Hall effect shoes speed sensitive?

[suttonbr]

I am using Hall Shoes to measure flux leakage in longitudinally magnetized tubing (for inspection purposes). I have noticed the signal put out by the hall shoes is much higher when the hall element is moved across the same area of flux leakage at a faster rate of speed. Why is this, and is there any way to regulate it?

 

[BrianG]

I was curious as to what a "hall shoe" was so I did some Googling.
The link below has some useful background on the general technique for inspection. Section 2.5 may give the clue where it states that incorrect choice of direction of magnetisation, or orientation of the hall device, can produce the undesirable effect of the output signal being sensitive to changes introduced by eddy currents induced by the moving magnetic field: these will be proportional to speed of movement over the surface, as in your question.

http://www.silverwinguk.com/en/technical pdfs/magnetic_flux_leakage_technology.htm

 

[suttonbr]

Ah. That is a very good start and eddy currents gives me a good idea of where to begin researching why this happens. The hall shoes in question are in fact horizontally mounted and in a longitudinal field. They are non-contact sensors however. They stay quite far from the material. So I am not sure if the explination of eddy current noise generated by the magnetic carriage is exactly what I am experiancing. But eddy currents being a function of velocity (having to do with speed) is a good start for me. Thank you for that so far. Anyone have anything to add?