Advantages, Disadvantages of PLC over DCS.


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What are the advantages & disadvantages of a PLC over DCS?
In my opinion; PLCs are pretty good at controlling machines with a lot of discrete devices like limit switches, motor starters, etc., that are often involved in automation such as material handling, sequencing, state machines,
and the like. While DCS systems are good at controlling analog devices and lend themselves toward process control.

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With modern PLC's & DCS's there is very little either cannot do in a plant automation setting. However PLC's are better with discrete control (on/off) and DCS's tend to be better with Analog control (PID etc). Unless you have a large proportion of analog vs discrete you are generally
better off with a PLC system. There is also a huge range of variation in quality and capability both of DCS and PLC systems.

Atul Vispute (Pune - India)

(First of all this question should be : what are advanteges of PLC+PC based systenm over DCS. In fact PLC is a part of DCS so you can not have such
comparision. PLC is just controller and DCS is controller + MMI)

Advantages of PLCs : For small systems PLC+PC based system is best solution which supersids DCS requirement. DCS is costly than PLC+PC based system. PLC scan time is very less so critical loops you can execute very fast. Lots of
diagnostics are possible.

Disadvantages : We can not use it for large systems. It is not a rugged architecture.

Atul Vispute
Control System Engineer
- PLC hardware & software usually cheaper;

- PLC digital I/O always cheaper, but analogue I/O may not be

- third-generation DCS (e.g. DeltaV, ABB Freelance, Rockwell ProcessLogix) often not much more expensive to buy than PLC/SCADA

- applications engineering can cost MUCH less than for PLC/SCADA (degree depends upon nature of application)
RESPONSE: DCS offers an integrated development environment (applications written on a central host using closely coupled programming tools).

·DCS offers more powerful remote process control computers (more memory, sophisticated control loops, etc.).

· DCS takes a more systems oriented approach to solving production problems. By focusing on the enterprise, rather than on just the control strategy, a more unified solution to the problems of running a business is created. For example, built into every DCS system are the following components that typically must be purchased from other vendors for PLC or SoftPLC solutions;
a) Continuous and regulatory control software
b) Batch and ISA SP88 compliant software
c) Dynamic simulation and advanced control capabilities
d) Redundant networking
e) Process alarm reporting and logging
f) Material and product tracking software
g) Statistical Process Control (SPC)
h) Automatic loop tuning
i) Support for fault tolerant and disaster recovery operation
j) Multiple I/O drivers
k) Graphical user interface software supporting multiple re-sizable
l) Trending to relational databases
m) Interfaces to desktop applications such as spreadsheets

·The design of the DCS is to a large degree focused on reliability and security. The real time executive in the process controller is tailored to provide bumpless failover in the event of a hardware failure. Any data that is
used in batch or sequential logic, complex analog control schemes, sympathetic alarm filtering, operator equipment allocation, and other equally esoteric activities, is available for safe plant operation despite the failure. This is not always the case when the control activity is split between a PLC and the application running on the operator station.

·Unit relativity--Unit relative batch programming is a technique for creating a single application that is capable of run time binding to multiple units of equipment. Total cost of ownership is significantly reduced with this technique since only one program has to be created, modified, and tested. All data points connected to a DCS controller can be an input or output to an
equipment unit and, therefore, under the direct control of the unit's application program. Splitting the control program among multiple units, especially those that do not offer transparent redundancy, require exception skill on the part of the application programmer to plan for all contingencies.
PLC's vs DCS the term Distributed Control System was to mean a distribution of I/O sensors and 'local' controllers that really never came about from the vendors who use the DCS term. Instead the sensors were mainly connnected by
long home runs of wiring through many marshalling cabinets and junction boxes back to CENTRAL processor. Also the Distrbuted model was to move data processing outward from the control room to the point of control. This also is mostly a misnomer.
Bailey, FoxB, Honeywell and others all claimed, at one time, to offer DCS but they really sold more of a CCS or Centralized control.
PLC's can be either DCS or CCS by layout and location of processors. The true gain to PLC over DCS is the ability to buy exacty the I/O count you need and put it where you want it. This is true DCS where the I/O and CPU/controller
could be truely Distributed.

