1. Introduction Tangshan Iron and Steel Company's medium-sized plant is a steel section production enterprise. It originally had 4 sets of drilling and milling machines for light rail finishing. Its electrical control system was composed of relays and contactors. The control methods were relatively backward. The control effect depended entirely on the operator's experience and mental state. The continuity between each process was poor, which was time-consuming and labor-intensive. The failure rate was high and maintenance was difficult, which affected production efficiency. Therefore, it was necessary to carry out technical transformation. PLC control has the advantages of high reliability, good flexibility and short development cycle. It is particularly suitable for machine tool control and fault self-diagnosis system. It can greatly reduce the number of relays and other components and improve the stability and reliability of electrical control system. Therefore, replacing the large-volume, high-investment and high-energy-consuming relays with PLC control system is the trend of electrical control system development. In view of the above reasons, we used PLC technology to carry out a comprehensive technical transformation of the original electrical system. 2. System Functions The light rail finishing PLC intelligent control system includes milling machine and drilling machine control. The basic functions are as follows: (1) Switching function: It can realize the switching between manual and automatic control. Automatic mode is used under normal circumstances. When maintenance or debugging is required, switch to manual mode. (2) Automatic alarm function: An alarm can be triggered at any time in case of an abnormality. When any of the four parts of the clamping head fast down, power head fast forward, power head working feed and power head fast back are abnormal, the corresponding sound and light alarm will be activated, allowing the on-site staff to take measures quickly to avoid or reduce the losses caused by the accident. (3) Automatic memory function: Equipped with "stop" and "continue running" buttons. When some problems occur during the operation and it is necessary to stop the operation, press the "stop" button and the machine tool will stop running, and all parts will remain in place. Press the "continue running" button again and the machine tool will continue to run. (4) Emergency stop reset function: Equipped with an "emergency stop reset" button. When an abnormality occurs during the operation, or when the power is suddenly interrupted and then restored, press this button to return the machine tool parts to the initial state before processing. In order to realize the above functions, it is necessary to make intelligent judgments on the operation process and perform corresponding control. At the same time, considering the limitations of the PLC's computing function, a fault diagnosis module needs to be added and displayed accordingly. 3. System Composition The PLC is implemented using Mitsubishi's FX2N series programmable controller [1]. The structure of the light rail finishing intelligent control system composed of programmable controller is as follows: Figure 1. Structure diagram of light rail finishing intelligent control system The system consists of three parts: input, control operation and output. 1) Input part includes operation buttons and signal detection. a. Operation buttons are used to manually set parameters or perform manual operation to handle emergencies. b. Signal detection is automatically monitored by sensors to monitor the working status of machine tools on the production line. Once an abnormal situation occurs, an alarm will be immediately triggered to prompt the operator to perform corresponding fault handling, such as emergency shutdown, so as to avoid accidents. 2) Control operation part The control operation part is mainly completed by PLC. The main functions of signal input, processing and control output are completed by the application software of the control system. 3) Output part includes alarm device, conveying and power device, and fixed device. a. The alarm device consists of flashing red, yellow and green lights and alarm bell. The three colors correspond to three different alarm levels. Green indicates that the system is normal, yellow indicates that the system parameters are out of range but can still work and need to be dealt with; red alarm with alarm sound, and emergency shutdown is required. b. The conveying device is controlled by the signal output of PLC to control the main circuit, send instructions to the motor, and make it automatically complete the material conveying and power transmission. c. The hydraulic device is a fixed device. The signal given by the PLC controller is controlled by the solenoid valve to fix the material in a certain position and serve the material processing. 4. System software design 4.1 Issues to be considered in PLC software design When using ladder diagram to compile control programs, the following issues should be considered in PLC software design: (1) Priority principle of high power shutdown: In milling machine software design, as long as there is a high power shutdown signal in the control signal, the high power should be shut off regardless of other signals. As shown in Figure 2, as long as the shutdown signal XO2=1, the intermediate relay M100 should be shut off. (2) Action interlock principle: Some controls cannot be operated at the same time, so interlocking is required. For example, spindle forward and reverse control, Figure 3 is a schematic diagram of spindle interlock control. After any loop is started, the other loop must be shut off at the same time to ensure that the two cannot act at the same time. [align=center] Figure 2 Figure 3[/align] (3) Sequential interlock control principle: that is, some control requirements cannot be reversed. This requires that the normally open contact of the previous action be connected in the next control action, and the normally closed contact of the next action be connected in the control loop of the previous action, as shown in Figure 4. [align=center] Figure 4[/align] In short, there are many factors that affect the PLC control system. As long as we fully consider all aspects of the factors in the software design, we can avoid failures and the operation of the control system will be more stable[2]. 4.2 PLC basic control program design The specific milling machine control function block diagram is shown in Figure 5. The drilling machine control function is similar. [align=center] Figure 5. Functional block diagram of milling machine control sequence[/align] 4.3 Program design of fault diagnosis module For PLC system, due to limited memory resources, complex intelligent diagnosis is difficult to implement. Therefore, a fault diagnosis intelligent module is added. This module is based on a microcontroller and uses C51 programming, which can easily implement various control algorithms. The fault tree reasoning and expert experience rule reasoning are combined to use the I/O function and internal information of the intelligent module for fault diagnosis. [3][4] (1) Fault structure analysis When designing fault diagnosis, it is necessary to first analyze the possible faults of the entire system to obtain the fault hierarchy of the system. The fault diagnosis part is designed using this hierarchy. Figure 6 shows the fault hierarchy of the system. [align=center] Figure 6. Fault hierarchy block diagram[/align] (2) Program design The hierarchy of the system fault structure provides a reasonable hierarchical model for fault diagnosis. When designing the system program, the hierarchy of the fault structure should be fully considered and the logical flow should be arranged reasonably. Two points should be noted when introducing fault input points: a. All detection points that may cause faults in the system must be introduced into the PLC. This is mainly for the safe and reliable operation of the system, so that the system can handle faults in a timely manner; b. As much low-level fault input information as possible should be introduced into the PLC program under the conditions allowed by the system, so as to obtain more fault detection information to provide services for the system's fault self-diagnosis. 5. Conclusion After online debugging and industrial trial operation, the control system was officially put into operation in 2004. Since its operation, it has performed well, achieving the predetermined control function requirements, overcoming the limitations of relay and contactor control, and avoiding the problems of many auxiliary components, high failure rate, high operating noise, single control mode, and difficult maintenance of the original control system. It is convenient to switch between manual and automatic modes, has strong anti-interference ability, is suitable for the harsh working environment of steel plant production lines, and is easy to communicate with computers to realize network monitoring. The innovation of this paper is: combining PLC and microcontroller to design a control system for light rail finishing drilling and milling equipment, and making it have fault diagnosis and alarm functions. The system structure is simple and easy to operate.