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 idea is to turn on all the pilot lights for a couple of seconds so you can verify that all the lights work. This feature is very handy when you are troubleshooting a machine.
When the Emergency stop is first cleared, Timer 0 is started. The result is that all the pilot lights will turn on for two seconds after the E-Stop is cleared. This bit is then used throughout the program. You want to enable certain machine functions in Auto Mode, and disable some in Manual Mode, and vice versa.
Notice how the System Running bit is used. If we lose that bit, such as when the emergency stop is pressed or the machine guard is opened, neither mode is valid.
A manual mode is provided to allow ease of set-up. These photoeyes are positioned so that if they "see" a part, they will turn on the input. A part will break the beam, the input will turn on and you know you have a part present. In this rung, we want to make sure there is no part in the machine.
The rung will only be true if all the photoeyes indicate there is not a part present. This is standard on most machines. When the operator goes to Auto Mode, and there are no parts in the machine, and the machine components are at home, the cycle will begin.
You might ask, "If there is a part in the press, wouldn't the machine start running as soon as the operator took the part out? You don't want this machine to start running when someone clears a part.
In our case, to remove a part the operator would have to open the machine guard door in order to physically remove the part, and that would kick the machine out of automatic mode. He would have to close the guard and start the machine again. Bit coil22 will stay on during the entire drilling cycle and drop out when an end of cycle signal is generated. Note that the PLC will stop the conveyor after the part has been clamped in place Rung 16 , but until that happens, the conveyor will run.
This rung will let the operator know that. Also, a manual method of holding the part has been provided for machine set-up. This particular drive has two inputs; one to make the motor go forward move the spindle down and one to make the motor go in reverse move the spindle up. So, when the part is held in place, the PLC will command the drive to lower the spindle until it reaches the lower limit Spindle Lowered proximity switch.
The feed rate is determined by a variable speed drive. A manual method of raising the spindle is also provided. The drilling is done and the spindle is back in its home position, so the End of Machine Cycle bit is latched. When this bit goes high, it causes the Machine In Cycle latch to drop out Rung This in turns raises the stop gate Rung 13 , releases the part Rung 15 and turns on the main conveyor Rung When the part clears all the photoeyes, the stop gate lowers and the machine waits for another part.
The machine would still run without these rungs, but they make life a lot easier for the operators and could prevent the machine from damaging itself. You might think this should be obvious, but it is nice to have a red light come on when a safety gate is open. It makes people take a little more care when working around the machine. The machine will not operate properly if there is insufficient air pressure. You can put it on a fairly long time delay, because you don't care about momentary drops in pressure, like a second or two.
But when it does drop beyond the lower limit for enough time, you want the machine to stop. You also want it to latch so you know why the machine stopped running.
The air pressure could come back up before the operator sees the pilot light, and he would be left scratching his head. The latch is reset by the Reset System pushbutton. 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.
Train and test your skills with the built-in quiz 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. Valuable, interactive software for those who need to quickly master PLC handling 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.
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