Programmable Logic Controllers - Ijcaonline

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International Journal of Computer Applications (0975 – 8887)Volume 77 – No.11, September 2013Programmable Logic ControllersRicha NettoAditya BagriThadomal Shahani Engineering College, MumbaiUniversityBandra, Mumbai - 400050Maharashtra, IndiaThadomal Shahani Engineering College, MumbaiUniversityBandra, Mumbai - 400050Maharashtra, IndiaABSTRACTThe analysis carried out in this technical paper highlights theconcepts, working, advantages and practical applications ofprogrammable logic controllers, along with a comparison withother control systems. A PLC aids in automation of a processby monitoring inputs and controlling outputs after making adecision on the basis of its program. It is commonly used forcontrolling many mechanical movements of heavy machineryand to control the voltage and frequency of power supplies.PLCs offer an array of advantages over other control systems,and have hence evolved as an important controller inindustries these days owing to its large number ofapplications.General TermsControl Systemstiming, counting, and arithmetic to control, through digital oranalog input/output modules, various types of machines orprocesses.”[1]Programmable Logic Controllers are hardware and softwareengineered microcomputers, used to provide industrial controloperations. A PLC audits inputs, makes decisions according toits program, and uses these decisions to administer outputs toautomate a process or machine. The PLC is proficient invarious tasks, including storage of instructions pertaining tosequencing, timing, counting, arithmetic, data manipulationand communication with the objective of controllingmachines and processes.3. BASIC COMPONENTS OF A PLCThe basic components of a PLC are as follows:KeywordsAutomation, Ladder Logic, SCADA, Control Loop, ScanCycle, EEPROM1. INTRODUCTIONIn the past, humans were the main methods for controlling asystem. Over the years, control engineering has evolved toinclude electricity in control systems. Previously,electromechanical relay based control systems were used.These systems obviated the use of switches for power on andpower off actions. Elementary logic control decisions arepredominantly undertaken by relays.The advancement of economical computers has brought abouta modern revolution, the Programmable Logic Controller(PLC). The PLC has its origins around the 1970’s in theautomation and motor manufacturing industries.2. DESCRIPTIONProgrammable Logic Controllers (PLCs), also referred to asprogrammable controllers, are a classification in computers. Itis a device that performs discrete or continuous control logicin process plant or factory environments. PLCs are used incommercial and industrial applications. They have becomepopular since their inception in a wide variety of operationsfrom boiler control to robot control.The Allen-Bradley Company (Rockwell Automation) coinedthe term “Programmable Logic Controllers” as well as thecorresponding acronym “PLC”. Both of these are thereforeregistered trademarks of the company. The National ElectricalManufacturers Association (NEMA) defines a programmablelogic controller as:“A digitally operating electronic apparatus which uses aprogrammable memory for the internal storage of instructionsfor implementing specific functions such as logic, sequencing,Power OutputSignals tosolenoids,motorsetc.Programming DeviceFigure 1: Block diagram of a PLC3.1 I/O ModulesI/O modules physically connect to field devices. Since thereare different types of input and output devices, the processdetermines the type of input or output module selected, digitalor analog.Inputs give the controller real-time status of variables in theform of real world signals. These variables can be analog,register or discrete. Typical analog inputs can be fromthermocouples, RTDs, flow, pressure, and temperaturetransmitters. These inputs are transmitted over the I/O bus tothe central processor unit after being converted into digitaldata. The input module is entrusted with the operation ofconverting electrical signals flowing in from input fielddevices like push buttons to electrical signals that the PLC canunderstand.[2] Examples of input modules include limitswitches, proximity switches and push buttons.27

