This course will teach you how to do that. 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.
It can be an input, output or internal coil, among others. 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. An internal coil is labeled COIL in our example. When used with a hardwired input, this instruction is "true" until there is a voltage applied to the input. 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.
It requires the use of its "timer finished" bit, like a time delay relay uses its contact. 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. 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.
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. Each PLC manufacturer gives you the details of wiring their particular modules.
Therefore, the PL1 output is off. 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. 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.
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.
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.
These terminals will usually have the capability of programming online or offline. 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. 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. The inputs are updated during the program scan. In high-speed applications, such as bottling or pharmaceutical lines, this can cause problems. 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.
The entire process needs to be automatic. The mechanical and electrical engineers bring you an isometric drawing like the one shown here. 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. 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.
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 main conveyor. Though the device name, such as PB1, would not show up on the actual station, it is a good idea to show them on your drawing.
Do not skip this crucial step. The text in the fixed font is basically the information that you would see if you were looking at the monitor of the computer or a printout.
In actual practice, the fonts used in PLC software vary widely. For the purposes of this book, we want to easily differentiate the program logic from our explanations of the logic. For purposes of this manual, I have placed additional explanations between rungs. Use a title to name the program and include any general information.
This type of bit is what we call an internal coil. It has no hardwired connection to the outside world. If the emergency stop is clear, and the machine guard is in place, and there is no system fault the operator may press the start button to set the latch. Most of the time, the order of the bits in a rung doesn't matter.
We could have rearranged any of the bits in this rung, though we would still have to put the latch around the Start pushbutton. The PLC wouldn't care and the output coil would still respond the same. However, to make the rung easier to read, I try to place bits from left to right in order of importance. If the E-Stop is not cleared, then nothing else should matter anyway. Having the safety guard in place is more important than a system fault.
Now, if those requirements have been met, we can press the start button. And we don't care about the stop button until we have pushed the start button. Note the instruction used for the input of PB3, the Stop System bit.
It may seem backwards at first, since a hard-wired circuit would use the normally closed contacts of the switch. The E-Stop and the guard limit switch are called "safety interlocks. Use additional contacts from the switches and wire them to inputs of the PLC so that it knows the machine is to be stopped, or has stopped.
It is very important to label the bits properly. Arrange the verbs and nouns correctly. If you do, the rung will read like a sentence. There are some simple rules that I always follow when I am writing a description for a bit: - Descriptions for bits portray an action. We can then place just that bit in the rung that controls the spindle motor and know that we have met all the criteria to allow the spindle motor to run.
The app bases its entire functionality on a tutorial that is made out of different sections, all accessible through a dropdown menu. A useful feature is the fact that users can navigate through the different sections even when they are within the training itself, thanks to a set of dedicated playback controls. Beside the training data and learning through interactive slides, users can also resort to a knowledge quiz, which will put their knowledge to the test. All questions feature item-choice answers and are very intelligible, with no distracting elements.
Overall, this program is a simple, accessible solution for learning what programmable logic controllers entail and how to handle such assemblies when working into an industrial environment. It features a no-fuss, interactive tutorial interface, which lets users both learn and test their knowledge through a dedicated quiz. Featuring really old, basic illustrations, it might not appeal to demanding users, but since it does a good job at providing a PLC learning platform, one can overlook that minor issue.
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