Historical background of PLC

Industrial automation in process control and manufacturing plants leads to introduce the era of manufacturing plants leads to introduce the era of programmable logic controller in short we call it PLC. In the early 1960s and 1970s relays, timers, counters, and sequencers were all consisting of electro-mechanical in nature. Many control panel contained hundreds of these electro-mechanical devices and, in some cases, a mile or more of wire.The primary negative aspect of electro-mechanical control was that reliability was low, in contrast to the maintenance costs associated with keeping the maintenance costs associated with keeping these panels operating, which were extremely high.The automation in industry complained that there all cost to purchase and replace a single relay could run as high as fifty dollar or more.

A second major factor was the time, expense, and labor required when a change in control needs dictated a control panel modification.dictated a control panel modification. In fact, during the model year changeovers, the auto industry discarded entire control panels and replaced them with new ones as the quickest and cheapest solution.The programmable controller is a solid-state electronic device designed in the early 1970s to replace electro-mechanical relays, mechanical replace electro-mechanical relays, mechanical timers, counters, and sequencers. Keep in mind that the first PLCs were little more than relay re-placers

Functional description of PLC

lHardware of PLC:

  1. Power supply module

  2. Central Processing Unit (CPU)

  3. Input / Output modules

  4. Programming device or Programmer

lPLC Input module:

lPLC Output module:

lScanning sequence of PLC:

lAdvantages of PLC:

  1. Smaller physical size than hard-wire solutions.

  2. Easier and faster to make changes.

  3. PLC has integrated diagnostics and override                   

        functions.

  4. Diagnostics are centrally available.

  5. Application can be immediately documented.

Sensors & Transducers

lPressure sensing & measuring elements

lLevel sensing & measuring elements

lTemperature sensing & measuring elements

lFlow sensing & measuring elements

Concept of different programming software

lCX-Programmer for Ladder diagram

lCX-Process tool for Function block development

lNS-Designer for Operator station

lCX-Supervisor for SCADA application

lCX-Simulator for Program testing

CX-Programming Environment

CX-Process tool Environment

NS-Designer Environment

CX-Supervisor Environment

Maintenance of PLC

l Preventive Maintenance

l On line Maintenance

l Off line Maintenance

Before maintenance of PLC an Engineer must have:  

l Training

l Proper documentation

l Spares parts

l Practical working experience on PLC

Introduction to PLC

l Control:

By control we mean that the physical variables/process variable are kept constant as the desired value.

l Type of control:

          1. Manual control

          2. Automatic control

l Type of Automatic control:

        1. Open loop control

        2. Close loop control

l Type of Close loop control:

        1. Feed-forward control

        2. Feed-back control

l Advantage of Automatic control:

       1. Increased production

       2. Improve quality

       3. Saving in Raw material, Energy, Manpower.

       4.Increased Safety, etc.

l PLC:

         A Programmable Logic Controller is a special form of microprocessor based controller that uses a programmable memory to store instruction and to implement function such as logic, sequence, timing, counting, comparing and the processing of analog signals in order to control machines and process. 

l Advanced PLC:

        The modern advanced PLC’s can handle various types of analog signals. It can handle Thermocouple (TC), Resistance Temperature Detector (RTD), Pulse, High speed count signal, Voltage, Current, etc.

         So the modern PLC can provide a complete solution for operating and process plant.

l General specification of PLC:

  1. Input output specification

  2. Control specification

  3. Peripheral function

  4. Maintenance function

l SCADA system:

It means Supervisory Control and Data Acquisition. SCADA is graphical software for Human Machine Interface (HMI).