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Lift off compensation of eddy current crack detection system by controlling damping resistance of oscillator

  • US 4,609,870 A
  • Filed: 09/13/1984
  • Issued: 09/02/1986
  • Est. Priority Date: 03/27/1981
  • Status: Expired due to Fees
First Claim
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1. An eddy current crack detection system comprising an inductive probe coil forming part of an oscillatory circuit and a measuring device arranged to monitor the amplitude of the oscillating voltage across the probe coil whereby to detect cracks, characterised in that in order to substantially nullify the effect of lift-off on the amplitude of said voltage there is provided a variable electrical resistance in the oscillatory circuit and control means responsive to the instantaneous magnitude of lift-off to provide a control signal for continuously varying the value of said resistance and hence the amount of damping in the oscillatory circuit so that the ampliltude of said voltage does not vary substantially with lift-off.

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