In response I have trotted out the titles of various standard text books but I have never come across a book that really develops the skill of PLC programming instead of telling the reader what PLCs are all about. I have finally decided to fill this gap in the market myself. What sort of PLCs do you use? It implies that familiarity with one make and model of PLC will leave the programmer struggling when asked to use a different type.
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I have been writing PLC programs for over 20 years. I often get asked what is the best way to learn PLC programming. Programming in the way I was taught in college was with the Motorola Yes, I know that I am dating myself This was microprocessor programming, but it was the best way to sometimes explain the methods behind PLC programming. Manufacturers of PLCs had proprietary software that was not even related in their appearance and methods of programming.
Today we have a few standards that have changed the look and feel of the programming software packages so each manufacturer is similar. The following is the best recommendation that I have for beginners to start to learn PLC programming today. He also includes some test questions along the way in order for you to retain and understand the important points that he is making. Topics covered include:.
After learning the basics from the above manual, practice. Create programs yourself and test what you have learned. You can accomplish this by using simulators. Allot of the programming software will have simulators. The simulator will mimic the PLC hardware so you can test your programs before installing in the field.
It is the Do-More Designer Software. This software simulator includes the entire instruction set Not Just a Bit Logic as well as communication protocols. It can be downloaded and installed for free from the above link. The next step I recommend is then to advance into some of the advanced instructions. An understanding of the numbering systems in the PLC will be a benefit. All of these and more instruction information can be obtained from reviewing the documentation from the PLC manual that you are programming.
Once again all of these instructions are included in the Do-More Designer Software. The program structure is the next topic. Allot of programmers would stop here and can do well with developing software, however, there is much more than you can lean. Sequencers give programmers the methods to change the logic on the fly and allow troubleshooting the system easier. This method of programming can benefit you greatly and reduce the development time of your logic. The last step that I recommend learning is the sharing of information.
This refers to the understanding of the ways in which information can be gathered from the PLC and displayed in different ways. Here are a couple of previous articles that have been written on this subject:. As you can see, there is a lot of information available to you to begin and lean PLC programming without spending a dime!
Systems are expanding and changing every day. Happy programming. Do you know of additional tips or methods to share? All of the programmable logic controllers have similar basic features. Here is how I would approach learning about basic PLCs. If you have any questions or need further information please contact me. We will walk through the numbering systems used in PLCs. To get this free article, subscribe to my free email newsletter. First Name: Last Name: Email address:. The link is included when you subscribe to ACC Automation.
Timers are used in the majority of PLC programs. There are also a wide variety of off the shelf industrial timers that you can use. A timing chart is a secret behind understanding of the timer that you need in your application. Making a timing chart before writing the program will ensure that all of the information will be accounted for. The timing chart is mapped out on an x and y plain.
This is the basic operation for an Omron H3BR industrial timer. Start — In this case, the start signal is momentary to start the time cycle. Output — The output will show when it turns on.
This can also indicate the opposite, and show when it turns off. Time — Time is shown by the relationship between the start signal and the output. Our example shows timing starts on the leading edge of the Start. This could have also been on the trailing edge. Here is the same on-delay timing chart with some more detail. Several conditions are added to the chart.
As you can see the timing chart is vital in determining how the sequence will be performed. This is the exact same method that I use when determining timing sequences in a PLC program. When we hit the start button, the warning light then comes on. After a fixed time the warning light goes off and the motor starts.
The motor will run until the stop button is hit. You will notice that we have added an internal memory bit C0 as our Start Sequence. If the power goes off, or the PLC is put into program mode the circuit does not remember the last state. It will default to be off. The sequence is as follows:. Every PLC has timers. They all have different types depending on what you are trying to achieve. It will all start with your Timing Chart. Looking at a stop-start jog circuit in the PLC will help us in understanding the differences in hard wiring the circuit and programming.
Here is what it looks like hard wired. The start pushbutton can now be released because the CR contacts allow the power to pass through to the CR. NO — Normally Open — This refers to the state of the input device if nothing acts upon it. NC — Normally Closed — This refers to the state of the input device if nothing acts upon it. This is because the actual signal wired in the input is NC and we do not want to inverse this signal. You can see that the stop input is currently on in the program. If we hit the start pushbutton then the circuit is complete and the output CR turns on.
Letting go of the start pushbutton, the output remains on because of the CR input seals in the start pushbutton. Pressing the stop pushbutton will break the circuit and turn off CR. Letting go of the stop pushbutton will return us back to the original state shown above. Adding a jog input to the hard wiring diagram will look something like this: You can see that the diagram will work the exact same as the circuit above with the start and stop pushbuttons.
The jog when pushed will break the sealing contact, and then make a bypass of the start pushbutton. This will keep the M coil on as long as the jog button is pressed. Letting go of the jog will stop the bypass of the start pushbutton which will stop M coil.
When the jog returns to the original state M input will already be off so it will not keep M coil on. The action on the jog is referred to as a Break before Make device. The jog pushbutton will break the circuit before making another connection. Inputs are made before the previous ones are broken. The programmable controller will scan the program from left to right, top to bottom. The outputs from the rung above are available to the rungs below. Here is a previous article on PLC scanning.
Lets take a look at PLC program with a jog that will not work. Even though this looks like it would work… Remember that the contacts in the PLC are making before breaking. You can jog the unit and it will turn on but as soon as you release your finger off of the pushbutton the not jog input will seal the CR in. The output will not be able to turn off.
We must consider the delay from on to off when looking at the PLC program for this circuit. Here is a circuit that will work:. The set will have all of the conditions to turn on a bit in memory and the reset will have all of the conditions to turn off a bit in memory.
These instructions are used to make the program easier to view and troubleshoot. Here is the same logic above using the set and reset instructions. Notice that X10 Jog2 is in parallel with the Start. We use a trailing edge one shot in parallel with the Stop.
This sets our delay so the output will turn off.
PLC Programming for Industrial Automation. Kevin Collins
All Rights Reserved. This book provides a basic, yet comprehensive, introduction to the subject of PLC programming for both mechanical and electrical engineering students. It is well written, easy to follow and contains many programming examples to reinforce understanding of the programming theory. The student is led from the absolute basics of ladder logic programming all the way through to complex sequences with parallel and selective branching.
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The PLC scans its inputs and, depending on the program, switches on or off various combinations of outputs. The logic state of the output depends on the input conditions and so the term conditional logic is used. A machine switches on if either of two start switches are closed and all of three stop switches are closed. The conditions could be realised by a hard wire solution as shown in Figure 1. The two start switches are connected in parallel. Current will flow if one or the other or both are closed.
PLC Programming for Industrial Automation