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Who is this course for?

OUR MOST POPULAR COURSE - BUY BOTH Level 1 & 2 together and SAVE approx 14% OFF the normal individual course prices.
This course also includes FREE the content from our “INTRODUCTION TO PLCs”

TRAINING IS FOR - BEGINNERS - EMPLOYED - UNEMPLOYED - STUDENTS
Companies employ engineers who have had professional accredited
PLC (Programmable Logic Controller) Training
SCANTIME are an INDUSTRIAL TRAINING & ENGINEERING COMPANY.

This training course is for Maintenance or Control Engineers and anyone wishing to become a PLC Automation Designer. The training includes industrial programming techniques with introduction to programming standards and how PLC technology is used in manufacturing processes.


What you will receive with this course

• Industry approved PLC Programming training
• Access to your Online Course Area 24 Hrs a day
• Study in your own time and at your own pace
• FREE PLC software for your PC - Syswin34
• Online Course Viewer includes AUDIO & VIDEO
• Offline Manuals to read offline
• Assistance from Professional Engineer Tutor
• FREE course Certificate - Training CD & Delivered FREE worldwide

Introduction

In today’s manufacturing industries, it is essential to ensure that your Engineering Skills and Knowledge  of PLCs (Programmable Logic Controller) is up to date - Industrial PLC Programming knowledge is fundamental for ALL engineers worldwide - You can gain this knowledge via our Industry Accredited eLearning PLC training courses.

PLC Programming Part 1 (Programmable Logic Controller) - Learn how to use Timers - Counters - create Ladder Logic to industrial standards - Hardware Configuration - Addressing - Basic PLC programming exercises -  PLCs and Operator HMI & SCADA Interfaces -  introduction to other programming languages, be capable of reading ladder logic in process control routines, identify faults in ladder logic routines.

Part2 of our PLC Programming series is the hand’s on practical training, working with your course tutor, he will teach you programming, and how to write Programmable Logic Controller control routines for industrial processes.  Learn about: Data Handling, using Comparators, creating Automatic Control of a process, Safety Interlocking, Monitoring process limits, Program Structure, Alarm Handling, How to write a Full program to Industrial Standards to reduce manufacturing Downtime and Safety.

Learn about PLCs (OMRON - MITSUBISHI - SIEMENS & ALLEN BRADLEY PLCs). 

Study in your own private Online course area.  You also have a private course tutor who is available to you over the entire course, anything that you do not understand he will help you with. There is no pressure to complete the training by a certain time or date, once you buy a course it is yours to utilize whenever you wish. Study & Practice Programming OnLine OR OffLine whenever you wish, entirely flexible to suit you.

(*PLC software req 32 Bit PC or use our free Remote Access software)

Course 2 - RSLogix500 / SLC500 Advanced Programming

Section 1     Advanced Math Instructions
1.1         Objective
1.2         Exercise – Square Root
1.3         Exercise – Compute
1.4         Exercise – Negate

Section 2     Advanced Timer and Counter
2.1         Objective
2.2         Exercise – Retentive Timers
2.3         Exercise – Time off Delay
2.4         Exercise – Count Down Timer

Section 3     Advanced Comparison Instructions
3.1         Objective
3.2         Exercise - Limit Test
3.3         Exercise – Masked Equal

Section 4     File Operations
4.1         Objective
4.2         Exercise – File Fill
4.3         Exercise – Copy File

Section 5     Processor Faults
5.1         Objective
5.2         Exercise – Causing and Clearing Faults
5.3         Exercise – Preventing Faults

Section 6     Indirect Addressing
6.1         Objective
6.2         Exercise – Indirect Addressing
6.3         Exercise – Indirect Addressing Faults
6.4         Exercise – Preventing Indirect Addressing Faults

Section 7     Advanced Instructions
7.1         Objective
7.2         Exercise – Shift Registers
7.3         Exercise – Sequencer
7.4         Exercise – Labels and Jumps
7.5         Exercise – First In/Out

In addition, access to "Inhibiting I/O in the SLC500 for simulation and fault recovery" video is included in this training to allow the student to learn simulation principles and methods.

