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PLCAn Introduction
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Basic Component of PLC
i. The PLC Processor & controller
ii. I/O module
iii. Chases or backplane
iv. Power supplyv. Programming Software
In addition to these 5 components, most PLCs also have
Network Interface.
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Processor, Controller or CPU
Stores the control program and data in its memory
Reads the status of connected input devices
Executes the control program
Commands connected outputs to change state based on programexecution
For example: Turn a light on, start a fan, adjust a
speed, or temperature
Comes in various physical forms.
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I/O Modules
Physically connect to field devices
Input modules convert electrical signals coming in from input field
devices such as pushbuttons, to electrical signals that the PLC can
understand.
Output modules take information coming from the PLC and convert it
to electrical signals the output field devices can understand.
For example: a motor starter, or a hydraulic solenoid valve.
I/O comes in various forms
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Input Modules
Input modules interface directly to devices such as switches and
temperature sensors.
Input modules convert many different types of electrical signals such
as 120VAC, 24VDC, or 4-20mA, to signals which the controller can
understand.
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Input Modules
Input modules convert real world voltage and currents to signals the
PLC can understand. Since there are different types of input devices,
there is a wide variety of input modules available, including both
digital and analog modules.
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Discrete vs. Analog Modules
Discrete modules use only a single bit to represent the state of the
device. For example, a switch is either open or closed. Therefore, the
bit is either a 0 (switch is open) or a 1 (switch is closed). Discrete
modules are also known as Digital modules.
Analog modules use words to represent the state of a device. An
analog signal represents a value.. For example, the temperature could
be 5, 9, 20, 100, etc degrees. Analog modules use a value, such as 52,
rather than a 0 or 1 to represent the state of the device.
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Output Module
Output modules interface directly to devices such as motor starters
and lights
Output modules take digital signals from the PLC and convert them to
electrical signals such as 24VDC and 4 mA that field devices can
understand
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Output Module
Output modules take a signal from a PLC and convert it to a signal
that a field device needs to operate. Since there are different types of
output devices, there is a wide variety of output cards available,
including both digital and analog cards.
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Chassis/Backplane
All PLCs need some method of communicating between the controller,
I/O and communications modules. Here are three ways used to
accomplish this communications between the various components that
make up the PLC system.
I. Modules are installed in the same chassis as the PLC and
communicate over the chassis backplane.
II. Modules are designed to pluginto each other. The interconnecting
plugs form a backplane. There is no chassis
III. Modules are built into the PLC. The modules come together in one
physical block. The backplane in this case is transparent to the user.
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Chassis/Backplane
Below is an example of a backplane in a chassis based system. You
can see the backplane in the area where the modules are not inserted.
The modules have connectors that plug into the black connectors on
the backplane.
All of the connectors on the backplane are connected together
electrically.
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Chassis/Backplane (example)
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Backplane & Chassis
Great flexibility in choice of modules.
Modules can be easily installed orremoved without affecting other modules
Great flexibility in choice of modules.
In some cases modules cannot be
removed without breaking the chain
and affecting all modules downstream.
No chassis cost.
Low cost solution but limited flexibility.
Generally used in smaller, simpler
systems.
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Power Supply
A power supply is needed to provide power to the PLC and any other
modules. Power supplies come in various forms:
Power supply modules that fit into one of the slots in a chassisExternal power supplies that mount to the outside of a chassis
Stand alone power supplies that connect to the PLC or I/O through a
power cable
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Programming Software
Software that runs on a PC is required to configure and program PLCs.
Different products may require different programming software.
Software allows programs to be written in several different languages
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Network Interface
Most PLCs have the ability to communicate with other devices.
These devices include computers running programming software, or
collecting data about the manufacturing process, a terminal that lets an
operator enter commands into the PLC, or I/O that is located in a
remote location from the PLC.
The PLC will communicate to the other devices through a network
interface.
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PLC Programming
Every PLC has associated programming software that allows the user to
enter a program into the PLC.
Software used today is Windows based, and can be run on any PC.
Different products may require different software: PLC5, SLC, andControl-Logix each require their own programming software.
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Example
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Example
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Linking PETRA to ML1500
Petra Output Symbol PLC Input Wire Color Comment
Hole detection sensor 3 of 6. H3 I0 Black Aligned to detect a hole. ONif materialis in b/w the sensors.
Hole detection sensor 6 of 6. H6 I1 Brown Aligned to detect a hole. ONif material
is in b/w the sensors.
Cut out length Detector 1 L1 I2 Red Aligned to detect cut out. ON if
material is in b/w the sensors.
Cut out length Detector 2 L2 I3 Orange ONif Material Below the sensor.
Part Thickness Sensor T I4 Yellow ON if Component is of correct
thickness.
Slot sensor S I5 Green Aligned to detect slot. ON if material
below the sensor.
Carriage Position Status CS I6 Blue ONif the carriage is not at the pick and
pace position. Determined by CPL &
CPH
Arm Position AP I7 Purple OFFarm reaches belt 2. stays ON until
arm return to belt 1.
Dispenser Empty DE I8 Gray ONwhen there is no component in thedispenser.
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Connection b/w PETRA control I/P and PLC O/P
Petra Input Symbol PLC Output Wire Color Comment
Hole detection sensor 3 of 6. PV O4 Brown When energized, It applies vacuum to the
plunger pad.
Hole detection sensor 6 of 6. PA O5 Red Move the Plunger down when energized.
Carriage Position Low CPL O6 Orange CPL & CPH determined Position of
carriage.
Carriage Position high CPH O7 Yellow 00 = Dispenser 01 = Belt 1
10= Reject Bin 11 = Belt 2
Conveyor Belt 1 C1 O8 Green Belt 1 moves when energized.
