PLC: Basics, Operating Systems and User Program

By | October 20, 2020

As industrial demand outstrips supply, there is a need for our production to keep up. Sometimes the pace of it all requires a machine that can churn out products and keep up with the explosion of demand. This is where a Programmable Logic Controller or PLC comes in. This is a part that allows automation in industries and helps control the operations. 

It is at the heart of all controlling systems and can be something as simple as a push button to a layered structure, requiring input at every level. In a nutshell, it is a combination of textual or graphical instructions that can guide applications in an industry in real-time. These can control one function, a set of processes, and in some cases, the entire gamut of operations as well. 

How Does It Work?

The system receives dedicated inputs that it processes and then gives out specific output based on pre-decided objectives- all this almost at the same time. A program is written on a separate computer, which forms the basis of the PLC. The program also offers the benefit of customization as per the user’s requirements and is fixed with some specific parameters. 

Typically, such a program is written in the traditional ‘Ladder Logic’ or C. This follows the same logic as the circuit diagram where each ‘rung’ of logic is read left to right. The rungs represent the program’s sequence of actions, and by extension, the PLC is supposed to follow.

Once this program is fed into the system, it is placed in a Run mode where it repeats itself according to the specifications. 

The Fundamentals Of PLC Programming

A PLC program has two main components- the operating system and the user program. The former takes care of the functions that are not related to the control task. On the other hand, the latter is a user-generated system of functions required for the automation of tasks. These include processing input data to give out the final command for the operation to be carried out. 

The PLC programming language falls into two broad categories- textual and graphical. Together, they make up for the five different languages of PLC programs- Ladder Diagram (LD), Sequential Function Charts (SFC), Function Block Diagram (FBD), Structured Text (ST), and Instruction List (IL). 

Human Machine Interface

While a software keeps the PLC running, it does require another aspect to keep its users in the loop. This is where a Human-machine Interface (HMI) is useful. This is an interface used by the user to view the tasks and feed in more real-time information. It can be a simple display or keypad or can be a more elaborate system with various touch screen panels. 

A PLC that comes with an HMI with it is commonly referred to as the ‘all in one PLC. This is a complete solution that helps the user cut down on costs and save time as they do not have to program different devices separately. 

How To Program PLC Using Ladder Logic?

The Ladder Logic is one of the often-used programs when it comes to the PLC and is preferred for its easy and logical approach. This is how you can use to program the same to fit the needs of the user:

 

  • What Is The Main Use?

 

The main idea or function behind the program is the control application and is thus embedded into the PLC’s core. 

 

  • Explore The Different Variables

 

Once you know more about the program, you must list the variables and design a flowchart representing the sequence of activities. 

 

  • Configuration

 

Once the flowchart is done, configure the software with the appropriate input and output modules. 

 

  • Add The Logical Sequences

 

The next step will entail keying in the rungs based on the central task. These rungs represent the sequence of operations based on each input and output. 

 

  • Check For Errors

 

After your program is in place, spot the errors, and run a program simulation before it goes live. 

 

  • Download It To PLC CPU Memory

 

The program is now ready and is set for the final step, which is to download the program into the brain of the PLC.  

The PLC has many uses, automation, one of them, and has applications in various industries. So, if you are unsure of what and how of PLC programming, it is advisable to approach a specialist who can help and guide you through the process to find the right solution for you. 

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