Photocoupler

Programming a PLC involves translating control tasks into a structured programming language. Two common methods used in PLC programming are the empirical design approach and the sequential function diagram method. This section explores how to develop PLC control programs using both techniques, focusing on practical implementation and logical flow.

The empirical design method is essentially an extension of traditional relay-based electrical schematic design. It relies on known control circuits and uses auxiliary relays to implement memory, interlocking, and other functions based on specific system requirements. However, this method lacks a strict set of rules, making it more exploratory and subjective. As a result, the program often requires multiple iterations and adjustments before it meets all the necessary criteria. The lack of standardization can lead to inconsistencies between different programmers, which makes future maintenance and system improvements more challenging. This approach is typically suitable for simple ladder logic diagrams and demands that the designer have significant experience and a deep understanding of basic control units and systems.

When using the empirical design method, the process usually follows these steps:

(1) Understand the controlled equipment and its operational process, then analyze the control system's requirements and select an appropriate control strategy;

(2) Identify the main components, sensors, and auxiliary relays based on the system’s functional needs;

(3) Design the start, stop, and protection functions using input signals;

(4) Incorporate auxiliary components, timers, and counters to manage complex timing or counting operations;

(5) Apply function instructions to enhance system performance;

(6) Add interlock and safety conditions to ensure proper sequence and prevent errors;

(7) Finally, review, test, and refine the program to ensure reliability and efficiency.

On the other hand, the sequential function diagram (SFC) method is specifically designed for sequential control systems. It provides a clear, structured way to represent the control logic, making it easier for beginners to understand and implement. Programs created with this method are more standardized, readable, and easier to modify and debug. In fact, the FX series of PLCs includes step-by-step instruction programming tools tailored for SFC design, significantly simplifying the development process. This approach not only improves clarity but also enhances the maintainability of the final control system.