plc
TRANSCRIPT
ABSTRACT
A programmable logic controller (PLC) or programmable controller is a
digital computer used for automation of electromechanical processes, such
as control of machinery on factory assembly lines, amusement rides, or
lighting fixtures. PLCs are used in many industries and machines. Unlike
general-purpose computers, the PLC is designed for multiple inputs and
output arrangements, extended temperature ranges, immunity to electrical
noise, and resistance to vibration and impact. Programs to control machine
operation are typically stored in battery-backed or non-volatile memory. A
PLC is an example of a real time system since output results must be
produced in response to input conditions within a bounded time. The PLC
was invented in response to the needs of the American automotive
manufacturing industry. Programmable logic controllers were initially
adopted by the automotive industry where software revision replaced the re-
wiring of hard-wired control panels when production models
changed.Before the PLC, control, sequencing, and safety interlock logic for
manufacturing automobiles was accomplished using hundreds or thousands
of relays, cam timers, and drum sequencers and dedicated closed-loop
controllers. The process for updating such facilities for the yearly model
change-over was very time consuming and expensive, as electricians needed
to individually rewire each and every relay.
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1. INTRODUCTION
The National Electrical Manufacturers Association (NEMA) as 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 operations to control
machines and processes” currently defines a programmable Logic
Controller. The PLC was invented in response to the needs of the
American automotive manufacturing industry. Programmable logic
controllers were initially adopted by the automotive industry where
software revision replaced the re-wiring of hard-wired control panels
when production models changed.Before the PLC, control, sequencing,
and safety interlock logic for manufacturing automobiles was
accomplished using hundreds or thousands of relays, cam timers, and
drum sequencers and dedicated closed-loop controllers. The process for
updating such facilities for the yearly model change-over was very time
consuming and expensive, as electricians needed to individually rewire
each and every relay.In 1968 GM Hydramatic (the automatic
transmission division of General Motors) issued a request for proposal
for an electronic replacement for hard-wired relay systems. The
winning proposal came from Bedford Associates of Bedford,
Massachusetts. The first PLC, designated the 084 because it was
Bedford Associates' eighty-fourth project, was the result. Bedford
Associates started a new company dedicated to developing,
manufacturing, selling, and servicing this new product: Modicon, which
stood for Modular Digital Controller. One of the people who worked on
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that project was Dick Morley, who is considered to be the "father" of
the PLC. The Modicon brand was sold in 1977 to Gould Electronics
and later acquired by German Company AEG and then by French
Schneider Electric, the current owner. One of the very first 084 models
built is now on display at Modicon's headquarters in North Andover,
Massachusetts. It was presented to Modicon by GM, when the unit was
retired after nearly twenty years of uninterrupted service. Modicon used
the 84 moniker at the end of its product range until the 984 made its
appearance.
1.2 DEVELOPMENT
Early PLCs were designed to replace relay logic systems. These PLCs
were programmed in "ladder logic", which strongly resembles a
schematic diagram of relay logic. This program notation was chosen to
reduce training demands for the existing technicians. Other early PLCs
used a form of instruction list programming, based on a stack-based
logic solver.
Modern PLCs can be programmed in a variety of ways, from ladder
logic to more traditional programming languages such as BASIC and C.
Another method is State Logic, a very high-level programming language
designed to program PLCs based on state transition diagrams.
Many early PLCs did not have accompanying programming terminals
that were capable of graphical representation of the logic, and so the
logic was instead represented as a series of logic expressions in some
version of Boolean format, similar to Boolean algebra. As
programming terminals evolved, it became more common for ladder
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logic to be used, for the aforementioned reasons. Newer formats such
as State Logic and Function Block (which is similar to the way logic is
depicted when using digital integrated logic circuits) exist, but they are
still not as popular as ladder logic. A primary reason for this is that
PLCs solve the logic in a predictable and repeating sequence, and
ladder logic allows the programmer (the person writing the logic) to
see any issues with the timing of the logic sequence more easily than
would be possible in other formats.
