3. It uses a programmable memory to
store instructions that include
1. on/off control,
2. timing,
3. counting,
4. sequencing,
5. arithmetic and
6. data handling.
3
4. The PLC is an assembly to
solid state digital logic elements
designed to make
logical decisions and
provide outputs.
4
5. PLCs are used for the
control and operation of
1.manufacturing processes,
2.equipment and
3.machinery.
5
6. The PLC is, then, basically a
computer
designed for use in machine
control.
6
7. PLC has been designed to
operate in the industrial
environment and is
*equipped with special
input/output interface and a
*control programming
language.
7
8. The common abbreviation
used in industry for these
devices, PC,
can be confusing because it is
also the abbreviation for
‘Personal Computer’.
8
9. Therefore, most of the
manufacturers refer to their
Programmable Controller (PC)
as a
Programmable Logic Controller
(PLC).
9
10. 1. Flexibility: Relays have to be
hardwired to perform a specific
function. When the system
requirement change, the relay
wiring has to be modified or
changed. In extreme cases,
complete control panels had to be
replaced.
10
11. The PLC has eliminated much
of the hardwiring associated
with conventional relay control
circuit.
It is easier to create and change
a program in a PLC than to wire
and rewire a circuit.
11
12. 2. Increased reliability: Since
all the logic is contained in the
PLCs memory there is no
chance to making a logic wiring
error.
PLCs also offer the reliability
associated with solid state
components.
12
14. 4. Lower cost
Generally, if an application has
more than about a half dozen
control relays, it will probably
be less expensive to install a
PLC.
14
15. 5. Communication capability:
A PLC can communicate with other
controllers or computer equipment to
perform such functions as
1. supervisory control,
2. data gathering,
3. monitoring devices and process
parameters, and
4.download and upload of program.
15
16. 6. Faster response time:
PLCs are designed for
high speed and
real time applications.
16
17. Machines that process
thousands of items per second and
objects that spend only a fraction
of a second in front of a sensor
require the PLCs
quick response capability.
17
18. 7. Easier to troubleshoot:
PLCs have resident diagnostics and
over ride functions
that allow users to
easily trace and correct
software and hardware problems.
18
19. To find and fix problems, user can
display the control programs on a
monitor and watch it
in real time as it executes.
19
21. These components are
Central Processing Unit (CPU),
Input/ Output (I/O) section,
Power Supply and
Programming Devices.
21
22. 1. I/O section: There are two
ways in which I/O is
incorporated into the PLC,
a) Fixed,
b) Modular.
22
23. Fixed I/O is typical of small PLCs
that come in one package with no
separate, removable units.
23
24. The processor and I/Os are
packaged together and the I/O
terminals are available but
cannot be changed.
24
25. The main advantage if this type
of packaging is lower cost.
The number of available I/O
points varies and usually can be
expanded by buying additional
units of fixed I/O.
25
26. Disadvantage of fixed I/O is its
lack of flexibility.
For some models, if any parts of
units fail, the whole unit has to be
replaced.
26
27. Modular I/O is divided by
compartments into which separate
modules can be plugged.
27
28. This feature greatly increases
options and units flexibility.
The basic modular controller
consists of a
1.rack, 2. power supply,
3. processor module (CPU),
4. input/ output (I/O) modules &
5.operator interface.
28
29. When a module is
slide into the rack, it makes an
electrical connection with a series
of contacts called backplane
(similar to mother board of PC),
located at the rear of the rack.
29
30. The PLC processor is also
connected to the backplane and
can communicate with all the
modules in the rack.
30
31. 2. Power supply
supplies dc power to
others modules that
plug into rack. For
large PLC system, this
power supply doesn’t
normally supply
power to the field
devices.
31
32. For small and micro PLC
systems, the power supply is
used to power field devices.
32
34. A typical processor usually consists
of a Microprocessor for
implementing the
logic and controlling the
communications
among the modules.
34
35. The processor requires memory
for storing the results of the
logical operations performed by
the microprocessor.
Memory is also required for the
program EPROM or EEPROM plus
RAM.
