Overview of ProgrammableLogic Controllers (PLCs)Dr. Fernando Rios-GutierrezECE4951- Design WorkshopSpring 2007

Lecture ObjectivesExpose basic characteristics of PLC.Describe the various subparts of a PLChardware and modules.Outline the basic sequence of operationfor a PLC.Describe the hardware input/outputsections available.ECE 49514951-Fernando Rios

PLCA Programmable Logic Controller (PLC) isa specialized computing system used forcontrol of industrial machines andprocesses.A PLC is a computer designed to work inan industrial environmentPLCs are equipped with specialinput/output interfacesPLCs are programmed using a controlprogramming language.ECE 49514951-Fernando Rios

PLC ApplicationsOriginally hardwired arrays of relays wereused to control the operation of heavymachines that contain motors and otherhigh power devices.PLCs were originally used to substitute theswitching relay networks used in industrialapplications, but now they can also be usedimplement other tasks such as timing,delaying counting, calculating, comparingand processing of analog signals.ECE 49514951-Fernando Rios

Relay NetworksECE 49514951-Fernando Rios

PLC AdvantagesIncreased ReliabilityMore FlexibilityLower CostFaster ResponseEasier to troubleshootRemote control capabilityCommunication CapabilityECE 49514951-Fernando Rios

PLC DisadvantagesIn contrast to microcontroller systems thathave what is called an open architecture,most PLCs manufacturers offer onlyclosed architectures for their productsPLC devices are proprietary, whichmeans that parts and software from onemanufacturer can’t easily be used incombination with parts of anothermanufacturer, which limits the design andcost optionsECE 49514951-Fernando Rios

PLC Basic ArchitectureECE 49514951-Fernando Rios

PLC ConfigurationsThere are two PLC basic configurationsthat commercial manufacturers offer. Fixed ConfigurationModular ConfigurationECE 49514951-Fernando Rios

Fixed ConfigurationECE 49514951-Fernando Rios

Modular ConfigurationECE 49514951-Fernando Rios

Input/Output ModulesECE 49514951-Fernando Rios

Programming the PLCECE 49514951-Fernando Rios

Principles of OperationTo get an idea onhow PLC operateslets consider thefollowing simpleprocess controlproblem.ECE 49514951-Fernando Rios

Process Control DescriptionECE 49514951-Fernando Rios

PLC Inputs ConnectionECE 49514951-Fernando Rios

PLC Output ConnectionsECE 49514951-Fernando Rios

Programming the PLCTo implement the described system usingthe PLC, a description in ladder logicshould be provided.Next the ladder logic is complied andtranslated to basic instructions and downloaded to the internal memory.During programming the PLC should be inthe Terminal or Programming mode.ECE 49514951-Fernando Rios

Ladder Logic ProgramECE 49514951-Fernando Rios

Running the ProgramFor the program to operate the PLCshould be put on the Run mode orOperating cycle.In the operating cycle first the controllerexamines the inputs and their status isrecorded in the PLC memory, then theladder diagram is evaluated, and changesare send to the outputs accordingly.ECE 49514951-Fernando Rios

Scan TimeThe completion of a cycle of the controlleris called a Scan.The scan time needed to complete a fullcycle by the controller gives the measureof the speed of execution for the PLC.Generally outputs are updated in memoryduring the scan but the actual output isupdated until the end of the programduring the I/O scan.ECE 49514951-Fernando Rios

Process ModificationECE 49514951-Fernando Rios

PLC VS ComputerPLC ComputerDesigned for extremeindustrial environmentsCan operation in hightemperature and humidityHigh immunity to noiseIntegrated commandinterpreter (proprietary)No secondary memoryavailable (in the PLC)Optimized for Single task ECE 49514951-Fernando RiosDesigned mainly for dataprocessing and calculationOptimized for speedCan’t operate in extremeenvironmentsCan be programmed indifferent languagesLost of secondary memoryavailableMultitasking capability

PLC SizeManufacturers offer five sizes of PLCs:Nano (up to 16 I/O points)Micro (more than16 I/O points, up to 64 I/O points)Small (up to 960 I/O points)Medium (multitasking - control of severalprocesses)Large (control management of several PLCs)ECE 49514951-Fernando Rios

PLC Instruction SetThe instruction set for a particular PLCcould range from 15 instructions for smallunits up to 100 instructions for larger unitsECE 49514951-Fernando Rios