High resolution inductive sensor arrays for UXO
For the inspection of materials and the detection and characterization of hidden objects, features, or flaws sensors and sensor arrays are used to image form two-dimensional images suitable for characterizing the hidden features. Magnetic field or eddy current based inductive and giant magnetoresistive sensors may be used on magnetizable and conducting materials, while capacitive sensors can be used for dielectric materials. Enhanced drive windings and electrode structures permit nulling or cancellation of local fields in the vicinity of the sense elements to increase sensor sensitivity. The addition of calibration windings, which are not energized during measurements, allows absolute impedance and material property measurements with nulled sensors. Sensors, sensor arrays, and support fixtures are described which permit relative motion between the drive and sense elements. This facilitates the volumetric reconstruction of hidden features and objects.
1. A test circuit comprising:
a primary winding having at least one linear segment to impose a magnetic field in a test material when driven by an electric current;
at least two sense elements positioned parallel to a linear primary winding segment; and
a bucking coil proximate to and surrounding the sense elements and driven by an electric current so that a local magnetic field opposes the imposed field.
2. A test circuit as claimed in claim 1 wherein the sensing elements are inductive to sense a time varying magnetic flux.
3. A test circuit as claimed in claim 2 wherein the sensing elements are absolute measurement coils.
4. A test circuit as claimed in claim 3 wherein the sensing elements are differential measurement coils.
5. A test circuit as claimed in claim 1 wherein at least one sense element has a giant magnetoresistive sensor.
6. A test circuit as claimed in claim 1 wherein the bucking coil is driven in series with the primary winding.
7. A test circuit as claimed in claim 6 wherein the current through the bucking coil is attenuated with passive circuit elements.
8. A test circuit as claimed in claim 7 wherein the passive element is a resistor.
9. A test circuit as claimed in claim 1 wherein the bucking coil is driven separately from the primary winding.
10. A test circuit as claimed in claim 1 wherein the local magnetic field in the vicinity of the sense elements is nulled.
11. A test circuit as claimed in claim 1 further comprising a calibration coil located in the vicinity of the sense elements.
12. A test circuit as claimed in claim 11 wherein the calibration coil spans the area over the sense elements.
13. A test circuit comprising:
a primary winding having at least one linear segment to impose a magnetic field in a test material when driven by an electric current;
at least two sense elements positioned parallel to a linear primary winding segment;
a calibration coil located in the vicinity of the sense elements and spanning the area over the sense elements; and
a bucking coil proximate to the sense elements and driven by an electric current so that a local magnetic field opposes the imposed field.
14. A test circuit as claimed in claim 13 wherein the sensing elements are inductive to sense a time varying magnetic flux.
15. A test circuit as claimed in claim 14 wherein the sensing elements are absolute measurement coils.
16. A test circuit as claimed in claim 15 wherein the sensing elements are differential measurement coils.
17. A test circuit as claimed in claim 13 wherein at least one sense element has a giant magnetoresistive sensor.
18. A test circuit as claimed in claim 13 wherein the bucking coil surrounds the sense elements.
19. A test circuit as claimed in claim 13 wherein the bucking coil is driven in series with the primary winding.
20. A test circuit as claimed in claim 19 wherein the current through the bucking coil is attenuated with passive circuit elements.
21. A test circuit as claimed in claim 20 wherein the passive element is a resistor.
22. A test circuit as claimed in claim 13 wherein the bucking coil is driven separately from the primary winding.
23. A test circuit as claimed in claim 13 wherein the local magnetic field in the vicinity of the sense elements is nulled.