Abstract: This paper uses the circuit simulation software Multisim2001 to simulate the circuit of the electric control system of a missile launching vehicle. Through simulation , the fault parameters of the electric control system circuit of the missile launching vehicle are obtained, the correspondence between function and fault is obtained, and a knowledge base of intelligent diagnostic expert system is established. This provides a new idea for solving the "bottleneck" problem of knowledge acquisition of expert system. Key words: circuit simulation, knowledge acquisition, expert system At present, intelligent fault diagnosis expert systems are facing the problem of lack of fault knowledge and diagnostic and maintenance knowledge. As a result, many developed intelligent diagnostic systems are mostly in the theoretical exploration or prototype design stage and have low practical value. How to obtain effective and sufficient knowledge has become a bottleneck restricting the actual engineering application capability of intelligent diagnostic systems [1]. The intelligent diagnostic expert system is built with the knowledge base as its core. The most effective and feasible way to build a system and a complete knowledge base is to simulate the circuit. This paper intends to establish a dynamic operation model of the electrical system of a certain equipment by means of circuit simulation. The powerful analysis function of the software is used to perform DC and transient analysis on the circuit, including the voltage and current values ​​of some components and the setting of test points. The simulation parameters of the circuit under normal and fault conditions are obtained. The obtained parameters are imported into the database so that the system can call and retrieve them. Through reasonable algorithms and knowledge representation, the fault node is finally determined [2] [3]. 1 Simulation Platform Multisim2001 software is the most convenient and intuitive circuit simulation software to date. It has added a large number of VHDL component models, which can simulate more complex digital components. While retaining the advantages of EWB such as being visual and intuitive, it has enhanced the simulation testing and analysis functions of the software and expanded the number of components in the component library. In particular, it has added a large number of component models corresponding to actual components, making the simulation design results more accurate, more reliable and more practical. The knowledge base of the fault diagnosis expert system mainly relies on the various input and output interfaces provided by Multisim2001. Multisim2001 can input the Spice netlist created by other circuit simulation software such as Spice and automatically form the corresponding circuit schematic. The circuit schematic file created in the Multisim2001 environment can be output to common PCB software such as Protel for printed circuit board design. The simulation results can also be sent to applications such as MathCAD and Excel [4]. 2 Production of the electrical control system circuit The circuit of the electrical control system of a missile launch device is divided into two main parts: the power distribution circuit and the erection circuit. The entire circuit diagram is composed of various unit circuits. Figure 1 shows the circuit schematic of the deck chamber heater in the power distribution circuit. The erection circuit is used to control the electric mechanism of the support, clamp, limiter, jack, slewing machine and the hydraulic system electromagnetic valves and electromagnetic clutch of the launch platform and erection arm to work according to the set program. The program includes the following steps: 1. Power supply program for the erection circuit; 2. Program for loading missiles onto the launch vehicle; 3. Program for preparing to launch the missile; 4. Program for transitioning from missile launch to marching status. In the entire circuit, the motor's operation is mainly controlled by relay contacts, and the relay's energization is controlled by microswitches. Therefore, many manually controlled switches are defined in the circuit, and the above four programs will be simulated by sequentially operating according to each program. Figure 2 shows a part of the erection circuit. [align=center] Figure 1: Circuit diagram of the power distribution circuit Figure 2: Circuit diagram of the erection circuit[/align] 3. Fault Simulation and Generation of Diagnostic Knowledge On the battlefield, faults in the launch device's electrical control system are mainly repaired by replacing components. Therefore, finding the damaged components in the circuit is crucial for repair. Relying solely on the repairman's experience makes it difficult to quickly identify the fault source. In the intelligent diagnostic system, faults are set in the circuit through simulation, and analysis allows for rapid fault identification. The following is a fault diagnosis using the erection circuit as an example: 3.1 Fault setting The clamp cannot be opened, which is a common fault phenomenon. Here we assume that the motor is good and there is an open circuit fault in the circuit. In the erection circuit, the node before motor 7M1 is set as node 3 and its state is set to open. All other components are normal. The nodes before S1, S2, S4, S5, and S6 are set as nodes 1, 2, 4, 5, and 6 respectively. 