Beginner's Guide to PLC Programming How to Program a PLC (Programmable Logic Controller PDF

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Beginner’s Guide to PLC Programming How to Program a PLC (Programmable Logic Controller) By Neal Babcock modernmediaonline.com Copyright 2008 Modern Media Table of Contents Introduction to PLCs .............................................................................................................

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Beginner’s Guide to

PLC Programming How to Program a PLC (Programmable Logic Controller) By Neal Babcock modernmediaonline.com Copyright 2008 Modern Media

Table of Contents Introduction to PLCs .....................................................................................................................3 Ladder Logic.................................................................................................................................3 The Dialect of PLCs .....................................................................................................................5 Equivalent Logic ...........................................................................................................................7 Scan Time ....................................................................................................................................9 The Automated Drill Press..........................................................................................................10 Sequence of Operation...............................................................................................................11 Operator Station .........................................................................................................................13 I/O Listing ...................................................................................................................................14 Inputs ......................................................................................................................................14 Outputs ...................................................................................................................................15 Internal Coils...........................................................................................................................15 The Program ..............................................................................................................................16 Machine Safeties ....................................................................................................................16 Pilot Light Test ........................................................................................................................18 Indicate The System Is Operational ........................................................................................19 Machine Operation Mode........................................................................................................19 Run The Spindle Drive Motor..................................................................................................20 Indicate The Spindle Drive Is Running....................................................................................20 Run The Infeed Conveyor.......................................................................................................21 Ensure There Are No Parts In The Machine ...........................................................................21 Ensure All Components Are At Home.....................................................................................22 Begin The Cycle .....................................................................................................................22 Lower The Stop Gate..............................................................................................................23 Run The Main Conveyor .........................................................................................................23 Indicate The Part Is In Place...................................................................................................24 Clamp The Part In Place.........................................................................................................24 Lower The Spindle ..................................................................................................................25 Drilling Operation Is Complete ................................................................................................26 Return The Spindle To Its Home Position...............................................................................27 Machine Cycle Is Complete ....................................................................................................28 Fault Detection And System Diagnostics ................................................................................29 Personnel Safety Guard Door.................................................................................................29 Low Compressed Air Pressure ...............................................................................................30 Motor Starter Overload ...........................................................................................................31 Latch The Motor Overload Detection ......................................................................................32 Indicate A Motor Overload Condition ......................................................................................32 Jammed Part Detection ..........................................................................................................33 Latch The Part Jammed Detection .........................................................................................34 Indicate A Part Jammed Condition .........................................................................................34 Monitor The Drill Time.............................................................................................................35 Summarize The Fault Conditions............................................................................................36 13 Marks Of A Well Written Program .........................................................................................37 General PLC Tips.......................................................................................................................38 The Automated Drill Press in Rockwell Automation’s RSLogix 500 ...........................................39 Beginner’s Guide to PLC Programming How to Program a PLC (Programmable Logic Controller) Copyright  2008 Modern Media modernmediaonline.com

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Introduction to PLCs Nearly all the industrial equipment that you find in a modern manufacturing facility shares one thing in common - computer control. The most commonly used controller is the PLC, or the Programmable Logic Controller, using a programming language called Ladder Logic. The language was developed to make programming easy for people who already understood how switches, relay contacts and coils work. Its format is similar to the electrical style of drawing known as the “ladder diagram”. Originally, there were only a few functions available in this language, but as times have progressed, many more higher-level functions have been introduced. We are going to stick to the basic, commonly used functions here. Also, this text will not replace the user's manual that comes with a PLC, but it will give you a big head start if you have never programmed a PLC. This course is intended to provide an introduction to the programming methods used in PLCs and give the reader a solid, basic understanding of the language of Ladder Logic. After you complete this course, you may be interested in learning about hardware-specific software and programming techniques. Modern Media offers a book entitled PLC Programming Techniques How to Program an Allen-Bradley SLC 500 with Rockwell Automation’s RSLogix. This ebook shows, step-by-step, how to create a program from scratch in Allen-Bradley’s RSLogix 500. To learn more, please visit http://www.modernmediaonline.com.

Ladder Logic I have summarized the terms and techniques you need to know if you are going to work with ladder logic. It is not a comprehensive summary, as that would take volumes of text, but if you are just starting out, the information in this book will be very helpful. Every PLC programmer, no matter what skill level, must have learned the principles described in this book at one point in time. There is simply no way around it. I have included a program for a simple machine that lets you really understand how Ladder Logic works. To effectively write a program, or even edit one, the programmer must know how to visualize the effects of the changes he will make. In other words, you have to be able to look at the logic “on paper” and imagine how it will work when it is entered into the PLC. This course will teach you how to do that.

