Drive coil for eddy current inspection probe
A drive coil assembly to generate a specific spatial distribution of eddy currents within the walls of a conductive tube or pipe, which may be used in conjunction with a separate array of magnetic sensors to detect defects in the tubing wall. The drive coil assembly comprises a plurality of coils which are generally wrapped around the circumference of a cylindrical probe body, but which are further shaped with serpentine undulation in the axial direction. The undulation is characterized by a spatial amplitude, a spatial phase and a spatial frequency; typically, the spatial frequency results in an integer number of undulations around the circumference of the drive coil and the phase is chosen to uniformly distribute the lobes about the circumference. The temporal phase of the electrical current in each coil is chosen to null out net current of the assembly around the circumference.
1. An eddy current generation system for inspection of a tubular object comprising electrically conductive material, comprising
an alternating current source; and
a first current-carrying structure having a circular perimeter and comprising:
a plurality of current carrying paths, wherein said paths are substantially concentric about a common axis and are further spatially modulated in the axial direction, so that each of said one or more paths has an integer number of axial undulations around the perimeter of the structure, said axial undulations being spatially phased, and
wherein said alternating current source supplies each of said current carrying paths with an excitation current having a fixed temporal phase and said temporal phases of the currents supplied to the plurality of current carrying paths are configured so that net current about the common axis is zero.
2. The system of claim 1 wherein said undulations are shaped to optimize the distribution of eddy current in the wall of the tubular object.
3. The system of claim 2 wherein said undulations have a shape that is a sinusoidal shape or a cycloid shape.
4. The system of claim 1 wherein the spatial phases of the axial undulations of said current paths are arranged such the axial undulations are uniformly distributed about the circumference of the first current-carrying structure.
5. The system of claim 1 wherein a temporal frequency of the electrical current in each of the conducting paths is the same.
6. The system of claim 1 wherein temporal frequency of the different electrical currents in each of the current-carrying paths is different.
7. The system of claim 1 further comprising a first sensor array of magnetic field sensors.
8. The system of claim 7 wherein said first current-carrying structure produces a background magnetic field and where a spatial phase and pitch of the location of said array of magnetic field sensors is configured to provide symmetries in the background magnetic field.
9. The system of claim 7 wherein said first current-carrying structure produces a magnetic field and wherein said magnetic field sensors are oriented to detect a directional component of the magnetic field.
10. The system of claim 9 wherein said directional component is normal to a surface of the tubular object, parallel to the axis of the tubular object or tangential to a surface of the tubular object.
11. The system of claim 7 wherein the first sensor array comprises one or more of inductive coils, solid state sensors, AMR sensors, GMR sensors, Hall sensors, valve sensors, TMR sensors or magneto-optic sensors.
12. The system of claim 7 further comprising a second sensor array wherein said first sensor array is located axially adjacent to a first side of said current carrying paths and said second sensor array is located axially adjacent to a second side of said current carrying paths.
13. The system of claim 12 wherein the second sensor array is utilized to provide a signal reference for differential sensing.
14. The system of claim 12 wherein the second sensor array is utilized to increase a circumferential resolution of the instrument.
15. The system of claim 1 wherein said current carrying paths are fabricated from one or more of: discrete wires, photolithographically produced conductors, subtractive fabrication or additive fabrication.
16. The system of claim 3 wherein said current carrying paths are either fabricated on a flexible substrate which is subsequently formed into a cylindrical shape or directly formed on a rigid or semi-rigid probe body.
17. An eddy current generation system for inspection of an object of planar or arbitrary shape comprising electrically conductive material comprising
an alternating current source; and
a structure having a substantially planar disposition and a linear axis contained within the plane and comprising:
a plurality of current carrying paths, wherein said paths are generally disposed along a common axis and are further spatially modulated in the plane in a direction orthogonal to the common axis, so that each path has an integer number of axial undulations, and spatial phasing of the undulations relative to each of said current carrying paths is uniformly distributed along the common axis,
wherein said alternating current source supplies each of said current carrying paths with an excitation current having a fixed temporal phase and said temporal phases of the currents supplied to the plurality of current carrying paths are configured so that net current about the common axis is zero.
18. The eddy current generation system of claim 17 formed on a rigid flat substrate, a rigid shaped substrate, a flexible substrate, or an articulated substrate.
19. The eddy current generation system of claim 17 further comprising one or more magnetic sensors.