Eddy Current Probe Using Intersecting Coils and Solid-State Sensors
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Solution Overview
Problem
Conventional eddy current probes face limitations in spatial resolution and signal-to-noise ratio due to interference from excitation magnetic fields, which affects the accuracy of defect detection in conductive materials, especially with the use of magnetic field sensors that struggle to discriminate between the excitation and eddy current fields.
Innovation Solution
The design employs intersecting excitation coils to produce a rotating magnetic field, with magnetic field sensors positioned to avoid interference from the excitation field, allowing for high sensitivity and signal-to-noise ratio, particularly in flat surface and tubular inspections without the need for multiplexing circuitry.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional circularly wound inductive coils are used as receiving coils, then the probe can detect the out-of-surface magnetic field, but the spatial resolution is severely limited by the size of the receiving coil
Solution Approach 1:
The patent replaces the conventional mechanical receiving coil with a solid-state magnetic field sensor (such as GMR or TMR sensor). This substitution eliminates the need for a large circularly wound coil while achieving high spatial resolution, as the solid-state sensor can be made extremely small (tens of micrometers) while maintaining high sensitivity to magnetic field variations.
Solution Approach 2:
The patent changes the fundamental parameter of the receiving element from a macroscopic coil (with dimensions in millimeters or centimeters) to a microscopic solid-state sensor (with dimensions in micrometers). This parameter change enables significantly higher spatial resolution while reducing the physical size of the receiving element.
2Measurement precision
If magnetic field sensors are used to replace receiving coils, then sensitivity and spatial resolution are improved, but the sensors cannot discriminate between the excitation magnetic field and the eddy current magnetic field
Solution Approach 1:
The patent positions the magnetic field sensor at a specific height above the test object surface, creating a vertical separation between the sensor and the excitation coil plane. At this optimized height, the sensor detects the out-of-surface magnetic field component (vertical component) while the excitation coil primarily produces in-plane magnetic field components. This dimensional separation in the vertical axis enables the sensor to discriminate between the excitation field and the eddy current field.
Solution Approach 2:
The patent introduces the concept of the out-of-surface magnetic field component as an intermediary that carries the defect information. By detecting this specific field component that penetrates through the material surface, the sensor indirectly measures the eddy current disturbances caused by defects while avoiding direct detection of the excitation field, thus using the magnetic field structure itself as a mediator to separate signal from noise.
3Measurement precision
If the magnetic field sensor is placed close to the excitation coil, then the excitation magnetic field saturates the sensor and causes complete loss of sensitivity
Solution Approach 1:
The patent utilizes the vertical dimension (height above surface) to resolve the saturation problem. By positioning the sensor at an optimized height above the test object, the sensor is far enough from the excitation coil to avoid saturation from the strong in-plane excitation field, yet close enough to detect the weaker out-of-surface magnetic field component generated by eddy currents. This vertical positioning creates an optimal detection zone that avoids the harmful near-field region.
Solution Approach 2:
The patent creates a localized detection region with specific magnetic field characteristics by positioning the sensor at a particular height above the surface. In this local region, the magnetic field has a dominant out-of-surface component that is sensitive to defect-induced eddy current variations, while the in-plane excitation field component is minimized. This local optimization of the detection zone enables high signal-to-noise ratio without saturation.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration enhances the detection accuracy for transverse flaws in flat surfaces and both axial and circumferential defects in curved surfaces, improving the overall sensitivity and reducing interference, leading to more effective non-destructive evaluation.
Implementation Method 1
an excitation coil is used to produce a magnetic field which is able to induce an eddy current in the material that is being inspected
Implementation Method 2
sensing the variation of out-of-surface magnetic field resulted from the disturbance of eddy currents by the defects
Implementation Method 3
The discovery of magnetoresistive effect and the development of micro and nanofabrication technologies have led to an advancement of miniaturized solid-state magnetic field sensors, such as giant magnetoresistive (GMR) sensors and tunnel magnetoresistive (TMR) sensors
Data Source
AI summary
An eddy current probe for nondestructive evaluation of an object made of an electrically conductive material with a planar or curved surface, comprising: an excitation portion, consisting of a plurality of multi-turn conductive coils and a bar core, the bar core having a primary surface in proximity to the outer surface of the object to be inspected, each multi-turn coil formed by winding a conductor along the core with each wound equally spaced and parallel on the primary surface of the core, multi-turn coils intersecting from one to another on the primary surface of the core and forming a plurality of congruent grid shapes with the winding conductors, each multi-turn coil is energized by an AC current with each of AC currents at a phase difference from one to another; and a sensing portion, consisting of a plurality of magnetic field sensors with each of the sensors positioned in proximity to the symmetric center of congruent grid shapes, each magnetic field sensor having a sensitivity axis is placed with its sensitivity axis in the direction substantially orthogonal to the primary surface.