Nowadays it is commonly seen that students are often
confused between the terminologies of PLC, DCS and SCADA systems and consider
them as the alternatives of each other. However all of these systems are
developed to cater different needs. Since the PLC was integrated with Analog I/O it crosses the
boundary of being just digital and crosses to the realm of DCS in handling
Analogs, Bus Systems, Distributed I/O and etc. Also, since the DCS now handles
logics of Digital I/O it also crossed the boundary to the realm of PLC.
The detail analysis of both of these systems is discussed below:
PROGRAMMABLE LOGIC CONTROLLERS:
PLC is a specialized processor/ computer with all the hardware and software to do a
specific automation task, say running a plant/factory. This means that all the
Field/Real world Inputs are processed by the PLC (using Input interface
hardware ) and the same then issues commands to various devices and
actuators(via Output interface). This has some Pros and Cons.
Pros:-
1. As a single processor is working it can easily
access all the data / inputs and use it as it requires.
2. It has a simple architecture and is easier to
implement and maintain.
3. It has reduced hardware and reduced cost.
Cons:-
1. As all the functions are done by the same
processor it increases the loading on the controller. For large systems the
processing power required can be too high.
2. There is no physical segregation for critical
application such as protections.
3. Failure of the controllers (though redundant
controllers can be provided) will lead to complete shutdown of the control
system. For some applications a true standalone system is required.
4. If the various areas are far away the cabling
cost of inputs and outputs to the PLC can increase too much and for long
distance it can be impracticable.
If a standalone system is made for such
application then it will be difficult to integrate them in a common SCADA/ HMI/
interface and the spare for maintaining different systems will increase cost.
DISTRIBUTED CONTROL SYSTEM:
DCS can solve these problems. In
a DCS the entire logic and hardware is distribute or broken into different
microprocessors or Functional groups (FGs) for ease the implementation and
provide segregation.
These separate Functional groups (FGs) are
linked to the same HMI/SCADA/interface. The entire control of the plant is
broken in small groups with each groups having something similar.
For example:-
The AC/ventilation system can be separated
from the production part as it is not related functionally.
If separate FGs need to communicate some
signals, the same is done through Hardwire or Network/protocol such as OPC, Modbus,
profibus etc. As most of the processing is already done in the other FG the
cabling is also reduced.
Protection system and critical systems can be
made into a separate FG and this processor having low input and controls can
work with low loading and higher speed.
As an engineer working in Control systems, I
too find it very confusing when people use the term PLC and DCS
interchangeable. This is because DCS are an advanced or a larger implementation
of the PLCs. A number of vendors call the same processor as PLC or DCS
depending on the execution or architecture used for the application as
illustrated.
Now some important aspects of SCADA are
discussed below:
SCADA: Supervisory Control and Data Acquisition
1. DCS is process oriented while SCADA is data
acquisition oriented.
2. DCS is process state driven while SCADA is
event driven.
3. DCS is commonly used to handle operations on a
single locale, while SCADA is preferred for applications that are spread over a
wide geographic location.
4. DCS operator stations are always connected to
its I/O, while SCADA is expected to operate despite failure of field
communications.
