Plc 1st Unit.docx

PLC (PROGRAMABLE LOGIC CONTROLER) Definition-: PLC stands for Programmable Logic Controllers. They are basically used to control automated systems in industries. They are one of the most advanced and simplest forms of control systems which are now replacing hard wired logic relays at a large scale. OR A Programmable Logic Controller is simply a special computer device used for industrial control systems. 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. OR A programmable logic controller (PLC) is a small, modular computer with customized instructions for performing a particular task. PLCs, which are used in industrial control systems for a wide variety of industries, have largely replaced mechanical relays, drum sequencers and cam timers. PLCs are useful tools for repeatable processes because they have no mechanical parts and they can gather information. Each central processor unit continually through an input scan, program scan, output scan and housekeeping mode, repetitively performing a single task while monitoring conditions. The information gathers can be used as feedback to guide needed changes and improvements to processes, some of which can be performed automatically according to the device’s coding. PLCs take up less space, perform more complex tasks and are more customizable than the mechanical technologies they have replaced. They are known for their ability to operate continuously without maintenance and have had a great impact on digitizing a great many industries, particularly manufacturing. The first PLC, for example, was invented by Dick Morley in 1969 for General Motors and performed uninterrupted for 20 years before being retired.

Working Of PLC-:  The input sources convert the real time analog electric signals to suitable digital electric signals and these signals are applied to the PLC through the connector rails.  These input signals are stored in the PLC external image memory in locations known as bits. This is done by the CPU  The control logic or the program instructions are written onto the programming device through symbols or through mnemonics and stored in the user memory.  The CPU fetches these instructions from the user memory and executes the input signals by manipulating, computing, processing them to control the output devices

Working Schematic Diagram  The execution results are then stored in the external image memory which controls the output drives.  The CPU also keeps a check on the output signals and keeps updating the contents of the input image memory according to the changes in the output memory.  The CPU also performs internal programming functioning like setting and resetting of the timer, checking the user memory.

Block Diagram of PLC-:

A simplified block diagram of a PLC shown in above. It has three major units/sections.   

I/O (Input/Output) Modules. CPU (Central Processing Units). Programmer/Monitor. The input section converts the field signals supplied by input devices/sensors to logic-level signals that the PLC's CPU can read. The Processor Section reads these inputs, Processes the signal, and prepares the output signals. The output section converts the logic level output signals coming from processor section to high level signals and used to actuate various output field devices. The programmer/monitor is used to enter the user's program into memory and to monitor the execution of the program.

1) I/O Section:- The I/O section establish the interfacing between physical devices in the real world outside the PLC and the digital arena inside the PLC. The input module has bank of terminals for physically connecting input devices, like push buttons, limit switches etc. to a PLC. the role of an input module is to translate signals from input devices into a form that the PLC's CPU can understand. The Output module also has bank of terminals that physically connect output devices like solenoids, motor starters, indicating lamps etc. to a PLC. The role of an output module is to translate signals from the PLC's CPU into a form that the output device can use. The tasks of the I/O section can be classified as:    

Conditioning Isolation Termination Indication

2) CPU Section:- The Central Processing Unit, the brain of the system is the control portion of the PLC. It has three Subparts.   

Memory System Processor Power Supply

Memory System:The memory is the area of the CPU in which data and information is stored and retrieved .The total memory area can be subdivided into the following four Sections. 

I/O Image Memory The input image memory consists of memory locations used to hold the ON or OFF states of each input field devices, in the input status file.

The output status file consists of memory locations that stores the ON or OFF states of hardware output devices in the field. Data is stored in the output status file as a result of solving user program and is waiting to be transferred to the output module's switching device. 

Data Memory It is used to store numerical data required in math calculation, bar code data etc.

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User Memory It contains user's application program.

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Executive Memory

It is used to store an executive program or system software. Processor:The processor, the heart of CPU is the computerized part of the CPU in the form of Microprocessor / Micro controller chip. It supervises all operation in the system and performs all tasks necessary to fulfill the PLC function.

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It reads the information i.e status of externally connected input devices with input module. It stores this information in memory for later use. It carries out mathematical and logic operations as specified in application program. After solving the user's program, it writes the result values in the memory. It sends data out to external devices like output module, so as to actuate field hardware. It performs peripheral and external device communication. It Performs self diagnostics.

Power Supply:The power supply provides power to memory system, processor and I/O Modules.   

It converts the higher level AC line Voltage to various operational DC values. for electronic circuitry. It filters and regulates the DC voltages to ensure proper computer operations.

3) Programmer/Monitor:The Programmer/Monitor (PM) is a device used to communicate with the circuits of the PLC.The programming unit allows the engineer/technicians to enter the edit the program to be executed. In its simplest form it can be hand-held device with membrane keypad for program entry, and a display device (LED or LCD) for viewing program steps of functions.

Advantage of PLC The advantages of PLC are as follows: 1. Flexible in Nature: One model of PLC can be used for different operations as per requirement. 2. Easy to install and trouble shooting: In hard wired relay based systems, installation time is more as compared to the PLC based control panels.

3. Availability of Large contacts: PLC programming tools contain internal large number of contacts that can be used for any change induced in different applications. 4. Cost effective: Advanced technology and large production of PLC makes it cheaper than the other controller or relay based systems. 5. Simulation feature: PLC programming software comes with the simulation features by default. 6. Simple programming methods: PLC is provided with simple programming methods to program the PLC like Ladder or Boolean type of programming. 7. Ease of maintenance: As compared with the control systems like relay based or microcontroller based systems, maintenance cost of PLC is low. 8. Documentation: The programmer can program and print easily the programs of PLC for future use.

