Abstract : The electrical control of gantry milling machines is complex, the working environment is harsh, and failures are frequent. By combining PI controllers with frequency converters, the gantry milling machine was modified, reducing equipment, simplifying external wiring, and adding new functions, fully meeting the needs of process operation. Keywords : PLC, frequency converter, correction, automatic operation, continuous improvement. The gantry milling machine of Sichuan Tianhua Co., Ltd. had been unable to operate automatically for several years, requiring manual operation. Due to the harsh working environment, the operators faced high labor intensity. The original control system of the milling machine used relay control, resulting in numerous components. The components in the two large control cabinets were densely packed, hindering heat dissipation. During high-temperature or humid seasons, relays burned out in batches. Furthermore, the wiring was complex, and the original construction and manufacturing were not standardized; terminals and numbers were written by hand, which are now illegible, making maintenance difficult and costly. It was decided to design and modify the machine using a PLC and frequency converter, reducing the number of relays from nearly 200 to 40, eliminating 2 motors, 2 reducers, and 2 electromagnetic brake systems, lowering the failure rate, enabling automatic control, and reducing labor intensity. Compared with traditional relay control, PLC has many advantages: high reliability; simple wiring; different control functions can be achieved by simply changing the software, without rewiring; it can recognize and control both digital and analog signals with high precision; and it can be configured into one or more networks. The OMRON CJ series PLC is selected, with an independent CPU and input/output modules, facilitating the addition or removal of input/output points according to actual conditions. This also prepares the technical foundation for future integration with a back-end computer monitoring system. [b]1. Rake Machine Walking System 1) Variable Frequency Motor[/b] The variable frequency motor is used in conjunction with a frequency converter. Compared with ordinary motors, it adopts advanced electromagnetic calculation methods, fully considering the control characteristics of current SPWM technology and vector control frequency converters, ensuring that the motor has advantages such as low-frequency torque characteristics without crawling and a wide constant torque speed range. Generally, constant torque speed regulation is used from 3 to 50 Hz, and constant power speed regulation is used above 50 Hz. Before the modification, the high-speed motor had 6 poles, while the gantry mill mostly operates at slow speeds. Considering the ratio of fast to slow speeds, the frequency of slow movement should be increased as much as possible; therefore, an 8-pole motor was selected. [b][align=center]For details, please click: Application of Frequency Converters and PLCs in Gantry Mills[/align][/b]