Usually PLC's have several comm. layers both vertical and horizontal, while most socalled DCS are merely wide scope single-source proprietary sales schemes. The gain is that the control and programming can be much more agile and fault tolerant in a true PLC system. Most DCS products offer a more cohesive 'out-of-the-box' system when compared to PLC's and mulitple comm. layers. However the PLC will most likely offer the ability to defend legacy equipment which can be intergrated more easily into PLC based designs.
To tell you the truth I have worked on both systems, for similar applications and have noticed miner differences from the functional point of view between a DCS and PLC/HMI system. But it has taken me 3 times the time required to implement the application on the PLC system, and documenting the PLC system was much more difficult then the DCS system. The cost advantage for PLC HMI application is clear, approximately the cost is twice as much for a DCS IO vs. a PLC IO, keeping in mind that with DCS IO you can have redundancy off the shelf - but you will bay extra for it-, for a PLC if you want redundancy it is a bit more involved with most standard PLC - such as the Modicon Quantum or Micro or AB PLC-5 - unless you go for the more expensive types such as the Triconex or FSC systems, then there is no real cost advantage.

Any way to be totally honest with you it all depends on the budget you have been allocated to do the job, if you have adequate funds please do not hesitate in using a DCS.

If you want the best/ or maybe the worst of both world you might consider a highbred system such as the Honeywell Plant Scape, or Fisher-Rosemount Delta-V, but I have no practical experience on such systems.
There is very little difference between a current PLC/SCADA system and a DCS system from a functional standpoint. The DCS system tends to be more expensive and tends to use proprietary hardware and software. Engineering costs also are
typically higher with a DSC system.

Not every PLC has DCS like capabilities. However, in using a PLC/SCADA system of current design, such as the Schneider Electric Quantum PLC with Concept software, and a SCADA package such as from Ci Technologies, one achieves the
functionality and capabilites claimed to be exclusive to a DCS system.

Some of these features are:

-Redundant PLC Control
-Redundant Networking
-Redundant Cabling
-Client/Server Architecture
-Support of SQL Client, ODBC Client, DDE, DLL, SQL Server,ODBC Server, OPC Server
-Graphical User Interface Software
-Process Alarm Reporting and Logging
-Material and Product Tracking Software
-Statistical Process Control
-Trending to Databases
-Interface to Desktop Application
-Multiple I/O Drivers
-Automatic Loop Tuning
Likely taken direct from the DCS salesman's latest blurbs. :)

These are all nice things, and if an application needs these items and they were not available from a PLC/SCADA system combination (but except for the unit relativity thing I see nothing the combination could not do) than maybe a DCS is a better choice in a particular application.

OTOH - there are things a DCS just cannot do that PLCs do very well. Like high (relative to DCS's anyway) speed logic solving. Its unlikely anyone would be fool enough to try to run a transfer line off a DCS.

I guess the point is that if you have a nail to drive use a hammer, if its a screw use a screwdriver.

Bob Peterson

Yashar Entezarghofran

The big problem with PLC systems is that it is very hard to change them. When you prepare a PLC and implement it on a set of hardwire equipments to produce a product, in case there is any unexpected change in the product, you have to change the entire system which is very costly. On the other hand, PLCs may give better optimized results over DSCs.
> What are the advantages & disadvantages of a PLC over DCS?

Basically PLC comes into picture during the start up and safe shut down of the plant and DCS is used for continuous control during the normal plant operation. Now the question is why two different system. The answer to this is that many regulatory authorities demand for segregation of shut down system and control system. So its a statuary obligation at many applications. Second is cost..cost of PLC is much lesser as compared to DCS. Third is scan time. Scan time of PLC's are in the range of 30-80 msec while the scan time of DCS is 250msec upwards. Thumb rule is if your plant has more digital inputs and fewer analogue inputs go for PLC but in larger plant you have to go for PLC and DCS both. Hope this clears the doubts.

bob peterson

It is never as simple as PLC versus DCS.

These days there is nothing that a DCS can do that a PLC system cannot do at less cost and usually at least as well if not a bunch better, and there are many things DCS's just do not do very well at all.

It is about matching the right solution up with a specific problem that needs to be solved. Often the most important factors are not technical in nature.