1. Muhammad Atta ul Ghaffar
Senior Electrical Engineer
CENTURY PAPER & BOARD MILLS
PAKISTAN
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2. Programmable Logic Controller
 A digital electronic device that uses a programmable
memory to store instructions and to implement specific
functions such as logic, sequence, timing, counting and
arithmetic to control machines and process. “
 A PLC is a computer designed to work in an industrial
environment.
 It uses a programmable memory to store the instructions
and specific functions that include On/Off control, timing
counting, sequencing, arithmetic and data handling.
 PLCs are equipped with special input/output interfaces .
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3. PLC System
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4. What is Control?
“ CONTROL is the process in a system in which one or
several input variables influence other variables “
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5. A Simple Overview of Control
System
INFORMATION
COMMANDS
SENSORS
C
O
N
T
R
O
L
S
Y
S
T
E
M
ACTUATORS
P
L
A
N
T
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6. Types of Control System
CONTROL
SYSTEM
OPEN-LOOP
CONTROL SYSTEM
CLOSED-LOOP
CONTROL SYSTEM
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7. Open Loop Control System
 In an open loop system, the controller is told what the
desired process output is, and the controller sends a
command to the actuator to try and achieve this. The
process is shown below as a block diagram:
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8. Closed Loop Control System
 Systems that utilize feedback are called closed-loop
control systems. The feedback is used to make
decisions about changes to the control signal that
drives the plant. An open-loop control system doesn't
have or doesn't use feedback.
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9. PLC and Conventional Control
System
The essential difference between programmable control
and traditional control technology may be summed
up as follows:
 The functions are no longer determined by the wiring, but
rather by the program
 Programming is simplified to enable symbols familiar to
the control engineer to be used (contacts or logic graphic
symbols)
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10. Hard Wire & PLC Wiring diagram
L
24 VDC
S1
S2
S1
K1
K1
S2
PLC
K1
N
0V
Hardwire
PLC
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11. Comparison
Hardwired control systems
 The functions are determined by the physical wiring.
 Changing the function means changing the wiring
 Can be contact-making type (relays, contactors) or
electronic type (logic circuits)
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12. Comparison
Programmable control system
 The functions are determined by a program stored
in the memory.
 The control functions can be changed simply by
changing the program.
 Consist of a control device, to which all the sensors
and actuators are connected.
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13. History of PLC
 During the late 1960s, General Motors (USA) was
interested in the computer application to replace the
hardwire systems.
 Bedford
Associates (Modicon)
responded to General Motors.
and
Allen
Bradley
 The name given was “Programmable Controllers” or PC.
 Programmable Logic Controller or PLC was a registered
trademark of the Allen Bradley.
 Later, PC was used for “Personal Computer” and to avoid
confusion PLC for “Programmable Controller” and PC for a
personal computer
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14. PLC History
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15. PLC History
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16. PLC Timeline
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17. Advantages of PLC compared to
Hardwire
 Implementing changes and correcting errors
 Pilot run - trial / test run
 Visual observation - online monitoring
 Speed of operation
 Reliability
 Documentation
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18. Basic Structure of PLC
POWER
SUPPLY
INPUT
MODULES
CENTRAL
PROCESSING
UNIT (CPU)
PG/
PC
OUTPUT
MODULES
MEMORY
(EPROM/RAM)
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19. PLC Inputs & Outputs (I/Os)
Input
Devices
USER
PROGRAM
(LOGIC)
Output
Devices
PLC
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20. Input/output Modules
 Digital input modules adapt digital signals e.g. from
proximity sensors
 Digital output modules convert the internal signal level of
PLC into digital process signals e.g. relays
 Analog input modules adapt analog process signals e.g.
from transducers
 Analog output modules convert internal digital values of
the PLC to analog process signals e.g. temperature
controller
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21. Central Processing Unit (CPU)
What is a CPU?
 The “brain” of a PLC
 Controlled by a program called the executive or operating
system (OS)
 The executive is a collection of supervisory programs
permanently stored in memory
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22. CPU
Four basic types of CPU operations:
 Input and output operation
 Arithmetic and logic
 Reading or changing contents of memory locations
 Jump operations
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23. CPU
ACCUMULATOR
TIMERS,
COUNTERS,
Memory
PROCESSOR
PII
MEMORY
SUBMODULE
(EPROM/
EEPROM/
RAM)
INTERNAL
PROGRAM
MEMORY
(RAM)
SERIAL
INTERFACE
PIQ
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24. CPU
 The CPU reads in input signal states, processes the control
program and controls the outputs.
 The CPU provides internal Memory, timers and counters.
 Restart procedure can be preset and errors can be
diagnosed using the CPU’s LEDs.
 The overall Reset on the CPU is used to delete the contents
of the RAM.
 A PG or a Memory sub-module is used to transfer the
control program to the CPU.
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25. Power Supply Module
 The power supply module supplies the operational voltage
for the PLC and provides backup for the RAM with a
battery
 Backup battery
 The backup battery maintains the program and data when
the PLC is switch off
 The backup battery has a service life of approximately 2
years
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26. How does a Programmable
Controller Works?
24 VDC
Sensors
Program
Memory
Processor
Power
Supply
Input modules
Output modules
Actuators / Annunciators
GND
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