1.3 FUNCTIONALITY
The functionality of the PLC has evolved over the years to include
sequential relay control, motion control, process control, distributed
control systems and networking. The data handling, storage, processing
power and communication capabilities of some modern PLCs are
approximately equivalent to desktop computers. PLC-like programming
combined with remote I/O hardware, allow a general-purpose desktop
computer to overlap some PLCs in certain applications. Regarding the
practicality of these desktop computer based logic controllers, it is
important to note that they have not been generally accepted in heavy
industry because the desktop computers run on less stable operating
systems than do PLCs, and because the desktop computer hardware is
typically not designed to the same levels of tolerance to temperature,
humidity, vibration, and longevity as the processors used in PLCs. In
addition to the hardware limitations of desktop based logic, operating
systems such as Windows do not lend themselves to deterministic logic
execution, with the result that the logic may not always respond to
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changes in logic state or input status with the extreme consistency in
timing as is expected from PLCs. Still, such desktop logic applications
find use in less critical situations, such as laboratory automation and use
in small facilities where the application is less demanding and critical,
because they are generally much less expensive than PLCs.
In more recent years, small products called PLRs (programmable logic
relays), and also by similar names, have become more common and
accepted. These are very much like PLCs, and are used in light industry
where only a few points of I/O (i.e. a few signals coming in from the
real world and a few going out) are involved, and low cost is desired.
These small devices are typically made in a common physical size and
shape by several manufacturers, and branded by the makers of larger
PLCs to fill out their low end product range. Popular names include
PICO Controller, NANO PLC, and other names implying very small
controllers. Most of these have between 8 and 12 digital inputs, 4 and 8
digital outputs, and up to 2 analog inputs. Size is usually about 4" wide,
3" high, and 3" deep. Most such devices include a tiny postage stamp
sized LCD screen for viewing simplified ladder logic (only a very small
portion of the program being visible at a given time) and status of I/O
points, and typically these screens are accompanied by a 4-way rocker
push-button plus four more separate push-buttons, similar to the key
buttons on a VCR remote control, and used to navigate and edit the
logic. Most have a small plug for connecting via RS-232 to a personal
computer so that programmers can use simple Windows applications
for programming instead of being forced to use the tiny LCD and push-
button set for this purpose. Unlike regular PLCs that are usually
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modular and greatly expandable, the PLRs are usually not modular or
expandable, but their price can be be two orders of magnitude less than
a PLC and they still offer robust design and deterministic execution of
the logic.
2. THEORY
The main difference from other computers is that PLCs are armored for
severe conditions (such as dust, moisture, heat, cold) and have the
facility for extensive input/output (I/O) arrangements. These connect
the PLC to sensors and actuators. PLCs read limit switches, analog
process variables (such as temperature and pressure), and the positions
of complex positioning systems. Some use machine vision. On the
actuator side, PLCs operate electric motors, pneumatic or hydraulic
cylinders, magnetic relays, solenoids, or analog outputs. The
input/output arrangements may be built into a simple PLC, or the PLC
may have external I/O modules attached to a computer network that
plugs into the PLC.
2.1 TYPES OF PLC
There are two types of PLC
(a) Unitary PLCs and (b) Modular PLCs
2.1.1 Unitary PLC
A unitary PLC has a power supply, a CPU and a limited number of
inputs and outputs (20 inputs, 12 outputs, 32 I/O). It is sometime
called “Shoe-box type” and is mainly used for the control of a small
system.
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2.1.2 Modular PLC
A modular PLC is one that can be constructed using separate
modules of power supply, CPU, inputs, outputs, timers, counters,
ADC, DAC, expansion modules. These modular PLCs are sometimes
called “Rack-mounted type”. Modular PLC can be sub-divided into
the following types:
Small PLC
PLCs having less then 100 I/O are designed as a small PLCs. Out of
the I/Os, 20 inputs and 12 outputs are mounted locally with the
processor. Additional I/Os can be added through remote I/O racks to
accommodate the extra I/Os. These PLCs generally have a memory
of 2 KB to 10 KB to store the user’s logic programs.
