Abstract: To meet actual production needs, this paper proposes using photoelectric switches to achieve limit control and implement double-insurance control. Keywords: photoelectric sensor, relay, limit switch, limit mechanism, liquid level tracking 0 Introduction In 1995, a factory imported a SL-I2 type upward drawing oxygen-free copper continuous casting machine unit from Shanghai for the production of Φ14.4 mm copper rods. The production process diagram of this equipment is shown in Figure 1. [align=center] 1 Melting furnace 2 Holding furnace 3 Crystallizer 4 Rod drawing mechanism 5 Copper liquid float 6 Liquid level tracking 7 Guide wheel 8 Wire take-up limit switch 9 Wire take-up device Figure 1 Schematic diagram of oxygen-free copper rod furnace process[/align] The process involves adding electrolytic copper plates into a 300 kg industrial frequency induction melting furnace. The melting point of copper is 1063 ℃. After energizing, induction heating is applied to melt the copper material. Molten copper flows through a narrow trough between two furnaces into a holding furnace, where it is held at a temperature of (1150±3)℃. The copper rod is then drawn out by a guide rod mechanism using cooling water from the crystallizer, thus producing the copper rod. Finally, a take-up mechanism coils the copper rod into a coil for use in the next process. A limit device is installed between the guide wheel and the take-up machine. It consists of upper and lower limit switches. When the copper wire presses down on the lower limit switch, the take-up machine operates, taking the wire in. When the copper wire presses up on the upper limit switch and closes, the take-up machine stops. The take-up machine consists of a reducer, a coiling device, and a turntable. The copper rod is conveyed using two sets of clamping wheels to achieve coil take-up. A liquid level tracking mechanism is installed on the continuous casting machine. A limit device consisting of upper and lower limit switches is installed at the top of the float rod. When the liquid level in the furnace drops, causing the float rod to press down the lower limit switch, the control circuit is activated. The AC motor drives the lead screw through the worm gear reducer, using the forward and reverse rotation of the motor to raise and lower the mounting plate. Because this unit operates continuously without interruption, the corresponding limit switches frequently operate, causing the mechanical limit switches to become easily damaged. This not only affects normal production, reduces work efficiency, and increases maintenance, but also damages production equipment, resulting in economic losses for the factory. For example, when the limit switch on the take-up line is activated and cannot be reset, the take-up AC motor continues to operate without stopping, causing excessive tension on the copper rod, deforming the limit bracket, and preventing it from working properly. Furthermore, if the liquid level tracking switch is damaged, it cannot automatically track changes in the copper furnace liquid level, either causing the crystallizer to detach from the liquid surface, resulting in broken rods and scrap, or the crystallizer to be immersed too deeply in the liquid. The copper liquid temperature in the holding furnace is (1150±5)℃, which can easily burn the crystallizer sheath, and in severe cases, cause cooling water leakage from the crystallizer into the furnace, leading to an explosion. At the factory's invitation, photoelectric technology was used to modify the original equipment. [b]1 Improvement Measures 1.1 Improvement of the Take-up Limit Device[/b] A pair of E20-D5B3 infrared photoelectric switches (photoelectric sensors) are installed at the top and bottom (near the original limit switches) of the take-up device. These switches utilize the object's ability to block the infrared beam, and the synchronous circuit selects whether the object has passed through. This sensor signal is connected to the signal input terminal of a small relay JQX-13F, thus achieving non-contact take-up limit control. The original device control diagram is shown in Figure 2. The improved control diagram is shown in Figure 3, which has a double-safety function (i.e., both the photoelectric switch and the limit switch can operate effectively). [align=center]Figure 2 Original control diagram for wire retraction Figure 3 Improved control diagram for wire retraction[/align] 1.2 Improvement of the liquid level tracking and limiting device for the holding furnace A lever mechanism is added to the original liquid level tracking device. Its fulcrum is fixed to the frame. A pair of photoelectric switches (C17-3005NA integrated DC proximity switches) are placed at the other end of the lever. When the liquid level rises, the float rod changes accordingly, causing displacement at the end in contact with it. The copper plate at the other end of the lever approaches the lower photoelectric switch, which sends a signal to a small relay. The relay output connects to the corresponding terminal of the original control circuit to achieve photoelectric liquid level control. The original upper and lower distance at the beginning of the stroke is 12 mm, and the distance controlled by the photoelectric proximity switch is 10 mm, thus achieving double-safety control. The original control circuit diagram is shown in Figure 4, and the improved circuit diagram is shown in Figure 5. [align=center]Figure 4 Simplified diagram of the original liquid level tracking circuit Figure 5 Simplified diagram of the original liquid level tracking circuit after improvement[/align] 2 Conclusion Photoelectric switches are characterized by small size, high precision, high sensitivity, wide voltage range, high repeatability, strong anti-interference ability, polarity and short-circuit protection, and long service life. Since its commissioning, the photoelectric switch control device has achieved significant results, with no abnormalities reported to date. It effectively ensures normal production operation and completely eliminates the drawbacks caused by mechanical limit switches, receiving high praise from users. Market research has found that most small and medium-sized enterprises producing copper materials still use mechanical limit switches to control wire take-up and liquid level tracking in their production equipment. This improved technology has significant promotional value.