Industrial Control Panel In Depth Look Part 4: PLCs (Programmable Logic Controller)

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In today’s video we are going to be talking more about industrial control panels. Specifically, we are going to be taking an in depth look at the heart of the panel, the PLC, or programmable logic controller. The PLC is an industrial computer that is designed to be used in potentially rough environments that might be commonly exposed to the heat, cold, dust or moisture. They are designed to monitor and control a series in inputs and outputs. The programs running in the PLC’s evaluate the inputs and change the output signals based on the programmed response to the input signals. Several programming languages are available to program most PLC’s. The most common of these is called Ladder Logic. It is a visual programming language that evolved from relay logic diagrams which visually looks like a ladder because it consists of two vertical rails with rungs in between that contain the programmed logic. While ladder logic is the most common programming language used within PLC’s, there are others that are also common including: Function Blocks Diagrams, or Structured Text and Sequential Function Charts.
Most PLC’s are made up of very similar hardware even though they may be manufactured by different companies. Let’s talk about the hardware that typically makes up the PLC. First: we have the CPU. This is a microprocessor that acts as the brains of the operation. It will perform logic operations, control instructions and various other tasks that help keep the PLC running effectively. Next, the PLC will have some type of memory. This memory will allow the PLC to store data needed to run the program, and also the data that is being collected by the inputs wired to the PLC. The PLC will also have a section that is dedicated for the IO to be connected. (show) This is where the inputs and outputs are actually wired into the PLC itself.
Depending on the type of PLC you have, this section might have a fixed number of IO points, or it might be a modular based IO section that allows you to add the specific type of IO that you need. Many PLC’s will require some type of power supply as well. It is common to see the more modular based systems integrate this power supply. However, it is not always necessary to use a power supply provided by the original manufacture of the PLC and is some cases, the PLC will include a built in power supply so that a separate supply is not needed. Lastly, the PLC needs some kind of programming device, this allows you to feed to the program into PLC’s memory so that it can perform the desired operation. This is most commonly some type of computer.
Now that we have a solid understanding of the hardware the makes up the PLC. Let’s now talk about how it works. The PLC is designed to run the program repeatedly, over and over again. Normally many times a second. Each time the program runs, the PLC it goes through some very specific steps. While the order and specifics may vary from manufacturer to manufacturer, they all typically follow a pattern similar to the following.
First, the PLC performs internal self-checks to make sure that it is functioning correctly. Secondly, it reads the inputs that are connected to it. It will then store the status of each of those inputs in its memory. Then the PLC will execute the program logic. Finally, the PLC will write the new values to the hardwired outputs to match those determined during program execution. At some point in the cycle, communications requests will also be processed. Following the execution of the last step, the PLC will then cycle back to the beginning of the process and begin again. Depending on the complexity of the program and the amount of IO, this scan cycle can occur in just milliseconds, allowing the program to run, read the inputs and react very quickly. The speed and reliability of the PLC are some of the main reasons they are ideal for industrial applications.
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