Medium PLC
These have extended instruction sets that include mathematical
functions, file functions, PID process control etc. These PLCs can
have between 4000 to 8000 I/Os. They are also made to support wide
variety of special modules such as ASCII communication modules,
BASIC-programming modules, 16-bit multiplexing modules, analog
I/O modules and communication modules.
Large PLC
The purpose of introducing large PLCs was to provide enough user
memory space and I/O for complete factory automation. However, the
major disadvantage of these large PLCs is that the whole factory may
collapse if the PLC starts malfunctioning. The advent of Local Area
Networking (LAN) helped to introduce the concept of distributed
control, where small or medium PLCs are connected together through
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an appropriate network. The entire factory is brought under the control
of a number of PLCs, but failure in one system will not disturb any
other system.
2.2 BLOCKS OF PLC
It mainly consist of :-
CPU
I/O Module
Memory
Programmable device
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2.2.1 CPU
It understands input command instructions and status signal and
provides logic processing capability.
2.2.2 I/O Module
They are designed to interface directly with industrial equipment.
2.2.3 Memory
In a PLC the central program is stored in memory to tell itself output
commands. All I/O are updated per scan.
2.2.4 Program device
Transform the control scheme in to useful PLC logic understand by
CPU, then stored in memory. Ladder symbols are used for
programming.
CPU has following modules with their functions:
Memory module
It stores the user control program. It can be installed in rack sloat A
and B.
Register Module (RM)
It contains system I/O data table. Data table is delivered into three
areas I/O status, system status table and register tables.
Processor Module (PM)
It executes program stored in MM and handles arithmetic
computation and data movement instructions of all the modules in the
processor.
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System Control Module (SCM)
The functional extension of processor module co-orients the interaction
of all the modules in the processor control programs. Installed in rack
slot E.
I/O Control Module (IOCM)
Co-orients communication between processor and the 621 I/O systems
and formats the data flowing between I/O system and the processor. It
also monitors individual I/O modules and fault diagnostics.
Parallel Link Driver Module (PLDM)
The PLDM works with the IOCM module to control I/O
communications. The PLDM also controls system status by the mode
key switch and it determines I/O response to system fault and various
operating parameters.
Serial Link Module (SLM)
The SLM front has five LCDs indicating the status of module. Green
action light indicates that data being transmitted properly. The normal
state of the light is on during the transmission.
Communication Interface Module (CIM)
The CIM is optional in the 620-25/35 processor. They provide
interface to microcomputer or other serial device.
Highway Interface Module (HIM)
HIM produces a service facility for higher ordered device in the
system, such as computer and operator station to interface with the
IPC 620 processor and Honeywell’s TDC 3000 data highway.
Redundancy Control Module (RCM)
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The RCM is the primary component in the Honeywell IPC 620
processor redundancy system. The high availability control system
requires minimal hardware and installation effort.
Parallel I/O Module (PIOM)
The PIOM is located in the N slot in the full I/O rack or H slot of the
I/O half rack. This module acts as the interface to processor parallel
link driver module and to the PIOM. The PIOM has two 50-pin D-
type connectors. The male plug is the in port and female is the out
port. The green LEDs labeled active indicates proper communication
from preceding rack
2.3 Programming
PLC programs are typically written in a special application on a
personal computer, then downloaded by a direct-connection cable or
over a network to the PLC. The program is stored in the PLC either
in battery-backed-up RAM or some other non-volatile flash memory.
Often, a single PLC can be programmed to replace thousands of
relays. Under the IEC 61131-3 standard, PLCs can be programmed
using standards-based programming languages. A graphical
programming notation called Sequential Function Charts is available
on certain programmable controllers. Initially most PLCs utilized
Ladder Logic Diagram Programming, a model which emulated
electromechanical control panel devices (such as the contact and
coils of relays) which PLCs replaced. This model remains common
today.
Programming in the PLC can be sampled and straightforward when
approached in a systematic manner. This is achieved by:
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Draw a relay ladder schematic of the control scheme.
Assign an input address to each field input device (pressure switch,
temperature switch, etc). They are wired to input modules per their
respective address assignments.
Assign output relay coil numbers, where required for solenoid valve,
motors, indicators light alarms etc.
