Signal Processing for Impedance-Sensors

摘要

In 1953 F. Forster presented a method to separate the influence of two parameters on the impedance Z of a coil. He put the coil, applied as a sensor in non-destructive testing, on the specimens surface and was able to eliminate the influence of the distance a on the impedance. By that the various distance a between the sensor and the surface did not influence the measurement of the electrical conductivity σ of the specimen for example. In 1995 Y. Wang and W. J. Becker represented a "function S_a". They used this function to measure the distance a independently of the specimen material. This "function S" connects the real and the imaginary part of the impedance Z. That way you eliminate the influence of two materials on the impedance and reduce the influence of other materials at any distance. They were not forced to put the sensor on the surface of the specimen. From that Flaschke and Trankler developed a procedure for determination of the soil water content. They defined a "function S_a". This function connects the both parts of the admittance Y and minimizes the influence of three or more materials on the determination of the soil water content. The coefficients of the "function S_Ψ" were determined using a least square algorithm. In this paper the method of Wang and Becker is enlarged. A new "function T" connects two functions S at different frequencies. That way you can separate the influence of two and more parameters on the impedance Z of the sensor. You can generalize this procedure and connect the impedances at three and more frequencies for separation of four and more parameters. The conditions result out of the context between the "function S_a" and a method for determining of the concrete cover thickness and the bar diameter.