Discrete I/O Modules-:

Discrete I/Os has only two states of operation ON/OFF. Process switches, pushbutton switches, limit switches, and proximity switches are all examples of discrete sensing devices. The picture below shows an discrete input connection:

Discrete input card When the hand switch is turned ON, current passes through the circuit which turns ON a LED light. The light is sensed by a photosensitive device such as a phototransistor inside the module, which in turn activates a bit (a single element of digital data) inside the PLC’s memory. Each input channel has its own optocoupler, writing to its own unique memory register bit inside the PLC’s memory. Using photosensitive devices in the switching action protects the PLC processor from an external current circuit. It isolates the circuit from peak or excess current in the external circuitry. Now lets look at the Discrete output module:

Discrete output card The output is given out from a PLC using a LED and when the light is emitted and there is a photo sensing device at the other end. Which senses the light signal and turns ON the circuit. Alternatively, small electromechanical relays may be used in lieu of opto-isolating semiconductor switching elements such as transistors (DC) or TRIACs (AC). Each output channel has its own optocoupler, driven by its own unique memory register bit inside the PLC’s memory. Discrete output cards for PLCs also typically have 4, 8, 16, or 32 channels.

Analog I/O Circuits of this type sense or drive analog signals. Analog inputs come from devices, such as thermocouples, strain gages, or pressure sensors, that provide a signal voltage or current that is derived from the process variable. Standard Analog Input signals: 4-20mA; 0-10V. Analog outputs can be used to drive devices such as voltmeters, X-Y recorders, servo motor drives, and valves through the use of transducers. Standard Analog Output signals: 4-20mA; 05V; 0-10V.

The analog input interface module contains the circuitry necessary to accept an analog voltage or current signal from the level transmitter field device. This input is converted from an analog to a digital value for use by the processor.

Analog I/O is commonly available for modular PLCs for many different analog signal types, including: • Voltage (0 to 10 volt, 0 to 5 volt) • Current (0 to 20 mA, 4 to 20 mA) • Thermocouple (millivoltage) • RTD (Millivoltage) • Strain gauge (millivoltage)

INSTALLATION OF PLC-: There are many things to consider when installing a programmable logic controller (PLC). First, the PLC will require a proper enclosure and output loads, such as room lights, should have their own relay panel. The relay panel is installed above or below the PLC enclosure depending on the output wiring of the PLC. Mount the PLC horizontally to provide proper ventilation. You cannot mount the PLC vertically, upside down, or on a flat horizontal surface. Provide a minimum clearance of 40mm between the PLC and all sides of the enclosure. Remember to allow for other items, like a Lutron interface unit, to be mounted in the same PLC enclosure. There should also be at least 80mm of clearance between the PLC and wiring that runs parallel to the unit. PLCs can be secured to an enclosure by using mounting rails. They are approximately 35mm high, with a depth of 7mm. If you mount the PLC on a rail, do consider using end brackets on each side of the PLC. The end bracket helps keep the PLC from sliding horizontally along the rail, reducing the possibility of accidentally pulling the wiring loose.

The ground terminal on the PLC must be connected to a good common ground reference (Earth ground). One method of providing an adequate common ground reference is connection to the incoming power system ground. Most PLCs have no internal fuses for the input power circuits, so external circuit protection is needed to ensure the safety of personnel and safe operation of the equipment itself. When operating the PLC from 110/125 VAC, it is only necessary to fuse the line (L) lead; it is not necessary to fuse the neutral (N) lead. Most connections, indicators, and terminals on a PLC are located on its front panel. The communication ports are located on front of the PLC as are the option card slots and the mode selector switch. The PLC terminals are divided into two groups. Each group has its own terminal block. The outputs and power wiring are on one block, and the input wiring is on the other. In some instances, it may be desirable to remove the terminal block for easy wiring. The terminal block is designed for easy removal with just a small screwdriver. Minimum wire sizes, color coding, and general safety practices should comply with appropriate electrical codes and standards for your area. Each terminal connection of the PLC can accept one 16 AWG wire or two 18 AWG size wire. Avoid running DC wiring close to AC wiring where possible. The PLC systems are designed to be powered by 110/125 VAC. Electrical power in some areas where the PLCs are installed is not always stable and storms can cause power surges. Due to this, powerline filters are recommended for protecting the PLCs from power surges. Evaluate any installations where the ambient temperature may approach the lower or upper limits of the PLC specifications. If you suspect the ambient temperature will not be within the operating specification for the PLC system, measures such as installing a cooling/heating source must be taken to get the ambient temperature within the range of specifications.

TESTING OF PLC-: 

Checking that all cable connections between the PLC and the plant are complete, safe, and to the required specification and meeting local standards.

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Checking that all the incoming power supply matches the voltage setting for which the PLC is set.

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Checking that all protective devices are set to their appropriate trip settings.

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Checking that emergency stop button work.

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Checking that all input/output devices are connected to the correct input/output points and giving the correct signals.

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Loading and testing the software.

Application of PLC-: 1. Traffic Light Signals 2. Control of barrier at Toll Tax booth 3. Switching of generator as per the availability of electricity

4. Airway Runway lighting control 5. Filling and packaging of products at food and beverages industry 6. Roller Coasters 7. Conveyor belts 8. Automatic doors 9. Elevator 10. Automatic car wash 11. Energy management in boiler control for maximum efficiency