Material Property Estimation Using Inverse Interpolation

Abstract

Magnetic field sensor probes are disclosed which comprise primary or drive windings having a plurality of current carrying segments. The relative magnitude and direction of current in each segment are adjusted so that the resulting interrogating magnetic field follows a desired spatial distribution. By changing the current in each segment, more than one spatial distribution for the magnetic field can be imposed within the same sensor footprint. Example envelopes for the current distributions approximate a sinusoid in Cartesian coordinates or a first-order Bessel function in polar coordinates. One or more sensing elements are used to determine the response of a test material to the magnetic field. These sense elements can be configured into linear or circumferential arrays.

1 . A test circuit for measuring material properties comprising:
 a primary winding configured to carry an electric current;   a solid-state sensing element for sensing a property of a magnetic field produced when the primary winding is driven by the electric current;   a feedback coil that encircles the solid-state sensing element; and   a feedback control circuit configured to control a bias current provided to the feedback coil.   
     
     
         2 . The test circuit of  claim 1 , wherein the solid-state sensing element is a magnetoresistive sensing element. 
     
     
         3 . The test circuit of  claim 2 , wherein the solid-state sensing element is a giant magnetoresistive sensing element. 
     
     
         4 . The test circuit of  claim 1 , wherein the solid-state sensing element is a hall effect sensor. 
     
     
         5 . The test circuit of  claim 1 , wherein a segment of the primary winding most proximal to the solid-state sensing element is linear. 
     
     
         6 . The test circuit of  claim 1 , wherein the feedback control circuit is configured to control the bias current such that an output of the solid-state sensing element has a linear response to the property of the magnetic field. 
     
     
         7 . The test circuit of  claim 1 , wherein the property of the magnetic field is the amplitude of the magnetic field. 
     
     
         8 . The test circuit of  claim 1 , wherein the solid-state sensing element is one of a plurality of solid-state sensing elements forming a array. 
     
     
         9 . A method of characterizing a test material with test circuit of  claim 1 , the method comprising:
 placing the solid-state sensing element near the test material;   driving the primary winding with the electric current;   providing a bias current to the feedback coil;   measuring an output of the solid-state sensing element; and   estimating a property of the test material from the output of the solid-state sensing element.   
     
     
         10 . A test circuit comprising:
 a primary winding configured to carry an electric current;   an array of solid-state sensing elements for sensing a property of a magnetic field produced when the primary winding is driven by the electric current;   a feedback coil that encircles at least one of the solid-state sensing elements; and   a control circuit configured to control a bias current provided to the feedback coil.   
     
     
         11 . The test circuit of  claim 10  wherein the primary winding has a linear segment and the array is aligned with, but offset from, the linear segment of the primary winding. 
     
     
         12 . The test circuit of  claim 10  wherein the primary winding has a circular segment and the solid-state elements are distributed about a circle having a same center as the circular segment of the primary winding. 
     
     
         13 . The test circuit of  claim 10 , wherein the array further comprises at least one inductive coil sensing element. 
     
     
         14 . The test circuit of  claim 10 , wherein the primary winding has a linear segment and a distance between the linear segment and the solid-state sensing elements varies. 
     
     
         15 . The test circuit of  claim 10 , wherein each of the solid-state sensing elements is encircled by a respective feedback coil, each of the respective feedback coils having its own control circuit. 
     
     
         16 . The test circuit of  claim 10 , wherein the feedback coil encircles at least two solid-state sensing elements. 
     
     
         17 . The test circuit of  claim 10 , wherein the solid-state sensing elements comprise at least one magnetoresistive sensing element. 
     
     
         18 . The test circuit of  claim 10 , wherein the solid-state sensing elements comprise at least one giant magnetoresistive sensing element. 
     
     
         19 . The test circuit of  claim 18 , wherein the array comprises:
 a first giant magnetoresistive sensing element having a first orientation so as to measure the magnetic field in a direction; and   a second giant magnetoresistive sensing element having a second orientation, different from the first, so as to measure the magnetic field in a direction.   
     
     
         20 . The test circuit of  claim 10 , wherein the solid-state sensing elements comprise at least one hall effect sensor.