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Programming a PLC involves describing the control tasks using a specific programming language. Two common methods used in PLC programming are the empirical design method and the sequential function chart approach. This chapter explains how to develop PLC control programs using both techniques, focusing on their practical applications and benefits.

The empirical design method is based on traditional relay-based electrical schematics. It builds upon known control circuits and uses auxiliary relays to implement memory, interlocking, and other functions depending on the system's requirements. Programs created this way often require multiple iterations and refinements to meet performance standards. Since there are no strict rules, this method is more exploratory and less structured, which can lead to longer debugging times. The resulting code may vary significantly between designers, making it harder to maintain and understand, especially for complex control systems. This method is best suited for simple ladder logic diagrams and requires significant experience from the designer.

Here are the key steps involved in using the empirical design method:

• (1) Understand the controlled equipment and process, and analyze the system requirements to choose an appropriate control strategy;
• (2) Identify the main components, sensors, and auxiliary relays based on the system's needs;
• (3) Design start, stop, and protective functions using input signals;
• (4) Incorporate auxiliary relays, timers, and counters as needed;
• (5) Use function instructions where applicable;
• (6) Add interlocking and protection conditions;
• (7) Review, modify, and improve the program iteratively.

The sequential function diagram (SFC) method, also known as the step-by-step or function chart approach, is ideal for sequential control systems. It provides a clear, structured, and easy-to-read format that is particularly useful for beginners. SFC-based programs are standardized, easier to debug, and more maintainable. Many modern PLCs, such as the FX series, include dedicated step-by-step instructions to simplify the design process, making it even more efficient and user-friendly.