| A SQUID can be employed in an eddy-current NDE system, in which the
SQUID measures the secondary magnetic field generated by the eddy currents
in the material under test.
For such eddy-current NDE systems, we developed a control circuit which contains an ac-current generator (up to 1 A between 1 Hz and 250 kHz) used to generate eddy currents in the material under test. The secondary field produced by these eddy currents is then measured with the SQUID and further processed in the two-channel lock-in amplifier contained in the controller. In addition, the controller employs a circuit which allows for a compensation of the eddy current field at the location of the SQUID, to enhance the measurement dynamic range. This NDE controller employs three numerically-controlled oscillators with 16-bit DACs to achieve a very-low-distortion eddy-current excitation signal, which is necessary to make use of the full dynamic range a SQUID is offering. The controller can be controlled manually, or by a standard personal computer via the serial port. The size of the controller is 250 mm by 250 mm by 85 mm. If desired, the controller can of course also be used for conventional eddy-current NDE with induction coils as field sensors. |
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Eddy-current generator and lock-in amplifier WBS LKI 250 k. |
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This NDE controller uses three numerically-controlled
oscillators with 16-bit DACs to achieve a very-low-distortion eddy-current
excitation signal, which is necessary to make use of the full dynamic range
a SQUID is offering. One of the oscillators is used for the eddy-
current generation and the reference of the in-phase channel (X) of the
lock-in amplifier, the second is for the in-quadrature channel (Y) of the
lock-in, and the third generates the excitation-field compensation.
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The graph to the left shows the high spectral purity of the excitation and compensation currents. Because of the low level of the harmonics, the excitation field at the location of the SQUID can be suppressed to one part in 10,000. Due to the low noise of the excitation signal, the maximum achieveable dynamic range in 1 hertz bandwidth is about 170 dB. |