International Journal of Computer Applications (0975 – 8887)Volume 77 – No.11, September 2013The working of a processor is as follows: The signal to theinput modules is transmitted by the machine or process inputelements. The input modules then generate a logic signal thatis sent to the CPU. When it receives instructions from thememory and the feedback on the status of the I/O devices, theCPU produces commands for the output devices. Thesecommands control the devices on a machine or a process.[4]Typical loads to be controlled are devices such as solenoidvalves, indicator lamps, relay coils and motor starters.3.3 Power SupplyFigure 2: Input ModuleThe output module converts the signals containinginstructions of the desired status of outputs into real worldoutputs after they have been transmitted over the I/O bus tothe module. There are three categories of outputs: discrete,register and analog. Information flowing out from the PLC isagain converted into electrical signals that can be interpretedby the output field devices, by the output module. Examplesof these output devices are motor starters or hydraulicsolenoid valves.PLCs and their constituting modules are equipped with powergenerated from a power supply. The power supply convertspower line voltages into those required by the solid-statecomponents. It may be integral or separately mounted whichprovides the isolation necessary to protect solid-statecomponents from high-voltage line spikes. The power supplyalso provides the heat dissipation required for the plant flooroperation. It drives the Central Processor Unit, I/O logicsignals, memory unit and some peripheral devices.[5] Theexpansion of I/O has lead to increased power requirement ofsome PLCs. Hence, they are furnished with additional powersupplies.3.4 Memory UnitThe memory unit of a PLC is the registry where the programsare stored. The fundamental unit of memory is the word.Words are made up of bits. A bit is a single piece of data. Itcontains information on only two states (ON/OFF orYES/NO). Longer words contain more information within.Programs are combination of words that produce controllogic.Memory can be volatile or non-volatile. Volatile memory isundesirable as is results in loss of data when power isremoved. Battery backup is usually a feature of volatilememory systems. On the other hand, power outages do notresult in change of state of the non-volatile memory.Therefore, it is administered in cases where it is expected thatthe transportation time to the job site will be long or extendedpower outages will be frequent.Figure 3: Output Module3.2 ProcessorThe processor is the intellect behind the PLC. It is similar inconstruction to the microprocessor used in personalcomputers. The Central Processor Unit (CPU) performs taskssuch as self-diagnostics, memory management, data handlingexecution, scanning, peripheral and external devicecommunications, resource sharing, program execution and I/Obus traffic control.[3]Execution of various logic and sequencing functions is carriedout by the CPU, which operates on the PLC inputs todetermine the appropriate output signals. Operations such asadding, subtracting, multiplying, dividing and comparing areperformed by the Arithmetic Logic Unit which is usually apart of the CPU (but sometimes it is separate).Memory is of three types: Read Only Memory (ROM),Read/Write Memory (R/W) and Random-Access Memory(RAM).[6] Another type of memory is the electrically alterableprogrammable read only memory (EAPROM).3.5 Network InterfacePLCs are capable of communicating with other devices likecomputers running programming software, a terminal that letsan operator enter commands into the PLC, or an I/O that islocated in a remote location from the PLC. Thiscommunication is carried out through a network interface.There are two ways in which the network interface can be apart of the PLC: it can be built directly into the PLC or it canappear as a plug on the front of the PLC.[7]3.6 Programming UnitsProgramming units form an interface between the requirementof the programmer and the understanding of the PLC. In thecourse of program development, system initialisation andtroubleshooting, the programmer provides for an interfacebetween the PLC and the user. This programming device isused to code and insert into memory the instructions that areto be performed during the scan.Figure 4: Central Processing Unit28

International Journal of Computer Applications (0975 – 8887)Volume 77 – No.11, September 2013Programming devices vary greatly in size, ranging from smallunits that are hand-held to large, astute CRT-based units withthe ability to document, compute I/O status and work in bothonline as well as offline modes.