Key Technology Research On Displacement Measurement By Eddy Sensor

As one of the five conventional detection methods in nondestructive testing, eddy current testing is widely applied in fields, such as aerospace, nuclear power and manufacture, etc. However, based on the plate-coil eddy coupling effect, the displacement measuring by eddy sensor is only can be applied to extraction of geometrical characteristic for plane and little curvature curve at present time. In this paper, direct at the demands for measurement of discontinuous and irregular geometrical characteristic by eddy sensor, step is selected as the sample piece; through theory and experiment analysis, the influence of measuring speed to the eddy displacement measuring is studied, and the relation between eddy sensor’s displacement measuring signal and parts’ geometrical characteristic is also researched.An experimental system for eddy current displacement measurement is self-designed. There are two detection models in this system:continuation detection and stepping detection. The hardware of the system is composed of bidimensional AC servo NC workbench, motion control card, data acquisition card, eddy sensor, computer and other peripherals. In VC++, motion control functions and data acquisition functions are guided into dynamic link library; integration of motion control and data acquisition is completed by call these functions, meeting the demands that displacement output should correspond with geometrical position of the parts’ characteristic.Based on the electromagnetic field theory and finite element methods, a simplified model of eddy current testing system is built, and eddy distribution with coil placing in different positions of sample’s edge and step is analyzed. The influence of measuring speed to the eddy displacement measuring accuracy is researched, reaching a conclusion that in bigger measuring speed the measuring signal deviates further from the precise one. Then, this conclusion is verified by the experiment. Moreover, the reasonable range of continuation detection speed is defined by data’s fiducial error and sensor’s linearity.Three test samples with different width, different height and different size are designed; through the experiment analysis, the similarity between eddy sensor displacement measuring signal and parts’ geometrical characteristic is revealed. In different lift-off heights, the eddy sensor’s displacement output rule around edge is studied; the relation curve of lift-off height and point mutation position of measuring signal around edge is obtained by interpolation, realizing the function of edge identification by measuring signal.