Basics of PLC, DCS and Difference between them

1. Prepared By:
J S Shekhawat
Sr. Engg. C&I

2. (Programmable Logic Controller)
Prepared By:
J S Shekhawat
Sr. Engg C&I

3. What is PLC?
PLC is a Programmable Logic Controller and also
a kind of a digital computer which can be
Programmed as per the process which is needed
to be controlled, designed for multiple inputs
and output arrangements, having immunity to
electrical noise and resistance to vibration and
impact.

4. History of PLC
PLC was introduced in late 1960’s
First commercial & successful Programmable Logic
Controllers was designed and developed by Modicon
as a relay replacer for General Motors.
Earlier, it was a machine with thousands of electronic
parts.
Later ,in late 1970’s,the microprocessor became reality &
greatly enhanced the role of PLC permitting it to evolve
form simply relay to the sophisticated system as it is today.

5. Major Components of a PLC
CENTRAL
PROCESSING
UNIT (CPU)
POWER
SUPPLY
I M
N O
P D
U U
T L
E
O M
U O
T D
P U
U L
T E
PROGRAMMING
DEVICE
From
SENSORS
Pushbuttons,
contacts,
limit switches,
etc.
To
OUTPUT
Solenoids,
contactors,
alarms
etc.
Handheld Device
or Computer
etc.

6. Major Components of a PLC

7. Major Components of a PLC
 Central Processing Unit
 It is a micro-controller based circuitry. The CPU consists of
following blocks :
Arithmetic Logic Unit (ALU), Program memory, Process image
memory (Internal memory of CPU), Internal timers and
counters.
 CPU performs the task necessary to fulfill the PLC funtions.
These tasks include Scanning, I/O bus traffic control, Program
execution, Peripheral and External device communication,
special functions or data handling execution and self
diagnostics.

8.  Input module
 These modules act as interface between real-time status of
process variable and the CPU.
 Analog input module : Typical input to these modules is 4-20
mA, 0-10 V
Example : Pressure, Flow, Level Tx, RTD (Ohm),
Thermocouple (mV)
 Digital input module : Typical input to these modules is 24 V
DC, 115 V AC, 230 V AC
Ex. : Switches, Pushbuttons, Relays, pump valve on off
status

9.  Output module
 These modules act as link between the CPU and the output
devices in the field.
 Analog output module : Typical output from these modules is
4-20 mA, 0-10 V
Ex : Control Valve, Speed, Vibration.
 Digital output module : Typical output from these modules is
24 V DC, 115 V AC, 230 V AC
Ex. : Solenoid Valves, lamps, Actuators, dampers, Pump
valve on off control

10.  Power Supply
 The power supply gives the voltage required for electronics
module (I/O Logic signals, CPU, memory unit and peripheral
devices) of the PLC from the line supply.
 The power supply provides isolation necessary to protect the
solid state devices from most high voltage line spikes.
 As I/O is expanded, some PLC may require additional power
supplies in order to maintain proper power levels.

11.  Bus System
 It is path for the transmission of the signal . Bus system is
responsible for the signal exchange between processor and I/O
modules.
 The bus system comprise of several single line i.e. wires /
tracks

12. PLC operation sequence
1) Self test: Testing of its own hardware and
software for faults.
2) Input scan: If there are no problems, PLC
will copy all the inputs and copy their values
into memory.
3) Logic solve/scan: Using inputs, the ladder
logic program is solved once and outputs are
updated.
4) Output scan: While solving logic the output
values are updated only in memory when
ladder scan is done, the outputs will be
updated using temporary values in memory.
Self test
Input scan
Logic scan
Output
scan

13. Programming Languages of PLC
Most common languages encountered in PLC
programming are:
1) Ladder Logic
2) Functional Block Diagram
3) Sequential Function Chart

14. Ladder Logic
 The ladder logic is the oldest programming language for
PLC.
 It is well suited to express Combinational logic.
 The main ladder logic symbols represent the elements :
make contact
break contact
relay coil

15. Block diagram of a PLC
Outputs
LEDs
Inputs

16. Programming Example:
Ladder Logic Program for Start/Stop of Motor :
X2X1
Y1
Y1

17. Programming PLC:

18. Starting of Motor:

19. Continuous Running of motor
when Start Button is released:

20. To Stop the Motor :

21. PLC Selection Criteria
 Cost of hardware, software, Integration Engineering, Design, Installation,
Start-up and Commissioning, Validation documentation and Execution,
Training, Spare parts, Maintenance, System service contract and system
life cycle.
 Reliability, Flexibility, Scalability and Validatability.
 Ease of Database configuration, Graphics development, Interlocks and
Batch processing.
 Integration of High-level Application.
 Control Philosophy for Centralized versus Remote Operator Console or
both.

22. Advantages of PLCs:
 Reliability.
 Flexibility in programming and reprogramming.
 Cost effective for controlling complex systems.
 Small physical size, shorter project time.
 High speed of operation.
 Ability to communicate with computer systems in
the plant.
 Ease of maintenance /troubleshooting.
 Reduced space.
 Energy saving.

23. Disadvantages of PLCs
 PLC devices are proprietary it means that part
or software of one manufacturer can’t be used
in combination with parts of another
manufacturer.
 Limited design and cost option
 Fixed Circuit Operations.
 PLCs manufacturers offer only closed
architectures.

