In the manufacturing process, there are two types of control that can be employed. One is the Programmed Logic Controller, otherwise commonly referred to as PLC, and the other is the DCS, or the Distributed Control System. The Programmed Logic Control system is a standalone control and is developed to perform a specific task. The DCS, on the other hand, functions as a control system that can work through various levels for the end result to be achieved. This means that the DCS can be formed by several levels of PLC’s in creating a fully functional system.
Traditionally, the formulation of DCS’s was extremely expensive and was only recommended for batch processing industries, as they had various levels of production before the final product was delivered. This concept holds today, albeit with a few changes that have occurred along the way.
Multiple PLC and DCS solutions have been developed through time to ease automation and entire control processes. One of the PLC solutions developed was coupled with HMI (Human Machine Interface)/SCADA (Supervisory Control And Data Acquisition) which allows for user interaction. Furthermore, PLC is a management tool that has a rather analogous control function for process management. PLC ensures that the ladder logic is maintained. This is the preferred solution for use for Original Equipment Manufacturer (OEM) and special project needs. For user interaction, a HMI/SCADA panes must be provided.
For slightly larger processes, a DCS is preferred. This allows for easier management of processes that are beyond the scope of a single PLC management. A small DCS management is much better when compared to traditional DCS management systems, largely due to a smaller footprint. Also the system has a diagnostics database that lowers ownership costs.
For large processes a large DCS solution is recommended. This is a distributed control that has numerous features to help in meeting the production requirements. Functions such as collecting controlling output, alarming, processing, and data collection all are managed in the DCS system. There are specific measures instituted in the DCS solution to handle each process. With the entire system synchronized no single system failure can lead to the failure of another system of a different part.
The geographical distribution of the area is also a factor when deciding between PLC and DCS. If the control functions have been distributed over different geographic locations, use of PLC or even DCS can be considered, depending on the needs. Having different processes individually can help a single part of the system in its management, especially when failure arises, since the failure of a given system does not mean that the entire process should come to a grinding halt.
The control algorithms in DCS ought to be advanced, as the loop between the measured and manipulated input handles the entire process. When the word ‘system’ is mentioned in DCS, it means that one process is linked to the rest and there are different physical processes spread over a wide area. To ensure that the PLC control is working as expected, there should be two processes running in the PLC, one controlling the process in question, while the other safeguards the process. Smaller PLC’s can have the same unit running both the control and safeguarding processes.
PLC is mainly used as a controller of processes and mainly comes as a standalone program.
DCS is mainly used as a control system and will feature different processes which can be made up of merged PLC’s.
Both DCS and PLC can be configured or reconfigured.
DCS is a relatively large system while the PLC is a small system.
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