| Ultrasonic gas flow-meter has unique advantages in the measurement accuracy, reliability, pressure loss and maintenance costs compared with the orifice plate, turbine and other gas flow-meter. It is especially suitable for measuring the gas flow rate in the medium and large diameter pipes. In the field of natural gas transmission, ultrasonic gas flow-meter is becoming the best choice. However, there is energy attenuation in the propagation of the ultrasonic wave, and the ultrasonic wave is susceptible to the velocity distribution, gas pressure fluctuations and other factors. In addition, since the ratio between the speed of sound and the flow rate is low, the propagation path of the ultrasonic wave will shift with the flow rate increasing. This offset will lead to less energy received by the receiving transducer and further energy attenuation. All these above factors will lead to the amplitude of the ultrasonic echo signal is very weak, and volatility, even submerged in the noise, which will affect the measurement accuracy and turndown ratio of ultrasonic gas flow-meter. Therefore, the domestic and foreign scholars established various mathematical models of ultrasonic gas flow-meter, such as a damped exponential model, a double exponential model, etc. These models could accurately reflect the operating characteristics of ultrasonic gas flow-meter under certain conditions, for example, a fixed excitation signal or a fixed flow rate. However, when the excitation signal is changed or when the gas flow rate changes greatly, these models are often no more applicable. The domestic and foreign scholars also explored various signal pre-processing methods and achieved better filtering effect, these methods includes the adaptive filter based on Laguerre transform, wavelet filters and the adaptive FIR filter based on the multi-linear neural network, and so on. However, there are several widespread shortcomings, including slow convergence and excessive calculations. In terms of echo signal processing method, the digital signal processing method is adopted in the foreign products to obtain a higher measurement accuracy, but the technical details have not been disclosed. The analog threshold detection method is mainly adopt in the domestic products, whose measurement accuracy is lower, turndown ratio is limited and the anti-interfere ability is poor.Therefore, a gas flow-meter experimental setup is built in this paper. This experimental setup consists of two DN 100mm ultrasonic gas flow-meters, a blower, a number for pipelines, an oscilloscope and so on. Based on a large number of experimental data, it is found that the sine wave provides the best energy transfer ratio, and the corresponding echo signal has the best SNR and stability by comparing the driving effect of various types of excitation signal.Based on a large number of experimental data, an energy transfer model of ultrasonic gas flow-meter without/with the gas flow rate is established through the statistical analysis and curve fitting methods. This model reflects the energy conversion efficiency of ultrasonic gas transducers and the energy attenuation pattern with the flow rate increasing. Furthermore, based on the energy transfer model, a method to enhance the excitation signal energy is proposed under the output power of the transmitting circuit being a finite value so as to extend the turndown ratio of ultrasonic gas flow-meter.A pretreatment method of the ultrasonic echo signal based on the zero-phase filter is proposed. To deal with the time-varying, pit-like echo signal, firstly, put the original signal through a conventional ⅡR filter, secondly, invert the output of the previous step, and then put it through the conventional ⅡR filter again, finally, invert the output of the second step. This not only can filter the noise out, and can minimize the change of the envelope shape caused by the phase delay and other reasons.A new method called "variable threshold tracking the peak of the echo signal" is proposed to pick up the characteristic wave so as to calculate the propagation time. Since the characteristic wave of the echo signal determines the end of the propagation time, thus the pickup of the characteristic wave is the key to achieve the ultrasonic gas flow-meter with the propagation time-difference method. The new method is according to this feature that the normalized values of the peak point and each extreme point in front of the peak point are almost unchanged to select an appropriate ratio, so as to determine the corresponding extreme point and the characteristic wave. This method overcomes the weak anti-interference ability and other shortcomings of the traditional threshold method, and the data computation is small, which is conducive to the real-time implementation on a DSP chip.The random errors during the computation process can be reduced by using the average of a plurality of zero crossings to obtain the propagation time. During the process of using multiple ultrasonic propagation time to calculate the gas flow, the influence of the echo signal fluctuation, which caused by the changes of the velocity distribution, gas pressure and other factors, can be overcome by using the method of sorting and weighted.A signal processing unit of the ultrasonic gas flow-meter which based on a dual core architecture constituted by a FPGA chip and a DSP chip is developed to implement the above mentioned various digital signal processing methods, output high voltage excitation signal meeting the requirements of driving, and realize the amplification, filter and automatic gain control of the echo signal. Then, a measurement of gas flow can be achieved by combining with the two-acoustic-paths four-transducers direct-type meter body provided by Chongqing Chuanyi Automation Co., Ltd. to form a complete digital ultrasonic gas flow-meter.In order to verify the validity of the signal processing method and the system, the ultrasonic gas flow-meter was calibrated in the following three companies:Chongqing academy of metrology and quality inspection, Chongqing science and technology test center and Shanghai Yinuo instrument Co., Ltd.. The calibration results show that the precision meets the requirement for 1-level precision ultrasonic gas flow-meter in JJG1030-2007, achieving the highest prison level of domestic products. The upper limit of the flow reaches 1000 m3/h, beyond the product performance of the domestic ultrasonic gas flow-meter companies and Daniel Company affiliated to the Emerson Process Management, that is, it broadens the turndown ratio of ultrasonic gas flow-meter. |
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