PROJECT REPORT

ON

BASICS OF PLC

BY

ROHIT KESHARI

(ELECTRICAL AND ELECTRONICS ENGINEERING)

OF

COCHIN UNIVERSITY COLLEGE OF ENGINEERING

(KERALA-688504)

UNDER THE GUIDANCE OF

MR. S.V.SUBRAMANIAN

(DGM,BG-II(ISG),BHEL)

ACKNOWLEDGEMENT

I WISH TO EXPRESS MY GRATITUDE TO BHEL FOR PROVIDING ME WITH ME FOR MY PROJECT AND ALL REQUIREMENTS TO CARRY OUT THE PROJECT SUCCESSFULLY.

I WISH TO EXPRESS MY GRATITUDE TO MR. B.K.R PATNAIK (HR,BHEL) FOR PROVIDING ME WONDERFUL OPPORTUNITY TO WORK WITH THEM.

I WISH MY GRATITUDE TO MR. V.SUBRAMANIAM, MY PROJECT GUIDE FOR PROVIDING USEFUL INFORMATION FOR THE PROJECT DEVELOPMENT AND SUCCESSFUL COMPLETION.

I AM VERY GRATEFUL TO MR. KEERTHAN FOR PROVIDING THE IDEAS AND RESOURCES NECESSARY FOR ME TO COMPLETE MY PROJECT.

BIBLIOGRAPHY AND REFERENCES

WEBSITES:

WWW.GOOGLE.COM WWW.WIKIPEDIA.COM

PLC LADDER LOGIC PROGRAMMING REFERENCE MANUAL:

HTTP://W1.SIEMENS.COM

HISTORY OF PLC :-• First Programmable Logic Controllers were designed and developed by Modicon as a

relay replacer for GM and Landis.

• These controllers eliminated the need for rewiring and adding additional hardware for each new configuration of logic.

• The new system drastically increased the functionality of the controls while reducing the cabinet space that housed the logic.

• The first PLC, model 084, was invented by Dick Morley in 1969.

• The first commercial successful PLC, the 184, was introduced in 1973 which was designed by Michael Greenberg.

What is a PLC?

A PROGRAMMABLE LOGIC CONTROLLER is a solid state control system that continuously monitors the status of devices connected as inputs. Based upon a user written program, stored in memory, it controls the status of devices connected as outputs.

A Programmable Logic Controller, or PLC for short, is simply a special computer device used for industrial control systems. They are used in many industries such as oil refineries, manufacturing lines, conveyor systems and so on. Where ever there is a need to control devices the PLC provides a flexible way to "softwire" the components together.

The basic units have a CPU (a computer processor) that is dedicated to run one program that monitors a series of different inputs and logically manipulates the outputs for the desired control. They are meant to be very flexible in how they can be programmed while also providing the advantages of high reliability (no program crashes or mechanical failures), compact and economical over traditional control systems.

A Simple Example

Consider something as simple as a switch that turns on a light. In this system with a flick of the switch the light would turn on or off. Beyond that though there is no more control. If your boss came along and said I want that light to turn on thirty seconds after the switch has been flipped, then you would need to buy a timer and do some rewiring. So it is time, labor and money for any little change.

A PLC Saves the Day

Now consider the same device with a PLC in the middle. The switch is fed as an input into the PLC and the light is controlled by a PLC output. Implementing a delay in this system is easy since all that needs to be changed is the program in the PLC to use a delay timer.

This is a rather simple example but in a larger system with many switchs and lights (and a host of other devices) all interacting with each other this kind of flexibility is not only nice but imperitive. Hopefully a light bulb has now turned on over your head.

How PLCs Work

A programmable logic controller is a specialized computer used to control machines and processes. It therefore shares common terms with typical PCs like central processing unit, memory, software and communications. Unlike a personal computer though the PLC is designed to survive in a rugged industrial atmosphere and to be very flexible in how it interfaces with inputs and outputs to the real world.

The components that make a PLC work can be divided into three core areas.

The power supply and rack

The central processing unit (CPU)

The input/output (I/O) section

PLCs come in many shapes and sizes. They can be so small as to fit in your shirt pocket while more involved controls systems require large PLC racks. Smaller PLCs (a.k.a. “bricks”) are typically designed with fixed I/O points. For our consideration, we’ll look at the more modular rack based systems. It’s called “modular” because the rack can accept many different types of I/O modules that simply slide into the rack and plug in.

The Power Supply and Rack

So let’s start off by removing all our modules which leaves us with a naked PLC with only the power supply and the rack.

The rack is the component that holds everything together. Depending on the needs of the control system it can be ordered in different sizes to hold more modules. Like a human spine the rack has a backplane at the rear which allows the cards to communicate with the CPU. The power supply plugs into the rack as well and supplies a regulated DC power to other modules that plug into the rack. The most popular power supplies work with 120 VAC or 24 VDC sources.

