New Eddy Current Sensor Measurement Circuit Design

By studying the theory of equivalent circuit about eddy current probe coil, the conclusion is arrived that the output of sensor is relative closely to the electromagnetic properties of measured targets, which limits the use of eddy current sensor seriously. In order to eliminate the influence of the electromagnetic properties of measured targets on the output of sensor and improve the performance of sensor, the new eddy current measuring circuit is needed. In this thesis, based on the principle of phase-sensitive detection, according to the impedance vector projecting method, phase-sensitive detection technology is adopted to resolve the probe coil impedance signal into the resistant component and reactance component, to eliminate the influence caused by different targets. The multi-modulated method (including phase and amplitude) is adopted to design the measuring circuit independent of measured targets. The concrete work is as follows:1. Design and simulation of measuring circuitThe exciting frequency of probe coil is determined by analyzing the influence of coil impedance on frequency. The measuring circuit with waveform generator, impedance to voltage converter, X/R orthogonal resolver, impedance normalizer, associated with nonlinear compensation is designed to eliminate the influence caused by different targets.2. Design and simulation of non-linear compensation circuitThe Least Square method, associated with Fibonacci method, is adopted to determine the parameters of measuring circuit. The Open-loop method and the Closed-loop method are utilized to compensate the non-linearity characteristics caused by the properties of eddy current and the modulating method, and the 2nd order fitting function method and the 3rd order fitting function method are used to fit the function of non-linear circuit.3. The experiment on the measuring circuitThe 2-layer PCB of measuring circuit is designed by Altium Designer. After completing the PCB, the practical performances of all modules are tested. Finally, the test platform is built to do the functional test on the designed measuring circuit. It is tested by experiment results that the design makes it possible to measure distance under different targets. The conclusions are shown that:(1) The relationship between the resistance component and reactance component under different targets and the same distance tested by experiment is linear, and the fitting lines under different targets and different distances are parallel.(2) The measuring circuit can be used under six kinds of targets, and the maxim relative error is 2.5%. The impedance projecting method can be accepted as a useful method in eliminating the influence caused by measured targets.(3) The efficiency of different compensation methods is different. The Closed-loop method is better than the Open-loop method, and the 3rd order fitting function method is better than the 2nd order fitting function method.