Research On Signal Processing Method Of Gas Ultrasonic Flowmeter Based On Coded Excitation

Natural gas is a high-quality,efficient and clean fossil energy source.Compared with oil and coal,it has obvious advantages in terms of safety,emissions and mining costs.It has been widely used in various fields,and its using rate accounts for a large proportion of the global energy structure and continues to rise.Compared with the traditional flowmeter,the ultrasonic flowmeter has the advantages of high measurement accuracy,wide range ratio,low maintenance cost and small airflow resistance,which has caused great concern in the natural gas industry.The existing time-difference gas ultrasonic flowmeter uses threshold triggering and dynamic selecting zero-crossing to detect ultrasonic signal transit time,but under complicated and severe working conditions,it makes some serious shortcomings such as unreliable measurement result,excessively dependent on transducer performance and difficult cycle-skip problem.For this reason,this paper proposes a transit time measurement method based on coded excitation and characteristic waveform detection,and studies the reliability,stability and low power consumption of the ultrasonic flowmeter.The specific content is as follows:(1)Detailed analysising of the unreliability of traditional threshold method applied in complex conditions.This paper start comprehensive signal testing and cycle-skip exploration in different environments of temperature,pressure and flow.It is found that the root cause of the continuous cycle-skip problem in the threshold detection method,that is the gas medium,the flow rate and the temperature all change the performance parameters of the transducer,resulting in a large change in the envelope shape of the echo signal.(2)Coded excitation scheme design and theoretical analysis.According to the oscillation characteristics of the transducer,this paper proposed one "residual oscillation"phenomenon at the excitation end,and designed the "?m+?n" amplitude-phase modulation excitation pulse to obtain the characteristic receiving waveform.Then the simulation and analysis of the equivalent circuit model of transducer transceiver system is constructed by simulink model,at the same time experimental circuit is setted up for signal detection.Eventually,the specific patameters of the "?m+?n" amplitude-phase modulation pulse are determined.Then this paper design some experiments to varify the feasibility of the coded excitation scheme under different working conditions.Results show that if the excitation frequency is close to the resonant frequency of the transducer,the echo signal will inevitably produce stable and reliable coding feature.(3)Coded excitation circuit design.Amplitude-phase modulation pulse is modulated by use ePWM model in DSP,interrupt control function and hardware components such as analog switch chip,operational amplifier,etc.Transducer receiving circuit design.Ensure that the ADC module in DSP can acquire received signal data accuratery and undistored.Then characteristic pulse recognition algorithm and dynamic picking zero-crossing algorithm are prosed and used.(4)Coded excitation scheme system design.MSP430 is used as the host to build the main logic skeleton of the flowmeter and DSP acts as the slave to realize the functions of excitation pulse modulation,echo signal detection,signal frequency verification,etc.The host calls the function modules of the slave and achieves low power consumption through the GPIO peripheral port or serial communication model.Eventually,the working sequence of dual-core modules is regulated and a complete flow detection process of coded excitation scheme is designed.(5)Performance optimization for coded excitation scheme.The real-time characteristic frequency of the transducer is obtained by the echo signal frequency detection method,and at the next detection the excitation square wave pulse width is updated to ensure that the excitation frequency at any temperature is close to the transducer resonance frequency.This has effectively enhanced the temperature of the time-difference gas ultrasonic flowmeter.(6)Real-flow calibration test for coded excitation scheme.This paper designs the experimental scheme in accordance with the requirements of the national standard for gas flowmeters of 1.0-level accuracy.Under the installation conditions of the three front pipe sections,the verification results of the DN50 flowmeter have reached the accuracy requirements of the 1.0-level flowmeter.