35
36. The CPU is
designed so
that the user
can enter the
desired circuit
in relay
ladder logic.
36
37. 4. Interface device: The I/O
device itself act as interface
device.
To electrically isolate the internal
components from the input and
output terminals, PLC employ an
optical isolator, which use light
to couple the circuit together.
37
38. Interface devices are also referred
to as ‘field’ or ‘real world’ input
and outputs.
It is actual external devices that
must be physically wired from the
internal user program that
duplicates the functions of relays,
timers and counters.
38
39. 5. Programming devices or
terminals is used to enter the
desired program into the memory
of the processor.
39
40. The program is entered using
relay ladder logic, which is the
most popular programming
language used by all major
manufacturers of PLCs.
40
41. 5a. Hand held programming
devices are some time used to
program small PLCs
41
42. because they are inexpensive
and easy to use and easy to
carry. Once plugged into the
PLC, they can be used to enter
and monitor programs.
42
43. 5b. Compact hand held units
are frequently used on the factory
floor for trouble shooting
equipments, modifying programs
and transferring programs to
multiple machines.
43
44. 5c. A Personal Computer (PC) is
the most commonly used
programming device.
44
45. All leading brands of PLCs have
software available so that a PC can
be used as a programming device.
This software allows used to
create, edit, document, store and
troubleshoot ladder logic
programs and to generate printed
reports.
45
46. The computer monitor is able to
display more logic on screen than
hand held types are, thus
simplifying the interpretation of
the program.
The PC communicates with PLC
processor via serial or parallel data
communication link.
46
47. If the programming unit is not in
use, it may be unplugged and
removed.
Removing the programming unit
will not affect the operation of the
user program.
47
48. Additional optional PLC
components are often available,
including
Communication adapter for
remote I/O, so that a central
controller can be connected to
remote sensors and actuators.
48
49. Network interfaces to allow
interconnecting of PLCs and/ or
other controllers into
Distributed Control System
(DCS).
49
50. END OF PLC INTRODUCTION
NEXT TOPICS
PLC HARDWARE DETAILS
50
51.
52. The input and output interface
modules provide the
equivalent eyes, ears and
tongue to the brain of a PLC,
the CPU
52
53. The I/O section consists of an I/O
rack and individual I/O modules.
Input interface modules accept
signal from machine or process
devices and convert into signals
that can be used by the controller.
53
54. Output interface modules convert
controller signals into external
signals used to control the
machine or process.
54
55. The I/O system provides an
interface between the hardwired
components in the field and the
CPU.
The input interface allows status
information and allows the CPU
to communicate through the
output interface to the process
devices under its control.
55
56. Benefit of a PLC system is the
ability to locate the I/O modules
near the field devices to minimize
the amount of wiring required.
56
57. This rack is referred to as remote
rack when it is located away from
the processor module.
To communicate with the
processor, the remote rack used a
special communications network.
57
58. If fiber optic (OFC) cable is used
between the CPU and I/O rack, it
is possible to operate I/O points
from distances greater than 20
miles with no voltage drop.
Coaxial cable will allow remote
I/O to be installed at distance
greater than 2 miles
58
59. Fiber optic cable will not pick up
noise caused by adjacent high
power lines or equipment
normally found in an industrial
environment.
Coaxial cable is more susceptible
to this type of noise.
59
60. IDENTIFICATION
Each input and output device
must have a specific address. This
address is used by processor to
identify where the device is
located to monitor or control it.
Lights are also added to each
module to indicate the ON or OFF
status of each I/O circuit.
60
61. The most common type of I/O
interface module is the discrete type.
This type of interface connects field
input devices of the ON/OFF nature
(i.e. digital) such as selector switches,
pushbutton and limit switches.
61
62. Figure shows block diagrams for
one input of a typical ac discrete
input module.
62
63. The input circuit is composed of
two basic sections, the power
section and logic section. The
power and logic sections are
normally coupled together with a
circuit that electrically separates
the two.
