Research And Implementation Of Signal Processing Method For Ultrasonic Gas Flowmeter Based On Echo Envelope Fitting

Ultrasonic gas flowmeter is a new type of gas flow measuring instrument with high technology content.Compared with traditional instruments,it has the advantages of no pressure loss,large turndown ratio,high measurement accuracy,etc.Therefore,it is widely used in petroleum,chemical,natural gas and other fields.However,due to the large loss of energy when the ultrasonic signal propagating in the air,the stability of the echo signal and the signal-to-noise ratio are reduced,which affects the measurement accuracy of the ultrasonic gas flowmeter and the measurable flow range.The ultrasonic gas flowmeters produced by foreign companies adopt digital signal processing methods,which has high measurement precision and stable performance.Most of the products produced by domestic companies adopt analog measurement methods,and the measurement accuracy and stability are poor.Aiming at solving these problems,the echo signal envelopes under different flow rates are analyzed.It is found that the shape of the echo signal remains the same as the flow rate increases,and the envelope gradient of the middle parts of the upper envelope's rising section and the lower envelope's falling section remains the same at different flow rates.According to this,three kinds of signal processing methods based on echo signal envelope fitting are proposed.The mathematical models of the two sections are established to obtain the envelope gradient curves of the two sections.With the envelope gradient curves,the ranges of envelope and peak points that linearly distributed are obtained.The least squares fitting is performed on those peak points to obtain two feature straight lines for respectively representing the upper envelope's rising section and the lower envelope's falling section.According to the spatial characteristics of the feature straight lines,the points on the feature straight lines are selected as the feature points for quickly locating the echo signal.The three kinds of signal processing methods based on echo signal envelope are realized on the MATLAB.With the offline verification and comparison,the best one of three kinds of signal processing methods are selected and realized on the hardware system of the two-channel ultrasonic gas flow-meter.The transmitter of two-channel ultrasonic gas flow-meter is developed based on FPGA&DSP dual cores structure.Then,the repeatability of the system was verified in the laboratory with standard table methods.And the gas flow calibration experiments were also carried out in a national accredited testing agency to verify the effectiveness of the signal processing methods and system.The experiments results show that the two-channel ultrasonic gas flowmeter system with improved signal processing method based on echo signal envelope fitting has a measurement accuracy of 1 level,which achieves the highest measurement accuracy of the same type of products at home and abroad.And the measurable range of the ultrasonic gas flow meter based on this signal processing method is broadened to 10 m~3/h to 1,300 m~3/h,and the turndown ratio is broadened to 1:130.