Research On High Voltage Drive And Signal Processing Technology Of Ultrasonic Gas Flowmeter

Natural gas is one kind of clean and efficient energy resource.With the continuous improvement of energy structure in China,the market of natural gas is quickly surpassing other traditional energy sources.Compared with other gas flowmeters,ultrasonic gas flowmeter has many advantages.It has no blocking components,no moving parts,no pressure loss,wide range ratio,and is suitable for large diameter measurement etc.Therefore,the ultrasonic gas flowmeter has become the first choice in international natural gas trading.By the high accuracy,imported ultrasonic flowmeters dominate the national market,and are widely used in large and medium-size natural gas pipelines and pressure regulating stations.It is of great significance to develop domestic high-precision ultrasonic gas flowmeter.After reviewing literatures both at home and abroad,this thesis summarizes techniques used in ultrasonic gas flowmeter from different perspectives.Based on this,studies have been conducted to solve the poor measurement accuracy of transit time caused by low signal-to-noise ratio and waveform distortion of received ultrasonic signal,particularly under high flowrate condition.The main contribution and innovations of the thesis are as follows:1.A high-voltage driving ultrasonic gas flowmeter prototype is designed.The high-voltage driver module is designed as a three-stage driving voltage amplifying circuit,generating a driving voltage of up to±70V,which increases the power of the exciting ultrasonic signal compared with the former±12V driving voltage.The signal conditioning module is designed as a four-stage signal processing circuit.A programmable gain module is integrated in the signal processing circuit,which can automatically regulate the gain at different flow rate to ensure a good signal-to-noise ratio for the received ultrasonic signal.2.The sampling frequency of the ultrasonic signal is increased to improve the resolution of the transit time measurement.The accuracy of the cross-correlation algorithm highly depends on the sampling frequency.Generally,a higher sampling frequency contributes to the measurement accuracy of transit time.In this thesis,the internal resources of the DSP chip are optimally allocated to increase the sampling frequency to a lager extent.It is verified by experiments that the sampling frequency of 6MHz has reached the upper bound of the internal resources of the DSP.Therefore,the sampling frequency of the system is set at 6MHz.Compared with the sampling frequency of 4MHz,the measurement resolution of transit time is increased by 33.3%.3.The exciting waveform modes are studied,which can greatly influence the shape of the cross-correlation function of the ultrasonic signal.Thereby the ideal exciting waveform mode for the flowmeter is determined.The ordinate value difference among the global maximum point and the neighboring local maximum points of the cross-correlation function is the key to obtain the precise transit-time measurement.In the thesis,the combination of square wave and phase-reversed square wave sequence is adopted as the exciting waveform.The ordinate value difference among the global maximum point and the neighboring local maximum points is used as the performance indicator.The preferred combined waveform is determined as the exciting waveform of the system by experiments.The optimized exciting waveform mode can significantly increase the ordinate value difference among the global maximum point and the neighboring local maximum points,compared with other exciting waveform mode.Hence it has a strong ability to resist the fluctuations of the maximum points,which is beneficial to improve the stability and accuracy of the transit time detection.4.A method to calculate the ultrasonic transit time based on the envelope feature points of the cross-correlation function is proposed.In this method,the two neighboring local maximum points with the largest ordinate value difference in positive value,in the cross-correlation function waveform,are selected.The one with larger amplitude between the two selected points is used as the feature point to calculate the transit time.The position of the feature point is relatively stable in the waveform envelope of cross-correlation function ranging from low flowrate to high flowrate.Additionally,compared to the global maximum point,it has a larger ordinate value difference from its neighboring local maximum points,which increases the repeatability of the transit time measurement.Finally,to verify the effectiveness and practicality of the proposed method,an experiment platform is built.Many experiments are conducted to evaluate the accuracy and repeatability of the ultrasonic gas flowmeter prototype.The experiment results demonstrate that the proposed method is capable of improving the accuracy of gas flow measurement.