This is done by OPTO COUPLER
63
64. A simplified schematic diagram for
one input of a typical ac input module
is shown in figure
64
65. Output control is limited to
devices such as lights, small
motors, solenoids and motor
starters that require simple
ON/Off switches.
65
66. Figure shows block diagrams for
one output of a typical discrete
output module.
66
67. Like the input module, it is
composed of two basic sections,
the power section and the logic
section, coupled by an isolation
circuit. The output interface can
be thought of as a simple
electronic switch to which power
is applied to control the output
device.
67
68. A simplified schematic and wiring
diagram for one output of a
typical ac output module is shown
here.
68
69. Individual ac outputs are usually
limited by size of the triac to 1A or
2A.
For controlling larger loads e.g.
large motor, a standard control
relay is connected to the output
module.
69
70. When a control relay is used in
this manner, it is called an
interposing relay.
70
71. Discrete devices are input and outputs
that have only two states, on & off.
Analog devices are inputs and outputs
that can have an infinite number of
states. These devices send/ receive
complex signals to/ from a PLC.
71
72. Analog input interface modules
contain the circuitry necessary to
accept analog voltage or current
signals from analog field devices.
These inputs are converted from
an analog to digital converter
(ADC) circuit.
72
73. There are two basic type of analog
input modules available, current
sensing and voltage sensing.
Voltage input modules are
available in two types, unipolar
and bipolar.
73
74. Unipolar modules can accept only
one polarity for input.
For example, if the application
requires the card to measure 0-10
V (2-10V), a unidirectional card
would be used.
74
75. The bipolar card will accept input
of positive and negative polarity.
For example, if an application
produces a voltage between -10 V
and +10 V, a bidirectional input
card would be used because the
measured voltage could be
negative or positive.
75
76. Current input modules are
normally designed to measure
current in the 4 mA to 20 mA
range.
76
77. The analog output interface module
receives from the processor digital
data, which are converted into a
proportional voltage or current to
control an analog field device.
The digital data is passed through a
digital to analog converter (DAC)
circuit to produce the necessary
analog form.
77
78. The CPU houses the
Processor memory module(s),
Communication circuitry and
Power supply.
Central processing unit
architectures may differ from one
manufacturer to another, but in
general most of them follow
standard organization.
78
81. The CPU contains the same type
of microprocessor found in a
personal computer.
The difference is that the program
used with the microprocessor is
designed to facilitate industrial
control rather than provide
general purpose computing.
81
82. The CPU of a PLC system may
contain more than one
microprocessor.
The advantage of using
multiprocessing is that control
and communication tasks can be
divided up, and the overall
operating speed is improved.
82
83. The processor is the brain of
programmable logic controller. It is
the decision maker and controls the
operation of the machine.
The processor controls the output
devices based on the status of the
input devices and the program
entered into the PLC memory.
83
84. The size and number of
microprocessor used in the processor
depends upon the number of inputs to
be monitored and number of outputs
to be controlled.
Larger number of input and outputs,
more complicated the control
functions to be performed, the more
powerful the microprocessor required.
84
85. Various functions of microprocessor are:
Monitor the status (on/off) of the
input devices.
Solves the logic of the user program
stored in the memory.
Control the output devices (on/off)
based on the status of the input
devices and other program.
85
86. Communications with the other
devices like hand held
programmer, personal computers
etc and manages memory and
update timers, counters and
internal registers (house keeping)
86
87. Processor SCAN
When the microprocessor executes
the above task in sequence given
above it is called processor SCAN.
When the PLC is powered up, the
processor runs an internal self
diagnostic or self check, prior to
entering the mode; it was left in,
when power was last shutdown.
87
88. During the diagnostic test, it is
detected that any part of
processor not functioning such as
faulty memory, improper
communication with I/O section
etc. then a message displayed on
the display window of
programming terminal.
88
89. The scan time can vary from a
fraction of a millisecond to 100+
milliseconds depending upon the
size of program.
89
90. The processor performs other
functions such as timing, counting,
latching, comparing and
complicated math beyond the basic
functions of addition, subtraction,
multiplication and division.