Then enter the program into the processor using the CT
programming panel.
Early PLCs, up to the mid-1980s, were programmed using
proprietary programming panels or special-purpose programming
terminals, which often had dedicated function keys representing the
various logical elements of PLC programs. Programs were stored on
cassette tape cartridges. Facilities for printing and documentation
were very minimal due to lack of memory capacity. The very oldest
PLCs used non-volatile magnetic core memory.
More recently, PLCs are usually programmed using special
application software written for use on desktop computers, and
connecting between the desktop computer and the PLC such as via
Ethernet or RS-232 cabling. Such software allows entry and editing
of the ladder style logic, and then may provide additional
functionality to assist debugging and troubleshooting the software,
for example by highlights portions of the logic to show current status
during operation or via simulation. Finally, the software may allow
uploading and downloading of the program between the computer
and the PLC, for backup and restoration purposes. Alternately,
specific devices known as programming boards are used to hard wire
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the logic into the controller by the use of a removable chip, such as
an EEPROM, where the program is transferred to the programming
board from the workstation via serial or other bus logic.
2.4 Communication
PLCs have built in communications ports, usually 9-pin RS-232, but
optionally EIA-485 or Ethernet. Modbus, BACnet or DF1 is usually
included as one of the communications protocols. Other options
include various field buses such as Device Net or Profibus. Other
communications protocols that may be used are listed in the List of
automation protocols.Most modern PLCs can communicate over a
network to some other system, such as a computer running a SCADA
(Supervisory Control And Data Acquisition) system or web browser.
PLCs used in larger I/O systems may have peer-to-peer (P2P)
communication between processors. This allows separate parts of a
complex process to have individual control while allowing the
subsystems to co-ordinate over the communication link. These
communication links are also often used for HMI devices such as
keypads or PC-type workstations.
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Fig:-2.1
2.5 User Interface
PLCs may need to interact with people for the purpose of configuration,
alarm reporting or everyday control.
A Human-Machine Interface (HMI) is employed for this purpose. HMIs
are also referred to as MMIs (Man Machine Interface) and GUI
(Graphical User Interface).
A simple system may use buttons and lights to interact with the user.
Text displays are available as well as graphical touch screens. More
complex systems use a programming and monitoring software
installed on a computer, with the PLC connected via a communication
interface.
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2.7 PLC compared with other control systems
PLCs are well-adapted to a range of automation tasks. These are
typically industrial processes in manufacturing where the cost of
developing and maintaining the automation system is high relative to
the total cost of the automation, and where changes to the system
would be expected during its operational life. PLCs contain input and
output devices compatible with industrial pilot devices and controls;
little electrical design is required, and the design problem centers on
expressing the desired sequence of operations. PLC applications are
typically highly customized systems so the cost of a packaged PLC is
low compared to the cost of a specific custom-built controller design.
On the other hand, in the case of mass-produced goods, customized
control systems are economic due to the lower cost of the components,
which can be optimally chosen instead of a "generic" solution, and
where the non-recurring engineering charges are spread over thousands
or millions of units.
For high volume or very simple fixed automation tasks, different
techniques are used. For example, a consumer dishwasher would be
controlled by an electromechanical cam timer costing only a few dollars
in production quantities.
A microcontroller-based design would be appropriate where hundreds or
thousands of units will be produced and so the development cost (design
of power supplies, input/output hardware and necessary testing and
certification) can be spread over many sales, and where the end-user
would not need to alter the control. Automotive applications are an
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example; millions of units are built each year, and very few end-users
alter the programming of these controllers. However, some specialty
vehicles such as transit busses economically use PLCs instead of custom-
designed controls, because the volumes are low and the development cost
would be uneconomic.
Very complex process control, such as used in the chemical industry,
may require algorithms and performance beyond the capability of even
high-performance PLCs. Very high-speed or precision controls may also
require customized solutions; for example, aircraft flight controls.
Programmable controllers are widely used in motion control, positioning
control and torque control. Some manufacturers produce motion control
units to be integrated with PLC so that G-code (involving a CNC
machine) can be used to instruct machine movements.
