Last updated on Jun 28, 2024

What are the most common design patterns for PLC programs?

PLC programs are the core of many industrial automation systems, especially in safety-critical applications. They control the logic and behavior of sensors, actuators, and other devices that interact with physical processes. But how do you design a PLC program that is reliable, maintainable, and easy to understand? In this article, you will learn about some of the most common design patterns for PLC programs, and how they can help you improve your control engineering skills.

1 Sequential Function Chart

One of the most widely used design patterns for PLC programs is the sequential function chart (SFC). SFC is a graphical language that represents the sequence of actions and transitions that a PLC program follows. Each action is a block of code that performs a specific task, such as reading inputs, setting outputs, or calling functions. Each transition is a condition that determines when the program moves from one action to another. SFC is useful for modeling complex workflows, such as batch processes, state machines, or interlocks.

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Mr. Modou Lamin Ceesay

IT Support Specialist || Data Analyst || Software Engineer || Trainee || Project Manager
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PLC programs employ design patterns like Sequential Function Chart (SFC) for sequential tasks, State Machine for complex behavior management, Structured Text (ST) for advanced logic, Function Block Diagram (FBD) for modular functions, Ladder Logic Diagram (LLD) for intuitive automation, and both Sequential and Parallel Control for efficient operations. These patterns optimize PLC functionality across various industrial applications.

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2 Function Block Diagram

Another common design pattern for PLC programs is the function block diagram (FBD). FBD is a graphical language that shows the data flow and logic of a PLC program. Each function block is a symbol that represents a predefined or user-defined operation, such as arithmetic, logic, timer, or counter. The function blocks are connected by lines that indicate the input and output variables. FBD is useful for implementing control algorithms, such as PID loops, fuzzy logic, or neural networks.

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3 Structured Text

A third common design pattern for PLC programs is the structured text (ST). ST is a textual language that resembles high-level programming languages, such as C or Pascal. It allows you to write code using variables, operators, expressions, statements, and control structures, such as loops, branches, or subroutines. ST is useful for performing calculations, data manipulation, or communication tasks that are difficult or cumbersome to express in graphical languages.

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Alexandre Venito
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Without a doubt, ST is the most flexible and powerful language for programming PLCs. However, it is important to worry about the architecture for the code to be clear and easy to maintain. Other languages, such as SFC and FDB, already intrinsically have a more modular architecture.

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4 Ladder Logic

A fourth common design pattern for PLC programs is the ladder logic (LL). LL is a graphical language that mimics the wiring diagrams of relay circuits. It consists of horizontal rungs that contain contacts and coils that represent the inputs and outputs of a PLC program. The contacts can be normally open or normally closed, and the coils can be set or reset. The logic of a ladder program is determined by the continuity of the rungs from left to right. LL is useful for simple or discrete control tasks, such as switching, sequencing, or timing.

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Alexandre Venito
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Ladder Logic was the first language for programming PLCs. This is because the PLC was created to replace the complex control systems for automation in the automotive industry, at the end of the 60s. One of the requirements was "Easy to maintain and programmable in the same way as a system with relay logic (ladder).". The Ladder is an easy way to "transcribe" an electrical command and control circuit to a PLC.

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5 Hybrid Approach

A fifth common design pattern for PLC programs is the hybrid approach. The hybrid approach combines different languages and paradigms to create a modular and flexible PLC program. For example, you can use SFC to define the overall structure and flow of your program, FBD to implement the control algorithms, ST to handle the data processing, and LL to interface with the inputs and outputs. The hybrid approach allows you to leverage the strengths and avoid the weaknesses of each language, and to create a PLC program that is easy to read, debug, and modify.

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Alexandre Venito
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Each language has its advantages and disadvantages. For example, perhaps a simple motor start control would be much easier and quicker to implement in ladder than in ST. However, a more complex control, in which there is a need to analyze data collected by some kind of communication protocol, would be more complex to implement in ladder. A hybrid approach is much more interesting and can bring together the best of each language. For example, it is possible to create blocks in ST to be used in FBD. Nowadays, many manufacturers also integrate C++.

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6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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Alexandre Venito
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PLCs are embedded systems for industrial applications that control physical systems that require real-time responses. It is therefore important to programme from the beggining with these concepts in mind. A PLC is not like a personal computer that has lots of resources and no time requirements associated with the applications it runs. Therefore, keep the code as light as possible. Some basic precautions ... avoid loops that are difficult to predict the execution time and be careful with the use of pointers (ST). The simpler, the more stable and the more reliable.

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What are the most common design patterns for PLC programs?

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1 Sequential Function Chart

2 Function Block Diagram

3 Structured Text

4 Ladder Logic

5 Hybrid Approach

6 Here’s what else to consider

Control Engineering

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