Testing arrangement for ferromagnetic bodies including magnetic field detectors extending between two pairs of poles of magnetic field generators spaced longitudinally along the body

An apparatus for non-destructively testing for structural irregularities in a ferromagnetic body having a longitudinal extent comprising:

• first magnetic pole means, including at least two magnetic poles, for generating a first stationary magnetic field having a first central linear axis passing through said two magnetic poles and radially through said body;
• second magnetic pole means, including at least two magnetic poles spaced apart along said body from said first magnetic pole means, for generating a second stationary magnetic field having a second central linear axis passing through said two magnetic poles and radially through said ferromagnetic body, said first and second central axes being inclined with respect to one another at an angle .alpha. which is substantially equal to 180.degree./N, where N is the total number of magnetic fields generated by said apparatus;
• means for longitudinally displacing said body with respect to the poles of said first and second magnetic pole means;
• a plurality of rows of stationary Hall generators, each row extending either in a space between the magnetic poles of the first magnetic pole means or between the magnetic poles of the second magnetic pole means, for detecting a magnetic field over a zone corresponding to a section of said body perpendicular to the displacing direction of said body, said Hall generators providing detector signals indicative of the fields sensed thereby;
• means for time multiplexing said detector signals to form a time multiplexed signal;
• a pulse sequence control circuit for driving said Hall generators and to which said time multiplexed signal can be synchronized;
• means for generating compensation values for various signal levels of said Hall generators as a function of the position of the Hall generator whose output is being compensated;
• a compensation amplifier for compensating said time multiplexed signal with said compensation values;
• and
• means for analyzing said compensated time multiplexed signal.