What is a Programmable Logic Controller (PLC) ? - PowerPoint PPT Presentation


Title: What is a Programmable Logic Controller (PLC) ?


1
What is a Programmable Logic Controller (PLC) ?
2
Agenda
  • History
  • PLC Overview
  • Hardware
  • Inputs/Outputs
  • Operation
  • Programming Software
  • Communication
  • What you need to know when specifying a PLC
  • Glossary

3
History of the PLC
  • First Programmable Logic Controllers were
    designed and developed by Modicon as a relay
    replacer for GM and Landis.
  • These controllers eliminated the need for
    rewiring and adding additional hardware for each
    new configuration of logic.
  • The new system drastically increased the
    functionality of the controls while reducing the
    cabinet space that housed the logic.
  • The first PLC, model 084, was invented by Dick
    Morley in 1969.
  • The first commercial successful PLC, the 184, was
    introduced in 1973 which was designed by Michael
    Greenberg.

4
What is a PLC?
The Basic Block
CPU
A PROGRAMMABLE LOGIC CONTROLLER is a solid state
control system that continuously monitors the
status of devices connected as inputs. Based
upon a user written program, stored in memory, it
controls the status of devices connected as
outputs.
5
What is a PLC with Distributed I/O?
Getting Complex
CPU
6
What is a PLC with Distributed Control?
Getting Really Complex
Remote
7
Inside a PLC
Communications Port
Circuits
Circuits
Central Processor Unit (CPU)
Output
Input
CR
High Voltage
High Voltage
Isolation Barrier
Isolation Barrier
Low Voltage
or
DC Power Supply
AC Power Supply
8
Inside a PLC
  • The CPU contains an Executive program that
    tells the PLC how to
  • Execute the control Instructions
  • Users Program
  • Communicate with other devices
  • Other PLCs, Programming devices, I/O devices,
    etc.
  • Perform Housekeeping activities
  • Diagnostics, etc
  • This program is stored in nonvolatile memory
  • Meaning that the program will not be lost if
    power is removed

9
What are Inputs?
  • Switches and Pushbuttons
  • Sensing Devices
  • Limit Switches
  • Photoelectric Sensors
  • Proximity Sensors
  • Condition Sensors
  • Pressure Switches
  • Level Switches
  • Temperature Switches
  • Vacuum Switches
  • Float Switches
  • Encoders

10
What are Outputs?
  • Valves
  • Motor Starters
  • Solenoids
  • Actuators
  • Control Relays
  • Horns Alarms
  • Stack Lights
  • Fans
  • Counter/Totalizer
  • Pumps
  • Printers

11
PLC Operating Cycle
  • Four Steps in the PLC Operations
  • Input Scan
  • Scan the state of the Inputs
  • Program Scan
  • Processes the program logic
  • Output Scan
  • Energize/de-energize the outputs
  • Housekeeping
  • This step includes communications, Internal
    Diagnostics, etc.
  • The steps are continually repeated - processed
    in a loop

12
Programming the PLC
  • Types of Programming Software Languages
  • Ladder Logic (LL)
  • IEC Format
  • Function Block Diagram (FBD)
  • Structured Text (ST)
  • Instruction List (IL)
  • Ladder Diagram (LD)
  • Sequential Function Chart (SFC) - also known as
    Grafcet

13
Ladder Logic
Inputs
Outputs
Read / Conditional Instructions
Write / Control Instructions

( )
Start (Rung 1)

/
( )


/
( )

/
( )
End (Rung 4)

14
IEC - Function Block Diagram (FBD)
15
IEC - Structured Text (ST)
16
IEC - Instruction List (IL)
17
IEC - Ladder Diagram (LD)
18
IEC - Sequential Function Chart (SFC)
19
Communication
20
Communication Basics
  • Protocols
  • A set of rules for data exchange (format and
    timing of data) in a communications system
  • Stack / profile selected set of protocols for a
    communication application
  • Communication Relationships
  • Master / Slave - request / response response
    only
  • Peer-to-Peer - client / server, publisher /
    subscriber
  • Communication Architectures
  • Star (point-to-point)
  • Bus, Ring, LAN (multidrop)

