS systems. Selecting a controller for a particular application requires consideration of the size of the process, scalability and future upgrade plans, integration requirements, functionality, high availability, and return on investment over the entire life cycle of the plant facility, among many other factors.
Relevant elements that influence how to choose
Process size: How many input/output (I/O) points are needed? Smaller systems (<300 I/O points) may have a smaller budget and therefore be better suited with a PLC system. Trying to apply a DCS system to smaller projects is not really easy, on the contrary, it is more functional in large plant applications. With a global database, DCS systems are easier to manage and upgrade, and any changes are global in nature.
Upgrade planning: A PLC system can be applied to smaller industrial processes, but if the process needs to be expanded or upgraded, more PLC hardware and databases will need to be added and individually maintained. This is a time-consuming, labor-intensive and error-prone process. DCS systems are easier to upgrade, for example, user fiducials can be managed from a centralized hub and are therefore easier to maintain and service.
Integration Requirements: For stand-alone installations, PLC systems are ideal. When a plant is configured with multiple PLC systems, the requirement for interconnectivity arises. This is generally difficult to achieve as it usually requires mapping of data using communication protocols. Integration is of course no problem, but when there is a need for change, then the user is in trouble: if a change is made to one PLC system, the two PLCs may not be able to communicate properly because the data mapping is affected. For DCS systems, there is no need for mapping, configuration changes are a simple process; the controller comes with the system.
High Availability: For processes with high availability requirements, DCS systems can provide redundant configurations. Efficiency and ease of realizing redundancy are critical to keeping costs within budget.
Functional Requirements: Some industries and facilities require historical databases, streamlined alarm management, and centralized control rooms configured with common user interfaces. Others require Manufacturing Execution System (MES) integration, advanced controls, and asset management. DCS systems have these applications built in, making them easy to add to automation engineering applications without the need for additional standalone servers or increased integration costs. In this respect, DCS systems are more economical and can increase productivity and reduce risk.
Lifecycle ROI: The needs of the facility vary from industry to industry. For smaller process engineering, where there is no need for expansion or integration with other process areas, PLC systems offer a better ROI. DCS systems may have a higher installed cost, but the increased production and safety benefits of a DCS system offset some of these costs when viewed from a full life cycle perspective. Balancing short-term needs with long-term vision is critical for operational certainty and improved plant operation and maintenance.