Digitally controlled multifrequency eddy current test apparatus and method

An eddy current test apparatus including at least one coil adapted to be coupled to an object to be tested to induce eddy currents in said object which eddy currents induce voltages in said coil or in a sensing coil comprising

• at least one processing module adapted to excite said coil at frequency f.sub.1 and to process the voltages induced by the eddy currents in said object,
• means for providing a digital frequency control signal to said module to select the frequency f.sub.1,
• said module including a frequency synthesizer adapted to receive said digital frequency control signal and provide an output signal at frequency f.sub.1,
• a digital filter connected to receive said digital frequency control signal to be tuned to said frequency f.sub.1 and serving to receive said input signal at frequency f.sub.1 and provide sine and cosine output signals at said frequency,
• means for receiving one of said sine or cosine output signals and applying it to said coil to induce eddy currents in said object,
• means in said module for receiving the voltages induced by said eddy currents in said object and said sine and cosine signals and nulling out in-phase and quadrature quiescent voltages,
• a second digital filter responsive to said digital frequency control signal for filtering said nulled signals and providing an output signal E.sub.1 sine (.omega.t+.theta.) and
• phase sensitive detectors connected to receive said output signal and sine and cosine signals from said first filter and provide Fourier amplitude signals A.sub.1,B.sub.1 of the coil induced voltage at frequency f.sub.1 where A.sub.1 =E.sub.1 cosine .theta. and B.sub.1 =E.sub.1 sine .theta..

An eddy current test apparatus including at least one coil adapted to be coupled to an object to be tested to induce eddy currents in said object which in turn induce voltages in said coil or in a sensing coil comprising

• means for applying excitation signals at frequencies f.sub.1, f.sub.2 and f.sub.3, where f.sub.3 is substantially higher than f.sub.1 and f.sub.2 to said coil to induce eddy currents in said object,
• means for receiving the voltage induced by the eddy currents in said object in said coil or sensing coil to provide output signals E.sub.1 sine (.omega..sub.1 t+.theta.), E.sub.2 sine (.omega..sub.2 t+.theta.) and E.sub.3 sine (.omega..sub.3 +.gamma.),
• means connected to receive said output signals and provide Fourier amplitude coefficients A.sub.1 B.sub.1, A.sub.2 B.sub.2, A.sub.3 B.sub.3 representing the coil induced voltages at frequencies f.sub.1, f.sub.2 and f.sub.3 where A.sub.1 =E.sub.1 (cosine .theta.) B=E.sub.1, sine .theta., A.sub.2 =E.sub.2 cosine .phi., B.sub.2 =E.sub.2 sine .phi., A.sub.3 =E.sub.3 cosine .gamma. and B.sub.3 =E.sub.3 sine .gamma.,
• rotating means for receiving the Fourier amplitude coefficients A.sub.3 B.sub.3 to rotate them through an angle in an X-Y coordinate system in which they are coordinates so as to align the indications from near surface flaws as closely as possible with one axis of the coordinate system and summing means for receiving the probe wobble signal on the other axis at said frequency and the output coefficients from one of the other frequencies to subtract the probe wobble signal therefrom to provide a wobble free flaw detection signal which has been substantially unaltered by any flaw data at the frequency f.sub.3.