The CPU

The brain of the whole PLC is the CPU module. This module typically lives in the slot beside the power supply. Manufacturers offer different types of CPUs based on the complexity needed for the system.

The CPU consists of a microprocessor, memory chip and other integrated circuits to control logic, monitoring and communications. The CPU has different operating modes. In programming mode it accepts the downloaded logic from a PC. The CPU is then placed in run mode so that it can execute the program and operate the process.

Since a PLC is a dedicated controller it will only process this one program over and over again. One cycle through the program is called a scan time and involves reading the inputs from the other modules, executing the logic based on these inputs and then updated the outputs accordingly. The scan time happens very quickly (in the range of 1/1000th of a second). The memory in the CPU stores the program while also holding the status of the I/O and providing a means to store values.

I/O System

The I/O system provides the physical connection between the equipment and the PLC. Opening the doors on an I/O card reveals a terminal strip where the devices connect.

There are many different kinds of I/O cards which serve to condition the type of input or output so the CPU can use it for it’s logic. It's simply a matter of determining what inputs and outputs

are needed, filling the rack with the appropriate cards and then addressing them correctly in the CPUs program.

Inputs :

Input devices can consist of digital or analog devices. A digital input card handles discrete devices which give a signal that is either on or off such as a pushbutton, limit switch, sensors or selector switches. An analog input card converts a voltage or current (e.g. a signal that can be anywhere from 0 to 20mA) into a digitally equivalent number that can be understood by the CPU. Examples of analog devices are pressure transducers, flow meters and thermocouples for temperature readings

Outputs :

Output devices can also consist of digital or analog types. A digital output card either turns a device on or off such as lights, LEDs, small motors, and relays. An analog output card will

convert a digital number sent by the CPU to it’s real world voltage or current. Typical outputs signals can range from 0-10 VDC or 4-20mA and are used to drive mass flow controllers, pressure regulators and position controls.

Programming a PLC

In these modern times a PC with specially dedicated software from the PLC manufacturer is used to program a PLC. The most widely used form of programming is called ladder logic. Ladder logic uses symbols, instead of words, to emulate the real world relay logic control, which is a relic from the PLC's history. These symbols are interconnected by lines to indicate the flow of current through relay like contacts and coils. Over the years the number of symbols has increased to provide a high level of functionality.

The completed program looks like a ladder but in actuality it represents an electrical circuit. The left and right rails indicate the positive and ground of a power supply. The rungs represent the wiring between the different components which in the case of a PLC are all in the virtual world of the CPU. So if you can understand how basic electrical circuits work then you can understand ladder logic.

In this simplest of examples a digital input (like a button connected to the first position on the card) when it is pressed turns on an output which energizes an indicator light.

The completed program is downloaded from the PC to the PLC using a special cable that’s connected to the front of the CPU. The CPU is then put into run mode so that it can start scanning the logic and controlling the outputs.

The Birth of the PLC

The Original Challenge

The early history of the PLC is fascinating. Imagine if you will a fifty foot long cabinet filled with relays whose function in life is to control a machine. Wires run in and out of the system as the relays click and clack to the logic. Now imagine there is a problem or a small design change and you have to figure it all out on paper and then shut down the machine, move some wires, add some relays, debug and do it all over again. Imagine the labor involved in the simplest of changes. This is the problem that faced the engineers at the Hydra-matic division of GM motors in the late 1960's.

Fortunately for them the prospect of computer control was rapidly becoming a reality for large corporations as themselves. So in 1968 the GM engineers developed a design criteria for a "standard machine controller". This early model simply had to replace relays but it also had to be:

A solid-state system that was flexible like a computer but priced competitively with a like kind relay logic system.

Easily maintained and programmed in line with the all ready accepted relay ladder logic way of doing things.

It had to work in an industrial environment with all it's dirt, moisture, electromagnetism and vibration.

It had to be modular in form to allow for easy exchange of components and expandability.

The Race is On

This was a tall order in 1968 but four companies took on the challenge.

1. Information Instruments, Inc. (fully owned by Allen-Bradley a year later).

2. Digital Equipment Corp. (DEC)

3. Century Detroit

4. Bedford Associates

Bedford Associates, run by Richard Morley, won the contract and quickly formed a new company around the technology called MODICON after Modular Digital Control. By June of 1969 they were selling the first viable Programmable Controller the "084" (their 84th project) which sold over one thousand units. These early experiences gave birth to their next model the "184" in 1973 which set Modicon as the early leader in programmable controllers.

Not to be outdone, the powerhouse Allen-Bradley (all ready known for it's rheostats, relays and motor controls) purchased Information Instruments in 1969 and began development on this new technology. The early models (PDQ-II and PMC) were deemed to be too large and complex. By 1971 OdoStruger and Ernst Dummermuth had begun to develop a new concept known as the Bulletin 1774 PLC which would make them successful for years to come. Allen-Bradley termed their new device the "Programmable Logic Controller" (patent #3,942,158) over the then accepted term "Programmable Controller". The PLC terminology became the industry standard especially when PC became associated with personal computers.

