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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.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.
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.
DCS is a relatively large system while the PLC is a small system.
1. Performance
PLC:
Fast logic scan (approx.10ms) is required to perform motor or motion control
Redundancy may not be cost justified
System can be taken offline to make configuration changes
Analog Control: Simple PID only
Diagnostics to tell you when something is broken
DCS:
Control loops require deterministic scan execution at a speed of100 to500 ms
System redundancy is often required
Online configuration changes often required
Analog Control: Simple to advanced PID control up to Advanced Process Control
Asset Management alerts you to what might break before it does
2. Engineering & Customization
PLC:
High level programming languages are available for creating custom logic
Customized routines usually required
Standard libraries considered nice features
Provisions must be available to integrate functions/products into an integrated architecture
System designed to be flexible
Solution is generic in nature, to be applied on a wide variety of applications
DCS:
Custom logic created from existing function blocks
Many algorithms (i.e. PID) are complex and do not vary among applications
Standard application libraries are expected (function blocks and faceplates)
Entire system is expected to function as a complete solution
System designed to make it "easy" to engineer process applications
Use of pre-defined, pre-tested functions saves time
3. Application
PLC: Discreat Automation industry or Batch Control
DCS: Continious Controlling - Petrochemical Industry
DCS can control the entire system but PLC can control the individual part of the system. DCS has a rapid process when compared with PLC. DCS has communication with more CPUs where as PLC has the specified CPU.
Simple comparison:-
1) PLC may control one system, but DCS control many systems.
2) PLC contains only one processor, but DCS contains may processors (basically PLCs)
3) PLC has a limited numbers of I/Os, but DCS has much bigger I/Os.
4) PLC cheap, DCS very expensive.
Nowadays, PLC and DCS both are performing equally, But based on I/O counts ,type and importance of the datas we can choose the type of controllers it may PLC or DCS. if the industry having I/O which is mostly relative to digital then we supposed to go for PLC systems. if the maximum of I/O associated with analog then we should go for DCS systems, if the industry providing more importance for controlling then we should compulsory go with DCS because DCS having complete redundancy.
programmable logic controller can control one process at a time but when come to DCS it can control whole process at a time.
plc allows to rung the program one rung to another rung DCS can read all the rungs at a time
Plc's are high speed machine control, and Dcs's prevail in complex continuous process.
