Introduction to Structured Text

Structured Text, often abbreviated as ST, is a powerful text-based programming language. Its syntax is similar to Pascal or C, making it relatively easy to learn for programmers familiar with these languages. ST excels in handling mathematical calculations, complex logic, and data manipulation, making it suitable for advanced control applications.

Understanding ST Syntax and Structure

Mastering the syntax is fundamental to writing effective Structured Text code. Let's explore the key elements:

Data Types

Structured Text supports various data types, including:

• BOOL: Boolean (TRUE or FALSE)
• INT: Integer (e.g., -32768 to 32767)
• DINT: Double Integer (larger range than INT)
• REAL: Floating-point number
• STRING: Sequence of characters
• TIME: Represents a duration of time
• DATE: Represents a calendar date
• TOD: Time of Day
• DT: Date and Time

Operators

ST uses a variety of operators for arithmetic, comparison, and logical operations:

• Arithmetic: +, -, *, /, MOD (modulo), ** (exponentiation)
• Comparison: =, <>, <, >, <=, >=
• Logical: AND, OR, NOT, XOR

Control Structures

Control structures are essential for creating logic and controlling the flow of execution:

• IF-THEN-ELSE: Conditional execution
• CASE: Multi-way branching
• FOR: Looping a fixed number of times
• WHILE: Looping based on a condition
• REPEAT-UNTIL: Looping until a condition is met

Example Code

PROGRAM ExampleProgram
VAR
    InputSensor : BOOL;
    OutputRelay : BOOL;
    Counter : INT := 0;
END_VAR

    IF InputSensor THEN
        Counter := Counter + 1;
        OutputRelay := TRUE;
    ELSE
        OutputRelay := FALSE;
    END_IF;

    IF Counter >= 100 THEN
        Counter := 0; // Reset counter
    END_IF;

END_PROGRAM

Advanced Structured Text Concepts

Beyond the basics, Structured Text offers more advanced features for complex applications.

Functions and Function Blocks

Functions are self-contained code blocks that perform specific tasks and return a value. Function Blocks are more complex, containing internal state (variables) and can be used to encapsulate reusable logic. They are instantiated as objects.

Arrays

Arrays allow you to store multiple values of the same data type under a single variable name.

VAR
    TemperatureReadings : ARRAY[1..10] OF REAL;
END_VAR

Structures

Structures (or user-defined data types) allow you to group related variables of different data types into a single unit.

TYPE
    MotorData :
        STRUCT
            Speed : REAL;
            Current : REAL;
            Status : BOOL;
        END_STRUCT
END_TYPE

VAR
    MyMotor : MotorData;
END_VAR

Data Visualization

Understanding the performance of the PLC program is critical. The following chart demonstrates typical scan times for various PLC tasks.

Best Practices for Structured Text Programming

Following best practices ensures maintainable, reliable, and efficient code.

• Use Meaningful Variable Names: Choose descriptive names that clearly indicate the purpose of each variable.
• Add Comments: Explain complex logic and the purpose of different code sections.
• Modularize Code: Break down large programs into smaller, reusable functions and function blocks.
• Handle Errors: Implement error checking and handling to prevent unexpected behavior.
• Follow a Consistent Style: Use consistent indentation and formatting for readability.

Example Application: PID Controller

Let's illustrate the use of Structured Text with a simple PID (Proportional-Integral-Derivative) controller implementation.

FUNCTION_BLOCK PIDController
VAR_INPUT
    Setpoint : REAL;
    ProcessValue : REAL;
    Kp : REAL;
    Ki : REAL;
    Kd : REAL;
    dt : REAL; // Sample time
END_VAR
VAR_OUTPUT
    Output : REAL;
END_VAR
VAR
    Error : REAL;
    LastError : REAL;
    Integral : REAL;
    Derivative : REAL;
END_VAR

Error := Setpoint - ProcessValue;
Integral := Integral + Error * Ki * dt;
Derivative := (Error - LastError) / dt * Kd;
Output := Kp * Error + Integral + Derivative;
LastError := Error;

END_FUNCTION_BLOCK