【作者】 吕方；

【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， 硕士

【摘要】 超声波流量计作为无压损、易于安装维护的仪表在工业领域得到广泛的应用。尤其在天然气计量领域,虽然孔板流量计仍是最主要的手段,但超声波流量计具有无可动部件、无压损、测量精确度高等优点,它的潜力和后发优势是很明显的。随着相关标准的发布和实用问题的探索,超声波流量计一定会在这一领域广泛推广应用。本文将互相关时差法与脉冲压缩编码信号应用于超声流量测量系统的信号处理中。互相关时差法结合了传统时差法和相关法,继承了它们的优点,又克服了时差法触发时刻不容易确定、时间差难以直接精确测量,相关法被测对象不确定性、易受驻波影响等缺点。为了提高系统高精度,课题将雷达系统中常用的脉冲压缩信号应用于气体超声流量测量系统中。由于脉冲压缩信号具有足够大的脉冲宽度和带宽,因此信号能同时具有大的作用距离和高的距离分辨力,与原先使用的调幅信号和调频信号相比,脉冲压缩编码信号具有明显的优势。课题选用二进制移相键控编码信号以及线性调频信号进行气体超声流量测量系统的研究。硬件电路主要基于FPGA完成。电路主要包括发射模块、接收模块、存储模块以及一些辅助模块,可完成超声波的发射、接收、采样、存储以及上传等功能。其中发射模块基于DDS(直接数字合成)原理,产生不同带宽与时宽的脉冲压缩信号。课题实现了气体超声流量测量实验平台。数据采集使用NI6251数据采集卡,采集出的数据直接传送至PC机。上位机方面,课题基于LabVIEW软件完成上位机界面,上位机利用正交解调的计算方式实时在线处理数据,完成相关峰值以及延迟时间的计算。当实验平台完成后,课题对实验平台的性能进行了测试,测试结果证明实验平台能够正常工作,相关运算峰值位置的变化能够反映流量的大趋势。本文对测量误差进行了简要分析,并且提出了改进方案。更多还原

【Abstract】 Ultrasonic flowmeter is applied widely in industrial measurement fields in that it has no pressure loss and can be installed and maintained with ease. In the field of natural gas measurement, although orifice meter is still the uppermost means, ultrasonic flowmeter has potential advantages for its merits like without mobile component, without pressure loss and high precision in measurement. With relevant standards promulgated and practicality explored, ultrasonic flowmeter will be extended applied in this filed in the future.The correlation time-difference method and the pulse compression signal are applied to the ultrasonic flow measurement system. 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. The pulse compression signal is applied to ultrasonic flow measurement system. As Pulse Compression signal has wide bandwidth in both time domain and frequency domain, it has high resolution in ultrasonic ranging and long propagation distance. Compared with AM signal and FM signal, the Pulse Compression signal has obvious advantages. This research chooses 2PSK and LFM signal for the research of ultrasonic flow measurement system.This research completes hardware circuit of ultrasonic flow measurement system based on FPGA. The hardware circuit includes transmitter module, receiver module, storage module and some assistant modules. It has the function of transmitting, receiving, sampling and storing the signals. Transmitter module which based on DDS can generate Pulse Compression signal with different time length and frequency bandwidth.Experimental platform is also completed. In the platform, signals are sampled by NI6251 and the results of sampling are sent to PC directly. This research also completes man-machine interface based on LabVIEW. The host computer adopts the method of quadrature demodulation to process real-time data and has the function of calculating the peak value and delay time. When the platform is completed, some tests are done for it. According to the results, the platform can operate normally, and the change of the position of peak value can reflect the general trend of change in flow rate. At the end of the paper, the causes of error are analyzed and some suggestions are given to the readers.更多还原