【作者】 蒲诚；

【导师】 张涛；

【作者基本信息】 天津大学 ， 检测技术与自动化装置， 2005， 硕士

【摘要】 20世纪90年代末,超声波气体流量计正以其独特的优势进入天然气等气体计量现场。与传统流量计(如孔板、涡轮流量计等)相比,超声波流量计具有无可动部件,压损小,测量准确度几乎不受介质温度、压力影响等优点,尤其是在大管径流量测量方面,其优越性更加明显。传播速度差式超声波气体流量计应用最为广泛。由于实际计量中传播时间短、超声波在气体中衰减大,因此超声信号的精确测量是提高流量计整体性能的重点也是难点。本文在研究国内外近年来超声气体流量计的各种信号处理方法基础上,提出了一种新颖的将伪随机码和相关测量技术结合的超声信号处理技术——伪随机码相关测量技术(后简称伪码相关技术)。伪随机码具有优良的双值自相关函数特性,其形状类似冲激函数,而相关测量避免了对微小时间的直接测量。因此,对经过伪随机码调制的发射超声信号和接收超声信号进行时域的相关运算将会得到一个峰值十分尖锐而易于分辨的相关函数,此峰值对应的时间就是超声信号的传播时间,这就是伪码相关技术的原理。文章阐述了这一原理,并详细分析了它与经典的相关流量测量的区别。通过Matlab仿真进一步验证可行性,对编制伪随机码时将要选择的一些重要参数,如采样频率、码元宽度、伪随机序列长度、伪码幅值等进行了仿真实验,定性给出了选择标准。伪码相关技术的实现主要分为三个环节——超声信号的调制编码、捕捉解调以及软件相关算法,本文主要研究了前两部分。针对超声信号的捕捉解调分析了系统辨识、采样插值和延迟锁相环三种方案,在此基础上给出了最优设计;超声信号的调制编码是实验部分的重点,分换能器驱动接收电路实验、换能器基本特性实验和调制编码实验三个步骤完成。针对本课题使用的工作频率为125KHz的发射/接收型压电陶瓷超声换能器,研究了各种性能不同的振荡电路作为其激励源,同时设计了一个结构简单但性能良好的超声信号处理电路;为测试压电换能器的基本特性,进行了激励电压、激励波形、谐振频率等实验;最后在前期实验的基础上完成了压电换能器的移频和调幅编码实验,并提出了调幅编码的方法。更多还原

【Abstract】 At the end of the nineties of last century, ultrasonic gas flow meters set foot in natural gas measurement etc. with its remarkable advantages. Compared with those traditional gas flow meters such as orifice meters and turbine meters etc., ultrasonic flow meters have many merits like non-mobile component, little pressure loss and low sensitivity of medium temperature and pressure, which particularly features when measuring on large diameter pipes.The transit time ultrasonic flow meter is most widely used and the precise detection of ultrasound signal is the keynote of this kind of type due to the rapid transit time and the considerable attenuation of ultrasound in air when operating. Based on a variety of the domestic and foreign status and trends on ultrasonic flow meters’signal process, a novel method that combines the pseudo noise (PN code) code with correlation flow measurement technology, called PN code correlation method (PNC for short), is presented in this dissertation to solve the problem.The PN code has a‘good’ambiguity auto-correlation function like Delta function and correlation flow measurement can avoid direct time counting. So a correlation function shape with a sharp peak that is easy to identify will be obtained if the transmitting and receiving ultrasound signal modulated by the PN code get correlated in time domain and the transit time of the ultrasound signal is just the time value corresponding to the sharp peak. The theory of PNC method is detailed in this dissertation as well as the difference between the classical correlation flow measurement and the PNC method. Further Matlab simulation is carried out to prove its feasibility and meanwhile how to select when coding the parameters of PN code such as sampling frequency, bit size, PN sequence length and PN amplitude etc. is put forward.PNC method consists of three main parts: modulation and coding of ultrasound, capture and demodulation of ultrasound and correlation algorithm, the first two of which are discussed in this paper. For capture and demodulation part, three schemes (System Identification, Sampling and Interpolation and Delay Lock Loop) are analyzed and the final optimum design is given. The modulation and coding of ultrasound part is the key point of the project, which consists of three sub-experiments更多还原