_____________________________________________________________________________

SECTION 1 - (PREVIEW ONLY)

Advanced Math Instructions

This section will cover more advanced math instructions.

Exercise 1 - Square Root (SQR)

Exercise 2 - Compute (CPT)

Exercise 3 - Negate (NEG)

SECTION 2 - (PREVIEW ONLY)

Advanced Timer and Counter

This section will cover more advance timers such as the Retentive Timer and Time Off Delay Timer. Also covered in this section will be the Count Down counter.

Exercise 1 - Retentive Timer (RTO)

Exercise 2 - Time Off Delay (TOF)

Exercise 3 - Count Down Counter (CTU)

 

SECTION 3 - (PREVIEW ONLY)

Advanced Comparison Instructions

This section will cover additional comparison instructions such as the LIM and MEQ instructions.

 

Exercise 1 - Limit Test (LIM)

Exercise 2 - Masked Equal (MEQ)

SECTION 4 - (PREVIEW ONLY)

File Operations

This section covers file operation such as the FLL (Fill File) and COP (Copy) instructions used to move values to multiple register is on processor scan.

Exercise 1 - Fill File (FLL)

Exercise 2 - Copy File (COP)

SECTION 5 - (PREVIEW ONLY)

Processor Faults

This section will introduce processor faults showing how to cause a processor fault, clear a fault and methods for preventing a fault.

Exercise 1 - Causing and Clearing Faults

Exercise 2 - Preventing faults

SECTION 6 - (PREVIEW ONLY)

 Indirect Addressing

The section will introduce programming with indirect addressing. While indirect addressing is not recommended in some instances it is a necessity. It is very important to be cautious when using indirect addressing in an SLC to avoid faulting the processor. The section below will cover faulting the processor with indirect addressing and step that can be taken to avoid this.

Exercise 1 - Indirect Addressing

Exercise 2 - Indirect Addressing Faults

Exercise 3 - Preventing Indirect Addressing Faults

SECTION 7 - (PREVIEW ONLY)

Advanced Instructions

This section covers advanced instructions useless for program sequence and control.

Exercise 1 - Shift Registers (BSL)

Exercise 2 - Sequencer (SQO)

Exercise 3 - Label/Jump (LBL,JMP)

Exercise 4 - First In / First Out (FFL,FFU)

RSLOGIX500 COURSE 1 - INTRODUCTION TO PLC PROGRAMMING

Course 1 - RSLogix500 / SLC500 Programming

Section 1     Getting Started
1.1         Objective
1.2         Exercise – Creating a Program
1.3         Exercise – Defining I/O Configuration

Section 2     Adding Basic Logic
2.1         Objective
2.2         Exercise – OTE, XIC, XIO
2.3         Exercise – Adding Address Descriptions and Symbols
2.4         Exercise – RSLogix 500 View Properties

Section 3     Adding More than Basic Logic
3.1         Objective
3.2         Exercise - Programming with Symbols
3.3         Exercise – Branches
3.4         Exercise – Adding Motor Seal-in Circuit
3.5         Exercise – Add Latches (OTL, OTU)
3.6         Exercise – Rung Comments and Page Titles
3.7         Exercise – Advanced Diagnostics

Section 4     Getting to Know Your Processor Online
4.1         Objective
4.2         Exercise – Downloading to the Processor
4.3         Exercise – Going Online
4.4         Exercise – Processor Modes
4.5         Exercise – Program File Online Monitoring
4.6         Exercise – Data File Online Monitoring
4.7         Exercise – Project Organization

Section 5     Timers and Counters
5.1         Objective
5.2         Exercise – Introduction to Timers and Counters
5.3         Exercise – TON, CTU and RES

Section 6     Introduction to Addition and Subtraction
6.1         Objective
6.2         Exercise – Add and Subtract with Registers
6.3         Exercise – Add and Subtract with Registers and Constants

Section 7     Compare Commands
7.1         Objective
7.2         Exercise – GTR, GEQ, LES, LEQ, EQU, NEQ

