Research On Time-difference Ultrasonic Gas Flowmeters Based On Pseudo Noise Code Correlation

Ultrasonic flowmeter is applied widely in industrial measurement fileds for its advantages of non-contact and installation and maintenance with ease. In the filed of natural gas measurement, though orifice meter is still the uppermost means, ultrasonic flowmeters have evident potential and backwardness advantages with the merits like without mobile component, without pressure loss and high precision in measurement. With relevant standards promulgated and practicality explored, ultrasonic flowmeters must be extended applied at this filed in the future.Correlation time-difference method combines traditional time-difference method with the correlation method, and inherits their merits. The time-difference method's springing time is difficult to be fixed, and the difference time is difficult to measure exactly. The correlation method's object is uncertain, and it is easy to be influenced by standing wave. The new correlation time-difference method overcomes the shortcomings mentioned above. In order to settle the effects by the finite measuring time, Pseudo Noise (PN) code is introduced to the correlation time-difference method. Its fine diadic auto-correlation and periodic property make it easy to measure minute time. By theory analyse, it is obtained how to select the parameters such as sampling frequency, bit size, PN sequence and period number when coding PN code.Hardware system is composed of the emission, reception and procession of the ultrasonic signal. The emission part is designed two schemes: digital random waveform generator and direct digital synthesizer (DDS). They both can generate PN signal of amplitude shift keying (ASK) or frequency shift keying (FSK) directly, and have each merits, defects, and application situation. The reception and procession part is responsible to demodulate PN signal from the ultrasonic signal after amplifying and filtering, and restrict the amplitude in a range. The automatic gain control (AGC) amplifier is controlled by field programmable gate array (FPGA).Using the digital random wave generator, carry through the coding characteristic experiment on the SensComp ultrasonic transceiver piezoelectric transducer, found that the transducer is unaptly to Frequency Shift Modulation, and the appropriate parameters are needed to process Amplitude Shift Modulation. After system debugging, the PN code correlation time-difference ultrasonic gas flowmeter was tested on gas flow rate experimental device at the Process Detect and Control laboratory in Tianjin University. The result accorded with the trend that the difference time and the flow rate is linear relation. And with the increment of flow rate, the error was lower and lower. The error resources were analyzed detailedly, and the further schemes were araised to settle problems and perfect design.