PLC (Programmable Logic Device)

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PLC stands for programmable logic controller. It is basically a digital computer modified specially to perform control task. This system is user programmable. It is used for automation of electromechanical process such as control of amusements swings, assembly lines, control of machinery on factory or controlling of light fixtures or any activity that required high reliability, ease programming and process fault diagnosis.

What Is PLC?

PLC (programmable logic controller) is basically an automatic controlling system. It is developed for an electronic replacement for hard-wired relay logic system for machine control. It is used as a process control system, Distribution control system for industrial automation.

PLC is basically a special purpose computer specially design to perform control task. It has no display, no keyboard, no printer, no hard drive and hides in the control panel out on the factory floor, but it is still a computer.

Initially, the PLC was a replacement for panels of relay, devices that turn on and off.

PLC is a multiple input multiple and multiple output system. In PLC input is given in any from (physical, analog and digital ) temperature, pressure, humidity, electrical signal, digital signal, limit switches etc. On output side, PLC operate electric motors, pneumatic or hydraulic cylinder, lamp, magnetic relay. The size of PLC is depend on the number of input and output. PLC is available in three size small, medium and large depend how many task perform by PLC.

 Small PLC devices with tens of inputs and outputs (I/O), Medium PLC devices with hundred of I/O, large rack-mounted PLC devices with thousands of I/O, and which are often connected to other PLC and SCADA systems.     

History of PLC

The first PLC, named Modicon 084, was developed by Dick Morley in 1968 for “General Motors”. The term PLC are the register trademark (For manufacturing, marketing and selling) by Allen-Bradley Company (now a brand owned by Rock Well Automation).

Before PLC, control sequencing and safety, interlock logic for manufacturing automobiles composed of relays, cam timers, Drum sequencers and dedicated closed loop controller. Since these could number in hundreds or even thousands, So the process for updating such facilities was very time consuming and expensive, to change the logic electricians needs to individually rewire relays.  For example: Before PLC in railways to change rail track we use conventional relay (manually operated), but now a day this is done by automatically using PLC.

PLC is a microprocessor based controlling system. They are specially designed for industrial use and are used to control many automatic process in industries.

It use a programmable memory for internal storage of user oriented instruction for implementing specific function.

Major Type of Industrial Control Systems

There are number of controlling system currently operating in various industries

  1. PLC (programmable logic controller): PLC is a microprocessor based controlling system based on Boolean logic operation. Where as some module use timer and some have continuous control. These device are used to control various process. PLC control the component in DCS  and SCADA system.
  2. DCS (Distributed Control System): Distributed Control System is basically a decentralize  control system. It contain number of decentralize elements and all the process are controlled by these element. So, human interaction is minimize so, labor cost and injuries is also reduce.
  3. SCADA: Full form of SCADA is Supervisory Control and Data Acquisition system.It is a high performance controlling system for industrial automation. It is  basically a centralize  control system. This system is composed of various sub system like RTU (Remote Telemetry Unit), HMI (Human Machine Interface), PLC (programmable logic controller).

Advantages and Disadvantages of PLC

Advantages

  1. Simple and easy to program.
  2. Program change with out any system intervention and there is no internal rewiring.
  3. Simple and less expensive.
  4. Low maintenance and high speed.
  5. Smaller in size, easy to program, more reliable than conventional relay logic system.
  6. High accuracy, error detection and  isolation are easy.
  7. Interfacing for input and output already inside the controller.

Disadvantage

  1. Fault detection or isolation are not easy it required skill full work force.
  2. Too much time and work required in connecting wire.
  3. Difficulty face during change or replacement of any component in PLC.
  4. When a problem occur hold-up time is indefinite (Usually long).

Application Of PLC

PLC were designed to replace traditional relay logic system. It was programmed using “ladder logic”. Which is a replacement of traditional relay logic.

  1. PLC are specially designed for industrial use and are used to control many automatic process in industries. It is used for automation of electromechanical process such as control of amusements swings, assembly lines, control of machinery on factory or controlling of light fixtures or any activity that required high reliability, ease programming and process fault diagnosis.
  2. PLC control device such as push button, limit switch, pressure switches etc. These are the input device for PLC, provide input signal to the unit.

You tube video on Introduction of PLC

You tube video on Introduction of PLC in HINDI


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