PLC compared with other control systems

Allen-Bradley PLC installed in a control panel

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 economical. This is 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 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 buses economically use PLCs instead of custom-designed controls, because the volumes are low and the development cost would be uneconomical.[11]

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. Single-board computers using semi-customized or fully proprietary hardware may be chosen for very demanding control applications where the high development and maintenance cost can be supported. "Soft PLCs" running on desktop-type computers can interface with industrial I/O hardware while executing programs within a version of commercial operating systems adapted for process control needs.[11]

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.[citation needed]

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. The industry has standardized on the IEC 61131-3 functional block language for creating programs to run on RTUs and PLCs, although nearly all vendors also offer proprietary alternatives and associated development environments.

In recent years "Safety" PLCs have started to become popular, either as standalone models or as functionality and safety-rated hardware added to existing controller architectures (Allen Bradley Guardlogix, Siemens F-series etc.). These differ from conventional PLC types as being suitable for use in safety-critical applications for which PLCs have traditionally been supplemented with hard-wired safety relays. For example, a Safety PLC might be used to control access to a robot cell with trapped-key access, or perhaps to manage the shutdown response to an emergency stop on a conveyor production line. Such PLCs typically have a restricted regular instruction set augmented with safety-specific instructions designed to interface with emergency stops, light screens and so forth. The flexibility that such systems offer has resulted in rapid growth of demand for these controllers.

Discrete and analog signals:

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 0 - 10 V input or 4-20 mA 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.

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 by controlling the valve that refills the tank. Shown is a "ladder diagram" which shows the control system. A ladder diagram is a method of drawing control circuits which pre-dates PLCs. The ladder diagram resembles the schematic diagram of a system built with electromechanical relays. Shown are:

Two inputs (from the low and high level switches) represented by contacts of the float switches

An output to the fill valve, labelled as the fill valve which it controls

An "internal" contact, representing the output signal to the fill valve which is created in the program.

A logical control scheme created by the interconnection of these items in software

In ladder diagram, the contact symbols represent the state of bits in processor memory, which corresponds to the state of physical inputs to the system. If a discrete input is energized, the memory bit is a 1, and a "normally open" contact controlled by that bit will pass a logic "true" signal on to the next element of the ladder. Therefore, the contacts in the PLC program that "read" or look at the physical switch contacts in this case must be "opposite" or open in order to return a TRUE for the closed physical switches. Internal status bits, corresponding to the state of discrete outputs, are also available to the program.

In the example, the physical state of the float switch contacts must be considered when choosing "normally open" or "normally closed" symbols in the ladder diagram. The PLC has two discrete inputs from float switches (Low Level and High Level). Both float switches (normally closed) open their contacts when the water level in the tank is above the physical location of the switch.

When the water level is below both switches, the float switch physical contacts are both closed, and a true (logic 1) value is passed to the Fill Valve output. Water begins to fill the tank. The internal "Fill Valve" contact latches the circuit so that even when the "Low Level" contact opens (as the water passes the lower switch), the fill valve remains on. Since the High Level is also normally closed, water continues to flow as the water level remains between the two switch levels. Once the water level rises enough so that the "High Level" switch is off (opened), the PLC will shut the inlet to stop the water from overflowing; this is an example of seal-in (latching) logic. The output is sealed in until a high level condition breaks the circuit. After that the fill valve remains off until the level drops so low that the Low Level switch is activated, and the process repeats again.

| (N.C. physical (N.C. physical |

| Switch) Switch) |

| Low Level High Level Fill Valve |

|------[ ]------|------[ ]----------------------(OUT)---------|

| | |

| | |

| | |

| Fill Valve | |

|------[ ]------| |

| |

| |

A complete program may contain thousands of rungs, evaluated in sequence. Typically the PLC processor will alternately scan all its inputs and update outputs, then evaluate the ladder logic; input changes during a program scan will not be effective until the next I/O update. A complete program scan may take only a few milliseconds, much faster than changes in the controlled process.

Programmable controllers vary in their capabilities for a "rung" of a ladder diagram. Some only allow a single output bit. There are typically limits to the number of series contacts in line, and the number of branches that can be used. Each element of the rung is evaluated sequentially. If elements change their state during evaluation of a rung, hard-to-diagnose faults can be generated, although sometimes (as above) the technique is useful. Some implementations forced evaluation from left-to-right as displayed and did not allow reverse flow of a logic signal (in multi-branched rungs) to affect the output.

LEADING BRANDS OF PLC :-

AMERICAN 1. Allen Bradley

2. Gould Modicon

3. Texas Instruments

4. General Electric

5. Westinghouse

6. Cutter Hammer

7. Square D

EUROPEAN 1. Siemens

2. Klockner&Mouller

3. Festo

4. Telemechanique

JAPANESE 1. Toshiba

2. Omron

3. Fanuc

4. Mitsubishi