PLCs may include logic for single-variable feedback analog control loop,
a "proportional, integral, derivative" or "PID controller." A PID loop
could be used to control the temperature of a manufacturing process, for
example. Historically PLCs were usually configured with only a few
analog control loops; where processes required hundreds or thousands of
loops, a distributed control system (DCS) would instead be used. As
PLCs have become more powerful, the boundary between DCS and PLC
applications has become less distinct.
PLCs have similar functionality as Remote Terminal Units. An RTU,
however, usually does not support control algorithms or control loops. As
hardware rapidly becomes more powerful and cheaper, RTUs, PLCs and
DCSs are increasingly beginning to overlap in responsibilities, and many
vendors sell RTUs with PLC-like features and vice versa.
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2.7. Digital and analog signals
Digital or discrete signals behave as binary switches, yielding simply an
On or Off signal (1 or 0, True or False, respectively). Push buttons,
limit switches, and photoelectric sensors are examples of devices
providing a discrete signal. Discrete signals are sent using either voltage
or current, where a specific range is designated as On and another as
Off. For example, a PLC might use 24 V DC I/O, with values above 22
V DC representing On, values below 2VDC representing Off, and
intermediate values undefined. Initially, PLCs had only discrete I/O.
Analog signals are like volume controls, with a range of values between
zero and full-scale. These are typically interpreted as integer values
(counts) by the PLC, with various ranges of accuracy depending on the
device and the number of bits available to store the data. As PLCs
typically use 16-bit signed binary processors, the integer values are
limited between -32,768 and +32,767. Pressure, temperature, flow, and
weight are often represented by analog signals. Analog signals can use
voltage or current with a magnitude proportional to the value of the
process signal. For example, an analog 4-20 mA or 0 - 10 V input
would be converted into an integer value of 0 - 32767.
Current inputs are less sensitive to electrical noise (i.e. from welders or
electric motor starts) than voltage inputs.
2.8. Example
As an example, say a facility needs to store water in a tank. The water is
drawn from the tank by another system, as needed, and our example
system must manage the water level in the tank.
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Using only digital signals, the PLC has two digital inputs from float
switches (Low Level and High Level). When the water level is above
the switch it closes a contact and passes a signal to an input. The PLC
uses a digital output to open and close the inlet valve into the tank.
When the water level drops enough so that the Low Level float switch
is off (down), the PLC will open the valve to let more water in. Once
the water level rises enough so that the High Level switch is on (up),
the PLC will shut the inlet to stop the water from overflowing. This
rung is an example of seal-in (latching) logic. The output is sealed in
until some condition breaks the circuit.
| |
| Low Level High Level Fill Valve |
|------[/]------|------[/]----------------------(OUT)---------|
| | |
| | |
| | |
| Fill Valve | |
|------[ ]-----| |
| |
| |
Fig 2.3:-Ladder diagram
An analog system might use a water pressure sensor or a load cell, and
an adjustable (throttling) dripping out of the tank, the valve adjusts to
slowly drip water back into the tank.
In this system, to avoid 'flutter' adjustments that can wear out the valve,
many PLCs incorporate "hysteresis" which essentially creates a
“deadband” of activity? A technician adjusts this deadband so the valve
moves only for a significant change in rate. This will in turn minimize
the motion of the valve, and reduce its wear.
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2.9 Different types of PLC according to Manufacturer
2.9.1Siemens
The Simatic S5 PLC is an automation system based on Programmable
Logic Controllers. It is manufactured and sold by Siemens AG. Such
automation systems control process equipment and machinery used in
manufacturing. This product line is considered obsolete, as the
manufacturer has since replaced it with their newer Simatic S7 PLC.
However, the S5 PLC still has a huge installation base in factories
around the world and a number of companies such as Direct-
Industrial.com continue to provide support for users of the S5 range
Hardware
The S5 line comes in the 90U, 95U,101U,100U,115U, 135U, and
155U chassis styles.Higher the number, the more sophisticated and
more expensive the system. Within each chassis style, several CPUs
are available, with varying speed, memory, and capabilities. Some
systems provide redundant CPU operation for ultra-high-reliability
control, as used in the pharmaceutical manufacturing, for example.
