【作者】 郑丹丹；

【导师】 张涛；

【作者基本信息】 天津大学 ， 检测技术与自动化装置， 2009， 博士

【摘要】 涡街流量传感器是一种比较新型的速度式流量仪表,近30年发展迅速。因其具有可靠性高、压力损失小、量程比宽等优点,被广泛应用于化工、石油、冶金、轻工、食品等流程工业。对于目前工业现场常用的应力式涡街流量传感器,受其工作原理制约,在进行流量测量时易受到外界各种干扰,从而影响其测量精度及测量范围,该问题在小流量测量时尤为突出。本论文分别从数字信号处理方法、传感器结构优化——压电探头位置研究、纯硬件结构的仪表系数非线性修正几方面,对涡街流量传感器在小流量测量时的性能进行全面优化和改进。主要完成了以下工作:基于Hilbert-huang变换(HHT)的数字信号处理方法改进。通过对小流量时涡街信号进行时域和频域分析,总结有用信号、噪声的特征及形式。针对小流量时的信号特点,对HHT经典方法进行改进,使其更加适应涡街小信号去噪。通过与经典数字信号处理方法FFT的实验对比,验证了该方法在精确提取涡街有用信号、扩展测量下限方面的实用性。对目前工业中常用的应力式涡街流量传感器探头位置进行研究。分别在二维和三维涡街流场中对三种尺寸的梯形柱旋涡发生体进行实验,通过分析压力、速度信号在不同位置时的信号强度、信噪比、线性度等指标,最终分别确定了在二维、三维流场中压电探头的最佳检测位置,指出应将探头置于发生体下游涡的成熟区域,并推理出估算公式,揭示了影响该位置的因素。通过与涡街流量传感器原设计进行对比实验表明,不仅测量下限有所降低,测量精度也有提高。并借助流场数值仿真实验和理论分析对以下三个问题进行了详细讨论:二维、三维涡街流场差异;基于涡量输运方程的涡街流场研究;梯形柱与圆柱旋涡发生体流场差异。这些问题的研究与分析有助于更全面深刻地认识涡的产生、脱落机理、尾迹特征以及涡街流场的速度压力分布。提出基于CPLD的硬件式仪表系数非线性修正方法。利用三次样条插值法对小流量时超出线性度范围的仪表系数进行逼近,CPLD程序中嵌入查找表对仪表系数进行非线性修正,通过对查找表的优化,不仅节约了资源而且降低了功耗。实验证明,修正后的涡街流量计可以在保证精度的基础上有效地扩展测量范围。该方法对于存在仪表系数非线性问题的其他流量仪表也同样适用,具有良好的推广性。更多还原

【Abstract】 Vortex flow sensor is a kind of novel velocity-type flow instrument which has been developed fast in recent thirty years. It has many advantages such as low pressure loss, wide measurement range, stabilization and so on. Therefore, it is widely used in chemical industry, petroleum, metallurgy, food industry and other flow industry. To the piezoelectric vortex flow sensor that is commonly used in industry field, the measurement is easily disturbed by any external inferences because of its operation principle, which affects the measurement precision and range of vortex flow sensor. The problem is more important in low flowrate measurement. The digital signal processing method, structural optimization on vortex flow sensor—the location of piezoelectric probe and K-factor modified method with hardware implementation are investigated respectively in the thesis. The aim is to improve the measurement performance of vortex flow sensor at low flowrates.The mainly research works of this thesis are listed in the following.The improvement method of Hilbert-huang transform(HHT). Firstly, the characteristics of vortex signal at low flowrate are analyzed in time-domain and frequency-domain respectively. The source and representation of noise components are summarized. Secondly, according to these characteristics, the HHT method is improved in order to perform its better denoise function in weak vortex signal detection. Finally, by experimental comparison with the classic digital signal processing method FFT, it is indicated that the improved HHT method can not only extract the useful vortex signal exactly but also expand the low limit of measurement.Research on location of the piezoelectric probe in vortex flow sensor. Experiments are carried out in two-dimensional and three-dimensional vortex flow fields respectively with three trapezoidal cylinders. The optimum detection positions of the probe in the two flow fields are finally determined by analysis on signal intensity of pressure and velocity signals, signal-to-noise ratio, linearity and so on. Furthermore, a formula for the optimum position estimation is deduced, which discloses influencing factors on it. By experimental contrast to the original design of the vortex flow sensor, it is illustrated that the improved design has better measurement precision and lower limit of measurement. Moreover, three problems appeared in experiments are mainly discussed by numerical simulations and theoretical analysis: the differences of two-dimensional and three-dimensional vortex flow fields; study of vortex flow based on vorticity-transport equation; the variance of flow fields with trapezoidal cylinder and circular cylinder, those of which are useful for us to get more knowledge in vortex flow field, such as generation and shedding of vortices, characteristics of wake, distribution of velocity and pressure and so on.Nonlinearity modified method of K-factor based on CPLD, which is implemented by hardware. The spline interpolation method is adopted to approximate relation curve of flowrate and K-factor. The modified algorithm is realized by look-up-table(LUT) which is embedded in CPLD. Design and optimization on the LUT is paid more attention, becasure it is not only reducing the power of system but also saving the CPLD resources. Through experiments, it is indicated that the measurement range is expanded within accuracy level by vortex flowmeter with nonlinearity modified. This method is applicable in any other flowmeters which have the same nolinearity problem of K-factor.更多还原