0 Introduction Modular machine tools are specialized equipment designed for batch processing of specific workpieces according to specific processes. With the widespread application of PLCs and the continuous development of machine tool electrical control technology, utilizing PLCs to achieve automatic control of modular machine tools is undoubtedly the future development direction, making PLC program design for this type of control particularly important. This control belongs to sequential logic control, with various programming methods and languages ​​available, and there are some techniques and rules to follow in programming. The following is a detailed introduction to a PLC program design example for the automatic control of a modular machine tool. 1 Example Working Process and Program Design Ideas The example given in this paper is a vertical and horizontal three-sided boring machine with three working heads: right head, left head, and top head. It has four different working conditions: automatic cycle (simultaneous processing of all three heads) and single-head adjustment. When all three heads are processing simultaneously, an automatic work cycle is shown in Figure 1. Its characteristics are multi-head simultaneous processing and multiple steps, which manifests in the following control requirements: the transition conditions between steps are complex, there are parallel synchronization problems, and there are also many memory and interlocking issues. Therefore, the sequential function chart (SFC) programming method should be adopted: First, based on the analysis of the work process, each step, transition condition, and path should be fully defined to determine the action of each step. Then, according to the control requirements, instructions should be used to program each step and transition. Figure 1 shows the definition of the first step of the automatic work cycle process, which can be described by a sequential function chart. Figure 2 shows the main function flowchart of this example. It starts with the function and uses the function as the main line to decompose the production process into several independent continuous stages (steps). Each decomposed step can be an actual sequential step, such as step 1, whose corresponding action is to start the main pump motor, or it can be a stage of the production process, such as step 2, which is an automatic work process. Its function flowchart is shown in Figure 3. From these two function flowcharts, it can be seen that the operation, transition condition, and step progression of each step are simply and clearly displayed, and it reflects several basic structures, including single sequence, selection sequence, and parallel sequence. For example, steps 25 to 27 are a single sequence, realizing the sequential operation of multiple processes; steps 12, 13, 14, and 15 form a four-branch selection sequence structure, enabling the selection of four working conditions: simultaneous processing of three heads, right head adjustment, top head adjustment, and left head adjustment; while steps 28 to 30, 31 to 34, and 35 to 38 form three parallel branches, realizing the simultaneous processing of three heads; steps 21 and 22 are also parallel to steps 23 and 24, realizing the parallel operation of the workpiece upper position lowering and spindle positioning processes. There is a synchronization problem between these two parallel processes, that is, step 21 (workpiece upper position lowering) and step 23 (spindle positioning) start simultaneously but do not end simultaneously, requiring the merging of parallel sequences for synchronization (waiting for both actions to finish) so that they simultaneously transition to step 25. This problem also exists when processing three heads simultaneously. In the description of the sequential function flowchart, it is important to explain the transition conditions between each step, the corresponding commands and actions for each step, and the corresponding operating status. Figure 2. Main Functional Flowchart 2. Program Implementation Method The next step is to program the above functional flowchart using instructions from a programming language to implement its functions and operations. Currently, some PLCs offer direct functional flowchart programming. However, for PLCs without such a programming language, a pseudo-functional flowchart programming method can be used. This method utilizes the most common programming languages ​​such as ladder diagrams and instruction lists. Based on the description of the functional flowchart, the complex structure can be decomposed into several basic stages: single sequence, selection sequence, and parallel sequence. The patterns and programming rules of these basic stages are identified, and the program is broken down into smaller blocks for programming. This results in a structured modular program, making the programming logic clearer and the program design more standardized. The program implementation for each basic stage can use general logic instructions, set and reset instructions, or shift registers. These methods share the commonality of considering how to activate a step, maintain that step, and stop a step. If step instructions are used, these issues are eliminated, and the program becomes much simpler. The following shows the programming of Figures 2 and 3 using step instructions, and analyzes key issues. Figure 4 is the ladder diagram of the main function flowchart, and Figure 5 is the ladder diagram of the automatic operation function flowchart (only a part is shown). First, look at the single sequence from step 25 to step 27. The control rules for each step are as follows: If a step is active, then once the transition condition between it and the next step is met, that step becomes inactive, and the next step becomes active. When a step is active, the corresponding actions and commands are executed; for inactive steps, the corresponding actions and commands are not executed. Thus, when step 25 is active, a right-head fast-forward command is issued (energizing Y442), until the fast-forward reaches the position (limit switch SQ4 is pressed, and transition condition X412 is met). Step 25 then becomes inactive, the right-head fast-forward stops (de-energizing Y442), step 26 becomes active, and the workpiece begins to descend from the middle position (energizing Y447 and Y552)... The control rules for each step in the selection sequence are as follows: During branching, if a preceding step is active, the subsequent step becomes active when the transition condition between it and multiple subsequent steps is met, while the preceding step becomes inactive. During merging, if one of the multiple preceding steps is active, the subsequent step becomes active when the transition condition between that active step and a subsequent step is met, while the preceding step becomes inactive. In the example, when step 11 is active, the branch step becomes active when the transition condition of the four branches is met. If the automatic machining start button is pressed, making the transition condition X403 satisfied, step 12 will be entered, and the automatic machining process will begin until the transition condition X424 is satisfied. The branch merging loop returns to the initial step, starting a new cycle. According to the control requirements, the main pump motor needs to be running continuously throughout the machining process, so the Y430 set instruction is used in step 11. After step 11 becomes inactive, Y430 is not de-energized. The control rules for each step of the parallel sequence are as follows: During branching, if a preceding step is active, then when the transition condition is met, multiple parallel subsequent steps simultaneously become active steps, while the preceding step becomes inactive. During merging, if multiple parallel preceding steps are active, when the transition condition is met, one subsequent step becomes active, and multiple parallel preceding steps synchronously become inactive. In the example, when step 20 is active, the loading instruction is executed. After loading is completed, the transition condition X425 is met, and steps 21 and 23 simultaneously become active steps, i.e., loading stops, and the workpiece lowering and spindle positioning actions begin. Since these two actions do not end simultaneously, two empty steps without actions or commands are inserted—steps 22 and 24 (the corresponding stepper contacts in the ladder diagram are not connected to output relays)—to stop the two preceding steps respectively, end the corresponding actions, and wait for the moment when both actions stop. Once the moment arrives (conditions X410 and X427 are met), the two parallel steps merge and transition to step 25. Similar synchronization issues arise when three heads are processing simultaneously, which will not be elaborated upon here. Figure 3 Automatic Work Function Flowchart 3 Conclusion This PLC programming example demonstrates the advantages of using sequential function charts: a. Function flowcharts are closely integrated with the production process, providing clear design ideas and a clear understanding of system operation, facilitating communication between process engineers, automation engineers, and designers; b. Function flowcharts provide designers with a method for describing regular control problems, making it easier to obtain corresponding programming methods, design arbitrarily complex control programs, and making programming more standardized and regulated. Figure 4 Ladder Diagram of Main Function Flowchart Figure 5 Ladder Diagram of Automatic Work Function Flowchart (Partial)