Magnetic flaw detection apparatus

Claim 1

A system for detecting flaws in a relatively moving workpiece, comprising: a. means for moving the workpiece relatively along a path; b. an exciter circuit having a complex input impedance and being connectable to an electric power source for generating an alternating exciting field in the workpiece region and having a member inductively couplable to the workpiece to vary the value of the complex input impedance in response to the presence of a flaw in the inductively coupled moving workpiece; c. an open weld detector for detecting open welds in the coupled workpiece by sensing said variations in the complex impedance, comprising: i. circuitry for rendering the complex input impedance of the exciter circuit resonant when said member is inductively coupled with an unflawed workpiece, and ii. a circuit for sensing detuning of the complex input impedance from its resonant value responsive to the presence of a flaw in the inductively coupled workpiece; d. a detection circuit for sensing variations in the exciting field in response to the presence of flaws in the workpiece, said detection circuit comprising: i. a differential coil for producing a net voltage thereacross in the presence of a workpiece flaw, and ii. a balance circuit for nulling an error voltage produced by the differential coil in the presence of an unflawed workpiece, said balance circuit comprising: 1. a circuit for deriving a phase and amplitude controlled reference signal having a frequency equal to that of the alternating field, and 2. a bucking circuit for combining the reference signal and the error signal produced across the entire differential coil to producE an output having a zero value for indicating when the error signal is nulled by the reference signal; e. said detection circuit producing flaw indicating signals having a predetermined phase which is a function of the type and extent of workpiece flaws detected by the detection circuit; f. an end signal suppressor for temporarily disabling the detection circuit from generating flaw indicating signals in response to the passage of an end of the workpiece through the exciting field, the end signal supperssor comprising: i. an end sensing coil for generating an end signal of at least a predetermined value in response to the passage of a workpiece end through the field, and ii. an inhibiting circuit connected to the end sensing coil and the detector circuit for disabling the detector cirucit from producing the flaw indicating signals for a predetermined time period following the occurrence of the end signal; g. circuitry for generating a reference alternating signal having a frequency equal to that of the exciting field and a controlled phase equal to the phase displacement of the flaw indicating signal in response to a workpiece flaw, and h. a phase monitoring circuit comprising: i. first and second zero detectors connected respectively to the reference generating circuit and the detection circuit, ii. first and second monostable multivibrators connected respectively to the first and second zero detectors for each producing an output when the corresponding one of the reference signal and the flaw indication signal passes through a zero value in a predetermined direction, and iii. an AND gate having two inputs each connected to the output of a different one of the monostable multivibrators for providing an output when the values of the reference and flaw indicating signals pass through zero values simultaneously and are in phase with one another.

Claim 2

A system for detecting flaws in a relatively moving workpiece, comprising: a. means for moving the workpiece relatively along a path; b. an exciter circuit having a complex input impedance and being connectable to an electric power source for generating an alternating exciting field in the workpiece region and having a member inductively couplable to the workpiece to vary the value of the complex input impedance in response to the presence of a flaw in the inductively coupled moving workpiece; c. an open weld detector for detecting open welds in the coupled workpiece by sensing said variations in the complex impedance, comprising: i. circuitry for rendering the complex input impedance of the exciter circuit resonant when said member is inductively coupled with an unflawed workpiece, and ii. a circuit for sensing detuning of the complex input impedance from its resonant value responsive to the presence of a flaw in the inductively coupled workpiece; d. a detection circuit for sensing variations in the exciting field in response to the presence of flaws in the workpiece, said detection circuit comprising: i. a differential coil for producing a net voltage thereacross in the presence of a workpiece flaw, and ii. a balance circuit for nulling an error voltage produced by the differential coil in the presence of an unflawed workpiece, said balance circuit comprising: 1. a circuit for deriving a phase and amplitude controlled reference signal having a frequency equal to that of the alternating field, and 2. a bucking circuit for combining the reference signal and the error signal produced across the entire differential coil to producE an output having a zero value for indicating when the error signal is nulled by the reference signal; e. said detection circuit producing flaw indicating signals having a predetermined phase which is a function of the type and extent of workpiece flaws detected by the detection circuit; f. an end signal suppressor for temporarily disabling the detection circuit from generating flaw indicating signals in response to the passage of an end of the workpiece through the exciting field, the end signal supperssor comprising: i. an end sensing coil for generating an end signal of at least a predetermined value in response to the passage of a workpiece end through the field, and ii. an inhibiting circuit connected to the end sensing coil and the detector circuit for disabling the detector cirucit from producing the flaw indicating signals for a predetermined time period following the occurrence of the end signal; g. circuitry for generating a reference alternating signal having a frequency equal to that of the exciting field and a controlled phase equal to the phase displacement of the flaw indicating signal in response to a workpiece flaw, and h. a phase monitoring circuit comprising: i. first and second zero detectors connected respectively to the reference generating circuit and the detection circuit, ii. first and second monostable multivibrators connected respectively to the first and second zero detectors for each producing an output when the corresponding one of the reference signal and the flaw indication signal passes through a zero value in a predetermined direction, and iii. an AND gate having two inputs each connected to the output of a different one of the monostable multivibrators for providing an output when the values of the reference and flaw indicating signals pass through zero values simultaneously and are in phase with one another.