21
Communication Protocols
  • Ethernet
  • Modbus Plus
  • Modbus
  • S908
  • LonWorks
  • Interbus
  • Profi-bus
  • DeviceNet
  • Uni-Telway
  • CAN-Open
  • ASi
  • Seriplex
  • FIPIO/FIPway
  • RIO
  • HART
  • DIO
  • ControlNet
  • ASCII

Protocols are similar to languages - Devices
must support the same protocol to understand each
other
22
What you need to know when specifying a PLC
  • Quantity, Type and Location of I/O
  • Number of Inputs and output points
  • AC or DC voltage
  • Analog or Discrete
  • Concentrated or spread out (distributed)
  • Communication Requirements
  • Protocol/Network used
  • Devices to communicate with (HMI, other PLCs,
    etc)
  • Speed of Application
  • Response time required (throughput) of the system
  • How fast does the process change

23
What you need to know when specifying a PLC
  • Control Architecture Philosophy
  • Centralized Control, Distributed Control or
    combination
  • Redundancy - CPUs, Power Supplies, etc
  • Programming Software
  • IEC vs. 984
  • Installed base / what is currently being used
  • User Logic
  • Size and complexity of Program
  • Feedback control used
  • etc.

24
Alphabet Soup
HMI / MMI Human Machine Interface / Man Machine
Interface I/O Input(s) and/or Output(s) LSB /
MSB Least Significant Bit / Most Significant
Bit MODICON MOdular DIgital CONtoller PID Proport
ional Integral Derivative (feedback
control) RF Radio Frequency RTU Remote Terminal
Unit SCADA Supervisory Control And Data
Acquisition TCP/IP Transmission Control Protocol
/ Internet Protocol
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What is a Programmable Logic Controller (PLC) ?

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Title: What is a Programmable Logic Controller (PLC) ?


1
What is a Programmable Logic Controller (PLC) ?
2
Agenda
  • History
  • PLC Overview
  • Hardware
  • Inputs/Outputs
  • Operation
  • Programming Software
  • Communication
  • What you need to know when specifying a PLC
  • Glossary

3
History of the PLC
  • First Programmable Logic Controllers were
    designed and developed by Modicon as a relay
    replacer for GM and Landis.
  • These controllers eliminated the need for
    rewiring and adding additional hardware for each
    new configuration of logic.
  • The new system drastically increased the
    functionality of the controls while reducing the
    cabinet space that housed the logic.
  • The first PLC, model 084, was invented by Dick
    Morley in 1969.
  • The first commercial successful PLC, the 184, was
    introduced in 1973 which was designed by Michael
    Greenberg.

4
What is a PLC?
The Basic Block
CPU
A PROGRAMMABLE LOGIC CONTROLLER is a solid state
control system that continuously monitors the
status of devices connected as inputs. Based
upon a user written program, stored in memory, it
controls the status of devices connected as
outputs.
5
What is a PLC with Distributed I/O?
Getting Complex
CPU
6
What is a PLC with Distributed Control?
Getting Really Complex
Remote
7
Inside a PLC
Communications Port
Circuits
Circuits
Central Processor Unit (CPU)
Output
Input
CR
High Voltage
High Voltage
Isolation Barrier
Isolation Barrier
Low Voltage
or
DC Power Supply
AC Power Supply
8
Inside a PLC
  • The CPU contains an Executive program that
    tells the PLC how to
  • Execute the control Instructions
  • Users Program
  • Communicate with other devices
  • Other PLCs, Programming devices, I/O devices,
    etc.
  • Perform Housekeeping activities
  • Diagnostics, etc
  • This program is stored in nonvolatile memory
  • Meaning that the program will not be lost if
    power is removed