Section 8     More Addition and Subtraction
8.1         Objective
8.2         Exercise – Adding and Subtracting with One-shots
8.3         Exercise – Parking Lot, Part 1

Section 9     Multiply and Divide
9.1         Objective
9.2         Exercise – Introduction to Multiplying and Dividing
9.3         Exercise – Multiply and Divide with Registers and Constants
9.4         Exercise – Parking Lot, Part 2

Section 10        Move Command
10.1       Objective
10.2       Exercise – MOV

Section 11        Programming Project
11.1       Exercise - Traffic Light, Part 1
11.2       Exercise – Traffic Light, Part 2

Section 12        Final Programming Projects
12.1       Project - Bottling Line
12.2       Project – Lumber Yard

SECTION 1 - (PREVIEW ONLY)

This section will cover how to use the RSLogix 500 software to create a new project file, give the PLC processor a name and define the type of processor to be used in the project. This section also covers using the IO Configuration dialog window to define the rack type, identify the IO cards being used by indicating their slot position within the rack, and select the power supply for each rack.

Exercise 1 - Creating a program
Exercise 2 - Defining I/O configuration

SECTION 2 - (PREVIEW ONLY)

This section will cover how to add a rung to a program file and then insert simple instructions to the rung. This section will also cover adding descriptions and symbols to address registers.

Exercise 1 - OTE, XIC, XIO

Exercise 2 - Adding Descriptions and Symbols

Exercise 3 - RSLogix500 View Properties

SECTION 3 - (PREVIEW ONLY)

This section will cover how to add address registers to instructions using the address symbols. Next, you will learn different methods of adding branches to rungs and finally you will program a motor seal-in circuit.

 

Exercise 1 - Programming with Symbols

Exercise 2 - Branches

Exercise 3 - Motor Seal-in Circuits

Exercise 4 - Latches - OTL, OTU

Exercise 5 - Rung Comments and Page Titles

Exercise 6 - Advanced Diagnostics

SECTION 4 - (PREVIEW ONLY)

This section will cover how to download a project and go online with the processor. This section will also show the different processor modes and how the different ways to switch between them. Finally, you will learn how to monitor the program file and data files of you project online.

Exercise 1 - Downloading

Exercise 2 - Going Online

Exercise 3 - Processor Modes

Exercise 4 - Program Monitoring

Exercise 5 - Data Monitoring

Exercise 6 - Project Organization

Project - Garage Door

 

SECTION 5 - (PREVIEW ONLY)

This section will cover the timer and counter instructions, discussing the properties of the instructions and give examples of how to use them in your program.

Exercise 1 - Introduction to Timers and Counters

Exercise 2 - TON, CTU and RES instruction types

 

SECTION 6 - (PREVIEW ONLY)

This section introduces the ADD and SUB instructions.

Exercise 1 - Addition and Subtraction with Registers

Exercise 2 - Addition and Subtraction with Constants

 

SECTION 7 - (PREVIEW ONLY)

This section will cover the comparison instructions used to compare values of data.

Exercise 1 - Comparisons - GRT, GEQ, LES, EQU, NEQ

SECTION 8 - (PREVIEW ONLY)

The objective of this section is the show different ways to use the addition and subtraction instructions.

Exercise 1 - Addition and Subtraction with oneshots

Exercise 2 - Parking Lot Program, Part 1

SECTION 9 - (PREVIEW ONLY)

The objective of this section is to introduce the MUL and DIV instructions.

Exercise 1 - Introduction to Multiply and Divide

Exercise 2 - Multiply/Divide with Constants

Exercise 3 - Parking Lot Program, Part 2

SECTION 10 - (PREVIEW ONLY)

This section will introduce the MOV instruction which is used to move a copy of the source to the destination each scan.

