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  1. 1. <ul><li>What is a PLC? </li></ul>Introduction to Programmable Logic Controllers (PLCs) <ul><li>Hardware components of a PLC </li></ul><ul><li>Memory structure of a PLC </li></ul><ul><li>PLC Modes of Operation </li></ul><ul><li>Wiring Diagram </li></ul><ul><li>Digital Inputs/Outputs </li></ul>
  2. 2. What is a PLC? <ul><li>A Programmable Logic Controller (PLC) is </li></ul><ul><ul><li>a microprocessor based control system, </li></ul></ul><ul><ul><li>designed for use in an industrial environment, </li></ul></ul><ul><ul><li>programmed to sense, activate and control industrial equipment. </li></ul></ul>
  3. 3. Hardware components of a PLC
  4. 4. Memory structure of a PLC
  5. 5. PLC Mode of Operation <ul><li>A PLC operates in one of two modes: </li></ul><ul><ul><li>Program Mode: Download the user’s program from the programming device to the memory of the PLC. </li></ul></ul><ul><ul><li>Run Mode: Execute the user’s program using the scan-based execution model, in three phases: </li></ul></ul><ul><ul><ul><li>Read the state of the inputs and store them in the RAM referred to as the input image memory.. </li></ul></ul></ul><ul><ul><ul><li>Execute the user’s program and update the state of the outputs in the RAM (output image memory). </li></ul></ul></ul><ul><ul><ul><li>Copy the states of the outputs from the RAM to the physical output devices. </li></ul></ul></ul><ul><ul><li>The time needed to complete one scan cycle depends on the length and complexity of the user’s program. </li></ul></ul><ul><ul><li>PLC manufacturers specify the maximum number of program steps (say 1000) and the average scan time (few ms). </li></ul></ul>
  6. 6. Wiring Diagram
  7. 7. DC Voltage Digital Input <ul><li>The optoisolator isolates the internal components of the PLC and the external input devices. </li></ul><ul><li>Diode D1 provides protection during switching from inductive devices. </li></ul><ul><li>When the switch is closed, current flows through the status LED and the opto-isolator, giving a logic 1 to the input port. </li></ul>
  8. 8. Connecting DC Proximity Switches
  9. 9. AC Voltage Digital Input <ul><li>AC input devices switch On and Off the mains voltage. </li></ul><ul><li>The mains voltage must be reduced using a voltage divider. </li></ul><ul><li>The mains voltage must be converted to DC using a rectifier and a smoothing circuit. </li></ul><ul><li>An AC proximity switch switches On and Off the mains voltage by triggering (firing) a triac. The proximity sensor controls the gate current of the triac. </li></ul>
  10. 10. Digital Output (Relay and Transistor) <ul><li>Most PLCs have relay outputs. </li></ul><ul><li>Relay outputs have the advantage that they can control DC as well as AC loads at relatively high currents. </li></ul><ul><li>The disadvantages of relays are due to their mechanical contacts. </li></ul><ul><li>Transistor outputs provide higher switching speeds with more switching life. </li></ul><ul><li>Transistor outputs can only be used with DC loads and are polarity sensitive. A diode is usually used for reverse polarity protection. </li></ul><ul><li>Another diode is usually used to protect the output transistor when switching inductive loads. </li></ul>
  11. 11. Digital Output Using Triacs <ul><li>Triac outputs are used to avoid the problems due to the mechanical contacts of relays. </li></ul><ul><li>An opto-diac is used to fire the triac. </li></ul><ul><li>An RC circuit is usually used to avoid problems due to the phase difference between the voltage and the current when driving inductive loads. </li></ul>