During signal sampling process of steam turbine digital electric hydraulic control system(DEH) tests

situations that the instrument can not meet the requirements of Nyquist sampling due to the frequency of the primary components is over high may occur. To solve this problem

the envelope of the signal is calculated through Hilbert transform

and the valve closing time is calculated according to the envelope signal. However

the calculation accuracy is limited due to the time interval of the peak points. To improve the calculation accuracy

the undersampled signal is reconstructed

the primary frequency is analyzed by using fast Fourie transform(FFT)

and the primary frequency of the signal is determined based on the nature of the frequency domain. On this basis

the initial phase of the AC signals is calculated using the initial value

and the undersampled signals is reconstructed according to the frequency and initial phase. The closing time is calculated according to the difference between the reconstructed and sampled signals based on Akaike information criterion(AIC). In comparison with the Hilbert transform method

the reconstruction method can improve the calculation accuracy. The reconstruction and analysis method can be used in all kinds of undersampled periodic signals

which can make up the shortcomings of hardware in DEH tests.