Method and apparatus for measuring magnetic properties of sheet material with both low and high magnetic particle density

摘要

The method for measuring magnetic properties of sheet material is able to deliver reliable signals at both low and high magnetic particle density in the sheet material. First a measuring head converts the magnetic properties of the sheet material into electric signals. The electric signals produced by the measuring head are amplified in a certain signal range such that the lower-amplitude electric signals produced by the areas with low magnetic particle density of the sheet material are amplified to a greater extent than the electric higher-amplitude signals produced by the areas with high magnetic particle density of the sheet material. For this purpose the signal range is divided into at least three ranges which are each amplified constantly. The amplification in the two outer ranges is selected equal, and the amplification in the middle range greater than the amplification in the outer ranges. The electric signal is preferably shifted by a constant value before amplification so that the zero point of the signal range is in the middle of the middle range.