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What is a Programmable Logic Controller (PLC) ?
What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 2
Agenda
• History
• PLC Overview
• Hardware
• Inputs/Outputs
• Operation
• Programming Software
• Communication
• What you need to know when specifying a PLC
• Glossary
What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 3
• 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.
History of the PLC
What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 4
What is a PLC?
Inputs Outputs
The Basic Block
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.
CPU
What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 5
What is a PLC with Distributed I/O?
CPU
Inputs
Inputs
Outputs
Outputs
Remote
Getting Complex
What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 6
What is a PLC with Distributed Control?
CPU
Remote
Getting Really Complex
CPU
Inputs
Inputs
InputsOutputs
Outputs
Outputs
What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 7
CR
IsolationBarrierIsolation
Barrier
CentralProcessor
Unit(CPU)
MEMORY
program data
HighVoltage
HighVoltage
Low Voltage
AC Power Supply
Output
Circuits
DC Power Supplyor
CommunicationsPort
Input
Circuits
Inside a PLC
What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 8
Inside a PLC
The CPU contains an “Executive” program that tells the PLC how to:
• Execute the control Instructions
• User’s 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
What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 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
What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 10
What are Outputs?
• Valves
• Motor Starters
• Solenoids
• Actuators
• Control Relays
• Horns & Alarms
• Stack Lights
• Fans
• Counter/Totalizer
• Pumps
• Printers
What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 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
START
Input Scan
ProgramScan
Housekeeping
OutputScan
What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 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”
What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 13
Ladder Logic
Read / Conditional Instructions
Write / Control Instructions
| | |/|
| | |/|
| |
| | |/| ( )
| |
| |
Start (Rung #1)
End (Rung #4)
( )
( )
( )
Inputs Outputs
What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 14
IEC - Function Block Diagram (FBD)
What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 15
IEC - Structured Text (ST)
What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 16
IEC - Instruction List (IL)
What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 17
IEC - Ladder Diagram (LD)
What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 18
IEC - Sequential Function Chart (SFC)
What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 19
Communication
What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 20
• 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)
Communication Basics
What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 21
• Ethernet
• Modbus Plus
• Modbus
• S908
• LonWorks
• Interbus
• Profi-bus
• DeviceNet
• Uni-Telway
Communication Protocols
• 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
What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 22
• 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
What you need to know when specifying a PLC
What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 23
• 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.
What you need to know when specifying a PLC
What is a PLC presentation.ppt February 2003 G. Ziegler, Marketing Support Manager 24
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 Proportional Integral Derivative (feedback control)
RF Radio Frequency
RTU Remote Terminal Unit
SCADA Supervisory Control And Data Acquisition
TCP/IP Transmission Control Protocol / Internet Protocol
Alphabet Soup