This document provides an overview of programmable logic controllers (PLCs). It describes the basic structure of a PLC including input/output modules, a central processing unit, memory, and a programming unit. The document outlines how PLCs are used to automate industrial processes through input/output processing and programming using ladder logic and mnemonics. Additional features of PLCs like timers, counters, internal relays, and data handling are also summarized. The document concludes with factors to consider when selecting a PLC for an automation application.

1. UNIT 4
PROGRAMMABLE LOGIC CONTROLLER

2. Content
• Introduction
• Basic structure
• Input and output processing
• Programming
• Mnemonics
• Timers, counters and internal relays
• Data handling
• Selection of PLC

3. PROGRAMMABLE LOGIC
CONTROLLER
• A Programmable Logic Controller(PLC) is a
digital computer used for automation of
typically industrial electromechanical
processes, such as control of machinery on
factory assembly lines, amusement rides or
light fixtures.

4. Applications
• Automated manufacturing process
equipment and machinery
• Packaging and filling equipment
• Chemical mixing
• Conveyor systems and distillation etc.,

5. Features and specification
• They are rugged and designed to withstand
vibration, temperature, humidity and noise
• The interfacing for inputs and outputs is inside
the controller.
• They are easily programmed and have an
easily understood programming language.
• Programming is primarily concerned with
logic and switching operation.

6. Hardwired motor circuit

7. Hardwired motor circuit with PLC

8. Basic structure

9. • PLC is designed as a replacement for the
hardwired relay and timer logic, where PLC
provides ease and flexibility of control based
on programming and executive logical
instruction.
• The internal functions such as timers, counter
and shift registers making sophisticated control
possible using even the smallest PLC.

10. • PLC capable of performing function such as
– counting,
– logistics,
– numerical application,
– comparing and processing of signals.
• A PLC is divided in to 4 parts. They are
– Input/output module (I/O)
– Central processing Unit (CPU)
– Memory
– Programming unit

11. i) Input/output module (I/O)
• It is used to transfer the data between
external devices and CPU
• It is incorporated into PLC in two ways
I. Fixed I/O – it is a small unit that comes in one
piece with processor i.e., the I/O terminals
cannot be changed in fixed I/O
II. Modular I/O – it is packed together i.e., there
are several compartment of I/O module are
plugged together.

12. Central processing Unit (CPU)
• It is consisting of a microprocessor which
interrupts the input signal and carries out the
control actions according to the program stored
in the memory, communicating the decision as an
action signal to the output.
• It scan the total information package stored in
the memory and input and output devices
continuously.
• During the scan the CPU executes instruction
based on input data, sends appropriate output
responses to the output devices, updates data
acquisition systems and indicate condition
changes

13. • Scan time for larger unit depends on the
size of the memory and configuration of
the system
• Power supply unit is needed to convert
the AC voltage to the low DC voltage
necessary for the processor and to supply
power to other circuit in the input and
output interface module.

14. Memory Unit
• The memory in PLC stores the digital control
logic, the process program and the necessary
instruction to operate the system.
• The memory used in PLC are
• Non-volatile memory
• Volatile memory
• According to purpose of usage
• RAM –volatile memory
• ROM- permanent storage (Non-volatile memory )

15. Programming unit
• It is used to enter the required program
into the memory of the processor
• There are normally 3 approaches followed by
the program
–Use of hand held programmer
–Terminal with video display unit
–PC with appropriate software

16. Architecture of PLC

17. • BUSES
–Data buses – It is used for communicating
data b/w elements
–Address buses-It is used to read the address
of locations for accessing stored data
–Control buses- It is used for internal control
action carried by the CPU
–System buses- It is used for communication
b/n Input/output ports and input/output units

18. • Memory
– RAM (User program and data storage & volatile)
– ROM (Permanent storage)
• PROM ( Written once in the chip once received from the
manufacturer)
• EPROM ( Program Erased by the use of ultra voilet
light source and it can be reprogrammed and stored
permenantly)
• Electrically EPROM ( Program erased by Electrical
pulses)

19. Optoisolator
• Electrical Isolation from the external world is
usually by means of optoisolator
• When a digital pulses passes through the LED, a
pulse of Infrared radiation is produced.
• This pulses is detected by the phototransistor and
gives rise to a voltage in that circuit.