In addition to their control
functions, PLCs can be networked
to do supervisory control and data
acquisition (SCADA).
90
91. Memory is a physical space inside
the CPU where the program files
and data files are stored and
manipulated.
Data is typically stored in a file by
address. The information stored
in the memory relates to how the
input and output data should be
processed.
91
92. The program is stored in a
memory as 1s and 0s, which are
typically assembled in the form of
16 bit words.
92
93. Memory sizes are commonly
expressed in thousands of words
that can be stored in the system,
thus 2K is a memory of 2000 words,
and 64K is a memory of 64,000
words. The memory size varies
from as small as 1K for small
systems to 2000K (2M) for very
large systems.
93
94. Although there are many types,
memory can be placed into two
general categories volatile and non
volatile.
Volatile memory will lose its
stored information if all operating
power is lost or removed.
94
95. Non volatile memory has the
ability to retain stored
information when power is
removed accidentally or
intentionally. Although non
volatile memory generally is
unalterable, there are special
types in which the stored
information can be changed.
95
96. Memory types
Read only memory (ROM)
Random access memory (RAM)
Programmable read only memory
(PROM)
Erasable programmable read only
memory (EPROM)
Electrically erasable programmable
Read Only Memory (EEPROM)
96
97. The programming device provides
the primary means by which the
user can communicate with the
circuits of the controller. The
programming device is used to
input the desired instructions.
These instructions determine
what the PLC will do for a specific
input.
97
98. The simplest type of proprietary
programming unit is the hand
held programmer. A hand held
programming device has a
connecting cable so that it can be
plugged into a PLC’s programming
port. Hand held programmers are
compact, inexpensive and easy to
use.
98
100. There are two types of handheld
programmers, 1. dumb terminal
and 2. smart terminal.
A dumb terminal or programmer
has no built in intelligence or
memory of its own. It must be
physically attached to a PLC in
order to be used to program, edit
or monitor a program.
100
101. Most newer models of hand held
programming terminals are the
smart type. A smart hand held
programming terminal has its
own onboard microprocessor,
which allows it operate
independently from the PLC.
Sometimes called a stand-alone
terminal.
101
102. A smart programming terminal
can be used to develop programs
offline without being connected
to a PLC, a communication link is
being created that can be used to
download the program from the
smart terminal to the CPU of the
controller.
102
103. Currently, the most popular
method of PLC programming is to
use a personal computer in a DOS
or windows environment to run
the manufacturer’s software for a
specific PLC.
Some of the advantages of using a
personal computer for
programming are as follows:
103
104. 1. Large amount of logic can be
displayed on the monitor, which
simplifies the interpretation of the
program. Scrolling through a
program rung by rung is easily
accomplished by pressing the up
or down arrow key.
104
105. 2. A color monitor can highlight
the circuit elements in different
colors to indicate status.
3. More than one program can be
stored on the computer’s hard
drive.
105
106. 4. The computer can be used to
document the PLC program. This
documentation may be in the form of
labels for each element or comments
that may be useful for
troubleshooting and maintenance.
5. PC software provides cut-and-paste
features for program development
and editing.
106
107. 6. A PC allows easy monitoring of
data tables.
7. Copies of the program can be
made easily of CD-ROM or hard
disc.
107
108. 8. The added graphics capabilities
of some software packages allows
for the development of flow
diagrams of the controlled
equipment.
9. A laptop or notebook PC is
small and portable.
108
109. The PLC’s original purpose
was the replacement of
electromagnetic relay with a
solid state switching system
that could be programmed.
110. designed to replace the
physically small control relays
that make logic decisions but
are
not designed to handle heavy
current or high voltage.
110
111. An understanding of
electromagnetic relay operation
and terminology is important
for correctly converting relay
schematic diagrams to LADDER
LOGIC PROGRAMs
(DIAGRAM)
111
112. We consider a basic hard wired
circuit of a DOL STARTER
Here a seal-in-contact is referred as
HOLD ON CONTACT
112
113. When above circuit is programmed
in PLC the circuit become as below
113
114. This is called
LADDER LOGIC DIAGRAM
Here inputs and outputs shown are not
the part of Ladder Logic Diagram. This
is used to indicate the physical I/O’s
from and to the actual controlling field.
