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Figure 4-2.  Block Diagram of Eddy Current Inspection System.

 
  
 
T.O. 33B-1-1 4-4 Figure 4-2.  Block Diagram of Eddy Current Inspection System. 4.1.10.3 Bridge Circuit. The  bridge  circuit  converts  changes  in  eddy  current  magnitude  and  distribution  into  signals  that  are  ultimately processed and displayed.  A common mode of operation is to have the output of the bridge equal zero for a “good” or ‘“non-flaw” condition.  Presence of a flaw or an “other-than-good” condition results in an unbalance of the bridge, thus producing a relatively small signal.  This signal becomes the input to subsequent circuits. 4.1.10.4 Signal Processing Circuits. The processing of the signal from the bridge circuit depends on the type of information to be displayed.  Simple eddy current  devices  can  be  built  which  detect  and  amplify  the  signal  or  convert  the  signal  into  digital  format  (i.e.,  a conductivity value).  More sophisticated systems can process the complex electromagnetic signal into signal amplitude and signal phase, and provide filtering to suppress unwanted signals.  Details of the processes are discussed further in later sections. 4.1.10.5 Output Display. Eddy current test data can be presented as an analog or digital meter readout, a strip chart, an X-Y recorder plot, an oscilloscope  display  or  a  video  screen  presentation.    Meters  are  suitable  for  performing  specific  types  of  tests  such  as crack  detection,  alloy  sorting,  coating  thickness,  or  other  types  of  testing  that  require  a  measurement  of  signal amplitude  only.    Strip  charts,  X-Y  recorders  and  digital  storage  allow  the  signal  amplitude  to  be  displayed  and correlated  with  some  other  parameter  such  as  time  or  position.    Eddy  current  instruments  with  a  2  dimensional graphical  display  are  used  where  both  the  eddy  current  signal  amplitude  and  phase  must  be  measured.    These  are becoming the most common instruments available and provide the inspector with the greatest capability in interpreting the results of an eddy current inspection.  They are also very portable and can be battery operated, but are a few pounds heavier than those with a meter display. 4.1.11 Limitations of Eddy Current Method. There  are  several  limitations  to  the  eddy  current  inspection  method.    First,  it  is  limited  to  electrically  conductive materials  or  materials  with  electrically  conductive  components  such  as  carbon  fiber  re-enforced  composites.    Second, flaws parallel to the surface inspected are difficult to detect.  The most serious limitation, however, is the necessity to be able  to  prevent  or  suppress  eddy  current  responses  to  non-flaw  conditions.    This  becomes  a  particularly  difficult problem with ferromagnetic materials.


   


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