Table 2: Extracted features of the output signals of hydraulic servo system.

FeatureFeature magnitude
NormalAmplifier faultSensor faultHydraulic pump wear fault

Average−0.0446~−0.04110.0004~0.0036−0.0734~−0.0698−0.0100~−0.0063
Standard deviation0.2715~0.27280.2745~0.27560.2715~0.27270.3068~0.3080
Mean square root0.2746~0.27600.2742~0.27540.2806~0.28190.3067~0.3079
Skewness0.0080~0.01480.0028~0.00990.0082~0.01490.0085~0.0174
Crest factor1.4603~1.46081.4620~1.46851.4599~1.46751.8089~1.8189
Maximum amplitude of FFT95.8092~96.28896.8611~97.277895.8319~96.2598107.5320~107.94
Total energy of wavelet99.4925~99.74699.4695~99.735099.5243~99.766498.5909~99.0743
Wavelet singular entropy0.7682~0.81930.7701~0.81130.7857~0.84010.8579~0.9178
Maximum coefficient of wavelet0.0012~0.00350.0005~0.06690.0004~0.00340.0005~0.0543