This system is a production control system for automotive steering wheel mold frames. The previous system used relay control, which resulted in low production efficiency, poor anti-interference capabilities, and a high failure rate. To improve production efficiency and meet the growing demand for automotive parts, the control system for the automotive steering wheel mold frames was upgraded using a PLC. System Hardware Composition The automotive steering wheel production equipment mainly consists of molds, mold frames, and a high-pressure foaming machine system. The molds are divided into upper and lower sets; the mold frame is used to hold the upper and lower molds and complete all the necessary actions for steering wheel production; the high-pressure foaming machine system is used to high-pressure mix two chemical raw materials, isocyanate (isocyanate) and PO (polyether polyol), onto the steering wheel frame to form a self-skinning foam layer. The manufacturing process of the automotive steering wheel mold frame is shown in Figure 1: [align=center] Figure 1 Manufacturing process of automotive steering wheel mold frame[/align] First, prepare the raw materials for the steering wheel, namely the steering wheel frame and polyurethane. Open the lower mold and then the upper mold, spraying a release agent into the mold cavity. Place the pre-prepared original steering wheel frame inside, then close the upper and lower molds. Next, pour the foaming material from the high-pressure foaming process into the closed mold space. After curing in a specific environment for a period of time to achieve the desired effect, open the upper and lower mold frames again, using ejector pins in the mold frames to eject the product. Since there are still some rough edges in the mold frames, the finished product undergoes some trimming and shaping before being stored. The steering wheel control system is designed as one control cabinet controlling two hydraulic mold frames, with a total of 34 inputs and 18 outputs. Input signals include limit switches for the upper and lower mold platforms to open and close, limit switches for the upper mold platform safety pin insertion and withdrawal, product ejection proximity switch signals, mold opening and closing buttons, and automatic, manual, and mold changing mode selection switches. Output signals include solenoid relief valves controlling oil pressure, solenoid valves for the upper and lower mold platforms to open and close, solenoid valves for product ejection, solenoid valves for safety lock pins, timers, audible and visual alarms, and indicator lights. Based on production processes and mechanical control requirements, a Siemens S7-200 series PLC: CPU226 (24i/16o) and its expansion module EM223 (16i/16o) were selected. Both inputs and outputs have expansion capabilities, and if the PLC's external output contacts are damaged in the future, only a program modification is needed, without disassembling the PLC. The solenoid valves in this system are controlled by mold holders number one and two. The first mold frame is the main module, with an S7-200 series PLC-CPU226 (24i/16o) controlling the first solenoid valve. Its input/output terminal functions are shown in Table 1. The second mold frame is an expansion module, with an S7-200 series PLC-EM223 (16i/16o) controlling the second solenoid valve. Its input/output terminal functions are shown in Table 2. [align=center] Table 1 Function table of CPU226 input and output terminals Table 2 Function table of EM223 input and output terminals Figure 2 Flowchart of steering wheel mold frame[/align] Safety is of paramount importance. Mechanically, in addition to installing a balance valve to allow the upper and lower mold platforms to stop at any position and inserting a safety lock pin after the upper mold is opened to prevent it from falling, the electrical hardware design takes the following measures: A two-hand button is set for mold closing to prevent the operator from putting their hands into the mold cavity due to reasons such as the frame not being placed properly during mold closing, which could cause an accident; When opening the mold in automatic mode, it is not set to a jog mode, that is, it is designed to press the mold opening button until the lower mold is fully opened before releasing the button, to prevent the operator from leaving the operating table to the vicinity of the mold frame after jogging the mold opening button, which could easily cause injury when the upper mold is opened; A proximity switch for detecting whether the "product" has been "ejected" is used to detect the position of the hydraulic cylinder piston rod. If there is oil on the installation position, and if the signal is incorrect, the ejector will not return to its original position before the mold is closed, which will cause the ejector to damage the upper mold and cause equipment accidents. Software Design According to process requirements, the program has three working modes: automatic, manual, and mold-changing. Automatic mode is used for normal production to improve efficiency; manual mode is mainly used for testing the reliability of all mold frame movements and for fine-tuning; mold-changing mode is mainly used for changing molds. The programs for mold frames 1 and 2 are identical. Since one control cabinet controls two hydraulic mold frames, and the production processes for both mold frames are the same, the program flow for one mold frame is shown in Figure 2. In the program design, because the steering wheel production process requires a sufficiently large initial 10-second clamping force after the foaming material is injected into the mold cavity, and a very small gap between the upper and lower molds, otherwise the product will have large flash, wasting raw materials and potentially producing other defective products due to poor foam structure. Since even the best balance valves and cylinders can experience wear and leakage due to mechanical defects, the solenoid valve q1.1, which controls the closing of the lower mold, can be de-energized after a 5-second delay after closing, continuously supplying high-pressure oil to the cylinder to compensate for leakage losses. This effectively solves the problem and greatly improves the product qualification rate. Partial Program Design Figure 3 shows the ladder diagram of the system's operation. The mold closing mechanism is set with two-hand buttons to prevent operators from accidentally inserting their hands into the mold cavity during mold closing, causing accidents due to improper frame placement. The on-delay timer ton is set to t=50×100ms=5s. It is mainly used to delay the power supply for 5s after the solenoid valve of the lower mold is in position, ensuring continuous high pressure supply to the hydraulic cylinder, compensating for leakage losses, effectively solving problems, and greatly improving the product qualification rate. Figure 4 shows the hydraulic unit control valve and control polymerization indicator. Foam polymerization is the final timing step of this mold frame system. Polymerization requires a polymerization timer; when the ideal polymerization value is reached, polymerization is complete, the polymerization completion indicator light on the control panel illuminates, and the program ends. Figure 5 shows the ladder diagram of the system's automatic operation mode. Since the ejector pin ejection uses automatic mode, the sb4 key in the main circuit I/O configuration is used to press the ejector pin ejection key on the mold frame. The program also includes operations for closing the upper mold and opening the lower mold in mold changing mode. Figure 6 shows the ladder diagram of the mold unlocking/locking control. Unlocking and locking the upper mold involves adding a delay to the locked limit position of the upper mold to allow the foaming material sufficient time to polymerize. The ton connection delay is 1 second. There is also a proximity switch to detect whether the product has been ejected, used to detect the position of the hydraulic cylinder piston rod. If there is oil contamination at the installation location, or if the signal is incorrect and the ejector fails to return to its original position before mold closing, the ejector will damage the upper mold, causing an equipment accident. Conclusion After the system was modified, it not only improved production efficiency and anti-interference performance and reduced the scrap rate, but also features a user-friendly Chinese interface, allowing for parameter setting and modification, and excellent monitoring and display of faults.