Exercise 1 - MOV Instruction

SECTION 11 - (PREVIEW ONLY)

This is the first of three programs that you will complete to make sure you fully understand the material in this course.  This is a fairly simple and standard program for PLC programming students.  Everybody understands a stop light so it is a "system" that needs little explaination to understand the required operation.  Make sure you take the time to submit your final programs to the training area of the forum for instructor review.  We will make sure that the program works as required and follows proper programming principles.  Keep in mind that there are many different ways to program the same operation.  While some of the methods may produce proper results and some others may give you proper results most of the time - a good program requires more.  It must be well organized and well documented and programmed for your audience.  That means that you need to consider who will be taking care of the system you are programming.  Consider their level of programming.  The best plan?  Keep it simple.

Exercise 1 - Traffic Light, Part 1

Exercise 2 - Traffic Light, Part 2

SECTION 12 - (PREVIEW ONLY)

The objective of this section is to become more proficient with the instructions covered in course 1.  These final two programs will stretch your understanding and help you to actually be able to use what you have learned.  This is the most important part of this class and may take longer to complete than all of the other sections.  When you have completed a program, submit it on the training section of the forum for an instructor review.  This will allow us to comment on your methods and organization of your program.  It is possible that you may need to work through the programs more than once to meet good programming requirements, but upon completion you should feel very comfortable with the material covered. 

When writing your program make sure you add simulation code  to test your program. 

Please, please, please leave questions on the forum or bring them to the Webinar classroom.  It is important that you fully understand the material presented and understand projects as presented.

Project 1 - Bottling Line

Project 2 - Lumber Yard

 

FINAL THOUGHTS

PLC Considerations

When selecting a PLC or similar control engine there are many questions:

1. How much I/O?
2. What type of I/O?
3. What type of control logic -- simple ON/OFF or is there PID and data analysis?
4. What type of data is monitored and captured?
5. Are there recipes (databases) involved?
6. Is there an operator interface involved?
7. Are there special communication interfaces required? For example, flow meters, scales, thermocouples, or other signals that are not a regular discrete or analog signal.
8. Does the application require links to an external network, database, or some type of MES system?
9. Does the application require motion control, bar coding, machine vision, etc?

PLC and / or Computer

Sometimes we do not use a typical PLC for the control engine. Typically what we do is:

1. If the application is small (less than 50 I/O), no databases (only a few choices), and simplistic HMI then use a PLC.
2. If the application is small, slow (response time greater than 50 milliseconds) and requires computer functionality (machine vision, networking, databases, multiple axis motion control, etc) we prefer to do the entire application in Visual Basic (VB).
3. If there are large amounts of I/O (over 100) or you need fast, real-time response, then you will probably appreciate the PLC handling your real-time and direct I/O tasks and letting the computer handle the non-real-time tasks (such as HMI, databases, etc). There are a lot of gray areas in between.

Although today’s Pentium III running Windows NT or 2000 at 1 GHz with 512 KB RAM is very fast in comparison to technology only three years ago, it is still nice, in large systems, to use a PLC to help segment the system functionality. You can write subroutines to segment functionality -- you can also segment using different controllers.

Frequently Asked Questions (FAQs)

1. What about soft PLCs? Soft PLCs are where the PLC is actually software that resides on a computer. Although this is way that the industry is headed for the future, as with all new technology, we would recommend that your first test of new technology not be a critical application.
2. What brand of PLC is the best? It depends. J If you have a plant full of GE PLCs then it does not make sense to change to AB. If you have a plant full of some manufacturer that is no longer in business and you have to switch anyways then it may be time to reevaluate. If you have small applications that can be linked by computer networks then an AutomationDirect PLC may be fine. If you have large processes (thousands of I/O) requiring integration of drives and redundancy then you may want to consider AB or Siemens.
3. How can you program so many different PLCs? Most good PLC programmers, after learning three different PLC programming languages, can program most any PLC since the basic functions are the same. In fact there is an International PLC programming standard (IEC 61131-3) that your best PLC manufacturer’s are adopting.

To learn more

There is a lot you can learn about PLCs.  For example, there are books written on PLCs and some of these books do not seem complete.  If you want to learn more, there are many good resources.  As always try searching the best source of information -- the internet.

