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Conductors within insulating coatings or uniformity

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conductors within insulating coatings, or uniformity of conducting coatings on 155 Probes for Electromagnetic Testing F IGURE 40. Multidimensional arrays of hall effect detectors used to measure directional components of magnetic field intensity: (a) two-dimensional array of hall effect detectors, in which components of magnetic field in X,Y plane are sensed individually; (b) three-dimensional array of hall effect detectors, in which each detector senses magnetic field component perpendicular to face of semiconductor. (a) (b) Y B X B E H1 = K H I c B cos θ E H2 = K H I c B sin θ θ = 90 degrees θ Y X Z E HZ = K H I c B Z E HY = K H I c B Y E HX = K H I c B X EB X B z B Y B B X B Y θ Legend B = magnetic flux density E H = electric field detected in X, Y or Z direction I C = control current K = hall coefficient F IGURE 41. Linear arrays of hall effect detectors in eddy current magnetizing coils: (a) linear array of hall elements within single magnetizing coil, for multichannel longitudinal scanning of fusion weld zones; (b) linear array of hall effect detectors within circumference of encircling magnetizing coil, for multichannel longitudinal scanning of bars or tubes. For a 25 mm (1 in.) diameter coil, there must be about 100 hall elements each about 0.75 mm (0.03 in.) long for 100 percent coverage. (a) (b) Semiconductor detector array Elongated magnetizing coil Direction of scanning Semiconductor detector locations Magnetizing coil current Eddy current flow in tube 1 2 3 4 5 6 7 8
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concentric cores, may be evaluated with this type of probe. Longitudinal discontinuities, such as seams and laps in hot rolled bars or tubes, can also be detected continuously, rather than intermittently by, for example, orbiting probe coil test systems. Bridge Arrays An alternative arrangement of small area hall effect detectors within a single test coil corresponds to the wheatstone bridge circuit used in many electrical measurements. As sketched in Fig. 42a, the detectors are in the corners of a small square in the test surface and are excited by a single large magnetizing coil. Four local measurements are made simultaneously. Signals from the four discrete channels can be compared in various combinations in the external circuitry of the electromagnetic test instrumentation. Such an arrangement is responsive to the degree and orientation of anisotropic properties in test materials or to the directions and severity of discontinuities such as cracks. An alternative is to align coils precisely with the flat surfaces of test materials to measure tilt or surface waviness. Other arrangements of multiple hall effect detectors over an area of the test object surface can be applied for special measurement problems. Differential Systems Two hall effect detectors can be located at various spacings and their signals subtracted from one another in the external circuitry, to provide differential measurements. A single coil can provide excitation for both detectors (Fig. 42b).
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  • Fall '19
  • Wind, The Land, Magnetic Field, Eddy Current Probes, electromagnetic testing

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