Each chassis consists of a power supply, and a backplane with slots for
the addition of various option boards. Available options include serial
and Ethernet communications, digital input and output cards, analog
signal processing boards, counter cards, and other specialized interface
and function modules.
Software
The S5 product line is usually programmed with a PC based software
programming tool called Step 5. Step 5 is used for programming,
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testing, and commissioning, and for documentation of programs for S5
PLCs.
The original Step 5 versions ran on the CPM operating system. Later
versions ran on MS-DOS, and then versions of Windows through
Windows XP. The final version of Step 5 is version 7.2. No further
development of this product line has occurred since that time, due to its
announced obsolescence.
In addition to Step5, Siemens offerred a proprietary State logic
programming package called Graph5. Graph5 is a sequential
programming language intended for use on machines that normally run
through a series of discrete steps. It simulates a State machine on the
S5 platform.
Several third-party programming environments have been released
for the S5. Most closely emulate Step5, some adding macros and
other minor enhancements, others functioning drastically differently
than Step5. One allows Step5 programs to be cross-compiled to and
from the C-programming language and BASIC.
Structured text
STEP 5 allows the creation of structured or unstructured
programming, from simple AND/OR operations up to complex
subroutines. A STEP 5 program may, therefore, contain thousands of
statements.
To maintain maximum transparency, STEP 5 offers a number of
structuring facilities:
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Block technique
A linear operation sequence is divided into sections and packed into
individual blocks.
Segments
Within blocks, fine structuring is possible by programming subtasks in
individual segments.
Comments - Both a complete program as well as individual blocks or
segments and individual statements can be directly provided with
comments. m,k
Methods of representation
STEP 5 programs can be represented in three different ways:
Statement List (STL) - The program consists of a sequence of mnemonic
codes of the commands executed one after another by the PLC.
Ladder Diagram (LAD) - Graphical representation of the automation task
with symbols of the circuit diagram
Function Block Diagram (FBD) - Graphical representation of the
automation task with symbols to DIN 40700/ DIN 40719.
Absolute or symbolic designations can be used for operands with all
three methods of representation.
In LAD and FBD complex functions and function block calls can be
entered via function keys. They are displayed on the screen as graphical
symbols.
Additional functions
Saving user-specific project settings
Symbol editor
Automatic generation and updating of cross-reference lists
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Comparison of user programs
Transferring blocks to EPROM and EEPROM memory modules for
programmable controllers
Rewiring inputs, outputs, flags, timers and counters
Testing and service functions for startup and maintenance
2.9.2 GE Fanuc
GE Fanuc's Computer Numerical Control business provides machine
tool controls and solutions, servo drives and motors, computer
numerical control productivity solutions and industrial CO2 lasers. In
Europe Fanuc GE CNC Europe is responsible for the CNC business.
Logix5000 controllers are true 32-bit controllers, meaning the
memory words are 32-bits wide. No matter what, a tag always
reserves 32 bits of memory even if it is a Boolean or integer data
type. For this reason, it is best to use a DINT when dealing with
integers. Furthermore, a Logix5000 controller typically compares or
manipulates values as 32-bit values (DINTs or REALs).
A Logix5000 controller lets you divide your application into multiple
programs, each with its own data. The Scope of the tag defines if a
tag is global (controller tags) and therefore available to all programs
or local (program tags) to a select program group. Pay careful
attention to this field as creating it in the wrong area may lead to
some confusion later on as to its location.’
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RSLogix 5000 Tips and Tricks
Everybody enjoys nifty little tips and tricks to get their work done
faster. This listing is for Allen Bradley's RSLogix 5000 software. Feel
free to add your own tips and tricks using the 'add comment' link.
General
To access Release Notes for this version of software, choose Release
Notes from the Help menu.
The Quick View Pane, located below the Controller Organizer,
provides "thumbnail" information for the selected component.
The Watch Pane, located below the language editor window, provides
monitoring for all tags referenced in the active routine window.
