Handling Phase Ambiguity in Full Spectrum from FFT

Abstract

Condition monitoring of rotating machines essentially utilises frequency domain analysis of the vibration signals, to expose features otherwise hidden in the time domain. Generally, full spectrum of the frequency domain obtained from Fast Fourier Transform (FFT) is used for analysis. The FFT process introduces a phase ambiguity in the harmonics due to random selection of the span of the time domain signal used for decomposition. As the span of the time domain signal cannot be determined a priori; such that phase ambiguity is not introduced, alternate methods are required to remove this ambiguity. A phase compensation based on the FFT of the multi harmonic complex reference signal in unison with the vibration signal may be used for this purpose. This paper presents a mathematical background and a practical way for its implementation. Though the example presented in the paper is based on applications related to rotating shaft, the methodology is generic and applicable to all fields of application where, the phase of the harmonics obtained from FFT are important.