[8] A personal computer is themost commonly used programming device.PLCs are configured using the software that runs on personalcomputers. Different products may require differentprogramming software. Software allows programs to bewritten in several different languages and allows users tocreate, edit, document, store and troubleshoot programs.CHECK INPUT STATUSEXECUTE PROGRAMUPDATE OUTPUT STATUS4. PROGRAMMING A PLCPLCs are programmed in two ways. One is to directly connectthe PLC to a computer and the other is to make use of aprogramming panel. Logical commands are used to programthe PLC in both scenarios. The panels used in programming aPLC differ greatly in their intricacies, ranging from simplekeypads to hand-held computers with a screen for graphics.The programming software is operated on computers withgraphics, simulators, diagnostics and auditing. After thesoftware programmer develops the program, it is tested andthen sent to the PLC. This can be done using magnetic tapes,compact disks, and communication links. However, the mostcommon method for the transfer of a program to the PLC isby using a chip to which the program is written, commonlyknown as an EEPROM. After this, the chip is plugged into thePLC. The memory can be erased by exposure to UV light andreused but it cannot be overwritten.[6]The PROGRAM mode of the processor is used when theprogram has to be entered into the PLC. The instructions areentered sequentially, one at a time with the help of theprogramming device. The controller is made to work in theRUN state (or operating cycle) when the program has to becarried out. As a consequence of each operating cycle, thecontroller reviews the status of the input devices, executes theprogram that has been entered and alters the outputsaccordingly.5. WORKING OF A PLCThe main components of a PLC are input modules, a CentralProcessing Unit (CPU), and output modules. A variety ofdigital or analog signals are accepted from various fielddevices by the input module. They are then converted into alogic signal that the CPU can use. The program instructionsstored in the memory form the basis upon which the CPUmakes decisions. These decisions eventually result inexecution of the control instructions as required by theprogram. Control instructions from the CPU are convertedinto a digital or analog signal by the output module. Thissignal can be used to control various field devices. A PLCworks by scrutinizing a program, instruction by instruction,continuously. This process is also known as scanning. Onescan time is defined as the time taken to complete the threesteps. The control loop is a continuous cycle of the PLCreading inputs, solving the ladder logic, and then changing theoutputs.[9]Figure 5: Scan CycleControl circuits have been conventionally described usingladder diagrams. Ladder diagrams consist of rungs; each oneof these is a program statement. A program statement isbasically a condition, or a set of conditions (input). Each oneof these conditions has an action (output) associated withthem that is executed if the condition is satisfied. Elementarylogic operators like AND, OR and NOT are used to performlogical operations required by the PLC. They combine theinstructions in the program statement (present on a PLC rung)thus making the result of each run either true or false (1 or0).[10]The instructions entered using a programming devicedetermine what action will be performed by the PLC for aparticular input. A device acting as an operator interfacedisplays process information and provides for the addition ofnew process parameters to increase or decrease level ofcontrol.6. COMPARISON OF PLCs WITHOTHER CONTROL SYSTEMSPLCs are predominantly used in industrial processes wherethe development and maintenance costs of the automationsystem exceeds the total cost of automation and where itmight be necessary to alter the system during its operationallife, examples of these include motion and positioning control.Also, they cater to applications where the system required ishighly customized. However, PLCs lack the capabilityrequisite of highly complex algorithmic processes, like in thechemical industry.Different techniques entailing the use of microcontrollers,relay systems, minicomputers, solid-state controls, digitallogic control systems and electronic continuous controlsystems, among many others, are preferred for high volume orsimple fixed automation tasks.Relays can construct complicated control systems by linkingan array of logic components. On the other hand, a typicalrelay system has hundreds of switching contacts, presentingthe design engineer with a daunting task especially when thecontrol function of a panel has to be changed, eventuallyresulting in complete re-wiring of the system. They do notprovide the troubleshooting flexibility offered by modernprogrammable control systems. Relays also take up morespace and require regular maintenance.[11] In addition, thehigh cost, low speed and lack of reliability has resulted in thereplacement of relay systems by modern substitutes. Relaysremain optimal for reconstruction of small control signals tohigher current or higher voltage driving signals. This explainstheir comprehensive usage as output devices for other controlsystems.29

