Research On Time And Frequency Domain Based Signal Processing Methods And System Of Vortex Flow Meter

The vortex flowmeter is widely used to measure liquid, gas and steam in the process industrials, and has many advantages, such as no moving parts, lower pressure loss, and the pulse output. However, two key problems have remained to be solved. First, the measurement accuracy in the field cannot be ensured because the vortex flowmeter is easily subjected to the influence of noise, e.g. the vibration of the pump and pipe line. Second, the vortex flowmeters cannot detect a low velocity flow in low-density fluids.This paper focuses on the studies of the digital signal processing methods of the vortex flowmeter.(1) The vortex signal model was studied in order to evaluate the digital signal processing methods. The characteristics of the actual vortex signal were analyzed with the probability density function (PDF) and the power spectral density (PSD). The vortex signal model under the normal flow condition was established. Furthermore the vortex signal model under the pulsating flow condition was proposed.(2) The power spectrum analysis based signal processing method was studied. The power spectrum of the sample data was calculated using the fast Fourier transform (FFT). Then, the signal frequency was obtained. In order to reduce the systematic error introduced by non-integer-period sampling, the spectrum correction methods were studied. The phase difference correction method was proposed, which was based on the transfer center of window function. The simulation result shows that this correction method can improve the measurement accuracy under the condition of the lower amplitude noise.(3) The phase difference synthesis correction method was studied. The fundamental of directional angle delimitation for the phase difference principal value was proposed, and the principle value function was given. With the principle value function, the formula of the phase difference synthesis correction method was deduced, and the value range of parameters wass analyzed. In order to improve the measurement accuracy of the intensive spectrum, the zoom spectrum analysis was adopted based on complex analytic band pass filter.(4) The wavelet filter based signal processing method was studied. The characteristic of wavelet transform and wavelet filter was analyzed. The Butterworth wavelet filter with better amplitude-frequency characteristic was given. The signal is divided into various frequency bands using the Butterworth wavelet filter. The vortex signal is selected with the principle of optimal frequency band. The frequency is calculated with the average period. The simulation result shows that, the IIR Butterworth wavelet filter possess higher measurement accuracy under the conditionof the higher amplitude noise.(5) The wavelet filter based signal processing method for pulsating flow was discussed. The frequency of the vortex signal is time-varying according to the pulsating vortex model. The wavelet transform based on the IIR filter banks was used to decompose the vortex signal. The recursive wavelet algorithm based on IIR filter banks was proposed according to the need of real-time. The decomposed vortex signal was processed by the Hilbert transform and then the instantaneous frequency (IF) was estimated. The simulation result shows that this method can process the vortex signal under the normal condition accurately and in real-time, and also deal with process the vortex signal under the pulsing flow condition effectively.(6) A DSP-based digital signal processing system was developed for the vortex flowmeter. The wavelet filter algorithm is realized in real time, and calibration experiments of gas and water flow rate were performed in the Luda instrument corporation of Jiangsu province and the flowmeter calibrating station of Anhui province separately. The results show that the digital signal processing method and system posses the better performance and the adaptability than those of the conventional method and system.