# Multi-Source Information Fusion Diagnosis Method for Aero Engine
## 2 Methodology
### Subsystem Analysis Method
#### 2.1.2 Vibration Analysis Method  
Vibration analysis is conducted in both the time and frequency domains to extract relevant fault features. Aero engine vibration arises from structural responses to multiple simultaneous excitation forces acting on the engine. The characteristics of each exciting force, strength, role, and mode of action determine the vibration characteristics of the engine. It is impossible to thoroughly study the characteristics of each exciting force through the vibration sensor; during actual work, it can only determine the dominant \begin{table}
\begin{tabular}{l l} \hline \hline
**Gas Path Fault Features** & **Description** \\ \hline DEF & Efficiency degradation of fan \\ DGF & Flow rate degradation of fan \\ DEHC & Efficiency degradation of high-pressure compressor \\ DGHC & Flow rate degradation of high-pressure compressor \\ DELC & Efficiency degradation of low-pressure compressor \\ DGLC & Flow rate degradation of low-pressure compressor \\ DEHT & Efficiency degradation of high-pressure turbine \\ DGHT & Flow rate degradation of high-pressure turbine \\ DELT & Efficiency degradation of low-pressure turbine \\ DGLT & Flow rate degradation of low-pressure turbine \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Description of gas path fault features. Figure 2: Model-based GPA method. excitation components, the main excitation sources, and the main parts reflecting the vibration performance of the whole engine. Below are several typical vibration faults and their characteristics [23]. (1) Rotor unbalance  
Rotor unbalance is a fault caused by mass eccentricity of rotor components or defects of rotor components. Engine vibration caused by rotor unbalance is quite common. The reasons for rotor unbalance are as follows: unreasonable structure design, manufacturing and assembly errors, uneven material, uneven heating, corrosion, wear and scaling of the rotor during operation, etc. The time-domain waveform is a periodic single-frequency signal, typically sinusoidal in shape, reflecting the rotational nature of the excitation force. The frequency domain characteristic of the unbalanced rotor is that the energy is concentrated in the fundamental frequency; in addition, some other low-frequency components will be excited, such as 1/2 frequency and 2 frequency [24]. (2) The rotor is not centered  
Misalignments between rotor and support are divided into two types: angular misalignment and parallel misalignment. Angular misalignment refers to an inclined angle between the shaft and support axes, while parallel misalignment means their axes are parallel but offset. Both types generate dynamic axial and radial forces during rotation. Parallel misalignment predominantly excites second-harmonic radial vibrations, whereas angular misalignment induces both synchronous axial vibrations and second-harmonic radial vibrations. Axial vibrations are more pronounced in angular misalignment due to imposed bending moments. (3) Foreign inhalation  
Foreign objects first damage the rotor blades of the primary compressor. Metallic foreign objects will rebound after ingestion, repeatedly striking inlet guide vanes and rotor blades, causing cascading damage to multiple compressor stages. The broken foreign matter and the broken blade will also pass through the first stage rotor and further damage the subsequent levels of the blade. Foreign object ingestion triggers severe engine vibration, characterized by transient impact-induced amplitude spikes in vibration signals, and the vibration characteristics are related to the damage form and degree of foreign objects. (4) Oil film oscillation  
When the bearing is working, an oil film is formed in the gap between the journal and the bearing bush. When the bearing capacity of the oil film is balanced with the external load, the journal is in a balanced position. When the rotating shaft is subjected to some external disturbance, the center of the journal will vortex near the static equilibrium position.