A Planar Eddy Current Sensor With Koch Differential Excitation In Series Topology

With the requirement of function,the shape of mechanical parts becomes more and more complex,which brings new challenges of the nondestructive testing for such parts.Flexible planar eddy current sensors can be customized according to the shape of the tested components or can change their shape with the surfaces of the tested component.It is expected to become a new technology for detecting surface and near-surface cracks of such mechanical parts.In this paper,through the finite element analysis and experimental method,the optimal matching of signal pick-up coil with series topological fractal Koch curve exciting coil is explored:(1)By analyzing the topological structure of the exciting coil connection mode of the three-dimensional differential exciting eddy current sensor,the series topology Koch exciting coil structure is obtained for the design of the planar flexible eddy current sensor.In order to explore the optimal signal pick-up coil matching with series topology Koch exciting coil,according to the geometric size and area of Koch exciting coil,ten matching schemes of an exciting coil and signal pick-up coil are proposed,which are: Koch excitingKoch pick-up,Koch exciting-inscribed circle pick-up,Koch exciting-circumscribed circle pick-up,inscribed circle exciting-Koch pick-up,inscribed circle exciting inscribed circle pick-up,inscribed circle exciting-circumscribed circle pick-up,circumscribed circle exciting-Koch pick-up,circumscribed circle exciting-inscribed circle pick-up,circumscribed circle exciting-circumscribed circle pick-up,equal-area circle exciting-equal-area circle pick-up.(2)In order to quantitatively evaluate the eddy current distribution induced by a series topology Koch exciting coil in a test piece,a quantitative evaluation method of information entropy,relative entropy and cross entropy of the angle spectrum,tangential angle spectrum and annular energy spectrum of the eddy flow field in the fan-shaped region of the coil is proposed.The evolution law of eddy current distribution on the surface of the specimen with the change of exciting frequency and lift-off distance is studied through each evaluation index.(3)According to Maxwell's equation,the finite element simulation model of each structural sensor is established,and the real part and imaginary part of the response signal of each sensor to the different lengths,width,depth and direction cracks are obtained.Through the analysis of the response information,it is found that: when the size and shape of the signal pick-up coil are consistent with the excitation coil,the sensor with this structure has the best detection performance;the response signal of the Koch excitation Koch pick-up structure sensor to 3 mm and 5 mm cracks is significantly better than that of the other three optimal matching structures;the response signal of the Koch excitation Koch pick-up structure to short cracks in different directions.The range of variation is the smallest,which can reduce the missing detection rate to a certain extent.(4)The performance of the sensor is verified by the crack scanning experiment.The experiment results showed that: the sensor structure,in which the shape and size of the excitation coil and the signal pick-up coil are consistent,has the best detection performance;the range of response signal of Koch excitation Koch pick-up to short cracks in different directions is significantly smaller than that of inscribed circle excitation inscribed circle pick-up,circumscribed circle excitation circumscribed circle pick-up,equal-area circle excitation equal-area circle pick-up To reduce the occurrence of missing inspection.