What is a Programmable Logic Controller (PLC) - In Layman's terms
What is it?
A programmable logic controller, or PLC, is a type of computer that has been adapted for use in more rugged situations that require high reliability. For instance, industrial manufacturing and automation where you don't want your stuff to blow up or have robots chucking heavy car parts across the building. Extreme heat/cold, moisture, and or shock are only a handful of conditions that can exist in these environments and as we all know, regular computers normally choke under such conditions like when you wanted to ask your crush to senior prom. Generally speaking though, most run of the mill laptops and desktop computers blow the doors off of PLC's when it comes to raw computing potential and performance but that is in exchange for a rather unstable system though some high end PLC's are pretty on par with modern personal computers.
Ever had your computer crash and you lose all your work? Or a program keeps freezing up? Both those things are inconveniences for most people but can be disastrous in an industrial automated role. A program that refuses to load on an assembly line could cost millions of dollars in revenue. A program crash in a chemical factory could cause dangerous amounts of products to overflow tanks or cause heaters to overheat and ignite fumes. So while it may be nice to have a lot of power and versatility, stability is much more important. If you take anything away from this post etch into your mind that stability is safe and safe is bueno.
Setup:
Just a quick rundown of what PLC's have when it comes to hardware. All PLC's will have some sort of Input/Output capability where signal wires can be wired in to retrieve/send data. Some low end PLC's will have this built right into them offering a limited amount of inputs/outputs. High end PLC's however will have their inputs and outputs delegated to separate input and output modules making them very expandable. There is also a power supply (PSU) that powers all the modules, communications module (CSM) and an interface module(IM). These components are usually set up on the rack starting from left to right as PSU, Processor Unit, IM, CSM and then the I/O modules.
Typically speaking a PLC will be connected to some sort of process visualization software running on either a panel mounted screen or desktop computer. This allows an operator to monitor and adjust settings as needed. We'll get into this more when we cover Human Machine Interfaces(HMI) and Supervisory Control And Data Acquisition (SCADA) programs.
A programmable logic controller, or PLC, is a type of computer that has been adapted for use in more rugged situations that require high reliability. For instance, industrial manufacturing and automation where you don't want your stuff to blow up or have robots chucking heavy car parts across the building. Extreme heat/cold, moisture, and or shock are only a handful of conditions that can exist in these environments and as we all know, regular computers normally choke under such conditions like when you wanted to ask your crush to senior prom. Generally speaking though, most run of the mill laptops and desktop computers blow the doors off of PLC's when it comes to raw computing potential and performance but that is in exchange for a rather unstable system though some high end PLC's are pretty on par with modern personal computers.
Ever had your computer crash and you lose all your work? Or a program keeps freezing up? Both those things are inconveniences for most people but can be disastrous in an industrial automated role. A program that refuses to load on an assembly line could cost millions of dollars in revenue. A program crash in a chemical factory could cause dangerous amounts of products to overflow tanks or cause heaters to overheat and ignite fumes. So while it may be nice to have a lot of power and versatility, stability is much more important. If you take anything away from this post etch into your mind that stability is safe and safe is bueno.
Setup:
Just a quick rundown of what PLC's have when it comes to hardware. All PLC's will have some sort of Input/Output capability where signal wires can be wired in to retrieve/send data. Some low end PLC's will have this built right into them offering a limited amount of inputs/outputs. High end PLC's however will have their inputs and outputs delegated to separate input and output modules making them very expandable. There is also a power supply (PSU) that powers all the modules, communications module (CSM) and an interface module(IM). These components are usually set up on the rack starting from left to right as PSU, Processor Unit, IM, CSM and then the I/O modules.
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| Left to Right: Power Supply, CPU, Interface Module, Communications Module |
Typically speaking a PLC will be connected to some sort of process visualization software running on either a panel mounted screen or desktop computer. This allows an operator to monitor and adjust settings as needed. We'll get into this more when we cover Human Machine Interfaces(HMI) and Supervisory Control And Data Acquisition (SCADA) programs.
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| A typical HMI screen that can be mounted on a panel. |
Programming:
PLC's are generally programmed using ladder logic which is a visual(graphical) programming language that resembles the rungs on a ladder with the left and right rails being power and ground respectively. Though ladder logic is one of the most popular there are several other ways to program a PLC depending on the manufacturer and programming environment you use. Those are Functional Block Diagrams, Structured Text and Sequential Function Chart. There is one more which has been deprecated a.k.a. ignored until it goes away called instruction list. Instruction list is still common among older systems so it's wise to gain some familiarity with it in case you run across it.
Conclusion:
That's it for now. I'll start adding more content soon. If you see any errors please let me know, I strive to be accurate but I have an outdated glucose powered mushy logic gate processor and it does make mistakes from time to time. Until next time.
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