At present, most maintenance personnel are still not used to graphic analysis after encountering difficult faults, or have not mastered graphic drawing methods. This paper is the author's understanding of three graphic uses and drawing methods based on years of experience, for reference only.
1 causality diagram analysis method
Causality diagram analysis method is a branch expression method, which draws causality diagram with the fault phenomenon as the result and the factors leading to the fault as the reason. Through the graphic causality, we can comprehensively analyze all kinds of reasons leading to failure and find out the main reasons leading to failure. Causality diagram is very similar to fishbone diagram, also called fishbone diagram or fishbone diagram.
Cause and effect diagram of engine failure to start, as shown in figure 1. Causality diagram is used to show the relationship between various factors that cause the fault phenomenon. The fault phenomenon in this picture is that the engine can't start, which is caused by many factors, including the starter doesn't rotate, the ECU doesn't work, the crankshaft camshaft signal is abnormal, the spark plug doesn't ignite, the injector doesn't inject oil, and the valve timing is abnormal. These reasons form the first branch. Further down this branch, we can find that the second branch, such as the control unit is not working, may be caused by the control unit not supplying power or the control unit itself being damaged.
As can be seen from figure 1, there is a trunk in the causality diagram, and the arrow at the end of it points to indicate the fault phenomenon, which is the result; On both sides of the trunk, several branches form branches, and each branch represents the leading cause of the fault (such as the spark plug not firing). For complex fault phenomena, draw several small branches on the branch to represent different secondary reasons (such as ignition coil not receiving ignition trigger signal and ignition coil being damaged).
The significance of causality diagram analysis in automobile fault diagnosis is that every common automobile fault phenomenon is based on the inevitable connection of causality, clearly distinguish the different influencing factors of automobile parts or systems on this fault symptom, and master the basic context of finding out the fault cause from all directions, that is, the fault diagnosis skills along the network branches.
Causality diagram analysis shows the relationship between fault phenomena and fault causes concisely, and is especially suitable for finding fault phenomena caused by various multi-layer fault causes. However, the causality diagram analysis method can not express the quantitative relationship between the result and the cause, so this method can only be used for qualitative analysis of faults.
2 Fault tree analysis method
Fault tree analysis is a graphic deduction method, which takes the fault of the system as the analysis object, analyzes various reasons that may cause the fault phenomenon, and graphically represents the logical relationship between the reasons. List the correlation diagram between fault symptoms and fault causes.
The graph looks like an inverted tree, so it is called a fault tree. Fault tree analysis is used to analyze various possible ways and reliability characteristics of system failure. Fault tree analysis is a method to analyze fault events step by step from the whole to the part, from the system to the components, further identify the basic faults, determine the fault causes, fault effects and occurrence probability, and analyze the system reliability.
The fault tree of engine idling instability is shown in Figure 2. According to the hierarchical relationship between fault symptoms and fault causes, all the event lists from top to bottom and from middle to bottom are determined. In the fault tree, the system fault event to be analyzed (engine idling instability) is first called the top event. In automobile failure, the top event refers to the initial failure symptoms. Secondly, the basic event that cannot be decomposed (spark plug failure) is called the bottom event, and the bottom event refers to the minimum fault point. Finally, other events are called intermediate events, which refer to the causes of failures at all levels, so intermediate events have multiple layers. The fault tree consists of the top event of the first layer, the middle event of multiple layers and the bottom event of the last layer. However, it should be noted that the bottom event in the fault tree is not the ultimate cause of the fault, but only the smallest fault point.
The logical diagram between events is connected by the logical relationship between fault symptoms and fault causes. According to the logical relationship between fault symptoms and fault causes, a fault tree is established. First, the top and middle events are represented by rectangular symbols, and the bottom events are represented by circular symbols. Then determine the logical relationship between events at all levels, mainly composed of "and" and "or", and connect events at all levels with logical symbols. Only the logical relationship of OR is shown in Figure 2.
3 Flow chart analysis method
Flowchart analysis is a diagnostic step diagram, which is drawn on the basis of causality diagram or fault tree. The flowchart is similar to a fault tree. However, at the fork of the first floor and the second floor, test and analyze it, and then judge which way to enter the second floor, so as to optimize the detection route and save time, and so on between the second floor and the third floor.
The flow chart of engine idling instability is shown in Figure 3. Taking the engine idling instability as the fault object to be eliminated, the detection and analysis points are listed according to various fault causes that may cause fault symptoms, and the diagnosis is guided step by step.
The flow chart consists of a first-level top event, a multi-level intermediate event and a last-level bottom event. First, the system failure event to be analyzed (the engine can't start) is taken as the top event. Secondly, the most likely cause of failure (engine electronic control system failure) is placed in the first layer intermediate event, then the second layer intermediate event is listed, and finally the bottom event (ignition coil failure) is listed.
The top-level events and intermediate events are represented by rectangular symbols, the bottom-level events are represented by circular symbols, and the test and analysis points between events at all levels are represented by diamond symbols. Get "Yes" or "No" after testing and analysis, and then follow the line guide to a lower level until the bottom event is found.
In the maintenance manuals compiled by automobile companies all over the world, the flow chart of fault inspection often appears, so reading and consulting the flow chart frequently is beneficial to implementation. If there is no maintenance manual at hand, or if there is no flow chart in the maintenance manual, maintenance personnel need to have the ability to draw flow charts. For high-level maintenance personnel and students in higher vocational and secondary vocational schools, it is necessary to master three drawing methods of graphics. Practice makes perfect, which is certainly helpful for fault diagnosis.