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Electromagnetic Testing

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Test Coil Excitation<br />

The second consideration that was previously mentioned for determining the<br />

mode of operation of a test unit could be the way the probe is being energized.<br />

Figure 6.7 a typical surface riding pancake coil responses to an array of EDM<br />

notches on a calibration standard. Fig 6.8 shows a block diagram of a<br />

stepped, single frequency, phase amplitude instrument.<br />

The circuit in Fig 6.8 is capable of operating at any of the four frequency, if<br />

the four frequencies are spread over a wide range, several different test coils<br />

may be required to use the instrument over the entire range. Most modern<br />

single frequency instruments use this principle; however one variable<br />

frequency generator with a wide operating range usually replaced the four<br />

individual fixed generators. A typical frequency for such an instrument is in<br />

the low hertz range (50Hz to 100Hz) to several megahertz (8MHz to 10MHz).<br />

This large dynamic range gives these units a wide variety of possible<br />

applications.<br />

Charlie Chong/ Fion Zhang

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