Study And Experiments On Signal Processing Method Of Guided Wave Radar Level Gauge

Guided wave radar level gauge (GWRLG) is used to measure level of materials in storage vessels. While foreign productions have excellent performance, there exist many problems in the domestic productions, such as low measurement accuracy, large blind area and small measuring range. However, algorithm details of foreign productions are not disclosed.Working principles and two key technologies--time domain reflectometry (TDR) and equivalent time sampling (ETS) that are applied in the GWRLG are discussed in detail. Besides, the classification and implementation of ETS are introduced, and3methods of ETS are compared to get a full understanding of the feature of the compound equivalent time sampling. The composition and the function of each part of the GWRLG system are clearly explained. Different signal processing methods are put forward for materials with different relative dielectric constant.When measuring materials with large relative dielectric constant, the maximum peak point in the echo profile is obtained directly. A2-order Lagrange interpolation is applied to modify the peak point. According to this, a calibration experiment is done to determine the relationship between the position of maximum peak point and the corresponding level. A polynomial fitting or1-order Lagrange interpolation is employed to process the calibration results and a function expression is acquired. Based on this expression, a simulation of the signal processing algorithm on MATLAB is made, and software is built on the MSP430microprocessor system developed by our laboratory to realize the measurement of the material level. An experiment about water level measurement is carried out in Technical Center of Chongqing Chuanyi Automation Co. Ltd., whose result shows that for probe of72.4centimeters long, the measuring range is broadened from30-57cm to20-72cm and the measuring error can be controlled within±0.5cm, which is reduced from±1cm.When measuring materials with small relative dielectric constant, as the echo signal of the material surface is weak, the echo profile of the empty tank is considered. An echo profile of the empty tank is obtained and a certain amount of modifications is made. A subtraction of the modified echo profile of the empty tank from the echo profile obtained during normal measurement conditions is required. Then the maximum peak point in the echo profile resulted from the subtraction is obtained. A2-order Lagrange interpolation is applied to modify the peak point. According to this, a calibration experiment is done to determine the relationship between the modified position of the maximum peak point and the corresponding level. A polynomial fitting or1-order Lagrange interpolation is employed to process the calibration results and a function expression is acquired. Based on this expression, the material level measurement is realized. An experiment using baffle imitating level is carried out in the laboratory, and the result shows that for probe of2.5meters long, the measuring range can be achieved to the end of the probe from the20cm position under the flange surface, and the measuring error can be controlled within±0.5cm, which is reduced from±1cm.