Research On Pseudo Noise Code Correlation Method In Ultrasonic Gas Flow Meters

At the end of the nineties of last century, ultrasonic gas flow meters set foot in natural gas measurement etc. with its remarkable advantages. Compared with those traditional gas flow meters such as orifice meters and turbine meters etc., ultrasonic flow meters have many merits like non-mobile component, little pressure loss and low sensitivity of medium temperature and pressure, which particularly features when measuring on large diameter pipes.The transit time ultrasonic flow meter is most widely used and the precise detection of ultrasound signal is the keynote of this kind of type due to the rapid transit time and the considerable attenuation of ultrasound in air when operating. Based on a variety of the domestic and foreign status and trends on ultrasonic flow meters'signal process, a novel method that combines the pseudo noise (PN code) code with correlation flow measurement technology, called PN code correlation method (PNC for short), is presented in this dissertation to solve the problem.The PN code has a'good'ambiguity auto-correlation function like Delta function and correlation flow measurement can avoid direct time counting. So a correlation function shape with a sharp peak that is easy to identify will be obtained if the transmitting and receiving ultrasound signal modulated by the PN code get correlated in time domain and the transit time of the ultrasound signal is just the time value corresponding to the sharp peak. The theory of PNC method is detailed in this dissertation as well as the difference between the classical correlation flow measurement and the PNC method. Further Matlab simulation is carried out to prove its feasibility and meanwhile how to select when coding the parameters of PN code such as sampling frequency, bit size, PN sequence length and PN amplitude etc. is put forward.PNC method consists of three main parts: modulation and coding of ultrasound, capture and demodulation of ultrasound and correlation algorithm, the first two of which are discussed in this paper. For capture and demodulation part, three schemes (System Identification, Sampling and Interpolation and Delay Lock Loop) are analyzed and the final optimum design is given. The modulation and coding of ultrasound part is the key point of the project, which consists of three sub-experiments...