At present, the number of cars in China is increasing rapidly, and there are more and more imported cars, and the models are complex and various. Therefore, many maintenance units are unable to maintain or cannot meet the needs of maintenance due to the lack of technical guidance from maintenance experts. In order to improve the utilization rate of vehicles and reduce economic losses, it is necessary and inevitable for the development of the situation to develop an expert system for automobile fault diagnosis and use a computer as an advanced device for fault diagnosis.
Automobile fault diagnosis expert system is a computer intelligent software program that can simulate the diagnosis ideas of maintenance experts for fault diagnosis. To develop such a system, first of all, a large amount of actual maintenance experience of experts in the field of automobile maintenance needs to be summarized and refined into a knowledge base, which constitutes the core part of the expert system; secondly, an inference engine needs to be established. Abnormal phenomena, use the knowledge in the knowledge base to infer according to a certain inference strategy, and then get the diagnosis result. The structure of the expert system is shown in Figure 1. 
1 Overall design and construction of a practical automotive fault diagnosis expert system, not only must have a friendly man-machine interface, efficient reasoning strategy, convenient knowledge acquisition means, but also should have a complete database and image library, and a powerful help system, its The overall structure block diagram is shown in Figure 2. Among them, the fault diagnosis module uses different diagnostic methods according to the different types of deterministic faults and non-deterministic faults. For deterministic faults, the diagnostic tree method is mainly used to describe the diagnostic process. Starting from the fault phenomenon, list all possible The fault parts that cause such phenomena are eliminated one by one according to the most convenient judgment method according to the phenomenon, and the accurate fault parts are finally determined; and for non-deterministic faults, such as abnormal noise of the engine, the logical relationship between the fault phenomenon and the cause is not obvious If the situation is unclear, fuzzy diagnosis method is used to simulate the reasoning method of maintenance experts in identifying abnormal sound parts through fuzzy statistics, so as to find out the cause of the failure, so as to start from and carry out maintenance without having to rely on manual to make Diagnosis greatly saves time and improves the efficiency of vehicle maintenance. The database provides a large amount of various data about the car, including the general data of the car, usage data, engine and chassis maintenance adjustment data, etc., so that the maintenance workers can query in real time while working, without having to spend time to consult the book information. The image library provides users with a large number of exquisite car images, along with a brief description of various models. 
2 Design of each functional module
2.1 Design of the fault diagnosis module This module searches the fault clues grasped by the user in the form of human-machine dialogue, imitates the diagnosis ideas of the maintenance experts for reasoning, and then finds the faulty parts and causes step by step. The block diagram of the diagnosis process is shown in Figure 3. . According to the specific situation of the car, the diagnostic module is divided into three parts, and its detailed structural block diagram is shown in Figure 4. 
Taking the engine system as an example, when establishing the diagnosis sub-module of the diesel engine, a more vivid tree-like diagnostic scheme is adopted for its fault characteristics. When performing a diagnosis, the system determines the branch to be displayed next based on the user's answer to the query until the diagnosis proceeds to the leaves (finding the cause of the failure). At this time, the OK button becomes available from failure (the failure state is not completed after diagnosis), and the preview and print screen will pop up after pressing it. The human-machine interface is shown in Figure 5. 
2.1.1 Difficulties analysis in the design of diagnostic software To realize this fault tree diagnosis method, first draw all the tree diagrams on the form. When initializing, first set the visibility of all objects in the full screen VISIBLE = .F., And then according to each This button asks the logical relationship between files, and writes the action (CLICK attribute) to be executed when this button is pressed. For example: When a certain button is pressed, the program is executed:
thisform.line25.visible = .t.
thisform label11.visible = .t.
2.1.1.1 The design of wrong selection and repentance function Sometimes, when the user presses a button, after the corresponding diagnostic content has been displayed, it is found that the selection is wrong, and he regrets and presses back one or more levels A button. The system must allow this error to occur and fulfill his requirement to add some statements in the CLICK attribute of each button, so that in addition to the content VISIBLE (visible) that should be displayed, the attributes of other objects are changed back to VISIBLE =. F. (invisible), which requires controlling each object one by one. For example: after pressing the button on the right and popping up the next level of content, the user regrets and presses another button on the left, the system will reset the content on the right to VISIBLE = .F., Hide it, and The content on the left pops up (see Figure 6). 
When the user's diagnosis is completed, it is allowed to realize the browsing and printing functions. First, it is necessary to judge whether the user has completed the diagnosis, that is, whether the final result of a certain branch has been displayed. If not, browsing is not allowed. Then, take the content corresponding to each step as a record, add all the records in the diagnosis process of this branch to the USER. Fault field, and you can achieve browsing and other functions. After writing the added content of each branch, the design of a form is completed.
2.1.1.2 Design of the diagnostic record process The difficulty of this part is to add the design of the diagnostic process record. It was originally designed to add a statement to each selection button to add the part of the diagnostic process to the table in the form of records, but then considering that the user may regret it, how to identify the added error record and delete it, has been Is a big problem. After repeated research and testing, it was finally decided to use the existing method, that is, to ignore the user's diagnostic steps, only the diagnosis branch that was finally determined shall prevail. First determine which branch is based on the final result, and then add the content of the branch in the OK button.
2.2 Design of the database module The automobile database module mainly includes 5 parts: database, working interface, information retrieval, print output port, and system guidance information processing. The database part is the data source of the entire system and one of the most important components of the system. According to the extremely large and complex characteristics of car data. The structure of the whole system adopts the database method of "tree branch, module association, and partial structure". Its structure is shown in Figure 7. 
Corresponding to the database module, the work interface also uses an interactive associative calling method. Each working interface corresponds to the corresponding database and can be called each other. Due to the use of structured programming, each working interface appears as a sub-function, which not only increases the flexibility of the system, but also makes the whole system more organized. The entire working interface network is shown in Figure 8. The working environment of the entire system is a network structure connected to each other. In this working interface network, users can simply jump through each part of the data without having to care about the database where they are located. At the same time, each data interface is closely connected with the retrieval interface and the printing interface, and data acquisition is more convenient and fast. 
The data retrieval module is a central link of the entire system. Because, to measure the success of a database system, to a large extent depends on the speed and perfection of its retrieval function. According to the characteristics of the large number of automobile data record entries, the large number of data fields, and the large number of bytes occupied by fields, the automobile database system adopts a new method of view retrieval. Obtain the retrieval conditions through the work interface, pass them to the view in the form of parameters, use the view to retrieve, and then generate a temporary data table of retrieval results from the view. Map the temporary data table to the search interface to display the search results. This method uses structured query (SQL) view retrieval. The transmission and discrimination of retrieval conditions are relatively simple. When using RUSH MORE technology, it can show a satisfactory query speed. At the same time, the use of temporary table generation can greatly reduce the occupation of CPU and memory and speed up system performance. In the actual inspection, the system retrieved 298 records meeting the complex conditions in 8 databases (more than 50,000 data), which only took 0.27 seconds and the query results were satisfactory. In addition, the system can also use the view's temporary table to send updates, adding search result editing functions. Users can use the search interface to retrieve the required data records, and use this function to modify the data and send it to the original data table to better achieve data management.
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