Beginner’s Guide to PLC Programming How to Program a PLC (Programmable Logic Controller) Copyright  2008 Modern Media modernmediaonline.com

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There are many types of PLCs, and differences among PLCs, but what is discussed here should be common to all types. After you read and understand this, you will have a clear understanding of the structure of this type of programming. In the real world of industrial automation, the methods presented in this document may be all that many people will ever need to know.

Beginner’s Guide to PLC Programming How to Program a PLC (Programmable Logic Controller) Copyright  2008 Modern Media modernmediaonline.com

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The Dialect of PLCs Lets' define some terms and symbols: BIT - an address within the PLC. It can be an input, output or internal coil, among others. RUNG - A section of the PLC ladder program that terminates in an output function of some type. HARDWIRED INPUT - a physical connection to the PLC from an input device (switch or sensor, etc.) A hardwired input is labeled INPUT in our example. HARDWIRED OUTPUT - a physical connection from the PLC to an output device (relay or pilot light, etc.) A hardwired output is labeled OUTPUT in our example. INTERNAL COIL This is a programmable bit used to simulate a relay within the PLC. The internal coil has no connection to the outside world. It does not connect to an output card. Internal coils are used to store information. The “contacts” of this “relay” can then be used multiple times in other parts of the program. An internal coil is labeled COIL in our example. --] [-Normally Open Contact When used with a hardwired input, this instruction is off until there is a voltage applied to the input. The bit address then goes high, or on, and the instruction becomes “true.” It works the same way when it has the same address as an internal coil, except that the coil must be turned on by logic in the program. --]/[-Normally Closed Contact This is an inverted normally open contact. When used with a hardwired input, this instruction is "true" until there is a voltage applied to the input. It then goes low, or off, and becomes “false.” It also can be used with an internal coil, becoming true when the coil is off and becoming false when the coil is on. -( )Output Coil When used with a hardwired output, this function is off until the logic in the program allows it to turn on. It then becomes “true”, and will energize the device that is wired to the respective output. If it is used as an internal coil, it will toggle the instructions associated with it. That is, it will close a normally open instruction and open a normally closed instruction.

Beginner’s Guide to PLC Programming How to Program a PLC (Programmable Logic Controller) Copyright  2008 Modern Media modernmediaonline.com

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+---------+ TIMER | +-- SEC---+ Timer This function is used to supply a programmable delay. It requires the use of its "timer finished" bit, like a time delay relay uses its contact. +---------+ COUNTER | +-- 000---+ Counter The counter function is used to count events. It could be used to keep track of machine cycles, count parts, etc. It can be programmed with a preset value that triggers another event when the count is reached.

TRUE - An indication the a bit is “on”. If you press a pushbutton switch that is wired to an input, then the bit is said to be true. Also, if the logic in a rung turns on the output of the rung, then the rung is said to be true. FALSE - Without stating the obvious, this is the opposite of true.

Beginner’s Guide to PLC Programming How to Program a PLC (Programmable Logic Controller) Copyright  2008 Modern Media modernmediaonline.com

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Equivalent Logic In its elementary form, PLC logic is very similar to the hard-wired logic you would find in an electrical ladder diagram. For example, if you wanted to turn on a light with a momentary pushbutton, you would wire it like the circuit below.

When you press PB1, the pilot light PL1 lights up.

Now let's do the same thing in a PLC. To duplicate the hardwired circuit on a PLC, you would wire the switch PB1 to an input and wire the light PL1 to an output. Each PLC manufacturer gives you the details of wiring their particular modules. The I/O (hardwired inputs and outputs) is set up like this: - There is a “PB1” pushbutton switch wired to INPUT1 of the PLC. - There is a “PL1” pilot light wired to OUTPUT1 of the PLC.

PL1 | PB1 | INPUT1 OUTPUT1 [---] [------------------------------------------------------( OUT ) |

Now let’s examine the sequence of events. When you first turn on the PLC, the PB1 pushbutton is off, or false. Therefore, the PL1 output is off. Pressing PB1 will make INPUT1 true, Beginner’s Guide to PLC Programming How to Program a PLC (Programmable Logic Controller) Copyright  2008 Modern Media modernmediaonline.com

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OUTPUT1 will come on and the light will be energized. It will stay on only as long as you hold the button in. Just like electrical current has to flow through the switch to turn on the light in the hardwired circuit, the logic has to "flow" through the normally open instruction (which is closed when you press the switch) of INPUT1 to energize the output that turns on PL1. The programming terminal display will look something like this as you hold in PB1. The yellow highlight indicates the bit, or address, is “on” or “true”. | PB1 PL1 | INPUT1 OUTPUT1 [---] [------------------------------------------------------( OUT ) |