The Controller Organizer is dockable. That is, you can drag it to the left
or right side of the screen, or float it somewhere in between.
Hide/show the Controller Organizer via a toolbar button to make more
display area for editors.
RSLogix 5000 supports Cut/Copy/Paste/Drag/Drop of components
within the Controller Organizer as well as to other instances of
RSLogix 5000.
Double-clicking on error messages displayed in the Error Window will
navigate you to where the error was encountered.F4 and Shift-F4 can be
used to move between errors.
You can reorder the columns in the tag editor by clicking on the title
and dragging it to a new position.
To simultaneously display logic in multiple routines, select Window ->
New Window and then arrange the windows manually. Or select
Window -> Tile Horizontal.
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To remove a yellow triangle warning symbol on a device, first check
the connection status. If the status is "Connection is not scheduled", re-
open the RSNetWorx software. Return to RSLogix 5000 software and
the yellow triangle should be gone.
On one computer, you can install and simultaneously launch (run)
multiple translated versions of RSLogix 5000 software.
Once you do a partial import of rungs, add-on instructions, or user-
defined data types, you can't undo the import. If the import didn't work
as expected, close the project without saving.
When you select a partial import, make sure to select the correct rung or
trend file. Both files have L5X extensions and the software doesn't
prevent you from selecting the wrong file. If you try to import a rung
where a trend is expected, or vice versa, the software does display an
error that the import failed.
Partial import of rungs works in all ladder routines, including Add-On
Instructions.
I/O Configuration
Module icons in the I/O Configuration folder change to indicate the
module has faulted or the connection to the module has been
interrupted.
To remove a yellow triangle warning symbol, first check the connection
status. If the status is "Connection is not scheduled", re-open the
RSNetWorx software. Return to RSLogix 5000 software and the yellow
triangle should be gone.
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To easily find a module in the Select Module Type dialog, simply start
typing any part of the module name or description. When you start
typing, the Find Module dialog is launched automatically.
Use rack optimized communication formats for digital I/O modules to
minimize amount of controller memory and communications overhead
associated with these modules.
RSLogix 5000 automatically creates controller tags when you create an
input or output module. You can reference these tags directly in your
logic.
Use alias tags to assign names to specific input/output data and/or to
provide a short alternative to lengthy structure member names.
When you configure an analog I/O module, hold the shift key as you
move the slider to increment HH, H, L, and LL values in whole
numbers.
Copy I/O data to a User-Defined Type (UDT) so you can synchronize
I/O data with program scan. The UDT also enables easier mapping of
physical I/O.
Basic instructions
Be aware that specific nomenclature and operational details vary widely
between PLC manufacturers, and often implementation details evolve
from generation to generation.
Often the hardest part, especially for a beginning PLC programmer, is
practicing the mental ju-jitsu necessary to keep the nomenclature
straight from manufacturer to manufacturer.
Positive Logic (most PLCs follow this convention)
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True = logic 1 = input energized.
False = logic 0 = input NOT energized.
Negative Logic
True = logic 0 = input NOT energized
False = logic 1 = input energized.
Normally Open
(XIC) - examine If Closed.
This instruction is true (logic 1) when the hardware input (or internal
relay equivalent) is energized.
Normally Closed
(XIO) - examine If Open.
This instruction is true (logic 1) when the hardware input (or internal
relay equivalent) is NOT energized.
Output Enable
(OTE) - Output Enable.
This instruction mimics the action of a conventional relay coil.
On Timer
(TON) - Timer ON.
Generally, ON timers begin timing when the input (enable) line goes
true, and reset if the enable line goes false before setpoint has been
reached. If enabled until set point is reached then the timer output goes
true, and stays true until the input (enable) line goes false.
Off Timer
(TOF) - Timer OFf.
Generally, OFF timers begin timing on a true-to-false transition, and
continue timing as long as the preceding logic remains false. When the
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accumulated time equals set point the TOF output goes on, and stays on
until the rung goes true.
Retentive Timer
(RTO) - Retentive Timer On.
This type of timer does NOT reset the accumulated time when the input
condition goes false.