3.2 Fault finding (1) Fault simulation: Press the simulation switch and the circuit starts to simulate. In order to see the fault more intuitively in this circuit, a light-emitting diode is connected in series in the circuit of each DC motor. After the simulation is completed, the results are observed and it is found that the light-emitting diode of the motor is not lit. (2) Phenomenon comparison: In a normal circuit, the line where each motor is located is connected between the positive and negative busbars of the power supply. The motor can be determined to be working normally by comparing whether the light-emitting diode of each motor is lit or not through simulation. (3) Fault determination: In the process of determining the fault, we use transient analysis method. First, a transient analysis was performed on the normal circuit, obtaining the voltage values ​​of all nodes. Figure 3 shows the results of the transient analysis of the normal circuit, with the voltage value of node 3 being 29V. These parameters were imported into an Excel spreadsheet and displayed numerically, as shown in Figure 5. Next, a transient analysis was performed on the faulty circuit, again obtaining all voltage values. Figure 4 reflects the transient analysis results of the faulty circuit, showing that the voltage value of node 3 is 0V. Figure 6 shows the numerical representation of the node parameters. In these two tables, the values ​​for the first six steps were taken; column A represents the scan time, and column B represents the scanned voltage value. A comparison revealed a significant change in the voltage value at node 3; it was 29V before the fault was set, but 0V after. Therefore, it was determined that the fault was the result of the fault setting. By analyzing the entire circuit and carefully observing the voltage values ​​of each node, it was determined that the open-circuit fault occurred at node 3. [align=center] Figure 3 Transient analysis results of normal circuit Figure 4 Transient analysis results of fault circuit[/align] [align=center] Figure 5 Node parameters of normal circuit Figure 6 Node parameters of fault circuit[/align] 3.3 Generation of diagnostic knowledge By setting circuit faults, simulating all fault circuits, and comprehensively analyzing, a fault node is determined. In the circuit of the electronic control system, this type of fault is the most common. Solving this problem basically solves all faults caused by circuit breakage. Using the above three steps to determine and eliminate circuit faults is a very effective diagnostic method. Its advantage is that it can convert some parameters obtained after simulation into EXCEL tables for summarization and organization, forming a knowledge base, laying a good foundation for the establishment of intelligent diagnostic systems in the future. 3.4 Simulation results and corresponding fault phenomena By using the above fault diagnosis method, faults can be set and diagnosed. Here, common faults of the electronic control system are set and simulated. Taking the fault "the launcher does not rise" as an example, the corresponding results are as follows: Fault phenomenon: the launcher does not rise (1) Set fault: set the node parameter at the solenoid valve DF51 to open circuit. (2) Fault simulation: The LED connected to the solenoid valve is not lit. (3) Transient analysis: The scan waveform shows that the voltage at the node of this path is zero. (4) Conclusion: An open circuit fault occurred in this path. The transient analysis results of all nodes are imported into a spreadsheet for saving. 4 Conclusion The problem to be solved in this paper is to obtain intelligent diagnostic knowledge through the simulation of the electrical system circuit of a certain type of missile launcher, and to study the acquisition of fault knowledge of the intelligent diagnostic system of electromechanical equipment. Based on some problems that often occur in the launcher's electrical control system, fault settings and simulations were carried out, and fault diagnosis knowledge of the electrical control system was obtained, laying the foundation for the establishment of an intelligent diagnostic system in the future. If the electromechanical-hydraulic integrated software is used for co-simulation, the electronic system, mechanical system and hydraulic system of the launcher can be better simulated, and more systematic and complete data can be obtained. However, in this design, only the electrical circuit was attempted. References: 1. Lian Shiyou. Introduction to Artificial Intelligence Technology [M]. Xi'an University of Electronic Science and Technology Press. 2000. 2. Zhang Ping, Wang Guizeng. Fault Diagnosis Methods for Dynamic Systems [M]. 2000. 3. Mao Zhe, Zhang Shuangde. Circuit Computer Design Simulation and Testing [M]. Huazhong University of Science and Technology Press. 2003. 4. Zheng Busheng. Introduction and Application of Multisim2001 Circuit Design and Simulation [M]. Electronic Industry Press. 2002.