【作者】 陈学永；

【导师】 张涛；

【作者基本信息】 天津大学 ， 检测技术与自动化装置， 2004， 硕士

【摘要】 随着国民经济的迅速发展,冶金、化工等各部门对气体大流量的精确测量要求日益迫切,以达到进一步实现节能、安全生产及控制污染等目的。为了满足气体流量测量不断提高的要求,超声波技术被应用于气体流量检测,于是国际上就出现了一种新兴的气体流量仪表——超声波气体流量计。超声波气体流量计是通过测量超声波脉冲沿顺、逆流两个方向上声传播时间不同来测量气体的流速和流量的新技术。由于它对流体无阻力,无压力损失,受流体物理性质限制少,以及使用简单等特点,备受业界关注,具有很大的前景。根据本课题的特点,文中提出了适用于超声波气体流量计的高精度数字计时电路设计,计时精度可以达到0.25%,通过FPGA设计实现;系统硬件电路为微控制器+FPGA结构,微控制器是整个系统的控制核心,FPGA内部集成了系统中的大部分数字电路,这种结构设计思想大大简化了系统硬件电路的复杂性,增强了系统的稳定性与可靠性;本系统的数字电路采用柔性化设计,具有良好的硬件可升级性,改变FPGA配置电路中存储器中的程序即可改变系统中的部分硬件电路。本系统使用的传感器为发射/接收型超声波传感器,发射超声波的中心频率为125KHz。超声波传感器采用单声道方式,Z型安装于DN150管道上,声道与气体流动方向之间的夹角为45o。系统通过频差法方式对管道内气体平均流速进行测量,基本上消除了温度变化对系统流速测量带来的影响。模拟电路主要用于超声波发射、接收与处理,是硬件电路设计的重要组成部分。模拟电路整体上采用模块化设计,每个模块需要实现各自的功能,有很强的抗干扰能力。其次,每个模块与相邻模块之间有可靠的接口设计,保证模拟信号的可靠传输。系统调试成功后,为了检测其测量精度,在天津大学过程检测与控制实验室的气体流量实验装置上进行试验测试。测试结果为低流速时系统检测精度较低,随着流速的增加,检测精度不断提高。综合测试结果满足系统设计要求,且运行稳定可靠。更多还原

【Abstract】 With the development of national economy, the accurate measurement of largeflow rate gas becomes more and more necessary in metallurgy and chemicalengineering. The purpose of accurate measurement is energy conservation, safety inproduction, pollution control and so on. Because of more strict requirements aboutthe measurement of gas flow rate, ultrasonic technology has been used in gas flowrate measurement. Gradually, a new kind of gas flow meter is produced in the world,which is ultrasonic gas flow meter. The ultrasonic gas flow meter is a kind of latesttechnology, which measures gas current rate and flow rate by measuring two types ofpropagation times of ultrasonic pulse. The first time is what ultrasonic pulse costs totransmit in the direction of forward flow. The second one is what ultrasonic pulsecosts to transmit in the direction of opposite flow. The ultrasonic gas flow meterattracts a lot of attention and it’s blueprint is very beautiful, because it has fewrelation with physical feature of gas fluid and there is no resisting force and pressureloss in the pipe of flow meter.Based on the system feature, the high-accurate digital counter circuit has beendesigned in the topic, which is the same with ultrasonic gas flow meter and isdesigned in the FPGA. The errors of digital counter is 0.25%. The hardware structureof the system is microcontroller + FPGA. The microcontroller is control center, andmost of digital circuit has been designed in FPGA. This kind of design structuremakes the circuit more simple, and makes the system more dependable. The digitalcircuit design belongs to flexible design, and can be upgraded more easily. If youchange the program in the memory in the configuration circuit of FPGA, some partsof hardware can be upgraded.The ultrasonic transducer that is used in the system can transmit and receiveultrasonic wave. The center frequency of the ultrasonic transducer used in the systemis 125KHz. The transducer is fixed in the pipe wall of DN150, and the mode offixing are single track and Z type. The angle between sound track direction and flowdirection of gas is 45o. The frequency-difference method is used in the measurementof gas current rate, which can avoid the effect of temperature.Simulative circuit is responsible for the emission, reception and procession ofthe ultrasonic signal. It is very important in the system, one character of which ismodularization design. The modularization circuit can not only implement thesystem function, but also have steady interface with other modularizations.After system debugging, test has been completed in gas flow rate experimentaldevice in the process measurement and control laboratory in Tianjin University.When the flow rate is low, the uncertainty is high. With the increment of flow rate,the uncertainty is lower and lower. The test conclusion accords with systemrequirement.更多还原