8.0 Programmable Logic Controllers 1

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8.0 Programmable Logic Controllers 1
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Programmable Logic Controllers

• Introduction/background
• Connections
• Architecture
• Programming

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Books

• PLC Programming for Industrial Automation by
  Kevin Collins
• ISBN 978-1-84685-598-6
• Programmable Logic Controllers
• Festo basic level TP 301
• Order number093311

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8.1 Introduction/background
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Background

• The first Programmable Logic controller was
  developed about 1968 by engineers in General
  Motors
• The original task of a PLC involved the
  interconnection of input signals according to a
  program and to switch the corresponding output

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Background

• The task that the PLC can perform has rapidly
  multiplied
• The plc now performs the logic functions of
  relays, timers, counters and sequencers.

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Why use a PLC ?
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Why use a PLC ?
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Why use a PLC ?
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Applications of a PLC
PLC used to control a CNC lathe
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Introduction

• A plc is a digital computer with multiple inputs
  and outputs.
• Inputs are switches and sensors
• Outputs are actuators (solenoids, motors, lights,
  alarms etc.)

Toshiba plcs
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Operation of a PLC
Input
Processing
Output
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Operation of a PLC
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Introduction

• Advantages
• Low cost
• Reliability
• Reprogrammed

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PLC Architecture
Inputs
Input Module
CPU
Output Module
Outputs
Programming Input
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Input Modules

• Provide an interface between switches or sensors
  and the plc.
• Input module circuits have opto-isolators to
  protect the plc.
• Opto-Isolator

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PLC Architecture
Inputs
Input Module
CPU
Output Module
Outputs
Programming Input
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Output Modules
Output Module

• Relay (Volt-free) Signal from plc operates
  relay.
• Transistor Faster but only for d.c. and low
  current devices.
• TriacsUsed for a.c. devices. Must be protected
  against over-current.

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Output Modules

• Relay Output
• The power supply is connected to the output
  common port.
• The solenoid, light, etc is connected to the
  output main port.
• The plc switches the output on by internally
  connecting the common port to the output port.

Internal relay contact
plc
Common port
Output port
Power Supply ()
solenoid
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8.2 Architecture
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PLC Architecture
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Programming Methods

• Ladder Logic
• Sequential Function Chart (Grafcet)
• Others
• Statement List (mnemonics)
• Logic Blocks

Grafcet
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Ladder Logic

• Inputs
• n/o contact
• n/c contact
• Outputs

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Ladder Logic

• Series input connection
• Parallel input connection

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Ladder Logic

• Outputs can be connected in parallel
• But not in series

Output 1
Output 2
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Ladder Logic

• Series input connection

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8.3 PLC simulator
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PLC simulator

• During the lab sessions we will use the
  Matsushita NIAS FP1-C14 plc, this plc has been
  adapted to allow for quick connections of both
  inputs and outputs.
• Also a TRiLOGI plc simulator is available for
  download

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Trilogi Simulator

• Open the plc Program Editor

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• Click on red arrow to get toolbar

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• Select normally open (n/o) input symbol.

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• Click on the blue bar to get the
• Define Label Name dialog box

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• Call the input Start_Button or Start Button and
  press return.

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• Select another n/o contact and call it Stop Button

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• It appears in series with the Start Button contact

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• Clicking on the right hand box of the tool bar
  allows you to change the selected contact to n/c.

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Trilogi Simulator

• Now select an output and call it light

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Trilogi Simulator
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Trilogi Simulator
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• Highlight Start_Button again and select the
  parallel contact button.
• Call the contact light and it will act as a latch
  for the light.

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• Click on the Simulate menu and select Run (All
  I/O Reset)

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• The two inputs and one output can be seen on the
  simulator.
• Click on the Start_Button input to switch it on.

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• The output Light comes on and stays latched on,
  until the Stop_Button is pressed.

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Ladder Logic

• An output can also appear as a contact to the
  left of the ladder diagram

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Ladder Logic

• Similarly, in the example below, when Output one
  is off, Output 2 is on.

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8.4 Example
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PLC Inputs and Outputs
S4
N.O.
N.C.
N.O.
N.O.
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PLC Inputs and Outputs
S1
S2
S3
S4
N.O.
N.C.
N.O.
N.O.
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PLC Inputs and Outputs
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PLC Inputs and Outputs
S4
N.O.
N.C.
N.O.
N.O.
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PLC Inputs and Outputs
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PLC Inputs and Outputs
S4
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PLC Inputs and Outputs
S4
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PLC Inputs and Outputs
S1
S2
S3
S4
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8.5 Summary
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Summery

• History of plc controllers and an
  Introduction/background
• Connections (how they are made)
• Architecture (structure of a plc)
• Programming (outline of ladder logic)

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END