Research On Eddy Current Based On Cable Eccentricity On-Line Measurement

The online measurement and control of the eccentricity is important for the cable production. The cable eccentricity not only causes the huge waste of the material but also affects the performance parameters of the cable products. This thesis takes the development of the eddy current measurement system for cable eccentricity as background. Combining the basic theory such as similarity theory and electromagnetic field and eddy current non-destructive testing theory with the practice of online measurement for cable eccentricity, the theoretical and technological problems about it are deeply studied. The study of this subject has academic and practical value. The history and current status of domestic and overseas study about cable eccentricity online measurement technologies are reviewed. The principles and characteristics of these technologies are summarized. Based on that, eddy current tomography has been proposed. Aiming at it, the scientific and key technological problems to be solved are extracted. The whole system is composed of four parts: eddy current sensor, rotary machine organization, data collection system and PC. The sensor used in this system is designed according to the eddy current measuring distance principle. Due to the infinite length of the measured cable, the physical field model for cable eccentricity measurement is an open boundary 3D eddy current electromagnetic field problem in which more than one kinds of medium are infinite. The analysis and calculation of this physical field model haven't been reported up to now. Based on the correlative research, the 3D physical field model is founded. The field domain equations and interface continuity conditions are given. The potential variable and criterion adopted in the eddy current and source and nonconductive region are determined. The reasonable processing method of the boundary condition for 3D open boundary field is presented, and the mathematic model of FEM calculation is founded. The solution method for large-scale linear finite element discretization equations is given based on the analysis of various methods. By researching the effect of cut distance to dummy boundary on numerical calculation results, the cut distance that meet the need of engineering calculation is obtained. By numerical simulation of 3D eddy current field, the distribution nephogram and vector distribution diagram of magnetic induction intensity at typical positions created as the eccentricity measurement probe rotates round the cable are drew. The 3D color distribution diagrams of eddy current density are drew too. The feasibility of eddy current scan method and the effect of different structure rotary probe for eccentricity measurement are studied. Although FEM is instructive, it is hard to achieve the goal only with it during the research on the probe optimization because of too many times computations of 3D eddy current field to be needed. The similarity theory is introduced to the design of the sensor probe in this thesis. According to qualifications of phenomenon similarity, necessary and sufficient conditions are obtained by similarity conversion of the field model for cable eccentricity measurement, and the equations of similarity index are deduced. The similarity index method based on the parameters of the finite element character model is presented, which can reduce the times of computations for 3D eddy current field remarkably. The relations of eddy current loss with turns of excitation coil and current density, the optimization match of the excitation signal frequency and the size of sensor probe and the size of cable, and the optimization match of the size of the probe and the cable diameter are studied. The match relation of the excitation frequency with the cable core diameter, and the match relation of the structure size of the sensor with the insulation thickness are provided. By the transient simulation and experiment, the frequency characteristic and stability of the oscillating circuit of the eddy current sensor is analyzed and studied. The main parameters relations of the oscillating circuit are obtained. By the mutual certification of FEM and the similarity index method based on the finite element character model parameter, and the comparison of the calculation example with experiment, the correctness and feasibility of the calculation method presented in this thesis are proved. The whole design of the device for cable eccentricity online measurement is achieved. The hardware circuit of the data collector based on P89C51RD2 is designed which includes signal magnifying circuit, amplitude measuring circuit, filtering circuit, A/D converting circuit, position detecting unit and communication interface circuit. The flow charts of subprograms are provided. The real-time space-time separating and describing methods of the eccentricity signal are given. By using the impulse start signal of the position detecting unit as the accurate time synchronous signal, the circumferential signal sequence fulfil the two preconditions of accumulation average filtering method. So the practical and effective digital signal processing method is obtained. The subprograms of the superior system are designed with Visual Basic language, and the design methods of main subprograms are expatiated.