Muhammad Atta ul Ghaffar
Senior Electrical Engineer
CENTURY PAPER & BOARD MILLS
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
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 .
What is Control?
“ CONTROL is the process in a system in which one or
several input variables influence other variables “
A Simple Overview of Control
Types of Control System
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:
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.
PLC and Conventional Control
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
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)
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.
History of PLC
During the late 1960s, General Motors (USA) was
interested in the computer application to replace the
responded to General Motors.
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
Advantages of PLC compared to
Implementing changes and correcting errors
Pilot run - trial / test run
Visual observation - online monitoring
Speed of operation
Basic Structure of PLC
PLC Inputs & Outputs (I/Os)
Digital input modules adapt digital signals e.g. from
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.
Analog output modules convert internal digital values of
the PLC to analog process signals e.g. temperature
Central Processing Unit (CPU)
What is a CPU?
The “brain” of a PLC
Controlled by a program called the executive or operating
The executive is a collection of supervisory programs
permanently stored in memory
Four basic types of CPU operations:
Input and output operation
Arithmetic and logic
Reading or changing contents of memory locations
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.
Power Supply Module
The power supply module supplies the operational voltage
for the PLC and provides backup for the RAM with a
The backup battery maintains the program and data when
the PLC is switch off
The backup battery has a service life of approximately 2
How does a Programmable
Actuators / Annunciators