Study On The Mathematical Model And Signal Processing Methods Of Ultrasonic Gas Flow Meter System

Compared to other kinds of gas flow meters, ultrasonic gas flow meter contains no moving parts, almost creates no extra pressure drop and has relative high measurement accuracy. All those merits lead to the wide application of ultrasonic gas flow meter in the natural gas flow measurement field. With the development of digital signal processing methods, the measurement accuracy of ultrasonic gas flow meter is improved dramatically, and its development potential in the industry area is huge.Mathematical model of the ultrasonic gas flow meter system and digital signal processing methods are studied in this thesis. The whole mathematical model of ultrasonic gas flow meter is divided into three parts. They are amplitude sub-model, process sub-model, and delay time sub-model. The amplitude sub-model is established with curve fitting method, and reflects the relation between amplitude of ultrasonic echo, gas flow and excitation signal. Thus, it provides an evidence for the selecting of the suitable excitation signal in the ultrasonic gas flow meter. The process sub-model is obtained by using the system identification method, reveals the influence of excitation on the echo shape, and lays foundation for studying the signal processing method. The delay time sub-model is built by utilizing the piecewise linearization method. It can analyze quantitatively the propagation time of the ultrasonic echo under different flow rates. The whole mathematical model of ultrasonic gas flow meter reflects the effects of the excitation signal and gas flow rate on the ultrasonic echo so as to lays the foundation for the choice of excitation signal, studying of the signal processing method and enlarging of the measurement range.Then, signal processing methods are discussed in this thesis. After the features of the ultrasonic echoes are studied, a new method based on variable ratio threshold and zero-crossing method is proposed. In this method, the feature point is determined by the threshold value which is obtained through the peak value being multiplied by the ratio value, and then the corresponding zero crossing points are found with zero-crossing method. Their time points will be used to calculate the propagation time of ultrasonic wave, and then the gas flow will be calculated. As the amplitude of ultrasonic echo fluctuates, the threshold value used to find the feature points is variable. The ratio value used to calculate the threshold value is chosen according to the distribution of the extreme points of the normalized ultrasonic echo signal under the different flow rates, therefore the threshold value can be adjusted according to the application need and the flow rate variation. Moreover, the average value of the time points of zero points obtained by the zero-crossing method is used to calculate the propagation time in the calculation process, it avoids the random errors and then improves the measurement accuracy. Compared to the previous methods, this method has stronger anti-interference ability, and can do better in the expanding of the measurement range of ultrasonic gas flow meter.Finally, the gas flow calibration experiments were carried out to verify the effectiveness of the proposed digital signal processing method and developed ultrasonic gas flow meter. The calibration results show that the measurement accuracy of the ultrasonic gas flow meter is within ± 1.0% when the measurement range is between 30m3/h and 1000m3/h, which verify the validity of the proposed signal processing method and developed ultrasonic gas flow meter.