114
115. The best approach to developing a PLC
program from a relay schematic is to
understand first the operation of each
relay rung. As each relay ladder rung is
understood, an equivalent PLC rung
can be generated.
115
116. Most industrial processes require the
completion of several operations to
produce the required output.
Manufacturing, machining,
assembling, packaging, finishing or
transporting of products requires
precise coordination tasks.
116
117. The majority of industrial control
processors use
SEQUENTIAL CONTROL.
Sequential controls are required for
processes that demand that certain
operations be performed in specific
order.
117
119. In the filling and capping
operations, the tasks are
1. Fill bottle and
2. Press on cap.
These tasks must be performed in
proper order. Obviously we could not fill
the bottle after the cap is pressed, there,
requires Sequential control.
119
121. Here, the tasks are
1. Place the label 1 on bottle and
2. Place the label 2 on bottle.
The order in which the tasks are
performed does not really matter.
121
122. A simple sequential process can be
examined with reference to simple task
illustrated in following figure.
122
123. The sequential task is as follows:
1. START button is pressed
2. Table motor is started
3. Package moves to the position of
the limit switch and stops
123
124. 4. Red pilot lamp should glow
when motor stops
5. Green pilot lamp should glow
when motor starts
6. An emergency STOP button stop
the table for any emergency before
it reach to limit switch
124
126. A summary of control task for the process
could be written as follows:
1. START button is actuated; CR1 is
energized if emergency STOP button
and limit switch are not actuated.
2. Contact CR1-1 closes. It closes CR1
even if START button is released.
3. Contact CR1-2 opens, switching the
red pilot light from ON to OFF.
126
127. 4. Contact CR1-3 closes, switching
the green pilot light from OFF to
ON.
5. Contact CR1-4 closes to energized
the motor starter coil, starting the
motor and moving the package
toward the limit switch.
6. Limit switch is actuated, de-
energized relay coil CR1.
127
128. 7. Contact CR1-1 opens, opening the
hold on contact.
8. Contact CR1-2 closes, switching the
red pilot light from OFF to ON.
9. Contact CR1-3 opens, switching the
green pilot light from ON to OFF.
10. Contact CR1-4 opens, de-energizing
the motor starter coil to stop the
motor and end the sequence.
128
129. At this point it is wise to make an I/O
address chart for the circuit as
mentioned below
The address code, of course, depend on PLC model used
129
130. The four rungs of the relay schematic
of above fig can be converted to four
rungs of PLC language as illustrate
below .
In converting these rungs, the
operation of each rung must be
understood.
Here, internal relay is used to replace
control relay CR1.
130
132. To obtain the desired control logic, all
internal relay contacts are
programmed using the PLC contact
instruction that matches their
normal state (NO or NC).
Whereas, the original hardwired relay
CR1 required four different contacts,
the internal relay uses only one
contact.
132
133. Let us consider another sequential control
of three motors with a time gap.
A pictorial representation of the same is
given herewith.
133
134. Three conveyor are operating for
material handling.
Conveyor number 1 is getting material
from a feed hopper.
It is delivering same material to
intermediate conveyor number 2 for
carrying a long distance.
134
135. Conveyor number 2 feeding same
material to conveyor number 3 which
is feeding the material into blast
furnace or Stacker Reclaimer (say).
Sequence of operation should be such
that, conveyor 3 should run first.
After some time, conveyor 2 should
run.
135
136. After some time gap conveyor 1
should run to feed the material.
If reverse operation is done material
will heap on the next conveyor and
it will cause overload of the same.
136
137. On stopping sequence conveyor 1
should stop first.
2nd conveyor should stop after
clearing all the material from it
thus require some time.
And the same way 3rd conveyor
should stop after some time.
137
141. The proposed ladder logic diagram of above logic is like
below. (Here on delay timer is used).
