Correlation Ultrasonic Flowmeter Secondary Instrument

Flowmeter measurement by using method of cross-correlation will produce result by calculating statistically the similarity of two random fluid signals sensed by ultrasonic transducers in cross-correlation function. It has the advantages of high reliability; no moving components ; no pressure drop ; no interruption of fluid field. Its standout advantage can be seen in the measuring two-phase or three-phase and difficult fluid flow. It can also be used to measure small diameter, or low velocity, dramatic interferential fluid flow.When cross-correlation measuring technology is used to measure fluid flow, there will include several research fields: (a) In order to extract fluid signals, it is needed to research on transducers and circuit of driving and receiving ultrasonic signals; (b) Research on the basic principal of fluid flow ; (c) Research on on-line cross-correlators ; (d) Research on calibrating equipment; (e) Research on integrated qualification and performance of the whole testing system.This paper focused on the circuit of extracting and processing fluid signals.Firstly, this paper introduced the history, present status and trend of flowmeters. The theory and characters of ultrasonic flowmeters was explained. Especially introducing world-wide research status cross-correlation flowmeters, the paper brought out the main task of this project, which was researching on an ultrasonic gas flow measurement circuit system based on the cross-correlation theory and making it feasible in measuring fluid flow.Secondly, this paper explained the theory of cross-correlation measuring fluid flow in details. Having studied the characters of ultrasonic, it chose the appropriate air ultrasonic transducers for measuring.Thirdly, this paper focused on the circuit hardware of the system. A circuit of driving and receiving ultrasonic was designed. It is important to accurately extract fluid flow signals from ultrasonic signals, so the method of envelope demodulation was adopt. And the fluid signals were sent to an A/D converter, where the fluid signals were acquired and converted from analog signals to digital signals. Then the kernel of the hardware circuit, DSP obtained and processed the digital fluid signals.In the part of software of the circuit, this paper made a comparison among three kinds of algorithms and calculating the cross-correlation in frequency domain to decrease the amount of calculation without losing fluid flow information. A parabola interpolation was applied to reduce the discrimination error.Through the experiment of system, it was necessary to test the reliability of transducers and circuit. After that, fluid flow with different velocity was measured.The results well matched the theory. Factors that affects measuring accuracy were analyzed and a more complex mathematicmodel of fluid flow cross-correlation were brought up for further research.Finally, this paper put up several problems rising from this project and their resolutions as well as personal opinions about the points which are waiting for further research and improvement.