Popular PLC Programming Methods 2022

05/04/2022


A brief introduction to PLC’s is that it is a uniquely designed computer, capable of operating in strenuous industrial conditions, such as wet/dry conditions, or extreme temperatures.

A PLC programme can be broken down into two operating capabilities. The two are the operating system and the user program.

In respect to the operating systems, what exactly is the operating system?

Operating System:


The operating system is responsible for organising the functions, operations and sequences that the CPU has to carry out. The operating system is not directly responsible for the control of a task.

Some of the operating system functions include:

  • Carrying out the user program.

  • Establishing a communion channel with programmable devices.

  • Controlling memory areas.

On the other hand, there is the user program.

User Program:


It is a combination of various functions which are required to process an automated task. This must be created by the users and need to be downloaded to the CPU of the PLC. Some of the tasks of the user program include:

  • Commencing specific tasks with all conditions required.

  • Reading and evaluating all binary and analog input signals

  • Specifying output signals to all binary and analog output signals

  • Executing interrupts and handling errors


Ways to program a PLC:


Much like any automation solution, there are a variety of ways to program the robot. What programming does is it creates a sequence of action in the program. This will be elaborated further below.

PLC programming can be broken down into two various categories.

  • Textual language

Textual language which consists of instruction lists and structured text.

  • Graphical form

This consists of ladder logic (LD) and function block diagram (FBD) and sequential function chart (SFC).

A range of methods exists when considering programming a PLC. Each programming method varies but all of which does not require such complex programming knowledge. The list of the most common programming methods are Ladder Logic (LD), Function Block Diagram (FBD) and Structured Text (ST).

Structured Text (ST):


As explained in the earlier point, Structured Text falls under textual language.

Structured Text programming is essentially a high-level language. Utilised primarily to solve complex arithmetic operations.

Examples include maths functions, conditional functions and also loops.

Advice:


In most contexts, one can develop Structured Text without utilising PLC programming software. Given it is text-based, writing the project on a simple text file, and thus proceeding to copying and pasting into the PLC project.

Benefits of Structured Text:

  • No PLC programming background needed:

It is self-explanatory, using structured text does not require a PLC programming background.

  • Text-Based:

Using structured text allows one to write the project in a simple text file. It does not require a special application or file format. From then onwards it is just a copy-and-paste job into the PLC project.

  • A lightweight programming language:

As previously mentioned before, as text-based programming is text based, it is not memory intensive with any graphics etc, therefore this allows for a relatively smaller memory processor.

Ladder Logic (LD):


Ladder Logic is a graphical language. It is based on the circuit diagrams or relay logic hardware. Another way of defining Ladder Logic is that it is based on the electrical control wiring.

Ladder Logic, as explained before, which communicates logic operations with symbolic notation. This method consists of numerous predetermined functions that have to contain a limit dependent on the designated applications.

Advice:


Ladder Logic is typically the easiest programming language to learn to program a PLC.


Benefits of Ladder Logic:

  • Self-Documentation

As mentioned before, it is a graphical representation of the circuit design concept, therefore as one codes the circuit, they are essentially writing the design documentation itself.

  • Intuitive:

Ladder logic’s graphical representation of circuits makes it an intuitive coding environment for anyone without direct knowledge of industrial control circuits.

  • Easily Debugged:

Modern ladder logic debugging tools provide an animated showing of life “power flow” in the form of diagrams. Thus making it easier to understand where the faults lie and to therefore debug.

Function Block Diagram (FBD):


Function Block Diagram utilises sequences of logical blocks to make sense of the input and output variables. The PLC will carry out the function using true and false conditions which would then lead to the next action on the sequence.

FBD is a simple and graphical way to program and connect any functions together in a PLC program. The Function Block Diagram is relatively simple and easy to learn.

Advice:


The manufacturer that offers the PLC hardware often offers softwares to program the PLC’s. There aren't any missing functions if you were to choose from a different vendor. As previously mentioned in the earlier point, this method of action is changing.

An example of an easy to learn PLC is the Fanuc PLC Motion Control.

Benefits of Function Block Diagram:

  • Extensive code reuse:


Function Block Diagram allows system designers to re-use existing function blocks and encapsulated custom logic. This is due to the fact that each time function blocks are called, duplicate copies are made. This reduces the risks of overwriting the data.



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