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Electromagnetic flaw detection system and method incorporating improved automatic coil error signal compensation
| Details |
Inventors: Harris, Richard M.; Janos, Wilbert J.;
Assignee: Republic Steel Corporation (Cleveland, OH)
Primary Examiner: Corcoran; Robert J.
Assistant Examiner:
Attorney, Agent or Firm: Watts, Hoffmann, Fisher & Heinke Co.
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. The coil's total output, including the error, and the adjusted reference signal, are summed at a location electrically separate from the coil to provide a flaw detection coil output in which the error is completely nulled out. |
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DETAILED DESCRIPTION What is claimed is: 1.
An electromagnetic flaw detection system comprising: (a) circuitry for exciting an electromagnetic field; (b) circuitry including a coil in the region of the field and susceptible of producing an error signal when in the field, said error signal having a phase angle with respect to a reference; (c) apparatus for moving a metallic workpiece in the field relative to the coil, for causing the coil to produce an output in response to the passage of a flawed workpiece; (d) circuitry coupled to the coil for detecting the coil output, and (e) balancing circuitry responsive to the error signal including circuitry for automatically nulling error signals having a phase angle ranging over 360.
degree.
with respect to the exciting field.
2.
The system of claim 1, wherein said balancing circuitry comprises: (a) circuitry for generating a reference signal; (b) circuitry responsive to the error signal for adjusting the reference signal continually to have opposite phase and equal amplitude relative to said error signal, and (c) summing circuitry for combining said error and said adjusted reference signals.
3.
The system of claim 2, wherein said reference adjusting circuitry comprises: (a) circuitry for phase shifting said reference signal; (b) a phase comparator responsive to the relative phases of said combined and said phase shifted reference signals for regulating the phase shifter for maintaining the phase of said phase shifted reference signal opposite that of said error signal; (c) circuitry for attenuating said phase shifted reference signal, and (d) an amplitude comparator responsive to said combined and phase shifted signals for controlling the attenuator for maintaining the amplitude of the phase shifted and attenuated signal equal to that of said error signal.
4.
The system of claim 2, wherein said summing circuitry comprises: a precision differential amplifier having a first stage having substantially unity gain, and a second stage in series with said first state
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