LADDER LOGIC

PLCs are traditionally programmed in a notation known as Ladder Logic or Relay Ladder Logic. This notation is easy for technicians to read and understand. Remember that the first PLC’s were designed to replace large numbers of relays and they were designed to be easy for the electricians responsible for those systems to be able to analyze and troubleshoot.  You might want to note that a background and understanding of basic electrical circuits is helpful if you plan to program or work with PLC’s. 

An example of a simple motor start/stop circuit is shown below:

 Assume that A is the "start" button, B is the "stop" button and C is the output that tells the motor to run. Imagine power flowing through the ladder logic example. In this case when the operator presses the "Start" button (labeled A above) power flows through the contact labeled A. The [/] contact means "not on" or off. So assuming that the "stop" button (labeled B above) is not pressed then power flows through the B contact and powers the output labeled C and turns on the motor.

Note that we have another contact labeled C below the contact labeled A. This is called a "seal" circuit since contact C "seals" in contact A. Remember the power flow analogy. If it was only the top row: A, B, and C then every time the operator took their finger off the "start" button (contact A) then the motor would stop. But contact C is placed in parallel with contact A so that once the output C turns on then the input C is turned on in parallel with contact A. Therefore the only way to stop the motor is to press the "stop" button (contact B).

Relay ladder logic - RLL) is the basic PLC logic that converts inputs into outputs.  There are several other methods for writing PLC code.  Sequential Function Chart (SFC) is excellent for the programming sequential machines -- however most customers hate it.  Therefore we usually provide the same framework in RLL.   Structured text is another available programming language – much more like a standard high-level language.  Function blocks are also available now on many processors.  Seen used more in DCS (Distributed Control Systems – a completely new discussion), function blocks are somewhat like a “black box” in electronics.  They have a defined set of inputs to the block (don’t get them confused with the PLC inputs) and a defined set of outputs (ditto).  The block performs a manipulation of the inputs to elicit appropriate outputs.  This can greatly enhance the computational ability of the PLC and simplify some applications.  Once again – most clients want to stay in the RLL world.  The number 1 rule associated with PLC programming is “keep it simple.”  Most end users of PLC’s do not have PLC programmers on staff, or if they do many times those programmers are not the people that are called at 3 am at night when a machine is down.  Generally it’s somebody off the maintenance staff that gets pulled in to solve the problem.  The simpler the program – the more likely the problem is diagnosed and fixed quickly. 

There is an organization that is working to standardize the logic in all PLCs.  More information on this can be found at The PLCopen (IEC 1131-3)

SYSTEM ARCHITECTURE

PLCs are continuously expanding in capabilities and the architecture or hardware that is part of that is also expanding with new functionality.  There are some hardware components that are basic to every system.  A PLC system generally has the following components:

1. Processor – consists of the CPU, main memory, possibly some communications and interface capability.
2. I/O – short for Inputs and Outputs – sometimes included in small quantities on a block with the processor
3. Power Supply – Provides power for the various components of the system -sometimes built into a block with the processor.
4. Rack/Chassis – Modular systems generally need a common backplane to plug into.  Racks are designed to at least accept a processor and multiple I/O cards.  Communication from the processor to the I/O is generally achieved through connections on the rack and power from the power supply is generally distributed through the rack.
5. Communication interfaces – used to communicate with the programmer, operator interfaces, data monitoring tools, SCADA systems, remote I/O, etc.  Sometimes built into the processor.
6. Programming Tool – used to program and access the processor.

The processor is generally what most people refer to when discussing a PLC.  Many controls specialists (including myself) tend to interchange PLC, processor and controller when referring to the module that receives the program and handles processing the logic.  All of the components together are generally referred to as a PLC system.  The processor is where a PLC program would reside and is responsible for receiving data from the input cards and distributing it to the output cards

I/O cards or Input and Output cards are the connections to the real world for the PLC system.  As discussed there are discrete I/O and analog I/O.  Both come in many different types.  Discrete I/O can come in various voltages:  24 Volt DC or AC; 120 VAC; TTL; sourcing; sinking; and on and on.  Analog I/O has even more variety.  There are Voltage inputs/outputs, Current inputs/outputs and cards that handle both.  There are cards that just grab inputs or outputs and cards that have a mix of inputs and outputs.  There are cards that are specially designed to connect to Thermocouple and RTD sensors and to compensate for the non-linearity of the signal.  I/O can also be local or remote.  Local I/O is located in the same rack or chassis as the processor, whereas remote I/O can be located in a separate location that is more convenient to wire into. 

