Research On Crack Detection Mechanism Of Double Channel Plane Rotating Field Eddy Current Sensor With Cross Structure Exciting Traces

Efficient and reliable detection and monitoring of the quality of important parts and structural integrity of equipment is an effective way to prevent safety accidents,reduce economic losses and ensure personnel safety.Aiming at the problem that traditional eddy current sensors have a high rate of missed detection for short cracks in special directions on the surface and subsurface of metal components,this paper proposes a cross-structure excitation planar rotating field eddy current sensor with dual-channel output.The performance of the sensor is studied through quantitative evaluation of eddy current field in space-time domain based on information entropy,finite element analysis and experiment.The main research contents are as follows:(1)Starting from the principle of eddy current testing,firstly,the eddy current sensor's detection process of the inspected object is analyzed theoretically,and the interaction between the rotational eddy current field and the crack is clarified.Subsequently,according to the differential principle,the sensor coil structure is proposed,and the working principle of the sensor is discussed.(2)Aiming at the lack of quantitative evaluation methods for eddy currents in space-time domains,two quantitative evaluation indexes for eddy currents in space-time domains based on information entropy are proposed:the information entropy of the angular spectrum of the 2-dimensional vortex vector distributed in the 0-dimensional space and the 1-dimensional time-domain(0D1T),and the information entropy of the angular spectrum of the 2-dimensional vortex field distributed in the 2-dimensional space and the 1-dimensional time-domain(2D1T).The former quantitatively evaluates whether the eddy current vector distributed at a point directly below the cross point of the excitation coil in the specimen rotates at a uniform speed in all directions over time;the latter quantitatively evaluates whether the eddy current in the spatial neighborhood around the corresponding point directly below the cross point of the excitation coil in the specimen is uniformly distributed in all directions.Two indexes are used to quantitatively evaluate the distribution of the eddy current field of the proposed rotational field eddy current sensor in the test piece:Except 100k Hz,the₀₁change under other frequencies can be divided into three segments:a horizontal part,a descending segment,and a fluctuating segment.The₀₁of the horizontal segment is close to 4.91 bit.The correlation coefficient between the depth of the inflexion and the skin depth is 0.99.The₂₁of all frequencies are close to4.91bit when the radius is 0.5mm.(3)A finite element analysis model of the sensor is established based on Maxwell's equations in the time domain.The effects of the response signal of cracks with different lengths,widths,depths and directions and excitation frequency on the sensor are explored,and the difference between the output signals of the two channels is compared and analyzed.After analysis,the two channels can effectively pick up the response signals of cracks of different sizes and directions,but channel 2 is more sensitive to the change information of the crack width and depth than channel 1,which can supplement the response signal of channel 1.(4)The performance of the dual-channel output cross structure excitation planar rotational field eddy current sensor was verified by crack scanning experiments.The experimental results show that:Channel 1 can effectively pick up the response signals of cracks different specifications and distributions,but it does not express much specific information about the cracks,such as the length of the crack;the crack response signal picked up by channel 2 can supplement the information contained in the crack response signal picked up by channel 1.Via the comparison of the finite element analysis and the test results,the results show that the two-channel pickup signals of the dual-channel output cross structure excitation planar rotational field eddy current sensor are complementary.