ME 71 Lesson 3 and 4

• Programmable Logic Controllers (PLCs) were initially designed to replace relay logic boards to sequence device actuation and coordinate activities
• PLC


  Accepts input from a series of switches and sends output to devices or relays
• Functions of controllers


  • On-off control
  • Sequential control
  • Feedback control
  • Motion control
• Types of control devices

  • Mechanical control - cam, governor, etc.
  • Pneumatic control - compressed air, valves, etc.
  • Electromechanical control - switches, relays, a timer, counters, etc.
  • Electronics control - similar to electromechanical control, except uses electronic switches
  • Computer control
• PLC CPU System


  • User Ladder Diagram
  • Working memory registers
  • Input Flag
  • Output
• Types of PLC output modules

  • Relay (volt-free)
  • Transistor
  • Triac
• Devices a PLC interacts with

  • Input relays (contacts)
  • Internal utility relays (contacts)
  • Counters
  • Timers
  • Output relays (coils)
  • Data storage

  
• Types of switches


  • Basic switch, operated by a mechanical level
  • Push-button switch
  • Slide switch
  • Thumbwheel switch
  • Limit switch
  • Proximity switch
  • Photoelectric switch
• Relay


  A switch whose operation is activated by an electromagnet
• Counter


  Digital counters output in the form of a relay contact when a preassigned count value is reached
• Timer


  Consists of an internal clock, a count value register, and an accumulator, used for timing purposes
• For process control, it is desired to have the process start (by turning on a motor) five seconds after a part touches a limit switch, and the process is terminated automatically when the finished part touches a second limit switch. An emergency switch will stop the process any time when it is pushed.
• PLC Architecture


  Programmable controllers replace most of the relay panel wiring by software programming
• PLC Components


  • Processor
  • Memory
  • I/O
  • Power supply
  • Peripheral
• Processor


  Microprocessor based, may allow arithmetic operations, logic operators, block memory moves, computer interface, local area network, functions, etc.
• Types of memory


  • ROM (Read Only Memory)
  • RAM (Random Access Memory)
  • PROM (Programmable Read Only Memory)
  • EEPROM (Electronically Erasable Programmable ROM)
  • EPROM (Erasable Programmable Read Only Memory)
  • EAPROM (Electronically Alterable Programmable Read Only Memory)
  • Bubble Memory
• I/O modules


  • AC voltage input and output
  • DC voltage input and output
  • Low level analog input
  • High level analog input and output
  • Special purpose modules, e.g. high speed timers, Stepping motor controllers, etc. PID, Motion
• PLC


  A typical PLC
• PLC Components


  • Processor
  • Memory
  • I/O
  • Power supply
  • Peripheral
• Memory types


  • ROM (Read Only Memory)
  • RAM (Random Access Memory)
  • PROM (Programmable Read Only Memory)
  • EEPROM (Electronically Erasable Programmable ROM)
  • EPROM (Erasable Programmable Read Only Memory)
  • EAPROM (Electronically Alterable Programmable Read Only Memory)
  • Bubble Memory
• I/O types


  • AC voltage input and output
  • DC voltage input and output
  • Low level analog input
  • High level analog input and output
  • Special purpose modules, e.g. high speed timers, Stepping motor controllers, etc. PID, Motion
• Ladder diagram


  A means of graphically representing the logic required in a relay logic system
• A ladder diagram (also called contact symbology) is a means of graphically representing the logic required in a relay logic system
• A PLC resolves the logic of a ladder diagram (program) rung by rung, from the top to the bottom
• Usually, all the outputs are updated based on the status of the internal registers. Then the input states are checked and the corresponding input registers are updated. Only after the I/Os have been resolved, is the program then executed. This process is run in a endless cycle. The time it takes to finish one cycle is called the scan time.
• PLC Instructions


  • Relay
  • Timer and counter
  • Program control
  • Arithmetic
  • Data manipulation
  • Data transfer
  • Others, such as sequencers
• Logic States


  ON: TRUE, contact closure, energize, etc.

  OFF: FALSE, contact open, de-energize, etc.
• Do not confuse the internal relay and program with the external switch and relay. Internal symbols are used for programming. External devices provide actual interface.

  
• Relay


  Consists of two parts, the coil and the contact(s)

  
• Timers


  • Retentive on delay

  Retentive off delay

  Reset
• Counters


  • Counter up

  Counter down

  Counter reset
• Sequencer


  Used with machines or processes involving repeating operating cycles which can be segmented into steps
• Sensor


  An element which when subjected to some physical change experiences a relative change
• Transducer


  A device, usually electrical, electronic, or electro-mechanical, that converts one type of energy into another for various purposes including measurement or information transfer
• Types of sensors/transducers


  • Active
  • Passive
• Active sensors/transducers


  • Have an auxiliary source of power that supplies a major part of the output power
• Passive sensors/transducers


  • Output energy is supplied entirely or almost entirely by its input signals
• Sensor


  • Sensor you often encounter
• Static characteristics


  Values given when steady state conditions occur
• Dynamic characteristics


  Refer to the behaviour between the time that the input value changes and the time required given by a transducer to settle down to steady state values