What is a Programmable Logic Controller (PLC) ?

Agenda History PLC Overview Hardware Inputs/Outputs Operation Programming Software Communication What you need to know when specifying a PLC Glossary

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.

Inputs Outputs 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.

What is a PLC with Distributed I/O? Getting Complex CPU Inputs Outputs Remote

Getting Really Complex What is a PLC with Distributed Control? Getting Really Complex CPU CPU Outputs Inputs Remote

Inside a PLC Central Processor Unit (CPU) MEMORY Circuits Circuits Communications Port Circuits Circuits Central Processor Unit (CPU) Input Output CR High Voltage High Voltage MEMORY program data Isolation Barrier Isolation Barrier Primary components of virtually all PLC’s, no matter what size are: Input Circuitry Output Circuitry Central Processor Memory Communications Port(s) Power Supply PLC’s are industrially hardened devices capable of operating in extreme environments Wide Temp extremes 0-60º C High humidity High Vibration “Dirty” Electrical Power High “Noise” (electrical) environments Low Voltage or AC Power Supply DC Power Supply

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

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 Program Housekeeping Output

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”

Ladder Logic | | ( ) | | | | |/| ( ) | | |/| ( ) | | |/| ( ) | | Inputs Outputs Read / Conditional Instructions Write / Control Instructions | | ( ) Start (Rung #1) | | | | |/| ( ) | | |/| ( ) | | |/| ( ) End (Rung #4) | |

IEC - Function Block Diagram (FBD)

IEC - Structured Text (ST)

IEC - Instruction List (IL)

IEC - Ladder Diagram (LD)

IEC - Sequential Function Chart (SFC)

Communication

Communication Basics Protocols Communication Relationships 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 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

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

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.

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 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