A
Anonymous
Guest
Hi All,
Here is a plot that I did of some ferrous objects using a log amp front end. At first sight it does not appear much different to the non-ferrous object plots in that it shows a range of different decays, some exponential and some not. However, a bit of further explanation is necessary. Plot 1 which is the fastest and shows a straight line decay is piece of tin plated steel 3in x 2in in size. This was held face on to the coil with the result that the flux is perpendicular to the surface and the response is almost pure eddy current with a fast decay due to the poorer conductivity of steel. The same goes for the coffee tin (plot 2). I realised later that if the object is moved off toward the edge of the coil then the flux is running parallel to the surface and the response is then due to the magnetic properties and a curve results similar to 4 or 5. In practice, if a coil is scanned over a ferrous object, both the shape and time constant will change during the pass. In contrast a non-ferrous coin or ring with display a constant decay time during the pass. Also a cross scan will give different results again for a ferrous object but the non-ferrous coin or ring will still give the same as the first scan.
Eric.
Here is a plot that I did of some ferrous objects using a log amp front end. At first sight it does not appear much different to the non-ferrous object plots in that it shows a range of different decays, some exponential and some not. However, a bit of further explanation is necessary. Plot 1 which is the fastest and shows a straight line decay is piece of tin plated steel 3in x 2in in size. This was held face on to the coil with the result that the flux is perpendicular to the surface and the response is almost pure eddy current with a fast decay due to the poorer conductivity of steel. The same goes for the coffee tin (plot 2). I realised later that if the object is moved off toward the edge of the coil then the flux is running parallel to the surface and the response is then due to the magnetic properties and a curve results similar to 4 or 5. In practice, if a coil is scanned over a ferrous object, both the shape and time constant will change during the pass. In contrast a non-ferrous coin or ring with display a constant decay time during the pass. Also a cross scan will give different results again for a ferrous object but the non-ferrous coin or ring will still give the same as the first scan.
Eric.