The International Electrotechnical Commission (IEC) standard IEC 61131-3 defines five main PLC programming languages, each suitable for different types of control tasks and program complexities. Below is a detailed analysis of how PLC multilingual programming adapts to complex programs.
1. The Five Programming Languages Defined by IEC 61131-3 1. Ladder Diagram (LD) • Characteristics: A graphical language based on relay logic, similar to electrical wiring diagrams, intuitive and easy to understand. • Advantages: • Suitable for electrical engineers to quickly get started, easy debugging. • Very efficient for discrete logic control (such as binary control). • Applicable scenarios: Simple sequential control, binary logic processing, such as motor start/stop, sensor triggering, etc. • Adaptability to complex programs: In complex programs, LD is often used for low-level I/O processing or parts that interact directly with hardware, but modular design (such as subroutine calls) can enhance its ability to handle complex logic.
2. Function Block Diagram (FBD) • Characteristics: A graphical language based on function blocks, modular design, similar to circuit diagrams. • Advantages: • Modular programming, easy to reuse and maintain. • Suitable for describing complex functional modules (such as PID control, signal processing). • Applicable scenarios: Analog processing, complex algorithm implementation, such as temperature control, flow regulation, etc. • Adaptability to complex programs: FBD, through the encapsulation and invocation of function blocks, is suitable for building submodules in large systems, supporting the integration of complex algorithms.
3. Structured Text (ST) • Characteristics: A text language similar to high-level programming languages (such as C or Pascal), supporting conditional statements, loops, and complex calculations. • Advantages: • Strong logical expression capability, suitable for complex mathematical operations and data processing. • High code readability, easy to implement algorithm logic. • Applicable scenarios: Data analysis, complex algorithms (such as matrix operations, dynamic programming), state machine implementation. • Adaptability to complex programs: ST is the preferred language for handling complex logic, especially when dynamic calculations or large data processing are required.
4. Instruction List (IL) • Characteristics: A low-level text language similar to assembly language, based on instruction sequences. • Advantages: • High execution efficiency, low resource consumption. • Suitable for real-time control with high performance requirements. • Applicable scenarios: Resource-constrained embedded PLC systems, or scenarios requiring precise control. • Adaptability to complex programs: IL is rarely used alone in complex programs but can supplement the optimization of certain key performance modules.
5. Sequential Function Chart (SFC) • Characteristics: A graphical language based on state machines, used to describe sequential control processes. • Advantages: • Clearly expresses complex process control logic, easy to decompose and debug. • Supports parallel processing and state transitions. • Applicable scenarios: Complex sequential control, such as multi-step management in production lines, batch processes. • Adaptability to complex programs: SFC serves as the “command center” for complex programs, suitable for overall process planning and coordination.
2. How Multilingual Programming Adapts to Complex Programs Complex programs typically have the following characteristics: multi-module collaboration, various data types (binary, analog, communication data), high real-time requirements, and complex, dynamic logic. PLC multilingual programming adapts to these needs in the following ways: 1. Modular and Layered Design • Implementation: SFC is used for top-level process control, defining the working phases of the system; FBD or ST is used to implement specific functional modules; LD handles low-level I/O logic. • Advantages: • Layered design reduces code coupling, facilitating team collaboration and later maintenance. • Modular reuse reduces development time and improves code reliability. • Example: In an automated packaging line, SFC defines the “feeding-detection-packaging-discharging” process, with each stage’s functional block implemented using FBD (such as image processing for the detection module), while the underlying sensor signals are handled by LD.
2. Language Complementarity • Implementation: Different languages work together in the same project. For example, ST implements complex mathematical models (such as interpolation algorithms in motion control), FBD encapsulates control algorithms, and LD handles emergency stop logic. • Advantages: • Each language plays to its strengths, optimizing development efficiency and program performance. • Improves code readability, making it easier for engineers from different backgrounds to understand. • Example: In a wind power control system, ST calculates the dynamic adjustment of blade angles, FBD implements PID control, and SFC coordinates states such as start, run, and stop.
3. Support for Complex Data Processing • Implementation: ST supports complex data types such as arrays and structures, suitable for handling multidimensional data; FBD processes analog signals through function blocks. • Advantages: • Capable of meeting the big data processing needs involved in Industry 4.0, such as real-time collection and analysis of sensor data. • Supports communication with upper-level computers or cloud platforms (such as Modbus, OPC UA). • Example: In an intelligent warehousing system, ST processes sorting and querying of inventory data, while FBD implements path planning for AGVs (automated guided vehicles).
4. Real-time Performance and Reliability • Implementation: IL optimizes the execution efficiency of critical tasks; SFC ensures the reliability of state transitions; LD provides intuitive fault diagnosis. • Advantages: • Meets the millisecond-level response requirements of industrial control. • Multilingual collaboration reduces the probability of system errors. • Example: In steel rolling mill control, IL optimizes roller speed control, SFC manages the production process, and LD implements safety interlocks.
5. Debugging and Maintenance • Implementation: SFC provides an intuitive process view, facilitating program state tracking; LD and FBD support online monitoring; ST supports log recording. • Advantages: • High debugging efficiency for complex programs, quick fault localization. • Facilitates later functional expansion and code refactoring. • Example: In chemical production, SFC displays the current stage of the reactor, LD monitors valve status, and ST records process parameters for problem tracing.
Editor’s Summary: PLC multilingual programming efficiently adapts to the demands of complex programs through the complementarity of the five languages, modular design, layered structure, and real-time support. SFC provides global process control, FBD and ST implement key complex algorithms, LD ensures simple and reliable low-level logic, and IL serves as a supplementary tool for performance optimization. In practical applications, reasonable planning of language allocation, adherence to best practices, and full utilization of modern tools are key to successfully implementing complex programs.