Welcome to the World of Programmable Logic Controllers Chapter 1 Welcome to the World of Programmable Logic Controllers

Objectives Define PLC. Explain where the PLC came from. Explain why their use is valuable. Explain where they are used. Detail what PLCs can do. Explain how PLCs know what they are supposed to do.

What Is a Programmable Controller? A programmable logic controller, usually called a PLC or programmable controller, is a solid-state, digital, industrial computer. Simply, a programmable controller is a computer, much like a desktop personal computer. A PLC is an industrially hardened computer.

Where Did the PLC Come From? (1 of 2) In the 1960s, electromechanical relays, timers, counters, and sequencers were the standard. Many control panels contained hundreds of these devices and a mile or more of wire.

Where Did the PLC Come From? (2 of 2) Reliability was low and maintenance costs were high. Cost was high to modify or upgrade control panels. In 1968 the General Motors Hydramatic division specified a device that would become what we know today as the programmable logic controller.

Early PLCs Only relay replacers Did not have timers or counters No sequencer instructions No math instructions No data manipulation instructions

Why a PLC? (1 of 2) Easily changeable Programmable Reliable Smaller Fast switching

Why a PLC? (2 of 2) Able to withstand harsh factory environment Consumes less power Easier to troubleshoot Easy to install

Why Use a PLC? The question “why use a PLC?” should really be rephrased to “why automate?” The PLC is the tool that provides the control for the automated process.

Automating Helps a Manufacturing Facility (1 of 2) Gain complete control of the manufacturing process Achieve consistency Improve quality and accuracy Work in difficult or hazardous environments Increase productivity

Automating Helps a Manufacturing Facility (2 of 2) Shorten lead time to market Lower cost of quality, scrap, and rework Offer greater product variety Allow a quick changeover from one product to another Control inventory

A PLC Upon First Glance A black box with wires bringing signals in and other wires sending signals out Some sort of magic being done inside that somehow decides when field devices should be turned on or off

Actually There Is No Magic The PLC is a computer and someone has to tell it what to do. The PLC knows what to do through a program that was developed and entered into its memory. Without a set of instructions telling the PLC what to do, it is nothing more than a box full of electronic components.

What Makes a PLC Work? (1 of 4) The heart of any computer is the microprocessor. The microprocessor, also called the processor or central processing unit (CPU), supervises system control through the user program.

What Makes a PLC Work? (2 of 4) The processor reads input signals and follows the instructions that the programmer has stored in the PLC’s memory.

What Makes a PLC Work? (3 of 4) As a result of the solved program, the PLC writes information to outputs, or field-controlled devices, to turn them on or off. When the PLC is running and following the program’s instructions, this is called solving the user program. The PLC is running or in RUN MODE.

What Makes a PLC Work? (4 of 4) The user program (ladder program) is the list of instructions that tells the PLC what to do. The library of instructions available to the PLC is called the instruction set. The instruction set determines how much flexibility the programmer has.

Common PLC Inputs Pushbuttons Selector switches Limit switches and level switches Proximity sensors Photo switches Relay contacts Motor starter contacts

An Overview of a PLC System (1 of 2)

An Overview of a PLC System (2 of 2) Incoming signals, or inputs, interact with instructions in the user program to help the PLC determine when an input instruction is either true or false.

Conventional Circuit

Representation of a PLC Program

PLC Ladder Program Rung

Series 90-30 and 90-20 Hand-held Programmer Image courtesy of GE Fanuc Automation

Interfacing a PC to an Omron CQM 1 PLC Image courtesy of Omron Electronics, Inc.

Correlating Ladder Program Rung to Actual PLC Wiring

Programmable Controller Block Diagram

Product Sensed in Position Will Send an Input Signal

Allen-Bradley SLC 500 Fixed PLC Image courtesy of Allen-Bradley, a Rockwell Automation business

Allen-Bradley SLC 500 Modular PLC Image courtesy of Allen-Bradley, a Rockwell Automation business

SLC 500 Power Supply and a Four-slot Rack Image courtesy of Allen-Bradley, a Rockwell Automation business

Installation of an I/O Module Image courtesy of Allen-Bradley, a Rockwell Automation business

Allen-Bradley SLC 500 Modular Processor Image courtesy of Allen-Bradley, a Rockwell Automation business

Limit Switch Interface

Output Module Wiring to a Motor Starter Coil