Verlag des Forschungszentrums Jülich
JUEL-3979
Generally, many parameters of multichannel SQUID systems scale approximately linearly
with the number of channels: cost, size, power consumption, adjustment effort, number of
cables from the warm to the cold end, and coolant evaporation rate, due to thermal conduction
into the cryostat. With growing number of SQUID channels, it becomes increasingly
important to introduce multiplexing, i.e. sharing some parts of the system among several channels.
Using three standard HTS rf washer SQUID magnetometers, a multiplexed SQUID array was
implemented and operated with one read-out electronics. The SQUIDs are positioned in line
with 7 mm spacing. Air gaps of ~ 1 mm between neighboring magnetometers and tank
circuits have improved the coupling of the system considerably, thus enhancing return losses and
ameliorating operation stability of the system. In order to couple the magnetometers
inductively to the electronics, three tank circuits and three coupling coils were fabricated. The
coupling coils are connected in series. For joining them to the read-out electronics, one coaxial
cable was used.
Cross-talk between SQUID channels was determined to be negligible. To show the
performance of the SQUID array, EC measurements of aluminium aircraft samples were carried out.
A holder with magnetometers, tank circuits, and coupling coils was installed in a bath
cryostat. At the lower end of the cryostat, a double-D excitation coil of diameter 30 mm was
fixed. The SQUIDs were arranged exactly along the centerline of the excitation coil where the
compensation of the primary field is optimum.
Newly developed software controls the continuous switching of the SQUIDs during the scan
and allocates the recorded signal of each magnetometer to the proper signal trace. Thus three
EC signal traces of the sample are obtained simultaneously. These traces are lock-in
demodulated to yield in-phase and quadrature component, respectively. To suppress the eddy
current frequency fEC in the outgoing signal, the time constant of the lock-in has to be selected
sufficiently large.
As the result of an optimization procedure, an eddy current frequency fEC = 135 Hz was
chosen for the measurements. An artificial crack of 20 mm length in an aluminium sheet of 0.6
mm thickness could still be detected lying up to 10 mm below the surface of the sample. By
rotating the sheet in steps of 30 degrees, the angle between crack and scanning direction was
varied. Thus it was shown that the orientation of cracks can be determined from the flaw
signature. The quality of the recorded signal clearly depends on the number of active SQUIDs,
i.e. the density of data per signal trace.
Gärtner, Stefan
Entwicklung eines dreikanaligen SQUID-Sensorarrays für die Flugzeuginspektion nach dem Wirbelstromverfahren
119 S., 2002
Conventional eddy current (EC) systems employed for non-destructive detection of fatigue
cracks hidden deeply in aircraft fuselage are at their sensitivity limit. Due to their wide
dynamic range, Superconducting Quantum Interference Devices (SQUIDs) are well suited for
EC testing of layered aircraft structure. Offering high sensitivity at low excitation frequencies
and high linearity, they allow localization of deep flaws and quantitative evaluation of
magnetic field maps from the investigated components. However, the requirement to take maps of
the magnetic field, usually by meander-shaped scans, leads to unacceptably long measurement
times. Due to their inductive coupling to a tank circuit, several rf-SQUID sensors may be read
out sequentially by selectively coupling to their tank circuits reducing the measurement time
considerably.
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