9
What are Inputs?
  • Switches and Pushbuttons
  • Sensing Devices
  • Limit Switches
  • Photoelectric Sensors
  • Proximity Sensors
  • Condition Sensors
  • Pressure Switches
  • Level Switches
  • Temperature Switches
  • Vacuum Switches
  • Float Switches
  • Encoders

10
What are Outputs?
  • Valves
  • Motor Starters
  • Solenoids
  • Actuators
  • Control Relays
  • Horns Alarms
  • Stack Lights
  • Fans
  • Counter/Totalizer
  • Pumps
  • Printers

11
PLC Operating Cycle
  • Four Steps in the PLC Operations
  • Input Scan
  • Scan the state of the Inputs
  • Program Scan
  • Processes the program logic
  • Output Scan
  • Energize/de-energize the outputs
  • Housekeeping
  • This step includes communications, Internal
    Diagnostics, etc.
  • The steps are continually repeated - processed
    in a loop

12
Programming the PLC
  • Types of Programming Software Languages
  • Ladder Logic (LL)
  • IEC Format
  • Function Block Diagram (FBD)
  • Structured Text (ST)
  • Instruction List (IL)
  • Ladder Diagram (LD)
  • Sequential Function Chart (SFC) - also known as
    Grafcet

13
Ladder Logic
Inputs
Outputs
Read / Conditional Instructions
Write / Control Instructions

( )
Start (Rung 1)

/
( )


/
( )

/
( )
End (Rung 4)

14
IEC - Function Block Diagram (FBD)
15
IEC - Structured Text (ST)
16
IEC - Instruction List (IL)
17
IEC - Ladder Diagram (LD)
18
IEC - Sequential Function Chart (SFC)
19
Communication
20
Communication Basics
  • Protocols
  • A set of rules for data exchange (format and
    timing of data) in a communications system
  • Stack / profile selected set of protocols for a
    communication application
  • Communication Relationships
  • Master / Slave - request / response response
    only
  • Peer-to-Peer - client / server, publisher /
    subscriber
  • Communication Architectures
  • Star (point-to-point)
  • Bus, Ring, LAN (multidrop)

21
Communication Protocols
  • Ethernet
  • Modbus Plus
  • Modbus
  • S908
  • LonWorks
  • Interbus
  • Profi-bus
  • DeviceNet
  • Uni-Telway
  • CAN-Open
  • ASi
  • Seriplex
  • FIPIO/FIPway
  • RIO
  • HART
  • DIO
  • ControlNet
  • ASCII

Protocols are similar to languages - Devices
must support the same protocol to understand each
other
22
What you need to know when specifying a PLC
  • Quantity, Type and Location of I/O
  • Number of Inputs and output points
  • AC or DC voltage
  • Analog or Discrete
  • Concentrated or spread out (distributed)
  • Communication Requirements
  • Protocol/Network used
  • Devices to communicate with (HMI, other PLCs,
    etc)
  • Speed of Application
  • Response time required (throughput) of the system
  • How fast does the process change

23
What you need to know when specifying a PLC
  • Control Architecture Philosophy
  • Centralized Control, Distributed Control or
    combination
  • Redundancy - CPUs, Power Supplies, etc
  • Programming Software
  • IEC vs. 984
  • Installed base / what is currently being used
  • User Logic
  • Size and complexity of Program
  • Feedback control used
  • etc.

24
Alphabet Soup
HMI / MMI Human Machine Interface / Man Machine
Interface I/O Input(s) and/or Output(s) LSB /
MSB Least Significant Bit / Most Significant
Bit MODICON MOdular DIgital CONtoller PID Proport
ional Integral Derivative (feedback
control) RF Radio Frequency RTU Remote Terminal
Unit SCADA Supervisory Control And Data
Acquisition TCP/IP Transmission Control Protocol
/ Internet Protocol
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