Let's look at how a timer works. Suppose you want to delay running a motor for 2 seconds after you turn on a switch. You can use the input from the switch to run a timer. Program the timer for the duration you want and then use the "timer finished" bit to turn on your motor. In this instance, we have configured an "on delay" timing sequence. Two seconds after INPUT1 is on, the TIMER1 will turn on its "finished" bit and the motor will run. Note that there is no "off delay" here. As soon as the start switch is released, the "timer finished" bit will drop out and the motor will stop. With a little creativity, you can combine timers to provide any timing function you need. +--Motor -+ |Start |Motor | Start | |PB1 | Delay | TIMER1 | |INPUT1 [---] [---------------------------------------------------+--2SEC------+ | | |Motor |Start Run Motor |Delay |TIMER1 OUTPUT1 [---] [-------------------------------------------------------( OUT )

One nice feature of PLCs is that you can document each bit in the program. In the example above, “INPUT1” is somewhat meaningless on its own. After you add the descriptive text “Start Motor PB1”, things make more sense. Most PLCs are programmed via a Windows based terminal. Editing, deleting or adding to the ladder logic is usually pretty straightforward. You use the arrow keys or the mouse to add instructions, change addresses or comments, etc. We won’t cover the specifics of keystrokes here, but will concentrate on understanding the ladder logic. These terminals will usually have the capability of programming online or offline. If you are making changes in online mode, be aware that any changes you make and save (or upload) will Beginner’s Guide to PLC Programming How to Program a PLC (Programmable Logic Controller) Copyright  2008 Modern Media modernmediaonline.com

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alter the program that is being used to run the machine. This requires great care and a full understanding of what will happen when you make the change.

Scan Time One critical difference between a PLC program and the equivalent electrical circuit is the issue of scanning. It works like this in most PLCs: The PLC looks at the state of the inputs and stores that information in a temporary buffer. Then, it ignores what is happening electrically at the inputs. The PLC will use the information in the temporary buffer to execute the logic in the program. It will solve the logic from top to bottom, determining the truth of each rung, and turn on or turn off the appropriate addresses in the temporary buffer. When it reaches the last rung in the program, the PLC will use the data in the temporary buffer to turn on or turn off the corresponding outputs. The scan cycle is complete, and the PLC will once again look at the inputs. The amount of time this takes is called scan time, and is measured in milliseconds. Stated more simply, the PLC reads the inputs, performs the logic and adjusts the outputs as needed. In some newer PLCs, such as Rockwell’s ControlLogix platform, it doesn’t work that way. The inputs are updated during the program scan. In high-speed applications, such as bottling or pharmaceutical lines, this can cause problems.

Beginner’s Guide to PLC Programming How to Program a PLC (Programmable Logic Controller) Copyright  2008 Modern Media modernmediaonline.com

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The Automated Drill Press Now let’s jump right into a project. The best way to learn a programming language is to look at a real world example. However, before you can do any programming, you must have a clear understanding of how the machine works. Let’s say a furniture manufacturer needs to drill a 3/8” hole in a certain spot on a piece of wood. The entire process needs to be automatic. The mechanical and electrical engineers bring you an isometric drawing like the one shown here. Mechanical details have been omitted for clarity, as is often the case in a “concept” drawing.

The main conveyor will transport the part into the machine where the part will meet a pneumatically actuated stop gate. At that time, another pneumatic cylinder will actuate a clamp that will push the part back against the conveyor wall. This will hold the part in place during the drilling process. Photocells will verify that the part is in position; the spindle will lower and proceed to drill a hole in the part. After the hole has been drilled and the spindle has retracted to Beginner’s Guide to PLC Programming How to Program a PLC (Programmable Logic Controller) Copyright  2008 Modern Media modernmediaonline.com

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its home position, the clamp will release, the stop gate will raise and the part will exit. The cycle then repeats itself for each part that comes down the line.

Sequence of Operation Here is a more detailed explanation of the drilling process:

When the machine starts, the stop gate lowers and the part is moved into position by the main conveyor.

Optical sensors (photoeyes) determine when the part is in place. When the part is positioned correctly, a clamp extends to hold the part in place.

Beginner’s Guide to PLC Programming How to Program a PLC (Programmable Logic Controller) Copyright  2008 Modern Media modernmediaonline.com

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The spindle of the drill press is lowered, and the hole is drilled in the part. A sensor in the drill press spindle tells the PLC when the spindle has reached the end of its travel.

After the hole is drilled, the spindle retracts, the clamp retracts, the stop gate is lifted and the part is carried out of the machine by the m...