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.
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 or 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.
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.