【作者】 张玉艳；

【导师】 王莉田； 侯培国；

【作者基本信息】 燕山大学 ， 测试计量技术及仪器， 2002， 硕士

【摘要】 原油储罐是油田存贮油品的重要设备，储罐液位是原油集输工艺的重要参数之一，精确测量液位是贮运系统信息管理的重要依据。但测量现场环境恶劣，易燃易爆，监测难度大。因此，实现油罐液位的高精度自动化计量具有十分重要的意义。 本文针对特殊王况下的测量要求，提出一种以差动电容为敏感元件的光推动油罐液位测量系统的设计方案。基于静压法测量液位的原理，利用光功率推动测量探头工作，用光纤将被测信息传输至远离测量现场的二次仪表端进行处理。从根本上解决了防燃防爆问题，使测量系统具有本质安全性。 采用三只差动电容作为压力的敏感元件，并采用热敏电阻作为温度敏感元件，用于补偿温度变化对电容传感器测量精度的影响。建立了液位测量的数学模型，阐述了差动电容、热敏电阻的工作特性，推导出压力—电容的转换关系，为系统测量和标定提供了理论依据。 结合基本光电器件的特性分析，合理选择了光纤、光源和光探测器，构成光推动通道和光纤传输通道。设计了光源LD的驱动电路和光电转换器，为测量探头提供稳定、可靠的工作电压。测量探头低功耗工作是实现光推动的关键。因此采用全CMOS数字电路分别实现电容和电阻的脉冲宽度调制—脉冲位置调制，获得表征被测信息的脉冲位置信号，再进行电复合和光脉冲发射。通过光纤将光脉冲信号传输到二次仪表端。 采用高性能微控制器PIC16C65A，首次实现对脉位信号直接解调。利用CCP部件捕捉被测脉冲，通过软硬件设计有效地实现了各路信号的识别、脉宽测量、比值处理和液位、温度的计算及显示、报警等功能。 进行了光推动差动电容式油罐液位测量系统的相关实验，如压力—电容关系，液位、温度的标定等。结果表明，本系统具有测量精度高、稳定性好、抗强电磁干扰等优点。更多还原

【Abstract】 Crude oil tank is important equipment for storing oil in oil field. Oil tank liquid level is one of the important parameters of storing and transmitting crude oil technology. Precise liquid level measurement is the dependence of managing. But the measurement site is inflammable and explosive, it is difficult to detect in such severe site. Therefore high precision and automatic measurement of the oil liquid level play a great role in oil industry.Considering the requirements of the measurement site under specific working condition, this paper presents a scheme of oil tank liquid level measurement system based on differential capacitance sensing. In view of hydrostatic tank gauging method, the probe is optically powered, and then the information is transmitted through optical fiber to Micro-control unit far from the site. So the inflammable and explosive problem can be solved, the system is safe in nature.Three differential capacitance sensors are used to measure the applied pressure, and one thermistor is used to compensate the effect that the temperature changes have on measuring precision of the capacitance sensors. The mathematical model of measuring liquid level is established, the characteristics of differential capacitance and thermistor are conveyed, and the relationship between pressure and capacitance is deduced in the paper. The theory above is the basis of the system measurement and calibration.By analyzing the characteristics of basic photo-electric elements, optical fiber, light source and photo detectors can be proper chosen which construct the optically powered channel and fiber-optic transmission channel. The design of driving circuit of LD and photo detector ensures that stable and reliable operation voltage of the probe can be given. The design of micro power consumption probe is the key technique in the optically powered system. The probe uses complete CMOS circuit to realize pulse-width modulation to pulse-position modulation of differential capacitance and thermistor. The multiplex electric signal can be converted to light pulse and then betransmitted to micro control unit (MCU) through a single fiber.The pulse signal can be demodulated directly by high performance MCU. The capture and measurement of pulse-width and ratio processing of measured signal, parameters calculation and display, warning are accomplished by hardware and software.The experiments of oil tank liquid level measurement system based on differential capacitance sensing are carried out, such as experiments of pressure-capacitance relationship, calibration of temperature and liquid level, etc. The results prove that the system has high accuracy and stability, and is immune to electro-magnetic interference.更多还原