1. Introduction to PLC by
Dr. Amin Danial Asham

2. References
Programmable Controllers- Theory and Implementation, 2nd Edition, L.A. Bryan and E.A. Bryan

3. Definition:
Programmable logic controllers, also called programmable controllers or PLCs, are members of the digital computer family, used to implement control functions.
They are capable of storing instructions, such as sequencing, timing, counting, arithmetic, data manipulation, and communication, to control industrial machines and processes.
PLCs can be thought of in simple terms as industrial computers with specially designed architecture in both their central units (the PLC itself) and their interfacing circuitry to field devices (input/output connections to the real world).

4. A conceptual diagram of a PLC application.
Definition: (continue)
A conceptual diagram of a PLC application.

5. A programmable controller, consists of two basic sections:
PRINCIPLES OF OPERATION
A programmable controller, consists of two basic sections:
The central processing unit
The input/output interface system

6. PRINCIPLES OF OPERATION
The central processing unit
The central processing unit (CPU) governs all PLC activities.
The CPU has three components:
The processor
The memory system
The system power supply: provides all the voltages required for the proper operation of the various central processing unit sections.

7. PRINCIPLES OF OPERATION (continue) The input/output interface system
The input/output (I/O) system is physically connected to the field devices that are encountered in the machine or that are used in the control of a process.
These field devices may be discrete or analog input/output devices, such as limit switches, pressure transducers, push buttons, motor starters, solenoids, etc.
The I/O interfaces provide the connection between the CPU and the information providers (inputs) and controllable devices (outputs).
During its operation, the CPU completes three processes:
it reads, or accepts, the input data from the field devices via the
input interfaces,
It executes, or performs, the control program stored in the
memory system, and
it writes, or updates, the output devices via the output
interfaces.
This process of sequentially reading the inputs, executing the
program in memory, and updating the outputs is known as
scanning Cycle.

8. General Overall Block Diagram of the PLC
PRINCIPLES OF OPERATION (continue)
General Overall Block Diagram of the PLC

9. PRINCIPLES OF OPERATION (continue)
The input/output interface system (continue)
The input/output system forms the interface by which field devices are connected to the controller.
The main purpose of the interface is to condition the various signals received from or sent to external field devices.
Incoming signals from sensors (e.g., push buttons, limit switches, analog sensors, selector switches, and thumbwheel switches) are wired to terminals on the input interfaces.
Devices that will be controlled, like motor starters, solenoid valves, pilot lights, and position valves, are connected to the terminals of the output interfaces.

10. PRINCIPLES OF OPERATION (continue)
Programming Device
Although not generally considered a part of the controller, the programming device, usually a personal computer or a manufacturer’s miniprogrammer unit, is required to enter the control program into PLC’s memory .
The programming device must be connected to the controller when entering or monitoring the control program.
Miniprogrammer
Personal computer

11. PLCS VERSUS OTHER CONTROL SYSYETMS
PLC has may positive points over old relay systems:
Flexibility: PLC system function can easily be modified by changing the program to meet the new needs. Whereas, relay system needs a lot of efforts to change the wringing connections to implement new functions.
Expansion: PLC system can be expanded by adding new components in a modular way and modifying the main program. Whereas , relay systems may need a large hardware modification.
Reliability : Since the PLC is controlled by a stored program and backed up on external disks, the system is reliable and can be restored easily. Whereas, in the old relay system the function is implemented by a hardwired complex circuit which is exposed to bad contacts because of dust and vibration.

12. PLCS VERSUS OTHER CONTROL SYSYETMS (continue)
PLC has may positive points over old relay systems: (continue)
Cost Saving: Although relay control may cost less initially, this advantage is lost if production downtime due to failures is high.
Troubleshooting :Troubleshooting the PLC controlled system is much more easier and time saving process because all the detailed necessary data is stored in log files that can be reviewed anytime. In addition, we have the ability to built sophisticated programs to monitor any critical process and store as much signals as needed without with the same hardwired circuitry. On the other hand relay systems have much less monitoring signals and with high cost of circuit complexity.
Maintainability: PLC ‘s are in general easier to be maintained than relay systems because of much less hardwired connections

13. PLCS VERSUS OTHER CONTROL SYSYETMS (continue)
PLC’s versus computer control s
The architecture of a PLC’s CPU is basically the same as that of a general purpose computer; however, some important characteristics set them apart:
Unlike computers, PLCs are specifically designed to survive the harsh conditions of the industrial environment. A well-designed PLC can be placed in an area with substantial amounts of electrical noise, electromagnetic interference, mechanical vibration, and noncondensing humidity.
Hardware and software are designed for easy use by plant electricians and technicians.
The hardware interfaces for connecting field devices are actually part of the PLC itself and are easily connected.
The modular and self-diagnosing interface circuits are able to pinpoint malfunctions and, moreover, are easily removed and replaced.
Also, the software programming uses conventional relay ladder symbols, or other easily learned languages, which are familiar to plant personnel.
The personal computer is, however, being used as the programming device PLCs .

14. TYPICAL AREAS OF PLC APPLICATIONS
Since its inception, the PLC has been successfully applied in virtually every segment of industry,
including steel mills,
paper plants,
food-processing plants,
chemical plants,
and power plants.
Heavy industries
PLCs perform a great variety of control tasks, from repetitive ON/OFF control of simple machines to sophisticated manufacturing and process control.

15. PLC PRODUCT APPLICATION RANGES
The PLC market can be segmented into five groups, The differences include I/O count, memory size, programming language, software functions, and other factors.
micro PLCs are used in applications
controlling up to 32 input and output devices.
small PLCs control 32 to 128 I/O
medium PLCs control 64 to 1024 I/O
large PLCs control 512 to 4096 I/O
very large PLCs can handle 2048 to 8192 I/O
The A, B, and C overlapping areas
in the Figure reflect enhancements,
by adding options, of the standard
features of the PLCs within a
particular segment. These options
allow a product to be closely
matched to the application without
having to purchase the next larger unit

16. Thanks