20. Optoisolator

21. Input channel with optoisolator

22. • Common input voltage is 5V and 24V
• Output voltage is 24V and 240V
• Output are often specified as being of
– Relay type
– Transistor type
– Triac type

23. Relay type of output
•The relay type output is used for both ac and dc
switching
•Relay are slow to operate

24. Transistor type output
•The transistor type output is used for dc switching
•This give faster switching action

25. Triac type of output
•The triac type output is used for switching AC voltages
•It act as a TWO WAY SILICON CONTROLLED
RECTIFIER

26. INPUT OUTPUT PROCESSING
• Processing of Input and output through PLC
programming
• Basic programming is LADDER PROGRAMMING
• PROGRAMMING SEQUENCE:
– Scan inputs with one rung of the ladder program
– Solve logic operations
– Set or reset the output of the rung
– Move on the next rung repeat operations, Step 1, Step2,
step 3.

27. TWO METHODS OF INPUT
OUTPUT PROCESSING
• CONTINEOUS UPDATING.
• MASS INPUT/OUTPUT COPYING.

28. PROGRAMMING
• The programming of PLC is based on the ladder
diagram.
• Ladder diagram involve writing a program in a
similar manner to drawing a switching circuit.
• Each rung must start with an input, or serious
of input and end with an output.

32. Switch controlling Solenoid
e.g., solenoid valve
open to allow water
to enter a vessel
Ladder Diagram for
Switch Control

33. Temperature Control System

34. Logic functions
An AND System
An OR System
Works with Two inputs and single output
A and B both are closed gives AND logic operations
Many inputs provides one output.

35. NOT System
NOR System
One input and one output.
OR function is inverted by NOT function

36. NAND System
XOR System
NAND function is inverted by NOT function
OR functions , with no input there is no output,

37. Sequencing
Cylinder Sequencing
A+, B+, A- and B-

38. Latching
• Ladder Latch circuit hold an output energised.
• Latch circuit used for the circuit carry out the
operation
• Coil maintain its status until different
condition occur

40. MNEMONICS
• A programmer can enter the program in to
PLC using Keyboard
• With the graphic symbols and ladder
elements
• Computer translates symbol in to machine
language
• Entering program that translates a ladder
program in to mnemonics

41. List of Mnemonics used for the
Mitsubishi f Series PLC

42. Mnemonics for Logic system

43. Mnemonics for Logic system

44. Timers, Internal relay and counters
• Task required for Series and Parallel
connections of input contacts.
• Tasks can involve Time delays and event counting
• EXAMPLE:
– For MITSUBISHI F Series, numbers are:
– Timers Delays on 0.1-999s
– Internal relays – 8 points
– Counters – 1 to 999
– POINT means there are timer circuit in system
– DELAY-ON Timer waits for a delay period.

45. TIMERS
• Timer circuit is specified by starting the
interval
• Timed events to be Start/ Stop
• Normally coils are energized result open or
closing the circuit

46. Timer

47. Timer circuit programmed to cause an output to
go ON for 0.5s, then OFF for 0.5s, then OFF for
0.5s and so on
ON-OFF
cycle timer

48. INTERNAL RELAYS (MARKER)
• RELAYS are able to Switch ON or OFF for
internal purpose
• It is used for multiple input connections
• Implementing switching connections
• It is used for starting the multiple outputs

49. Internal relay

50. COUNTER
• Counter are used to count a specific number of
contact operations
• EXAMPLE :
– Conveyor in the boxes
– Fruits filling

51. Counter

52. Master control relay
•Due to some internal relays, Simultaneously ON or
OFF the system totally shut down
•The alternate way to achieving without affecting the
RUNGS by using master ralays

53. JUMP Instruction
•Program A
encountered with
INPUT 1
CONDITIONAL
RELAY JUMP
(CJP)
•It occurs the
program jumps at
END OF JUMP (EJP)

54. Data handling
• Data movement
• Data comparison
• Arithmetic operation
• Code conversion

55. Data Movement

56. DATA COMPARISON

57. ARITHMETIC OPERATIONS

58. CODE CONVERSION

59. ANALOG INPUT/OUTPUT
Controlling the speed of motor:

60. Selection of PLC
• System definition
• Choosing the I/O hardware
• I/O timing consideration
• Analog I/O module –resolution, voltage level
• Conversion speed
• Analog closed control
• Communication
• Counter, encoders and positioning
• Selecting suppliers
• Choosing a correct processor