Investigation On Vortex Signal Processing Method Based On Duffing Oscillator And Double-window Estimation

Vortex flowmeters are widely used in different fields due to its simple structure, no moving components, insensitive to media, long operating life and so on. However, it has poor performance at low flow rate where the useful vortex signal is submerged by stronger noises and hard to be retrieved. In this case, the vortex flowmeter can not measure the lower limit of flow rate, which leads to a smaller measurement range.The subject is presented against above problem. Other than traditional analog signal method, the research adopts digital signal method to estimate the weak vortex signal. Based on large numbers of references, the work is divided into three steps. First, the single-frequency signal estimate method in relaxing notch periodogram is improved due to problems in simulation. Different window functions'effect on estimation is compared and finally the double-window is presented to estimate single-frequency signal: frequency estimation using hanning window and amplitude estimation using triangle window. Second, in order to prove the practicability of the double-window relaxing notch periodgram, experiments are made on the DN25 and DN50 water equipments respectively. The lowest flow rate DN25 can detect is 0.317m3/h and DN50 is 1.009m3/h. The repeatability is smaller than 1%. Therefore, the improvement method can effectively expande low limit of flow rate.Third, through experiments, it is found that the relaxing notch periodgram does not have good performance when measuring much lower flow rate where noises distribute the whole frequency domain. It is difficult to retrieve vortex useful signal by maximum likelihood frequency estimation. Therefore, a new research method—weak signal detection based on chaos is presented. Duffing oscillator's dynamic feature, noise immunity and detecting effect on unknown frequency signal are investigated by simulation. Its noise immunity and estimation accuracy are better than relaxing notch periodgram. Therefore, the paper initially regards it as a proper method to measure vortex signal at much lower flow rate. However, investigations on chaos develop fast in recent years and chaos application in engineering is in initial step. There are lots of problems in the subject, such as how to distinguish chaos, how to calculate the cycle time of intermittent chaos and so on. Above mentioned need further research in the future.