ACOE347 – Data Acquisition and Automation Systems Introduction to Programmable Logic Controllers (PLCs) Automation Systems What is a PLC? Hardware components of a PLC Memory structure of a PLC PLC Modes of Operation Wiring Diagram Digital Inputs/Outputs ACOE347 Introduction to PLCs

What is an Automation System? Industrial Automation: A system that controls automatically the operation of equipment such as machines or production processes in factories ACOE347 Introduction to PLCs

What is an Automation System? Home Automation: A system that automatically controls the operation of home equipment and appliances such as lights, gates, doors, alarm systems etc ACOE347 Introduction to PLCs

Industrial Automation Components Controlled System: Mechanical Components (conveyor belts, arms, rotary discs, etc) Actuators (motors, solenoid valves, relays, heating elements, etc) Sensors (tactile switches, opto-switches, proximity switches, etc) Controller: A unit that reads the state of the sensors and activate the actuators according to the control program Control Panel: Interface between the operator and the system. Buttons, switches, lamps, displays, touch screens etc ACOE347 Introduction to PLCs

Manually Controlled Systems ACOE347 Introduction to PLCs

Custom made and programmed controllers ACOE347 Introduction to PLCs

Industrial Computers or PCs ACOE347 Introduction to PLCs

Programmable Logic Controllers (PLC) A Programmable Logic Controller (PLC) is a microprocessor based control system, designed for use in an industrial environment, programmed to sense, activate and control industrial equipment. ACOE347 Introduction to PLCs

Main Units of a PLC? The components that make a PLC work can be divided into three core areas. The power supply and rack The central processing unit (CPU) The input/output (I/O) section PLCs are “modular” because the rack can accept many different types of I/O modules that simply slide into the rack and plug in. ACOE347 Introduction to PLCs

The Power Supply and Rack The rack is the component that holds everything together.  The rack has a backplane at the rear which allows the cards to communicate with the CPU.  The power supply plugs into the rack as well and supplies to other modules that plug into the rack.  The most popular power supplies work with 120/240 VAC or 24 VDC sources. ACOE347 Introduction to PLCs

Hardware components of a PLC ACOE347 Introduction to PLCs

Memory structure of a PLC ACOE347 Introduction to PLCs

PLC Mode of Operation A PLC operates in one of two modes: Program Mode: Download the user’s program from the programming device to the memory of the PLC. Run Mode: Execute the user’s program using the scan-based execution model, in three phases: Read the state of the inputs and store them in the RAM referred to as the input image memory.. Execute the user’s program and update the state of the outputs in the RAM (output image memory). Copy the states of the outputs from the RAM to the physical output devices. The time needed to complete one scan cycle depends on the length and complexity of the user’s program. PLC manufacturers specify the maximum number of program steps (say 1000) and the average scan time (few ms). ACOE347 Introduction to PLCs

A Typical PLC from Allen-Bradley ACOE347 Introduction to PLCs

DC Voltage Digital Input The optoisolator isolates the internal components of the PLC and the external input devices. Diode D1 provides protection during switching from inductive devices. When the switch is closed, current flows through the status LED and the opto-isolator, giving a logic 1 to the input port. ACOE347 Introduction to PLCs

Connecting DC Proximity Switches ACOE347 Introduction to PLCs

AC Voltage Digital Input AC input devices switch On and Off the mains voltage. The mains voltage must be reduced using a voltage divider. The mains voltage must be converted to DC using a rectifier and a smoothing circuit. An AC proximity switch switches On and Off the mains voltage by triggering (firing) a triac. The proximity sensor controls the gate current of the triac. ACOE347 Introduction to PLCs

Digital Output (Relay and Transistor) Transistor outputs provide higher switching speeds with more switching life. Transistor outputs can only be used with DC loads and are polarity sensitive. A diode is usually used for reverse polarity protection. Another diode is usually used to protect the output transistor when switching inductive loads. Most PLCs have relay outputs. Relay outputs have the advantage that they can control DC as well as AC loads at relatively high currents. The disadvantages of relays are due to their mechanical contacts. ACOE347 Introduction to PLCs

Digital Output Using Triacs Triac outputs are used to avoid the problems due to the mechanical contacts of relays. An opto-diac is used to fire the triac. An RC circuit is usually used to avoid problems due to the phase difference between the voltage and the current when driving inductive loads. ACOE347 Introduction to PLCs

Typical Wiring Diagram ACOE347 Introduction to PLCs