141
142. Typical PLC ladder logic diagram for
Automatic STAR DELTA starter is as follows.
142
143. Typical PLC ladder logic diagram for
Fluid filling operation is as follows.
143
144. Several control schemes can be
performed by different PLC models.
This include
(1) ON/OFF control;
(2) Proportional (P) control;
(3) Proportional Integral (PI) control;
(4) Proportional Integral Derivative
(PID) control.
144
145. Each involves the use of some form
of closed loop control to maintain
a process characteristic such as
Temperature
Pressure
Flow or level at desired value.
A typical block diagram of close
loop control system is shown
below.
145
147. This measurement is then compared
to a reference point or set point.
If a difference (error) exists between
the actual and desired levels, the
PLC control program will take
necessary corrective action.
Adjustment are made continuously
by PLC until the difference
between the desired and actual
output is as small as practical.
147
148. An Illustration on ‘ON/OFF’ control (or two position
control) is explained below.
148
149. Above figure shows a system using ON/OFF control in
which a liquid is heated by steam.
If the liquid temperature goes below the set point, the
steam valve opens and the steam is turned ON.
When the liquid temperature goes above the set point,
the steam valve closes and the steam is shut OFF.
The ON/OFF cycle will continue as long as the system
is operating.
149
150. Figure below illustrates the control response for an
ON/OFF temperature controller.
150
151. The output turns ON when the temperature falls
below the set point and turns OFF when the
temperature reaches the set point.
Control is simple but overshoot and cycling
about the set point can be disadvantages in some
processes.
The measured variable will oscillate around the
set point at an amplitude and frequency that
depend on the capacity and time response of the
process.
To avoid unnecessary oscillation, a dead band is
usually established around the set point.
151
152. ON/OFF control is usually used in the following
circumstances:-
1. When precise control is not necessary.
2. In systems that cannot handle the energy being
turned ON and OFF frequently.
3. When the mass of the system is so great that
temperature change extremely slowly.
4. For alarm system.
152
153. Thus, ON/OFF control (also known as
two position or bang bang control) is
simple and inexpensive, it is not
accurate enough for many process and
machine control applications.
The controller will never keep the final
control element in an intermediate
position.
153
154. Proportional controls (P) are designed to
eliminate the hunting or cycling, associate with
ON/OFF control.
They allow the final control element to take
intermediate positions between ON & OFF.
This permits analog control of the final control
element to vary the amount of energy to the
process, depending on how much the value of
measured variable has shifted from desired
value.
154
155. A proportional controller allows tighter control of the
process variable because its output can take on any
value between fully ON and fully OFF, depending on
the magnitude of the error signal.
Figure below shows an example of motor driven
analog proportional control valve used as a final
control element.
Typically, the actuator receives an input current
between 4 mA and 20 mA from controller.
It provides linear movement of the valve.
155
157. The Proportional plus Integral (PI)
control combines the characteristics
of both analog as well as digital type
of control.
A step change in measurement
causes the controller to respond
proportionally, followed by the
integral response, which is added to
the proportion response.
157
158. Because the integral mode determines
the output changes as a function of
time, the more integral action found in
the control, the faster the output
changes.
Proportional-Integral-Derivative (PID)
control is the most sophisticated and
widely used type of process control.
PID operations are more complex and
are mathematically based as shown fig
below.
158
159. PID controllers produce outputs that
depend on the magnitude (P),
duration (I) and rate of change of the
system error signal (D).
159
160. Sudden system disturbances are met
with an aggressive attempt to
correct the condition.
A PID controller can reduce the
system error to zero faster than any
other controller.
Because it has an integrator and
differentiator, however, this
controller must be custom tuned to
each process control.
160
161. A common PID equation for
obtaining PID control is
Co = K(E) + 1/Ti Edt + KD[E-E(n-
1)]/dt + bias,
where, Co = control output,
K = controller gain,
1/Ti = reset gain constant,
KD = rate gain constant,
E = error,
E(n-1) = error from test sample.
161
