Improvement of the HHT method and application in weak vortex signal detection

The vortex signal is greatly disturbed by noises from external interference when the meter works at low flow rate, which results in a limited measuring range for the flowmeter. In order to solve the problem, a new signal processing method based on the Hilbert–Huang transform (HHT) is proposed. With its good performance on local adaptability and time–frequency analysis, noises are removed by the empirical mode decomposition (EMD) and the residue components are analysed by the Hilbert transform; then instantaneous frequency distributions are achieved. When the probability density of a certain frequency component exceeds 5%, the sifting process will be terminated. Subsequently, the vortex frequency can be calculated from the last residue component. Experimental studies were carried out to compare the improved method with the classic method FFT at low flow rate. A better linearity and lower limit of measurement are achieved by the proposed method.