Conveyor Belt 2 C2 O9 Blue Belt 2 moves when energized.
Arm Activate AA O10 Purple
Arm moves from belt 1 to belt 2 when
energized.
Gripper Activate GA O2 White Gripper hold the component whenenergized.
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Basic Instruction
Ladder logic Inputs XIC- Examine if closed
XIO- Examine if open
Ladder logic Outputs OTE - Output Energized
OTLOutput Latch
OUTOutput Unlatch
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LADDER LOGIC I/P
XIC / Normally Open:Symbol:
Definition:
Examine a bit for ON condition. Use XIC bit in your ladder logic to determine if a bit is ON
0 = False
1 = ON
Devices: Start / Stop Push Buttons
Limit Switches Selectors
Proximity Switches
Sensors
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LADDER LOGIC I/P
XIO/ Normally Closed:Symbol:
Definition: Examine a bit for an OFF condition.
Use an XIO instruction in your ladder logic to determine if a bit isOFF.
1 = True
0 = FalseDevice:
Start/ Stop push button.
Selectors
Limit Switches
etc.
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LADDER LOGIC O/P
Output Energize (OTE):Symbol:
Definition: Turns a bit ONN or OFF
Use OTE instructions in Ladder logic when rung condition is
evaluated as True.
Devices: Light
Motor
Internal bits
Actuators
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LADDER LOGIC O/P
OUTPUT Latch (OTL)Symbol:
Definition:
Turns a bit on when a rung is executed, & a bit is retain its stateWhen the rung is not executed.
Once an OTL bit has been set "on" (1 in the memory) it will
remain "on" even if the rung condition goes false. The bit must
be reset with an OTU instruction.
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LADDER LOGIC O/P
OUTPUT Unlatch (OTU)Symbol:
Definition: Turns a bit OFF when the rung is executed.
Use this output instruction to unlatch (reset) a latched (set) bit
which was set by an OTL instruction. The OTU address must
be identical to the OTL address which originally set the bit.
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Example
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Example
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The Slot Sensor
Detects whether there is a slot in a component or not.
Principle:
Optical-reflection wherein it gives a 1 whenever light
reflects back from a smooth surface into the sensor (i.e.when the solid part of the component is beneath the sensor)
and gives a 0 when light does not reflect back to the
sensor (i.e. when the belt or any non-reflective material is
beneath the sensor).
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LAB TASK
1- Create a program that does the following:
When Dispenser is empty, Belt 2 in PETRA should run.
When Dispenser is not empty, Belt 1 in PETRA should run.
Save your program. We need a mechanism to stop bothbelts if required. We will employ
the Slot sensor for this.
When there is a part beneath the Slot sensor it comes ON
Use Table 3.1 and 3.2 for the addresses of inputs and outputs.
2-Amend the program with the following condition:
Neither belt should be driven when the slot sensor is detecting
material beneath the sensor.
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TIMERS
Three basic types of Timers are.
Timer ON Delay
Timer OFF Delay
Retentive Timer ON
Retentive Timer OFF
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Timer ON Delay
This types of timers simply delay turning on.i.e. After our sensor (I/P) turns on we wait X-sec before activating
a solenoid valve (O/P)
Symbol:
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Timer ON Delay
It will count the time base interval when the instruction is true. The Timer on delay instruction begins to count time base interval when the rung
condition remains true.
The Timer adjust its accumulator value until it reaches its preset value.
Time = Preset * Time base
The accumulator value goes reset when rung condition got false. Timer on delay is consist of 3 words element.
Control word
Bit 0-12: Internal use
Bit 13: DN (true when ACCUMULATOR >= Preset Value )
Bit 14: TT (This bit is on when the timer is timing)
Bit 15: EN (On when TT is energized)
Store the Preset value
Usually this bit is from 0-32767
It cant be negative
Accumulator
This is the time elapsed since the timer was last reset.
TIME base is the timing update interval vary from 0-1 sec
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Timer ON Delay
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TIMER ON DELAY (Example-1)
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Timer ON Delay (Example-2)
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Timer OFF Delay
Counts time base intervals when the instruction is
false.
TOF instruction begins to count time base intervals
when a rung makes true to false instruction.
As long as the condition will remain false , the timer
increments its accumulator value .
The accumulator value reset when the rung
condition goes true regardless of weather the timer
has timed out.
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Timer OFF Delay
Symbol:
Timer address:
Control Address
Bit: 0-12 (internal use)Bit: 13 DN (Done)
Bit: 14 TT (Timer Timing)
Bit: 15 EN
Store the Preset Value
When accumulated value become =/> the presetvalue the done bit is set.
Preset value is from 0-32767
If the Timer preset value isev an error will occur.
Accumulated value
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Timer OFF Delay
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Timer OFF Delay
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Recursive Timer
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Counters
Types of counters:1. Count up (CTU)
2. Count Down (CTD)
Count up: (CTU)
It increments the accumulated value at each false to true transition
and retain the accumulator value when the instruction goes false.
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Count up: (CTU)
CTU is an instruction that counts false to true transition.
The transition causes the accumulated value to increment by
one.
It is reset by the RES instruction.
If the accumulation value is equal to Preset value then
DN = 1
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Count up (CTU)
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Count Down
Decrements the accumulator value at each false to true
transition and retain the accumulated value when instruction
goes false.
CTD is an instruction that counts false to true transition.
Transition causes the accumulated value by one count.
CTD is reset by the RES instruction.
If accumulator value is below the Preset value the DN bit gets
low.
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Count Down (CTD)
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