PLCProgrammable Logic Controllers

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PLC(Programmable Logic Controllers)

• Definition PLC is a general purpose control
  system that accepts input from such sources as
  push buttons, limit switches, temperature,
  pressure, and flow sensors. It is capable of
  generating outputs to such devices as load
  relays, motor starters, stepping motors, solenoid
  valves and even servo drives.

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History

• In the late 1960s PCs were first introduced. The
  primary reason for designing such a device was
  eliminating the large cost involved in replacing
  the complicated relay based machine control
  systems. Companies at the time proposed computer
  based schemes, one of which was based upon PDP-8.
  The MODICON 084 brought worlds first PLC into
  commercial production.
• In the mid 70s the dominant PLC technologies
  were sequencer state-machines and the bit-slice
  based CPU. The AMD 2901 and 2903 were quite
  popular in Modicon and A-B PLCs.

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History(cont.)

• Conventional microprocessors lacked the power to
  quickly solve PLC logic in all but smallest PLCs.
  As the conventional microprocessors evolved,
  larger and larger PLCs were being based upon
  them.
• The 80s saw an attempt to standardize
  communication with General Motors manufacturing
  automation protocol. It was also a time for
  reducing the size of the PLC and making them
  software programmable through symbolic
  programming on personal computers instead of
  dedicated programming terminals or handheld
  programmers. Today the worlds smallest PLC is
  about the size of a single control relay.

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History(cont.)

• The 90s have seen gradual reduction in the
  introduction of new protocols, and the
  modernization of the physical layers of some of
  the more popular protocols the survived the
  1980s. The latest standard has tried to merge
  PLC programming languages under one international
  standard. We now have PLCs that are programmable
  in function block diagrams, instruction list, C
  and structured text all at the same time!

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Basic working principle

• The program for a process is stored in the solid
  state memory, As a rule, a program is not changed
  (except for the corrections) while the controller
  is dedicated to a particular process. Program
  controls the PLC in a way that any 1 (ON
  signal, TRUE pulse) coming from input unit, it
  performs the necessary process. This process
  usually the controls of an output.
• PLC is cyclic controller in that it goes through
  all the inputs analysis and outputs on a
  repetitive basis. The length of time for a cycle
  is a function of the number of rungs in a ladder
  diagram().

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Basic working principle

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Why we use PLC instead of computers

• Cheaper
• Compact
• PLC logic can be changed much more easily and
  quickly
• Less sensitive than computers and can be designed
  to survive better in harsh and dirty industrial
  environment
• Reprogrammable (if machine is no longer needed,
  its PLC may be easily reprogrammed for another)

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Parts of PLC

• Processor Process information
• Memory Used to store the control program and the
  status of the input and output used in computing
• Input Check outs the devices that supplies
  information rapidly and repeatedly
• Output Supplies output signal to the necessary
  equipments
• Power Unit Supplies the necessary power for PLC
• Programming Unit Enables the user to enter and
  modify the software programme

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Parts of PLC
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Inner Parts of PLC

• Input relays These are connected to the outside
  world. They physically exist and receive signals
  from switches, sensors,etc. Typically they are
  not relays but transistors.
• Internal Utility RelaysThese do not receive
  signals form outside world nor do they physically
  exist. They are simulated relays that are
  dedicated to performing only one task. Some are
  always on while some are always off. Some are on
  only during power on and typically used for
  initializing data that was stored.

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Inner Parts of PLC(cont.)

• Counters These again do not physically exist.
  They are simulated counters and they can be
  programmed to count pulses. Typically these
  counters can count up, down or both up and down.
  Since they are simulated they are limited in
  their counting speed. Some manufacturers also
  include high speed counters that are hardware
  based.

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Inner Parts of PLC(cont.)

• Timers These also do not physically exist. They
  come in many varieties and increments. Increments
  vary from 1ms through 1s.

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Inner Parts of PLC(cont.)

• Output Relaysthese are connected to the outside
  world. They physically exist and send on/off
  signals to solenoids, lights,etc. They can be
  transistors, relays etc.
• Data Storage Typically these are registers
  assigned to simply store data. They are usually
  used as a temporary storage for math or data
  manipulation. They can also be used to store the
  data when power is removed from the PLC. Upon
  power up they will still have the same contents
  as before power was removed.

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Typical PLC Applications

• Packaging
• Robots
• Mixers
• Textile machines
• Printing machines
• Traffic lights
• Materials handling (etc)

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Programming

• We can show the relation between input and output
  by using different formats
• Boolean algebra
• Truth table
• Electrical schema (not necessary in our class)
• Computer functions
• Ladder diagram()

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Boolean Algebra and Truth Table

• OR operation ( )
• (ABC)
• Truth table

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Boolean Algebra (continue)

• AND operation ( . )
• (A.BC)
• Truth table

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Boolean Algebra (continue)

• NOT operation ( A )
• (A B)
• Truth table

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Boolean Theorems

• A B B A 12) (AB)CA(BC)
• A.B B.A 13) (A.B).CA.(B.C)
• A 0 A 14) A.BA.CA.(BC)
• A.1 A 15) (AB)A.B ()
• A1 1 16) (A.B)AB
• A.0 0
• A A A
• A.AA
• AA1
• A.A0
• (A) A

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Computer functions

• LOAD Loads input status from memory into the
  register A.
• STORE Stores the information from register A
  into memory.
• OUT Changes output status from memory.
• AND Operates AND function between memory and
  register A.
• OROperates OR function between memory and
  register A.
• NOTComplements the register A.
• JMP Loops to the given line name.

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Example

• Consider following set of equations
• LAMP1PB1.PB2 Rewrite LAMP1(PB1PB2) (Rule
  )
• LAMP2PB1.PB2
• LAMP3(LAMP1 LAMP2)
• Solution
• START LOAD PB1
• OR PB2
• NOT
• STORE LAMP1
• OUT LAMP1
• LOAD PB1
• AND PB2
• STORE LAMP2
• OUT LAMP2
• OR LAMP1
• NOT
• STORE LAMP3
• OUT LAMP3
• JMP START

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SCAN TIME

• Program controls the output with respect to the
  state of inputs,
• continuously it must control the status. One pass
  of the program
• is one operating cycle. During each cycle, all
  inputs to the
• controller are sampled, logic evaluated and
  outputs are set
• accordingly. Such a cycle is often referred as a
  scan, and the time
• required for one cycle is called the scan time.
• Ex AA
• Program segment (only 4 lines, tave average
  time to pass one line)
• LOAD A
• NOT
• STORE A
• OUTPUT A
• Scan time tscan 4.tave

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SCAN TIME(continue)

• Scan time is an indication of how often each
  logic function is
• evaluated.
• Let say we need to detect a short event by a
  program, we need
• to make our scan time lower than the events
  duration, otherwise
• some operations can not be detected.
• We can solve this problem by optimizing logical
  relations in
• our programme, which creates shorter scan time,
  or we can
• increase event time.
• Best case to make the event period twice longer
  than the scan
• time.