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PROGRAMMABLE LOGIC CONTROLLERS AND MICROCONTROLLERS
LEARNING OBJECTIVES:
Basics of programmable Logic Controllers, their working and programming
Instructions sets like latch master control self holding relays
Timer and counter instructions sets
Electrical drives which were controlled by relays
Design , modify and troubleshoot control circuit
UNIT 1: INTRODUCTION TO PLC
1.1 What is PLC?
A programmable logic controller (PLC) or programmable controller is an industrial digital
computer which has been ruggedized and adapted for the control of manufacturing processes,
such as assembly lines, or robotic devices, or any activity that requires high reliability control
and ease of programming and process fault diagnosis.
They were first developed in the automobile manufacturing industry to provide flexible,
ruggedized and easily programmable controllers to replace hard-wired relays, timers and
sequencers. Since then they have been widely adopted as high-reliability automation controllers
suitable for harsh environments. A PLC is an example of a "hard" real-time system since output
results must be produced in response to input conditions within a limited time, otherwise
unintended operation will result
PLCs can range from small modular devices with tens of inputs and outputs (I/O), in a housing
integral with the processor, to large rack-mounted modular devices with a count of thousands of
I/O, and which are often networked to other PLC and SCADA systems.
They can be designed for multiple arrangements of digital and analog I/O, extended temperature
ranges, immunity to electrical noise, and resistance to vibration and impact. Programs to control
machine operation are typically stored in battery-backed-up or non-volatile memory.
It was from the automotive industry in the USA that the PLC was born. Before the PLC, control,
sequencing, and safety interlock logic for manufacturing automobiles was mainly composed
of relays, cam timers, drum sequencers, and dedicated closed-loop controllers. Since these could
number in the hundreds or even thousands, the process for updating such facilities for the yearly
model change-over was very time consuming and expensive, as electricians needed to
individually rewire the relays to change their operational characteristics.
When digital computers became available, being general-purpose programmable devices, they
were soon applied to control sequential and combinatorial logic in industrial processes. However
these early computers required specialist programmers and stringent operating environmental
control for temperature, cleanliness, and power quality. To meet these challenges the PLC was
developed with several key attributes. It would tolerate the shop-floor environment, it would
support discrete (bit-form) input and output in an easily extensible manner, it would not require
years of training to use, and it would permit its operation to be monitored. Since many industrial
processes have timescales easily addressed by millisecond response times, modern (fast, small,
reliable) electronics greatly facilitate building reliable controllers, and performance could be
traded off for reliability.
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1.2Invention and early development
In 1968 GM Hydramatic (the automatic transmission division of General Motors)
issued a request for proposals for an electronic replacement for hard-wired relay
systems based on a white paper written by engineer Edward R. Clark. 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 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, later acquired by German
Company AEG, and then by French Schneider Electric, the current oOne of the very
first 084 models built is now on display at Schneider Electric's facility 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.
Modern PLCs can be programmed in a variety of ways, from the relay-derived ladder
logic to programming languages such as specially adapted dialects of BASIC and C.
Another method is state logic, a very high-level programming language designed to
program PLCs based on state transition diagrams. The majority of PLC systems
today adhere to the IEC 61131/3 control systems programming standard that defines
5 languages: Ladder Diagram (LD), Structured Text (ST), Function Block Diagram
(FBD), Instruction List (IL) and sequential function chart (SFC).
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 logic to be used, for the aforementioned reasons and because it
was a familiar format used for electro-mechanical control panels. 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 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.
Early PLCs, up to the mid-1990s, were programmed using proprietary
programming panels or special-purpose programming terminals, which often had
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dedicated function keys representing the various logical elements of PLC
programs Some proprietary programming terminals displayed the elements of
PLC programs as graphic symbols, but plain ASCIIcharacter representations of
contacts, coils, and wires were common. Programs were stored on cassette tape
cartridges. Facilities for printing and documentation were minimal due to lack of
memory capacity. The oldest PLCs used non-volatile magnetic core memory.