Rather, it keeps the last accumulated time in memory, and (if/when the
input goes true again) continues timing from that point. In the Allen-
Bradley construction this instruction goes true once setpoint (preset)
time has been reached, and stays true until a RES (Reset) instruction is
made true to clear it.
Latching Relays
(OTL) - Output Latch.
(OTU) - Output Unlatch.
Generally, the unlatch operator takes precedence. That is, if the unlatch
instruction is true then the relay output is false even though the latch
instruction may also be true. In Allen-Bradley ladder logic (and others)
latch and unlatch relays are separate operators.
Edit and Monitor Tags
To edit existing tags select the Logic > Edit Tags menu item. A spread
sheet like view lets you create and edit tags.
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Table 1:- Tags & Monitor
Clicking the + sign next to a tag reveals its structure. For a DINT tag this is
the 32 individual bits that make up the tag which will not be of interest if
you are using the tag as a number rather then individual bits. If you do
wish to use the individual bits then you can address them in this way with
the tag name followed by a period and then the bit position (e.g. MyTag.5).
Shown below is the expanded structure for a TIMER. Notice it is made of
two DINTs and three BOOLs. In this case, the Booleans are packed into
one DINT and therefore a timer uses three DINTs of memory.
An Easier Way to Create Tags
The easiest way to create tags is on the fly while programming. When
an instruction is first used a “?” will indicated the need for a tag. There
are three options at this point:
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1. Double click on the “?” and select an existing tag from the drop down
box.
2. Right click on the “?” and select new tag.
3. Double click on the “?” and type in the tag name. If it does not all ready
exist, then right click on the tag name and select Create
“NewTagName”. Be careful with this method not to use spaces or
special characters.
The nice thing about all these methods is that RSLogix5000 will
automatically fill in the correct data type according to the instruction
used.
Another quick method is to drag and drop an existing tag to a new
instruction. Make sure to click on the tag name rather then the
instruction.
Fig 2.9 :- Ladder diagram
Conclusion
These are the basics of tags. The advantages are:
1. Tags, if done right, create a level of documentation that is stored in the
PLC.
2. The software does an automatic housekeeping of memory locations.
There’s no more worrying about physical addressing and memory
conflicts.
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3. Structures can be more easily put together based on function rather then
data type.
Advance subjects include arrays, user defined data types (UDT) and
Add-On Instructions. Hopefully, you will continue to learn more about
the power of tags. There is no doubt that if you grasp the principles
presented here you will be well on your way to using and
troubleshooting any Logix5000 controller.
3.0 PLC Applications in Process Industries
PLCs are widely used in process industries; mainly there are three
types of applications:
DCS- PLC combination
PLC- Blind controller
PLC- substitute of DCS
4.0 Advantages
PLCs have gained popularity in industry as they offer specific
advantages, which are given below:
Faster scan time
Intelligent I/O
High-speed counters
Supervisory control capability
Reliability in operation
Flexibility in programming and reprogramming in the plant
Cost effective for controlling complex systems
Flexibility in control techniques
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Minimum maintenance
Small physical size
Superior computational capabilities to execute complex control
techniques
ASCII message handling capability
Ability to communicate with computer systems in the plant
Programmable troubleshooting aids which reduce down time
5. CONCLUSION
In this article The Programmable logic controller is Used in Industries &
also for the substitute of D.C.S. It can also be used as a part of D.C.S
While configured with D.C.S. The Various Manufacturers PLCs are used
according to the application. The presentation contains the application
where Traffic Light Control & Interlocking of the Motor has been done.
6.BIBILIOGRAPHY
[1] Cox, Richard D.technician”s guide to programmable controllers
NY:Delmar,1995
[2] Filer, Robert. Programmable controller Using Allen Bardley SLC-500
& control logix.Prentice Hall,2002
[3] Morris, Brian C. programmable logic controller NJ: Prentice Hall,2000.
[4] Programmable Logic controllers,Principle & applications. John W.
Webb&Ronald Reis Nj: 2007 .
[5] Stevenson, Jon. Fundamental of Programmable Logic Controllers, 2nd ed.
NY : Glencoe, 1998
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