International Journal of Computer Applications (0975 – 8887)Volume 77 – No.11, September 2013Table 1: Comparison of PLCs with other Control rocessorMinicomputerDigital LogicPLCsHardware costLowEqualLowHighAverage (can behigh in smallquantities)Dependson numberof controlsVersatilityLowLowYesYesTroubleshooting andmaintainabilityPoorPoorPoorPoorPoor if IC’s rithmetic capabilityNoNoYesYesYesYesProgramming cost(Wiring)High(Wiring) HighHighHighLowLowReusableNoNoYesYesSpace requiredLargestLargeSmallOkFairly compactSmallOperating speedSlowFasterthanelectromechanicalrelaysFairly fastFairly fastFairly fastFastPLCs have a logic called PID (proportional, integral,derivative) controller. Usually, PLCs are constructed withvery less analog loops; processes that require a large numberof loops are based on distributed control system (DCS). Withtime, as the power of the PLC increases, the gap betweenapplications of PLCs and DCSs has lessened greatly.[12]Digital Logic Control Systems use logic gates to processinformation (exclusively encompassing binary signals). Logicgates run at much higher speeds, cause lesser powerconsumption and occupy considerably lesser space than acorresponding relay circuit. Nonetheless, they are incapable ofswitching to higher power driving signals.Operational amplifiers, a part of Electronic ContinuousControl Systems involve the performance of mathematicaloperations like integration, differentiation, etc. They wereutilized in the field of continuous control and lead to thesimplification of complex control functions with discreteelectronic systems. Largely built on linear integrated circuits,analog control is efficient with high speed. The fine-tuning ofsuch systems during designing coupled with their fixed naturecontinues to cause hindrances.A design based on microcontrollers is pertinent in systemswhere hundreds or thousands of units are to be producedwhere the end-user would not have to alter the control and thedevelopment costs are large. They can be a part of sequenceand continuous control systems. Both microprocessors andmicrocontrollers simplify connection requirements due totheir small size, making them easily locatable at the point offinal control.7. ADVANTAGESTo implement changes in control logic of a PLC no rewiringis required which saves considerable time. PLCs can carry outcomplex functions such as generation of time relays, counting,comparing, arithmetic operations, etc. Increased technologyYesYesmakes it possible to condense more functions into smaller andless expensive packages. The cost of a PLC is recovered in ashort period, hence making it highly economical.When changes are made to the program circuit or sequencedesign or when a programming fault has to be revised in theladder diagram, the necessary amendments can be made easilyby just typing it in using a keyboard. This results in faster, andat the same time, convenient troubleshooting.If the need arises, an instantaneous printout of the originalPLC circuit can be made available effortlessly, hence makingdocumentation very efficient. The PLC prints out the actualcircuit in operation at a given moment.[7] There is no need tolook for the blueprint of the circuit in remote files. Due totheir solid-state nature, PLCs are extremely compact andmuch more reliable when compared with hardwiredcontrollers. Ergo, the costs of maintaining PLC based controlsystems are low.[13]A PLC can form networks to communicate with other deviceslike controllers. They can be correlated to perform operationssuch as supervisory control, data gathering, monitoringdevices and process parameters, and downloading anduploading of programs. PLCs consume only 10% of the powerconsumed by an equivalent relay logic controller and hencealso save energy.8. DISADVANTAGESThe stop button is employed to disconnect a circuit in relaysystem; the system stops if the power fails. The PLC can beprogrammed to perform this function; although, in certainprograms, to stop a device, one may have to apply an inputvoltage. Being fail-safe is not one of the features of thissystem.Some applications are single-function applications. It does notpay to use a PLC that includes multiple programmingcapabilities if they are not needed. One of the conspicuous30

