INTRODUCTION TO MECHATRONICS AND ROBOTICS

Unit V class 1

IntroductionWHAT IS A PLC?PLC- Programmable Logic ControllerIt is a digital electronic device that uses a programmable memory to store instructions and to implement functions such as logic, sequencing, timing, counting and arithmetic in order to control machines.Used for logical and switching operations.

PLCCONTROL PROGRAMINPUTS FROM DEVICESOUTPUTS TO DEVICESRugged and designed to withstand vibrations, temperature, humidity and noise.

Interfacing for inputs and outputs is inside the controller - Possible to modify a control system without having to rewire the connections to the input and output devices

Easily programmable.The Need for PLCsThe PLC is primarily used to control machinery. A program is written for the PLC which turns on and off outputs based on input conditions and the internal program. In this aspect, a PLC is similar to a computer. However, a PLC is designed to be programmed once, and run repeatedly as needed.Hardwired panels were very time consuming to wire, debug and change. GM identified the following requirements for computer controllers to replace hardwired panels.Solid - state not mechanicalEasy to modify input and output devicesEasily programmed and maintained by plant electriciansBe able to function in an industrial environment

PLC Size1. SMALL- it covers units with up to 128 I/Os and memories up to 2 Kbytes. - these PLCs are capable of providing simple to advance levels or machine controls.2. MEDIUM- have up to 2048 I/Os and memories up to 32 Kbytes.3. LARGE- the most sophisticated units of the PLC family. They have up to 8192 I/Os and memories up to 750 Kbytes.- can control individual production processes or entire plant.

BASIC STRUCTURE OF PLCIt consists of the followingCentral processing UnitMemoryInput / output circuitryBusBatteryProgram step is an instruction for an event to occurProgram task consists of number of stepsSmall PLC have 300 to 1000 steps

ARCHITECTUTRE OF PLC

Input / OutputProvides the interface between the system and outside worldInput given through keyboard or PC link with appropriate softwareSensors and actuators directly connected to itCommon output voltage are 24 V and 240 VOutput may be relay type, transistor type or triac typeRelay typeAble to switch current in external circuitIt is used for switching both d.c and a.cTransistor typeGives faster switching actionConventional current flow + to output receives current from output module and referred as sinkCurrent flows from output module to output load is referred as sourcingTriac typeUsed to control external load connected to a.c.supply

Input / Output processingBasic form of programming commonly used with PLC is Ladder ProgrammingScan the inputsSolve the logic operationsSet or reset the output for that rugMove on to the next rung and repeat the operationsFollows until the end of the programMethods for input/output processingContinuous UpdatingCPU scans the input channelsInput points examined individuallyThere is a built in delay for 3msThe output are latched until its next updatingMass input / output copyingEach input and output has address in memoryAt start of each program the CPU scans all input and stores in memoryWhen program is executed the input data is read from RAM and logic operation is carried outThe resulting output signal is stored in reserved input output section in RAM

Key WordsRugged-RoughSynchronization-CoordinationBuffer-barrierIsolation-separationRung-StepLatch-boltLadder-Step

Mind mapPLCCPUInput/output ProcessingBusBatteryMemoryMass i/o

Continuous updating

SummaryPLC used for logical and switching operations.PLC a digital electronic device that uses a programmable memory to store instructions.Solid - state not mechanicalAble to function in an industrial environment.Applications - Manufacturing / Machining, Food / Beverage, Metals, Power, Mining, Petrochemical / Chemical, etc.It is Flexible, Faster response time and Less and simpler wiring.It consists of Input module, output module, CPU, memoryPLC Size Small, Medium, LargeMCQ & Higher Order Questions1. Prior to 1960 a control function was mostly to be provided by:A. Personal Computer B. Programmable Logic Controller C. Electromechanical relays2. In 1968 GMs Hydromatic Division realized the need for a programmable logic controller. What company proposed and delivered the Modular Digital Controller?A. Allen-Bradley B. Gould Modicon C. IBM D. Chevy3. Isolation between I/O modules and the PLC central processing unit is typically performed by what device?A. Gyrator B. Optical isolator C. Transformer D. Ethernet4. PLC are program using what language?A. Natural language such as English B. C language C. Relay-ladder logic5. The period of time it takes the CPU to completely evaluate the logic of a PLC is called what?A. Execution time B. Ladder time C. Scan time D. Interrupt time6. What are the logic functions used for switches (a) in series (b) in parallel?