Abstract: As an application example of PLC control, this paper introduces the PLC control transformation of a combined machine tool used for machining sewing machine housings. The motion status and control requirements of the combined machine tool are described. The PLC control design and programming are explained in detail. Keywords: PLC; Ladder diagram ; Combined machine tool With the rapid development of microelectronics technology in China, PLC technology is increasingly widely used in the transformation of electrical equipment of various machine tools in machining and in the automatic control of new equipment. This paper will describe a PLC transformation example of a combined machine tool for machining household sewing machines. 1 Motion and Control of the Combined Machine Tool A sewing machine factory in Zhejiang Province has many specialized combined machine tools for processing sewing machine parts, used to produce household sewing machines. Figure 1 shows the distribution and movement of the power heads in a combination machine tool used for machining the casing of a household sewing machine. In Figure 1, power heads I and III are used for drilling the large needle hole and the thread clamp hole, respectively, while power head II is used for milling the thread take-up lever's thread groove. M1, M2, and M3 are all three-phase asynchronous motors, providing power for the main cutting motion of power heads I, II, and III, respectively. To facilitate control of the combination machine tool's movement, numerous limit switches are installed to obtain position signals. The pressure of the limit switches in Figure 1 is as follows: SQ51, SQ61, and SQ71 are activated when power heads I, II, and III are in their original positions, respectively; SQ52, SQ63, and SQ72 are activated when power heads I, II, and III have reached their working positions, respectively; during machining, power head II moves first, and power heads I and III can only leave their original positions when SQ62 is engaged. Additionally, the following functions are also present: the rear pin slide lowering and raising to its final position presses SQ11 and SQ12 respectively; manual pulling and inserting of positioning pins presses SQ21 and SQ22 respectively; pin hole rod pulling and inserting pins presses SQ31 and SQ32 respectively; and clamping and loosening of fixtures presses SQ41 and SQ42 respectively. The hydraulic system's oil pump in the combination machine tool is also driven by a three-phase asynchronous motor M0. The first function of the hydraulic system is to provide power for the feed and retraction of the three power heads. The feed and retraction of the three power heads are controlled by three three-position five-way reversing solenoid valves in the hydraulic system. The switching of the power head from rapid feed to working feed is controlled by a hydraulic stroke speed control valve, which is independent of electrical control. The second function of the hydraulic system is for clamping and loosening the workpiece—the machine housing. The clamping process for the machine housing is as follows: the machine housing (workpiece) is placed on the worktable. First, the rear pin slide is raised hydraulically, then manually positioned by the pin. Next, the pin is positioned hydraulically by the needle rod, and finally, the lower clamp is clamped by hydraulic pressure. The clamping process is controlled by three 2-position 5-way directional solenoid valves in the hydraulic system, and these three directional valves have mechanical self-locking. The unloading process of the machine housing (workpiece) is exactly the reverse of the clamping process. Similarly, the releasing process is also controlled by another three 2-position 5-way directional solenoid valves in the hydraulic system. Therefore, including the forward and backward movements of the three power heads, a total of 12 solenoid valve coils are energized and de-energized for control. The selection of the three working states of the combination machine tool—single cycle, cycle, and adjustment—is determined by the three positions of the combination switch SA. SB1 and SB2 are the start and stop buttons for the oil pump motor M0; SB3 and SB4 are the start and stop buttons for the combination machine tool when the cycle (including single cycle) working state is selected; the remaining buttons SB11 to SB62 (a total of 15) are jog adjustment control buttons for the combination machine tool when the adjustment working state is selected. Regardless of the selected working state, starting the oil pump is a prerequisite for the operation of the combination machine tool; stopping the oil pump immediately stops the operation of the combination machine tool. When the single cycle or cycle working state is selected, the jog adjustment buttons SB11 to SB62 are inactive; when the adjustment working state is selected, the start and stop buttons SB3 and SB4 are inactive. Therefore, the original control circuit of the combination machine tool has one combination switch, 19 buttons, 4 contactors, 12 limit switches, 12 solenoid valve coils, and numerous intermediate relays and time relays. Its wiring is very complex, and the electrical contacts are easily damaged. Therefore, with prolonged use, reliability deteriorates, failure rate increases, and maintenance becomes difficult, significantly impacting production. Thus, a technical upgrade of the electrical control circuit of this special-purpose combination machine tool is urgently needed. Analysis suggests adopting PLC control technology to meet production reliability requirements. 2. PLC Control Design 2.1 PLC I/O Allocation Table Based on the electrical control and hydraulic system principles of the combination machine tool, the required inputs/outputs are all switching quantities. Analyzing the number of input and output points, the CPM1A-40CDR PLC from OMRON Corporation of Japan was selected, with relay outputs. The PLC has 24 input points and 16 output points. The 16 output points correspond to the 4 contactors controlling 4 motors and 12 solenoid valve coils, which is sufficient. For input points, there are 22 input points in total: 3 gears of SA, start and stop buttons for the oil pump motor, start and stop buttons for cyclic operation, and 15 buttons for adjustment. There are also 15 limit switch input points, bringing the total to 37 input points. Therefore, the PLC's 24 input points are insufficient. To address this, an input common terminal switching method is used to expand the PLC's input point count. Inputs for single-cycle and cyclic control are placed on one common line, while inputs for jog adjustment are placed on another common line. Switching between these two common lines is accomplished by the combination switch SA. This ensures that the number of input points in each case does not exceed 24. The specific PLC I/O point allocation table is shown in Table 1. Table 1 I/O Point Allocation Table [align=center] [/align] 2.2 PLC Control Program Design The PLC control program is programmed using ladder diagrams. According to the control requirements, the overall block diagram is shown in Figure 2. The diagram includes a reset procedure. This is to address the issue that if the machine tool is not in its original position when the cyclic operation mode is selected after the oil pump motor M0 starts, pressing the start button may cause the power heads to collide. Therefore, power heads I, II, and III must automatically return to their original positions, the fixture must automatically release, and the pin rod must automatically pull out its pin. If the rear pin slide does not descend, the positioning pin must be manually pulled out to make the slide descend automatically. Figure 3 shows the ladder diagram of the PLC for the cyclic operation (including single cycle) of the combination machine tool. The cyclic operation mode and single cycle operation mode of the combination machine tool are selected by the SA combination switch. The difference is that in the single cycle operation mode, SB3 must be pressed at the beginning of each cycle, while in the cyclic operation mode, a 30-second delay is set after each cycle for picking up and placing workpieces. After the delay, the machine tool automatically starts the next work cycle without needing to press SB3 again. The internal auxiliary relays 20110, 20111, and 20112 in the ladder diagram represent Reset 1, Reset 2, and original position in the reset to original position program, respectively, while 20000 to 20010 are used for shifting and are controlled according to the processing flow. 3 Conclusion The PLC technology is applied to the special combination machine tool for processing the casing of household sewing machines. The input common point switching method is used to expand the number of input points. The PLC with fewer input points is replaced with the PLC with more input points, which saves the modification cost. The shift instruction is used to control the cyclic processing flow. The modified control system has high reliability, good stability, and is easy to maintain. The quality of the processed parts is also well guaranteed, resulting in good economic benefits. References [1] Zhang Panfeng, Zhang Kaisheng, Guo Guofa. Application of belt conveyor system based on PLC control [J]. Microcomputer Information. 2005 (15) [2] Dai Yiping. Programmable controller technology and application [M]. Beijing: Machinery Industry Press, 2004