Abstract: Based on the author's design and on-site commissioning of multiple casting cranes in recent years, this paper introduces the control and solutions for some common and important problems of programmable logic controllers (PLCs) in the electrical control system of safety brakes in casting cranes. Keywords: running brake; safety brake; PLC Safety brakes are very mature and reliable products. The choice of control method to ensure accurate operation and effective safety protection in accident situations places high demands on electrical control; that is, the reliability and accuracy of the control are crucial. 1 The Role of Safety Brakes in Casting Cranes Casting cranes are used in the metallurgical industry, and their task is to transport molten steel to smelting furnaces. They generally have five main mechanisms: main hoisting, auxiliary hoisting, trolley, main trolley, and auxiliary trolley. The motor is mounted on the high-speed shaft, and the drum is mounted on the low-speed shaft. The high-speed and low-speed shafts are connected by a reducer. Traditionally, the hoisting mechanism brake is located on the high-speed shaft, called the working brake. In this case, if a transmission link between the high-speed and low-speed shafts fails, the main hoisting brake will lose its effect on the drum. In response to this situation, a brake is installed on the drum, called a safety brake. In the event of a transmission failure or overspeed failure, the running brake will act first, and the safety brake will act after a delay. This ensures that the molten steel ladle being lifted can be protected and braked safely and reliably in the event of an accident. 2 Safety brake control method (1) An overspeed switch is installed on the drum. When the hoisting mechanism fails and the drum is overspeeding, the safety brake will act. The advantage is that the control principle is relatively simple and the debugging is convenient. The disadvantage is that the drum only works when it is overspeeding, and the safety brake will cause greater damage to the mechanical structure. (2) The transmission speed ratio between high and low speeds is a constant. This constant is continuously detected by the device. When this constant changes, it means that the transmission link of the machine is damaged, and the safety brake will act immediately. The advantage is that the fault can be detected in the early stage of the fault, so that the safety brake can act. The safety brake will cause less damage to the mechanical structure. The disadvantage is that the control principle is relatively complex and the debugging requires professional electrical technicians. 3 Safety brake control hardware composition (1) Two encoders are installed on the two drums. (2) Two encoders installed on the two main hoisting motors. (3) Safety brake control panel (or cabinet) (equipped with: programmable controller, circuit breaker, relay, contactor, regulated power supply, terminal block, etc.). (4) Shielded cable from the control panel to the four encoders. (5) Emergency stop button and fault reset button, safety brake release indicator light, encoder fault indicator light, etc. installed on the control panel. 4 Electrical installation and component selection Misoperation of the safety brake affects the normal use of the crane and may cause greater losses. If the safety brake frequently misoperates, users will doubt the role of the safety brake and lose confidence. In order to ensure that the safety brake does not misoperate, the selection of components and electrical installation are particularly important when the programmable controller program is completely error-free and can work normally. 4.1 Electrical installation (1) There should be no breaks in the shielded cable from the control panel to the four encoders. If there are breaks, the breaks should be shielded. (2) The shielding layer of the shielded cable from the control panel to the four encoders needs to be grounded at both ends and ensured to be the same ground. To ensure they are in the same location, the main trolley should be equipped with a grounding device. This will ensure that the main trolley and the crane are in the same location. (3) Calculate the length of the shielded cable and confirm that the transmission distance of the encoder pulse signal is within the effective range. (4) Use different shielded cables for the signal lines and power lines of the four encoders. The signal lines and various power lines on the crane should be arranged separately. (5) The regulated power supply should be installed near the programmable controller and grounded. (6) The signal lines should not have terminals and should be directly connected to the input module of the programmable controller. (7) If a mechanism of the crane is equipped with a frequency converter or other equipment with strong interference, it should be equipped with inductors or other devices to minimize interference to the safety brake control system. 4.2 Component Selection (1) The encoder should be matched with the programmable controller. The output mode should be push-pull output mode. The environment of the steel plant is relatively poor, with a lot of iron dust, so the encoder should be selected with a higher protection level. The output voltage of the encoder should match the power supply voltage. (2) The programmable controller should have 4 independent high-speed counters, and the high-speed counting capability of the high-speed counters should meet the requirements. This needs to be determined by calculation. In actual use, it is found that the high-speed counting capability of the high-speed counters should ideally reach 1.5 to 2 times the actual required counting capability, so as to ensure that the programmable controller program works normally. 5 PLC control system principle (1) PLC control system The system uses Siemens S7-200 PLC for control, which has extremely high cost performance, high reliability, rich instructions, strong real-time communication capability, easy to master, easy to operate, rich built-in integrated functions and expansion modules. The block diagram of the PLC control system principle is shown in Figure 1. (2) PLC program compilation The program is structured and programmed. The control task is decomposed into small tasks that can reflect the process, function or can be used repeatedly. These tasks are represented by the corresponding program parts, that is, known blocks. In order for the user program to work, the blocks that make up the user program must be called. The block call instruction can only be written and started in the logic. In order for the program to work normally, the following points should be noted when compiling: ① Make the interrupt program as short and simple as possible. Furthermore, the execution of the interrupt procedure should not delay other processes for too long; ② The activation, execution, and exit sequence of several interrupt procedures should be calculated carefully; ③ A certain margin should be left when comparing. 6 Conclusion This system's control of the safety brake has been applied in production practice, playing a protective role in the transportation of molten steel. In the event of an accident, it has prevented equipment and property damage and eliminated personal injury accidents. The application of the safety brake control in production is accurate, reliable, and effective.