More recently, PLCs are programmed using application software on personal
computers, which now represent the logic in graphic form instead of character
symbols. The computer is connected to the PLC through USB, Ethernet, RS-
232, RS-485, or RS-422 cabling. The programming software allows entry and
editing of the ladder-style logic. In some software packages, it is also possible to
view and edit the program in function block diagrams, sequence flow charts
and structured text. Generally the software provides functions for debugging and
troubleshooting the PLC software, for example, by highlighting portions of the
logic to show current status during operation or via simulation. The software will
upload and download the PLC program, for backup and restoration purposes. In
some models of programmable controller, the program is transferred from a
personal computer to the PLC through a programming board which writes the
program into a removable chip such as an EPROM.
UNIT2: Timers and counters
2.1 INTRODUCTION-The main function of a timer is to keep an output on for a specific length of time.
A good example of this is a garage light, where you want power to be cut off after 2 minutes so as to
give someone time to go into the house. The three different types of timers that are commonly used
are a Delay-OFF, a Delay-ON, and a Delay-ON-Retentive. A Delay-OFF timer activates immediately
when turned on, counts down from a programmed time before cutting off, and is cleared when the
enabling input is off. A Delay-ON timer is activated by input and starts accumulating time, counts up
to a programmed time before cutting off, and is cleared when the enabling input is turned off. A
Delay-ON-Retentive timer is activated by input and starts accumulating time, retains the accumulated
value even if the (ladder-logic) rung goes false, and can be reset only by a RESET contact.Counters
are primarily used for counting items such as cans going into a box on an assembly line. This is
important because once something is filled to its max the item needs to be moved on so something else
can be filled. Many companies use counters in PLC's to count boxes, count how many feet of
something is covered, or to count how many pallets are on a truck. There are three types of counters,
Up counters, Down counters, and Up/Down counters. Up counters count up to the preset value, turn
on the CTU (CounT Up output) when the preset value is reached, and are cleared upon receiving a
reset. Down counters count down from a preset value, turns on the CTD (CounT Down output) when
0 is reached, and are cleared upon reset. Up/Down counters count up on CU, count down on CD,
turn on CTUD (CounT Up/Down output) when the preset value is reached, and cleared on reset.
2.2 Programmable logic relay (PLR) -In more recent years, small products called PLRs
(programmable logic relays), and also by similar names, have become more common and accepted.
These are 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 needed, 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 8
to 12 discrete inputs, 4 to 8 discrete outputs, and up to 2 analog inputs. Size is usually about 4" wide,
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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 or RS-485 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 modular and greatly expandable, the PLRs are usually not modular or expandable,
but their price can be two orders of magnitude less than a PLC, and they still offer robust design and
deterministic execution of the logics.
2.3 Scan time: A PLC program generally loops i.e. executes repeatedly, as long as the controlled
system is running. At the start of each execution loop, the status of all physical inputs are copied to
an area of memory, sometimes called the "I/O Image Table", which is accessible to the processor.
The program then runs from its first instruction rung down to the last rung. It takes some time for the
processor of the PLC to evaluate all the rungs and update the I/O image table with the status of
outputs.[14] Scan times of a few milliseconds may be encountered for small programs and fast
processors, but for older processors and very large programs much longer scan times (on the order
of 100 ms) may be encountered. Excessively long scan times may mean the response of the PLC to
changing inputs or process conditions is too slow to be useful .As PLCs became more advanced,
methods were developed to change the sequence of ladder execution, and subroutines were
implemented. This simplified programming could be used to save scan time for high-speed processes;
for example, parts of the program used only for setting up the machine could be segregated from
those parts required to operate at higher speed. Newer PLCs now have the option to run the logic
program synchronously with the IO scanning. This means that IO is updated in the background and
the logic reads and writes values as it's required during the logic scanning.
UNIT 3: WORKING OF PLC
3.1Process of a scan cycle -There are 5 main steps in a scan cycle:
Reading inputs
Executing the program
Processing communication requests
Executing CPU diagnostics
Writing outputs
3.2User interface-User interface and List of human-computer interaction topics.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 man-machine
interfaces (MMIs) and graphical user interfaces (GUIs). 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 programming and monitoring software installed on a computer, with the PLC connected via
a communication interface.
3.2 Communications-Many models of PLCs have built-in communications ports, using RS-232, RS-
422, RS-485, or Ethernet. Various protocols are usually included. Many of these protocols are vendor
specific.Most modern PLCs can communicate over a network to some other system, such as a
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