International Journal of Computer Applications (0975 – 8887)Volume 77 – No.11, September 2013features of PLC that is reprogramming would not be requiredsince their operational sequence is very rarely altered.[7]In a process, the environment may experience high heat,vibration, and direct contact with electrical appliances withinthe PLC. Such events continuously disrupt the performance ofthe PLC. In addition, professional expertise is required toreplace the previously used control system employing relayladder by a PLC computer concept as the technology iscomparatively new.11. ACKNOWLEDGEMENTThis technical paper has been done under the support andguidance of Mr. G.K. Dey, Manager (Engineering Services,Instrumentation) at Godrej Industries Limited.12. REFERENCES[1]W. Bolton, Programmable Logic Controllers, FifthEdition, Newnes, 2009.[2]Clarence T. Jones, Programmable Logic Controllers:The Complete Guide to the Technology, Patrick-Turner,1996.[3]W.N. Clare, G. T. Kaplan, D. R. Saldon, A. C.Wiktorowicz, R. A. Gilbert, C. W. Wendt, ProcessControl and Optimization Volume II.[4]James A. Rehg, Glenn J. Sartori, Programmable LogicControllers, Prentice Hall Higher Education, 2009.[5]S.K. Singh, Industrial Instrumentation and Control,Second Edition, Tata McGraw-Hill, 2007.[6]D. J. Dunn, Selection and Application of ProgrammableLogic Controllers.[7]Dr. D. J. Jackson, Programmable Logic Controllers –Introduction to PLCs.[8]Sadegh Vosough, Amir Vosough, PLC and itsApplications, International Journal of MultidisciplinarySciences and Engineering, Vol. 2, No. 8, November2011.[9]Greg P. Zimmerman, Programmable Logic Controllersand Ladder Logic, April 2008.9. APPLICATIONSPLC is used as a widespread controller that has numerousapplications. With the program installed in its memory, itprovides the user with a simple method of changing,extending and optimizing control processes. In a PLC, inputsignals are interconnected in accordance with a specifiedprogram. If “true”, the PLC jumps to the resultant output. Themathematical base for the working is Boolean algebra thatacknowledges only two states for one variable: 0 and 1.Modern PLCs can perform timer and counter functions,memory setting and resetting and mathematical computingoperations.[14] Industries have a variety of production tasksthat are highly repetitive in nature. In spite of each stage beingrepetitive and monotonous, it requires careful attention of theoperator to ensure effective production. Whenever sequentialcontrol and automation is required, PLCs are best suited toaccomplish the task.Supervisory Control and Data Acquisition (SCADA) is a formof widespread industrial control structure that incorporatesnumerous sites and large distances. Here, PLCs are of use inindustrial processes like: manufacturing, production, powergeneration, fabrication and refining. Infrastructure processesrequiring PLCs are: water treatment and distribution, wastewater collection and treatment, oil and gas pipelines, electricpower transmission and distribution, wind farms, civil defencesiren systems and large communication systems.PLCs find applications in the tire industry, steel factories,food and beverages industry, pulp factory, petrochemicalindustry, mining, power, continuous bottle-filling system, 3stage air conditioning system, speed control of DC motor andother similar sequential task as well as process control.[15]10. CONCLUSIONThe requirement for higher quality and reliability in controltechniques has stemmed from a constant demand for betterand more efficient and process machinery. Smart, condensedsolid-state electronic devices have made it feasible to offercontrol systems that can ease maintenance, reduce downtimeand enhance productivity by a large amount. Flexible andproficient working has been offered to the user byprogrammable logic controllers. Starting out as a replacementfor relays, various math and logic manipulations weregradually added to the PLC along with the incorporation ofsmaller cases, faster CPUs, networking and various Internettechnologies. PLCs continue to develop and have becomevital to a great extent of automation processes. New valuesmake an addition to their potential incessantly.IJCATM : www.ijcaonline.org[10] Albert Falcione, Bruce H. Krogh, Design Recovery forRelay Ladder Logic, IEEE Control Systems, First IEEEConference on Control Applications, September 1992.[11] Gregory K. McMillan, Douglas M. Considine (ed),Process/Industrial Instruments and Controls HandbookFifth Edition, McGraw-Hill, 1999.[12] Sanjeev Gupta, S C Sharma, Selection and Applicationof Advance Control Systems: PLC, DCS and PC-basedsystem, Journal of Scientific & Industrial Research, Vol.64, April 2005, pp. 249-255.[13] Prof. Jose A. Rodriguez Mondejar, ProgrammableLogic Controller, UPCO ICAI Departmento deElectronica y Automatica.[14] John R. Hackworth, Frederick D. Hackworth, Jr.,Programmable Logic Controllers: Programming Methodsand Applications.[15] Vishal Kumar Alok, Ajay Goel, To study the differentindustrial applications of PLC through ladder diagrams,National Institute of Technology Rourkela, May 2011.31

a modern revolution, the Programmable Logic Controller (PLC). The PLC has its origins around the 1970’s in the automation and motor manufacturing industries. 2. DESCRIPTION Programmable Logic Controllers (PLCs), also referred to as programmable controllers, are a classification in computers. It is