Study On Signal Processing Methods Of Digital Vortex Flowmeter For Rejecting Strong Vibration Disturbance And Implementation Of Low Power

The vortex flowmeter is widely used to measure flow-rate in power plants, etc. because it has many advantages, such as no moving parts, being applied for the measurement of liquid,gas and steam, lower pressure loss, high measurement precision, large turn-down ratio. However, two key problems in the applications have remained to be solved. One is how to realize low power and two-wires when digital signal processing methods are adopted, and the other is how to obtain the accurate vortex flow frequency under strong disturbance. Digital signal processing methods have many advantages, e.g. strong anti-interference ability, high precision and large turn-down ratio, and they are implemented by DSP chips commonly. The power consumption of DSP chip is relatively high, and its operating current is more than 100mA. This cannot meet the requirement of low-power and two-wires. The meters, however, with low power and two-wires are needed in many process industries. At the same time, the vortex flowmeter is often influenced by vibration disturbance because of its operating principle. When the vibration disturbance is strong, the frequency of vibration disturbance will be regarded as vortex flow frequency, which affects the measurement accuracy. Therefore, it is significant to study on how to obtain the accurate vortex flow frequency under strong disturbance.The changing pattern of amplitude of vortex flow signal was analyzed, different characteristics of bandwidth between vortex flow signal and mechanical vibration noise were studied, and the output signal model of vortex flow sensor was built up so as to study digital signal processing methods for rejecting strong disturbance.A MSP430-based digital signal processing system was developed for the vortex flowmeter with low power and two-wires. In the process of development, some key technologies were improved, such as real number FFT algorithm, stability of measurement results and response speed of system, correction of meter factor, compensation of temperature and pressure, 4~20mA current output, and pulse output. Calibration experiments, including gas flow-rate and water flow-rate for the MSP430-based vortex flowmeter with low power and two-wires were performed in Zhejiang Diyuan Instrument Co. Ltd. The experimental results show that the system has strong rejecting-disturbance ability, high measurement precision and large turn-down ratio, and at the same time, the meter runs in the mode of low power and two-wires.Aiming at the vortex flowmeter with single sensor, two kings of digital signal processing methods were studied. One is algorithm of power extremum calculation, and the other is digital filter combining with autocorrelation algorithm. The algorithm of power extremum calculation has improved the conventional adaptive FIR wiener filter, and determines the vortex flow frequency by calculating power extremum. The digital filter combining with autocorrelation algorithm filters and computes autocorrelation, and calculates the vortex flow frequency according to the attenuation extent of autocorrelation. At the same time, a digital vortex flowmeter for rejecting strong disturbance basing single sensor was developed. The simulation and experimental results show that digital vortex flowmeter for rejecting strong disturbance basing single sensor can obtain the accurate vortex flow frequency even if the power of noise is larger than that of the vortex flow signal.Aiming at the vortex flowmeter with two sensors, the validity of adaptive noise-suppression algorithm was validated by simulations. At the same time, another digital signal processing method for rejecting strong disturbance was proposed, that was frequency-domain substation combining with the frequency-variance calculation. The specific algorithm was chosen and switched according to the situations of field. When the peak number of amplitude spectrum of flow sensor are different from that of vibration sensor, the frequency-domain subtraction algorithm will be adopted; when the peak number of amplitude spectrum of flow sensor is the same as that of vibration sensor, the frequency-variance calculation algorithm will be employed. At the same time, digital vortex flowmeter for rejecting strong disturbance basing two sensors was developed. The simulation and experimental results show that digital vortex flowmeter for rejecting strong disturbance basing two sensors can obtain the accurate vortex flow frequency even if the power of noise is larger than that of the vortex flow signal.