Abstract: An eddy current system and method for hot annealed weld inspection is disclosed for use on metallic workpieces such as steel pipe. An annealer produces strong alternating fields to heat the weld. A nearby exciter coil produces test eddy currents within the workpiece at a carrier frequency. Detector circuitry including a demodulator and a detector coil senses defects within the workpiece as evidenced by nonuniformities in the test eddy currents, in response to which the detector coil produces a flaw indicating signal modulated on the carrier frequency. The carrier frequency is automatically maintained at a non-integral multiple of the annealer alternating frequency. A variable band pass filter between the demodulator and detector coil is automatically maintained with its pass band substantially centered upon the carrier frequency.

Abstract: An electromagnetic flaw detection system and method having automatic dynamic error balancing is disclosed.The system includes means for producing an alternating electromagnetic field and for passing a metallic workpiece to be tested along a path through the field. A differential detection coil is disposed in the field near the workpiece path. The coil responds to nonuniformities in the field caused by workpiece flaws, such as voids, to produce flaw indications. Detection circuitry actuates alarm apparatus in response to the flaw indications to further indicate need for corrective action.Automatic balancing circuitry compensates for an error signal, which the differential coil generates due to minute undesired assymmetries in the coil's mechanical structure. The balancing circuitry includes means for generating a reference signal. The reference signal is continually adjusted, in response to characteristics of the error signal, to exactly counterbalance the error signal in phase and amplitude.