What Is A PLC

ControLogix Modular Design

ControLogix Modular Design

The Programmable Logic Controller, or PLC is an industrial computer designed to run specific tasks quickly and efficiently.

Its sole purpose is to automate processes such as assembly lines, manufacturing cells, waste water treatment, material handling and mining operations. Keep in mind these are just a few examples.

Contollogix

Generally most medium to large PLC’s available today are of a modular design. This approach allows control systems to be scalable. Need more Inputs, add an input card. Need another processor, add another one. Need to access your program over Ethernet, add a communication card. Need motion control, add a motion control card. Quick, simple, modular design.

PLC’s also come in small compact affordable containers for use on projects requiring just a few I/O points.

Most PLC programs run continuously, scanning inputs, executing logic and switching outputs.

Programs can be  run periodically at predetermined time intervals or triggered by an event such as a push button or alarm.

Inputs can be a transition from off -to-on or on-to-off, like a light switch. These inputs have two states (binary) and are known as Digital orCompact Discrete Inputs. Inputs can also be variable, such as a temperature input. These inputs are known as Analog Inputs and can have multiple values within a determined range. Outputs are similar to inputs; there are Digital or Discrete Outputs and Analog Outputs. A digital output might be a simple pilot light, while an analog output might be a current or voltage signal which controls the speed of an inductive motor.

Compare a Control system to a website, this website as an example.

If you click on a link you will be navigated to another page, the mouse click is a digital input that initiates a program or logic, the logic instructs your web browser to navigate to a new URL. The newly navigated webpage then becomes the output.

Now…. let’s replace the website analogy with an industrial example. The mouse click will be substituted with a push button. The output will be an obnoxious horn warning people a robot is about to move. Rather than navigating to a URL the logic simply turns an output/horn on. The logic turns the horn on and off rapidly for ten seconds. This is a simple, yet effective control system.

Typically a PLC is programmed (the program is written) using software installed on a computer such as the laptop I’m typing this article on. PLC’s can be programmed using several different languages with the most common being Ladder Logic.Micro Plcs

A PC can download, upload, or go online with a program in a PLC. Communication capabilities vary in regards to the manufacture and model of the controller. Some communication methods are Serial, USB, and Ethernet. There are many, many more, it all depends on the architecture of the control system.

This is a brief high level view of PLC’s. If you’re interested in learning Ladder Logic check out the basics here, or jump into some programming tutorials here. If you want the first chapter of a book I’m writing that teaches programming drop your name and email address in the box below. After an email verification you will be able to download the first part of the book for free. I wont spam your email address, I promise to only send you useful information pertaining to programming ladder logic.

 

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  • { 10 comments… add one }

    Pankil August 8, 2013, 8:54 am

    Hey there,
    I just wanted to know that I am using RSVeiw 32 softwere with RSLogix500 for programming Allen Breadly PLC 1200 C , Do i insert any arbitary image in SCADA and give animations to that…?
    I am doing project as part of my carricular objective and i need to know this as soon as possible.
    This is a kind REQUEST !!
    Thanx in Advance.

    Reply
  • { 10 comments… add one }

    leroy843@msn.com August 8, 2013, 5:40 pm

    Animation is created in your HMI or SCADA program and can be done with objects, such as Multi-State Indicators, String & Text displays, Polygons, etc.

    For instance a polygon can have Color Animation or Position Animation or both, as well as many others. Select the Polygon object, draw it on one of your screens, then right click the polygon and select animation. Select the animation option that best fits your needs. Link the animation to a tag or tags in your PLC. That’s all there is to it.

    Reply
  • { 10 comments… add one }

    SCADA August 30, 2013, 11:52 am

    Look above is Allen Bradley products, If you want to create configuration for AB you can download Free Program called Integrated Architecture Builder (IAB) from ab.com, But if you want to free PLC ebook or white paper you can visit controlmanuals.com

    SCADA engineer

    Reply
  • { 10 comments… add one }

    paris September 17, 2013, 11:19 am

    its not enogh for me

    Reply
  • { 10 comments… add one }

    Adepoju Olusola October 18, 2013, 4:36 pm

    Please update me on latest Plc. Thanks

    Reply
  • { 10 comments… add one }

    Milos November 1, 2013, 1:00 am

    I am 2nd engineer form the one vessel.We have on board PLC system for all aux engine.On one off them I have problem with PLC alarm,and i can not start engine.I try so many time to reset PLC alarm,but without result.On all fuel injector pump fuel rack is blocket by air,and pump can not make the presure on the fuel injectors.Pls if you have any idea what I can do pls let me now.Many thanks,and best regards

    Reply
  • { 10 comments… add one }

    arvind November 30, 2013, 2:05 pm

    learn programming instruction

    Reply
  • { 10 comments… add one }

    Morgan Wilebski December 8, 2013, 12:24 am

    Exceptional post however , I was wanting to know if you could write a litte more on this topic? I’d be very thankful if you could elaborate a little bit further. Thanks!

    Reply
  • { 10 comments… add one }

    Anurag kumar January 6, 2014, 4:28 pm

    I am highly interested to learn about plc

    Reply
  • { 10 comments… add one }

    Brandon January 9, 2014, 11:11 pm

    It’s very simple to find out any topic on net as
    compared to textbooks, as I found this article at this site.

    Reply

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