Accurate Steel Tube Axis Alignment in Nondestructive Evaluation Probe

We have developed a steel tube axis alignment method using magnetic sensors associated with a commercial nondestructive evaluation (NDE) probe. The probe consists of an eddy-current encircling coil and a magnetization yoke. The accurate steel tube axis alignment method uses eight commercial Hall sensors, placed in optimal gradiometric configuration. We present a theoretical analysis of the system by means of 2-D finite-element simulations and the expected analytical expressions. Our results predict a high theoretical accurate tube axis alignment of around 10-50 mum. Experimental results are in good agreement with simulations and show that, for a steel tube, the axis alignment measurement accuracy can reach to 100 mum in a true experimental laboratory magnetic environment, including the intrinsic magnetic noise of the magnetization yoke.