The Rack or Chassis is used to house the PLC processor, I/O cards, and other cards that are needed with the system.  Some smaller block style PLC systems don’t use racks and even some modular style systems have cards that stack together with each card having the required connectors to pass processor communications and power to each subsequent card.

There are many available communication interfaces ranging from cards designed to communicate via Ethernet to specialized cards designed to communicate on a manufacturers proprietary communication link.  Generally those cards are used to allow the programmer to interface with the controller, another device to interface with the controller, or to allow the controller to manipulate and read I/O located in a remote location.

Programming interfaces vary from handheld programmers (yuck) to more modern programs that can be installed on a laptop to interface and program the PLC.

BASIC SIGNAL

There are a lot of different types of signals that PLCs can read and write. The two most basic signals are discrete (digital) and analog. Discrete means on or off, 1 or zero, high or low, etc. Two possible states. Using discrete signals you could have a switch that when pressed starts a motor running.  Both the switch “input” and the motor start “output” would be discrete signals.  Analog signals are continuous signals varying between two limits.  Analog signals can have a multitude of different values between those two limits such as pressure, temperature, level, etc. 

Early PLCs were designed primarily as relay replacements.  Those PLCs were designed completely around discrete applications.  They were a fantastic improvement over the rooms of relays that were used to operate what would be considered now simple sequences.  You can imagine the improvement it would be to be able to add a new discrete input and output into your program as opposed to having to wire a new relay into an existing system. 

 A single discrete signal is also referred to as a "bit". Although they sound very simple, bits can be used in a lot of different ways. For example, by turning a bit on and off you can generate pulses. By using a bit going into a counter you can count the number of times the bit turns on. For example, if you have a motor and each revolution of the motor moves the crane one foot, and you need to move the crane six feet, then you would simply count six pulses. You can also have a bit go into a timer. So if the crane moves one foot per second, then the "crane is currently moving" bit starts a timer that in six seconds tells the PLC that the crane moved six feet.  Typically however, discrete signals are used for simple applications.  Starting a motor or opening and closing valves.  Not glamorous, but very useful

A single bit can have two states – one or zero. Two bits can have four states (00, 01, 10, and 11). Eight bits is known as a byte and can represent 256 states or the numbers from 0 to 255. This is how computers represent numbers and values – by cascading bits.  The PLC is no different.

Analog signals involve detecting different levels of a signal. For example, how fast you want to run a motor. With analog signals, the user can turn a potentiometer that generates a varying voltage or current (analog input) that tells the PLC to send an analog output to the motor indicating the speed that the motor should run. Technically you would need something like a motor inverter between the PLC analog output and the motor to essentially amplify the PLC analog output to control the motor voltage or frequency and vary the motor speed.

You can represent the position of the potentiometer using 12 bits (0 to 4095). Zero voltage (or current) would be represented by 0 in the PLC and 4095 would represent the maximum voltage (or current). Theoretically (assuming no noise, perfect linearity, and a few other things), the PLC can run the motor at one of 4096 different speeds.  Analog signals can be used to bring many types of continuous signals into the PLC – temperature, pressure, level, position, etc.  Once in the PLC memory, these signals can be manipulated to control motors, valves, and other field devices. 

Let me reemphasize that the function of a PLC is to read these discrete and analog inputs, run some logic based on those inputs, and then write discrete and analog outputs to control the available environment.  

