How to create user - defined function blocks in Siemens PLC?

Hey there! As a Siemens PLC supplier, I often get asked about creating user-defined function blocks in Siemens PLCs. It's a pretty cool and useful aspect of working with these PLCs, so I thought I'd share my insights on how to do it.

First off, let's understand what user-defined function blocks are. In simple terms, they are like custom programs or routines that you can create to perform specific tasks. These blocks can be reused multiple times in different parts of your PLC program, which saves a lot of time and effort. It's like having your own set of building blocks that you can use to construct more complex systems.

Step 1: Understanding the Basics

Before you start creating user-defined function blocks, you need to have a good understanding of the Siemens PLC programming environment. Familiarize yourself with the programming language you'll be using, such as Structured Text (ST), Ladder Diagram (LD), or Function Block Diagram (FBD). Each language has its own syntax and way of doing things, so choose the one that you're most comfortable with.

Let's say you're using Ladder Diagram. It's a graphical programming language that uses symbols to represent logic operations. You'll see things like contacts, coils, and timers. Understanding how these symbols work together is crucial for creating effective function blocks.

Step 2: Defining the Function Block

Once you're comfortable with the programming environment, it's time to define your function block. Think about what the block is supposed to do. For example, you might want to create a function block that controls the speed of a motor based on a setpoint.

To define the function block, you'll need to specify its inputs and outputs. Inputs are the values that the function block will receive, and outputs are the values that it will produce. In the motor speed control example, the inputs could be the setpoint speed and the current speed of the motor, and the output could be the control signal for the motor.

In Siemens PLC programming software, you can usually define the function block by creating a new block and specifying its name, inputs, and outputs. You can also add comments to make the block more understandable.

Step 3: Writing the Logic

Now comes the fun part – writing the logic for your function block. This is where you use the programming language to tell the PLC what to do with the inputs to produce the desired outputs.

Let's go back to the motor speed control example. You might use a PID (Proportional-Integral-Derivative) controller to calculate the control signal. In Ladder Diagram, you can use instructions like Compare, Add, and Multiply to perform the necessary calculations.

Here's a simple example of how the logic might look in Structured Text:

Siemens 6ES7414 3EM06 0AB0 plc

FUNCTION_BLOCK MotorSpeedControl
    VAR_INPUT
        SetpointSpeed : REAL;
        CurrentSpeed : REAL;
    END_VAR
    VAR_OUTPUT
        ControlSignal : REAL;
    END_VAR

    // Calculate the error
    Error := SetpointSpeed - CurrentSpeed;

    // Calculate the control signal using a simple proportional controller
    ControlSignal := Error * Kp;
END_FUNCTION_BLOCK

In this example, Kp is the proportional gain, which you'll need to tune to get the best performance.

Step 4: Testing and Debugging

Once you've written the logic for your function block, it's important to test it to make sure it works as expected. You can use the simulation features in the Siemens PLC programming software to test the function block without having to connect to a real PLC.

During testing, you might encounter some errors or unexpected behavior. This is where debugging comes in. Use the debugging tools in the software to step through the program, check the values of variables, and find out where the problem is.

Step 5: Using the Function Block

After you've tested and debugged your function block, you can start using it in your main PLC program. You can simply call the function block and pass the appropriate inputs to it. The function block will then calculate the outputs and return them to your main program.

For example, in your main program, you might have something like this:

PROGRAM Main
    VAR
        MyMotorSpeedControl : MotorSpeedControl;
        Setpoint : REAL := 100.0;
        Current : REAL;
        Control : REAL;
    END_VAR

    // Read the current speed from a sensor
    Current := ReadCurrentSpeed();

    // Call the function block
    MyMotorSpeedControl(SetpointSpeed := Setpoint, CurrentSpeed := Current);

    // Get the control signal
    Control := MyMotorSpeedControl.ControlSignal;

    // Send the control signal to the motor
    SendControlSignal(Control);
END_PROGRAM

Using Specific Siemens PLC Products

When working with Siemens PLCs, you might want to use specific products in your projects. For example, the Siemens LOGO! DM16 24 is a great option for small to medium-sized applications. It's easy to program and has a wide range of input and output options.

Another useful product is the Siemens 6GK7443-1EX11-0XE0 Industrial Ethernet Module. This module allows you to connect your PLC to an Ethernet network, which is great for remote monitoring and control.

And if you're working on a larger project, the Siemens 6ES7414-3EM06-0AB0 is a powerful CPU that can handle complex tasks.

Conclusion

Creating user-defined function blocks in Siemens PLCs is a great way to make your programming more efficient and organized. By following the steps outlined above, you can create your own custom function blocks and use them to build more complex control systems.

If you're interested in purchasing Siemens PLC products or have any questions about creating user-defined function blocks, feel free to reach out. We're here to help you with all your Siemens PLC needs. Whether you're a small business looking for a simple solution or a large corporation working on a complex project, we've got the products and expertise to support you.