| Precise measurement of the time difference between the downstream andupstream sound wave is one of the issues of the transit time ultrasonic gas flowmetering system. A method using coded excitations combined with correlation ispresented to estimate the time-of-flight in ultrasonic flow measurement, originatingfrom the pulse compression technique in radar but varying from it since the specialcharacteristic of the mechanical wave, i.e. ultrasound is different from that of theelectromagnetic wave, i.e. radar. The analysis by classical signal detection andestimation theory, the simulations by Matlab and the realistic experiments all showthat at low signal-to-noise ratio (SNR), the correlation algorithm has a highercorrect-detection probability of the transit time than that of the threshold method andthe system with coded exications has a lower standard deviation of the time delayestimation than that of the traditional single pulse excitation.An ultrasonic flow metering system with coded excitations is developed onFPGA configuration.The schemes and the timing controls of the submodules in thissystem are dedcribed. Due to its flexible structure, binary phase-shifted keying (2PSK)signals with various codes or different band widths as well as the linear frequencymodulated (LFM) signals with different time-bandwidth products can both begenerated.The2ASK system model is constructed on the transient effect theory of thepiezoelectric transducer and the procedures and test waveforms of2ASK areaddressed. The experimental results show that the maximum flow rate reading error is39.4%and the maximum reaptability is3.33%in the range of2.5~16m/s due to the2ASK ultrasound signal distortion caused by the piezoelectric transducers and thepropagation channel, i.e. the flow medium.Phase-modulated systems are comprehensively investigated, including the2PSK andthe LFM excitations. The dual standards, namely the downstream and the upstreamultrasonic signals received at zero flow are provide as the references in thecorrelation segment time delay estimation segment to estimate the transit time, whichcould compensate the delay caused by the transducers and the amplifier/filter. Furtherdiscussion based on the paired-echo theory demonstrates qualitatively that thepiezoelectric transducers generally introduce distortions in amplitude and/or phase into the ultrasound system and would lead to the excursion of the pulse compressionoutput.Then some realistic flow experiments are conducted on the air flow experimentalfacility in the range of3.0m/s~18.9m/s. The results show that the performances oftime delay estimation become worse as the echoes overlap seriously with thereceiving signal. And in contrast to the pre-correlation-envelope-extracting method(PREE), the average reading error of the flow rate obtained by thepost-correlation-envelope-extracting method (POEE) is about1/2of that obtained bythe PREE and the standard deviation is about1/3, as it takes advantage of the finestructure of the carrier and makes full use of the signal information. Also theprobability distribution of correlation peaks is given and the time gating at peak andthe temporal/spatial median filtering are introduced to solve the ambiguity problem ofcorrelation peak search. With these nonlinear processing techniques, the reading errorof flow rate at18.9m/s reduces from30%to3.3%and the standard deviation from2m/s to0.35m/s. Finnaly a camparitive study of coded exciations’ parameters such asthe type, the period number and the bandwidth are carried out in terms of the readingerror and the standard deviation of flow rate. It shows that at the same transmittingpower, the stsytem excited by the LFM signal with its duration of2.6ms and itsbandwidth of20KHz has better performances than that of the systems excited by2PSK signals modulated by m or barker sequences. |
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