PLC EXAMPLE

Let’s start off with a simple example. Suppose you are making a controller for an overhead crane. The operator has a simple control box with four push buttons: "left", "right", "up", and "down". When the operator presses the "left" button (note that the "left" button is an input to the PLC), then the PLC turns on the appropriate output to the motor that makes the crane move left. The other three buttons would operate similarly. Sounds pretty simple – right?

Suppose it takes ten minutes for the crane to reach the full left position. Soon the operator’s fingers start to hurt (holding that button down for ten minutes at a time hurts), and they are going to beg / bribe / threaten you, the programmer, to latch that output on and add a stop button. Instead of having to press the "left" button for ten minutes, the operator wants to momentarily press the "left" button and the crane keeps moving left till the operator presses the "stop" button. So you reprogram the crane and now the operator picks up a 10 ton container, presses the "left" button, realizes he forgot to get a drink (of water), and knowing that the crane will be moving for ten minutes, goes off to get a drink. Or suppose the crane hits the operator and knocks them out. Who is going to stop the crane? There are some major safety considerations since you have a 10-ton container moving around with no one to stop it.

So you start adding safety light curtains and mats around the crane’s operational area, so that if anything comes into the crane’s operational area the crane automatically stops. You would also add Emergency Stop (E-Stop) buttons around the area so that anyone can press one of these buttons to stop the crane. You would want to add end-of-travel limit switches so that when the crane moved as far as it can go then the PLC would automatically stop the motor. You would also want to add some more inputs (feedback) to the PLC so that when a motor fault occurred the PLC would detect the fault, turn off the motor, and sound alarms. There are many other safety and diagnostic inputs you should add.

Do you see how a very simple application can grow in inputs and outputs very quickly? The good news is that by using a PLC for this application the PLC is very quickly and easily reprogrammed for the new inputs. Other wise you have to go get more relays and do a bunch of wiring for each new input and output.

Even More Complexity

We can extrapolate this simple crane into more complex systems:

• A "crane" that automatically loads or unloads 55 gallon drums onto pallets, containers on or off a ship, or adds a finite amount of reagent to a matrix of test tubes.
• Multiple cranes that have overlapping work envelopes and require collision avoidance and cooperative handling
• "Cranes" that work in three-dimensional space to store and retrieve items. Applications from electronics to pharmaceuticals show that automated storage and retrieval systems reduce errors significantly.
• Two axis controllers that move a video camera around for inspecting parts

The control systems engineer sees a lot of similarities in these different applications. All of these applications can use a PLC but these applications are just a tiny subset of all the control schemes that employ PLCs.

PLC MINI-TUTORIAL

This document is designed to give an overview of what a PLC is, what it is good for, what it is not best at, an overview of its operation and where to find more information. A Programmable Logic Controller (PLC) is a rugged special purpose computer that reads a bunch of input signals; runs control logic, and then writes output signals. After reviewing this mini-tutorial please feel free to ask us questions or ask about a free seminar and demonstration at your site.

Pros

1. PLCs are good at turning outputs on or off based on the state of inputs. (control)
2. PLCs are good at bringing together and concentrating a lot of data and status that is uploaded into a computer in a compact form
3. PLCs are more rugged than computers and typically last five, seven, ten years without needing replacement

Cons

1. PLCs are not the best at handling large amounts of data or complex data.
2. PLCs are not the best at reading and writing databases.
3. PLCs are not the best at outputting resultant data to printers.
4. PLCs are not the best at displaying data and information to the operator.

History

Programmable Logic Controllers (PLCs) have been around since the dinosaurs. In fact it was PLCs that killed the dinosaurs. Don’t listen to those people that tell you that dinosaurs were killed off by global warming / cooling, meteors, or even smoking cigarettes. PLCs killed the dinosaurs. You see, millions of years ago the cavemen used PLCs for everything they did. All of the dinosaurs lost their jobs, couldn’t afford to buy food at the local stoney-mart and just died off.

Okay, okay I guess I have to admit that the last paragraph is not true. But the PLC has been around since at least the 1960s, which in technology years makes it a dinosaur. For the real history of the PLC follow this link  to the website of Dick Morley, the inventor of the PLC (just don’t forget where to come back to).