1. Methodology Overview

The wavelet feature extraction pipeline for mechanical vibration signals follows a multi-scale decomposition → feature quantification → feature fusion workflow, consisting of these core stages:

1. Wavelet Decomposition: Multi-level decomposition to capture frequency band characteristics
2. Time-Frequency Analysis: Extraction of wavelet coefficient energy, entropy, and other features
3. Feature Optimization: Dimensionality reduction using Principal Component Analysis (PCA)
4. Pattern Recognition: Support Vector Machine (SVM) classification for validation

2. MATLAB Implementation

1. Configuration and Signal Loading

%% Configuration parameters
waveletType = 'db4';      % Wavelet basis (db/sym/coif families)
decompLevel = 5;          % Decomposition depth
sampleRate = 10000;       % Sampling frequency (Hz)
timeVec = 0:1/sampleRate:1-1/sampleRate;  % Time vector

%% Load vibration data (example: bearing fault signal)
load('bearing_signal.mat'); % Expects 'signal' variable
normalizedSignal = signal / max(abs(signal)); % Amplitude normalization

2. Wavelet Decomposition and Feature Extraction

%% Multi-level wavelet decomposition
[coefStruct, lenArray] = wavedec(normalizedSignal, decompLevel, waveletType);
detailCoef = detcoef(coefStruct, lenArray, 1:decompLevel); % Detail coefficients
approxCoef = appcoef(coefStruct, lenArray, waveletType);   % Approximation coefficients

%% Feature extraction function
function featVector = extract_wavelet_features(coefData, fs)
    [numLevels, coefLength] = size(coefData);
    featVector = [];
    
    for levelIdx = 1:numLevels
        % Time-domain features
        signalEnergy = sum(coefData(levelIdx,:).^2);       % Energy
        rmsValue = sqrt(mean(coefData(levelIdx,:).^2));   % RMS
        
        % Frequency-domain features
        [freqMag, freqAxis] = spectrogram(coefData(levelIdx,:), 128, 120, 128, fs);
        spectralEnt = -sum(freqMag(freqMag>0).*log2(freqMag(freqMag>0)+eps)); % Spectral entropy
        
        % Statistical features
        kurtosisVal = kurtosis(coefData(levelIdx,:));     % Kurtosis
        skewnessVal = skewness(coefData(levelIdx,:));     % Skewness
        
        featVector = [featVector; signalEnergy, rmsValue, spectralEnt, kurtosisVal, skewnessVal];
    end
end

%% Extract multi-scale features
featureMatrix = extract_wavelet_features(detailCoef, sampleRate);

3. Feature Optimization and Visualization

%% PCA dimensionality reduction
[loadings, scores, eigenvalues] = pca(featureMatrix');
dimensionalFeatures = scores(:,1:3); % Retain top 3 principal components

%% Feature visualization
figure;
subplot(2,2,1);
spectrogram(detailCoef(1,:), 128, 120, 128, sampleRate);
title('First-Level Wavelet Coefficient Spectrum');
subplot(2,2,2);
histogram(dimensionalFeatures(:,1), 30);
title('Principal Component 1 Distribution');
subplot(2,2,3);
scatter(dimensionalFeatures(:,1), dimensionalFeatures(:,2), 'b', 'filled');
title('PC1-PC2 Scatter Plot');

3. Key Technical Innovations

1. Multi-Scale Feature Fusion

Concurrent extraction of time-domain (energy, kurtosis), frequency-domain (spectral entropy), and statistical features (skewness) to construct a 5-dimensional feature vector. 2. Adaptive Decomposition Strategy

Dynamic selection of decomposition levels based on signal length:

optimalLevel = floor(log2(length(normalizedSignal)));
decompLevel = min(5, optimalLevel); % Cap maximum decomposition depth

3. Frequency Band Energy Focusing

Energy enhancement for mechanical fault-sensitive frequency ranges (e.g., bearing faults at 2-4kHz):

targetBand = [2000, 4000] / (sampleRate / 2); % Normalized frequencies
[filterB, filterA] = butter(4, targetBand, 'bandpass');
bandpassFiltered = filtfilt(filterB, filterA, normalizedSignal);

4. Experimental Validation and Results

(* indicates significant difference from normal condition, p<0.01)

5. Engineering Optimization Recommendations

1. Real-Time Processing Optimiztaion

Accelerate computation using GPU parallel processing:

signalDevice = gpuArray(normalizedSignal);
coeffsDevice = wavedec(signalDevice, decompLevel, waveletType);

2. Noise Suppression Strategy

Combine wavelet threshold denoising:

cleanSignal = wdenoise(normalizedSignal, 4, 'Wavelet', waveletType, 'ThresholdRule', 'soft');

3. Feature Selection Strategy

Filter key features using mutual information:

[featureRank, ~] = mutualinfo(featureMatrix', classLabels);
optimizedFeatures = featureMatrix(:, featureRank(1:3));

6. Extended Application Scenarios

1. Early Fault Detection: Integration with LSTM networks for temporal feature analysis
2. Compound Fault Diagnosis: XGBoost multi-class classification algorithm
3. Remaining Useful Life Prediction: Fusion of wavelet features with degradation models

7. Implementation Guidelines

1. Wavelet Basis Selection: Prioritize testing compactly supported wavelets such as db4, sym4, coif3
2. Decomposition Depth Control: Avoid exceeding logarithmic relationship with signal length
3. Feature Normalization: Apply min-max scaling:

scaledFeatures = (featureVector - min(featureVector)) ./ (max(featureVector) - min(featureVector));