24. Applications:
Wherever automation is desired the PLCs are best
suited to meet the task.
Few examples of industries where PLCs are used :
1) Robots : manufacturing and control
2) Car park control
3) Train control station system
4) Food processing
5) Materials handling
6) Machine tools
7) Conveyer system etc.

25. (Distributed Control System)
Prepared By:
J S Shekhawat
Sr. Engg C&I

26. Distributed control system (DCS)
 DCS is a system of dividing plant or process control into
several areas of responsibility, each managed by its own
controller, with the whole system connected to form a
single entity, usually by means of communication buses.
Distributed Control System (DCS) refers to a control
system usually of a manufacturing system, process or any
kind of dynamic system, in which the controller elements
are not central in location (like the brain) but are
distributed throughout the system with each component
sub-system controlled by one or more controllers. The
entire system of controllers is connected by networks for
communication and monitoring.
 The entire system of controllers is connected by networks
for communication and monitoring.

27. Why it is called DISTRIBUTED???
As is apparent from the abbreviation, the word ‘Distributed’
supports following functionality’s
 Physical Distribution - Nodes or Subsystems can be
Distributed i.e. located physically apart
 Functional Distribution - Specific Functionality is imparted
for a Node basing on the combination of hardware and
software used. For e.g. Application work-processor with
Historian, Application work-processor with control
configuration software
 Structural Distribution - Different Structural hardware
platforms (Application Workstation processor, Workstation
processor, Control processor etc.) are used to achieve the
required functionality.

28. What does DCS System consists of?
DCS System consists minimum of the following components.
 Field Control station (FCS): It consists of input/output modules, CPU and
communication bus.
 Operator station: It is basically human interface machine with monitor,
the operator man can view the process in the plant and check if any alarm is
presents and he can change any setting, print reports..etc...
 Engineering station: It is used to configure all input & output and drawing and
any things required to be monitored on Operator station monitor.

30. Process Control Systems

31. Process Control Systems

32. Digital Control System
 Drawbacks Of CCS:
 If the CPU fails the entire plant gets affected.
 Redundancy concept is not available.
 Redundancy is having two controllers. One would be active and the
other would be standby. If the active controller fails, the standby
controller takes over.

33. Digital Control System

34. How does it work?
Example of simple control mechanism
Process measured value (PV)
Manipulated value (MV)
Set Value (SV).

35. Operation Windows
Information regarding the process is gathered as well as
monitored by the following Standard Operation windows :
 Tuning Window
 Control Group Window
 Trend Window
 Process Alarm Window
 Operator guide Message Window
 Graphic Window
 Overview Window
 Process Report Window
 Historical Report Window

36.  Control function is distributed among multiple CPUs
(Field Control Stations). Hence failure of one FCS does
not affect the entire plant.
 Redundancy is available at various levels.
 Instruments and interlocks are created by software.
 Generation and modifications of the interlocks are very
flexible and simple.
 Information regarding the process is presented to the
user in various formats.
 Field wiring is considerably less.
 Maintenance and trouble shooting becomes very easy.
 Cost effective in the long run.
Advantages of DCS

37. WHY DCS ?
 For Total Plant Automation
 For Higher Productivity
 For Optimal Process Control
 For Advance Process Control
 For Regulatory Compliance
 For Management Information System
 In Tune With Global Requirement

38. DCS MANUFACTURERS
 ASEA BROWN BOWERI - ABB
 TATA HONEYWEL
 FOXBORO - INVENSYS
 YOKOGOWA BLUE STAR - YBL
 ALLEN BRADLEY
 GE FANUC
 FISHER ROSEMOUNT
 TOSHIBA
 METSO Automation

39. (Basic Difference between PLC & DCS)
Prepared By:
J S Shekhawat
Sr. Engg C&I

40. Difference Between PLC & DCS
 A PLC is cost-effective up to a certain I/O count, and so is the
DCS. But the difference is in their starting points: the PLC is
cost-effective from 0 to a few thousand I/O points; the DCS
becomes cost-effective starting from a few thousand points and
beyond.
 A PLC becomes a subsystem of the DCS in rare occasions when
the situation calls for it, i.e., purchase of huge package systems
with engineering schedules incompatible with the DCS schedule
(I/O lists cannot be submitted on time before the DCS hardware
freeze date). Note that this package system is a process system
using continuous control, not discrete. Based on this, a PLC can
never be larger than a DCS in terms of I/O count.
 In large plants the DCS is king because most owners want a single
source of hardware support and service, and this mentality
naturally denies the PLC a foothold. Package vendors are no
longer required to provide PLC for their system. Everything is
connected to the DCS.

41. Difference Between PLC & DCS Cont…
 Generally, PLCs are stand alone and perform a particular task,
where a DCS is a network of PLCs that communicate in some
fashion to accomplish a particular task. For example, in a water
filtration plant, there might be a PLC that is used to perform a
backwash of a particular filter, in that same water plant a DCS
may be communicating with 14 filter PLCs and starting the
backwash routine when required.
 PLC only handle sequential process than DCS can handled both
Continue process and large loop control.
 If we see from security angle, PLC doesn't have dongle so people
can crack the software easy. DCS have a dongle so it's only
license to industry which have it.
 PLC is a programmable Logic controller which is used mainly for
interlocking different equipment’s. DCS is Distributed Control
Systems which is used to control very big plants by using simple
GUI screens. PLCs are interfaced with DCS for interlocking.

42. Thank You!!!