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• Programmable Logic Controller (PLC)


  A digitally operating electronic apparatus which uses a programming memory for the internal storage of instructions for implementing specific functions such as logic, sequencing, timing, counting and arithmetic to control through digital or analog modules, various types of machines or process
• PLC


  • Designed for (logic) control and regulation tasks
  • Programmed by non-specialists
  • Well adapted to industrial environment
• PLC was developed to replace relays in the late 1960s
• Costs of PLCs dropped and they became popular by 1980s
• PLCs are now used in many industrial designs
• Advantages of PLCs

  • Less wiring
  • Easier and faster to make changes
  • Trouble shooting aids make programming easier and reduce downtime
  • Reliable components make these likely to operate for years before failure
  • They are cost-effective
  • They are flexible, reliable and compact
  • They have significant advantages over traditional control systems based on relay or pneumatics
• PLC

  • Logic control tasks such as interlocking, sequencing, timing and counting (previously undertaken with relays or pneumatics)
  • Variety of calculation, communication and monitoring tasks
• Leading brands of PLC


  • American: Allen Bradley, Gould Modicon, Texas Instruments, General Electric, Westinghouse, Cutter Hammer, Square D
  • European: Siemens, Klockner & Mouller, Festo, Telemechanique
  • Japanese: Toshiba, Omron, Fanuc, Mitsubishi
• Areas of PLC application

  • Manufacturing/Machining
  • Food/Beverage
  • Metals
  • Power
  • Mining
  • Petrochemical/Chemical
• PLC sizes


  • Small: up to 128 I/O's and memories up to 2 Kbytes
  • Medium: up to 2048 I/O's and memories up to 32 Kbytes
  • Large: up to 8192 I/O's and memories up to 750 Kbytes
• Major components of a common PLC

  • Power Supply
  • I/O Modules
  • Processor
  • Programming Device
• Power Supply


  Provides the voltage needed to run the primary PLC components
• I/O Modules


  Provide signal conversion and isolation between the internal logic-level signals inside the PLC and the field's high level signal
• Processor


  Provides intelligence to command and govern the activities of the entire PLC systems
• Programming Device


  Used to enter the desired program that will determine the sequence of operation and control of process equipment or driven machine
• I/O interface


  Connects the PLC to external field devices and conditions the various signals received from or sent to the external input and output devices
• Input modules

  Convert signals from discrete or analog input devices to logic levels acceptable to PLC's processor
• Output modules


  Convert signal from the processor to levels capable of driving the connected discrete or analog output devices
• Functions of I/O modules

  • Prevent voltage transients from damaging the processor
  • Help reduce the effects of electrical noise
• Discrete input


  An input that is either ON or OFF, referred to as logic 1 or logic 0
• Analog input


  An input signal that has a continuous value, such as 0-20mA, 4-20mA or 0-10V
• Digital output


  An output that is either ON or OFF
• Analog output


  An output signal that has a continuous value, such as 0-20mA, 4-20mA or 0-10V
• Processor


  Contains the PLC's microprocessor, its supporting circuitry, and its memory system
• Memory system

  Has two parts: a system memory and an application memory
• System memory includes an area called the EXECUTIVE, composed of permanently-stored programs that direct all system activities
• PLO


  Programmable Logic Output
• 0 to 10V


  Voltage range
• Supply air


  Air supply
• Pneumatic control valve


  Valve that controls pneumatic systems
• Processor


  The PLC's microprocessor, its supporting circuitry, and its memory system
• Processor module


  • Contains the PLC's microprocessor, its supporting circuitry, and its memory system
• Main function of the microprocessor

  1. Analyze data coming from field sensors through input modules

  2. Make decisions based on the user's defined control program

  3. Return signal back through output modules to the field devices
• Field sensors


  • Switches
  • Flow
  • Level
  • Pressure
  • Temperature transmitters
• Field output devices


  • Motors
  • Valves
  • Solenoids
  • Lamps
  • Audible devices
• Memory system in the processor module

  Has two parts: a system memory and an application memory
• System memory


  Includes an area called the EXECUTIVE, composed of permanently-stored programs that direct all system activities
• System memory


  Contains the routines that implement the PLC's instruction set, which is composed of specific control functions such as logic, sequencing, timing, counting, and arithmetic
• System memory


  Generally built from read-only memory devices
• Application memory


  Divided into the data table area and user program area