ET06.pdf

Hugo libby laid his groundwork measuring the

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Hugo Libby laid his groundwork measuring the impedance of eddy current coils with which he performed his test. 1 He called the complex plane on which he displayed data the impedance plane. The impedance plane has become the most common display for electromagnetic test data, whether used with impedance probes, reflection (driver pickup or send/receive) probes or even display of remote field data. When Libby built his first eddy current instrument, he attached the cables to the oscilloscope to display the in-phase component (which he called imaginary ) on the horizontal axis and the out-of-phase component (called real ) on the vertical axis. He then rotated the field to get the real component back onto the horizontal axis. This rotation leaves a phase angle of zero on the left horizontal axis and phase angle increasing in a clockwise direction. This is in contrast to the true complex plane, which has a phase angle of zero on the right horizontal axis and phase angle increasing in the counterclockwise direction. This interesting artifact is often confusing for engineers new to nondestructive testing. In practice, most testing is done relative to a reference standard with the field rotated such that zero phase is defined to be a particular indication, most often the transducer liftoff signal. 174 Electromagnetic Testing F IGURE 1. Phasor in complex plane. Inductive reactance I (relative scale) A sin φ A A cos φ Legend A = amplitude φ = phase Resistance R (relative scale) φ
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For an eddy current system to provide information to an inspector, five functional steps have to be performed (Fig. 2): excitation, modulation, signal preparation, signal demodulation and signal display. An optional sixth step would be test object handling equipment. Excitation The excitation portion of an eddy current instrument consists of signal generation and amplifiers to drive the transducers. The signal generator (or oscillator) provides sine wave excitation for the test coil. Single-frequency systems have one fixed frequency whereas multifrequency systems can apply several frequencies to provide multiple-parameter options. The application determines the required frequency and the number of frequencies to be used. Additional frequencies are selected to provide additional test parameters for specimens with multiple variables. In this way, the number of measured parameters will equal or exceed the number of variables to allow discrimination of all desired discontinuity types. More complex test specimen problems require more sophisticated instrumentation to test the specimens adequately. Early systems had what were called frequency modules, designed to generate one frequency only. If a different frequency was wanted, then the physical module was replaced with another that would operate at a different frequency.
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  • Fall '19
  • Low-pass filter, Nondestructive testing, Eddy Current Instrumentation

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