Patent Publication Number: US-2006016277-A1

Title: Non-invasive magnetostrictive sensor

Description:
TECHNICAL FIELD  
      The present invention relates to magnetostriction-based force and torque sensors and, more particularly, to a non-invasive magnetostrictive sensor used to determine force and torque due to magnetostriction of magnetostrictive materials.  
     BACKGROUND OF THE INVENTION  
      Various materials are known in the art to be magnetostrictive by which their magnetic permeability varies with stress applied thereto, known as magnetostrictive materials. The physical effect is known as the “Villari” effect.  
      An example of a prior art method of determining the force acting upon a magnetostrictive material subjected to stress is depicted in  FIG. 1A . Magnetostrictive sensor  100  of  FIG. 1A  consists of magnetostrictive cylindrical rod  102  of radius R and length L wrapped with a coil  104  to which a time varying current is of a specified frequency is applied. Magnetostrictive sensor  100  is invasively embedded within a structural element  112  (shown in phantom in  FIG. 1B ) to determine the force applied to the structural element. A force applied to the structural element imposes a stress and force  106  upon the invasively embedded magnetostrictive cylindrical rod  102 , thereby varying the magnetic permeability of the magnetostrictive cylindrical rod. The varying magnetic permeability of the invasively embedded magnetostrictive cylindrical rod  102  produces a change in the inductance and impedance of magnetostrictive sensor  100 , which can be captured as a change in the voltage V S  across the coil  104 . The stress or force  106  applied to the structural element and, thus, upon magnetostrictive sensor  100 , can be determined by the produced change in inductance or impedance via the change in the voltage V S  by techniques well known in the art.  
      Magnetostrictive materials, such as nickel and nickel-iron alloys, are typically conductive. Therefore, the frequency of the time varying current is, typically in the kHz range to enhance bandwidth and response, in conjunction with the conductivity of magnetostrictive cylindrical rod  102 , results in eddy currents near the surface  108  of the magnetostrictive cylindrical rod by which the magnetic flux produced by the coil  104  is predominantly confined within the skin depth  110 , depicted in  FIG. 1B , of the surface. Therefore, magnetostrictive sensor  100  only responds to stress and force  106  near the surface  108  of magnetostrictive cylindrical rod  102 . Under planar conditions, the skin depth of a material, symbolized by δ, is known to follow: 
 
δ=1/√(π fμσ )=(π fμσ ) −1/2 .   (1) 
 
 The skin depth  110  of magnetostrictive cylindrical rod  102  in the example of  FIGS. 1A and 1B  is defined through equation (1) where f is the frequency of the time varying current is, μ is the magnetic permeability of the magnetostrictive cylindrical rod, and σ is the conductivity of the magnetostrictive cylindrical rod. Equation 1 is exact for a planar geometry, and approximate, but sufficiently close for design purposes, for other geometries such as the cylindrical case shown in  FIG. 1 . The skin depths of materials and the correlation of magnetic flux depth penetration to eddy currents and skin depth, are well known in the art. 
 
      What is needed is a simpler, cost effective method for determining force and torque acting upon structural elements utilizing magnetostrictive sensors which need not be invasively embedded within the structural element.  
     SUMMARY OF THE INVENTION  
      The present invention is a magnetostrictive sensor to sense force or torque (stress) applied to a structural element resulting in strain in the structural element to which the magnetostrictive sensor is non-invasively attached by an intimate contact with the structural element, whereby no air gap is present at the contact interface between the magnetostrictive sensor and the structural element.  
      The magnetostrictive sensor according to the present invention consists of, at least, a magnetostrictive layer, wherein the term “layer” is meant to include a “layer”, in intimate contact with a source of magnetic flux, whereby no air gap or an air gap as small as possible is present at the contact interface between the magnetostrictive layer and the source of magnetic flux, and wherein the source of magnetic flux is constructed to effectively and efficiently guide the produced magnetic flux to the magnetostrictive layer in order to maximize the magnetostrictive sensor response to strain. The air gap between the source of magnetic flux and the magnetostrictive layer should be as small as possible and is therefore preferably of zero length (no air gap). However, it must be recognized that under some circumstances there must be a clearance between the two, as for instance when the structural element and the magnetostrictive layer attached thereto are moving or rotating, and the source of magnetic flux is stationary. In the latter case, the reluctance of this air gap must be minimized, by reducing the length of the gap, or increasing its cross-section, in ways known in the art.  
      The non-invasive, fixed, intimate contact attachment of the magnetostrictive layer to the structural element can be accomplished by using kinetic spray, magnetic pulse welding of a sheet of magnetostrictive material to the structural element, or other techniques well known in the art, whereby no air gap is present at the contact interface between the magnetostrictive sensor and the structural element. The source of magnetic flux is, preferably, a coil (or coils), to which a, preferably, sinusoidally alternating current is applied to produce a magnetic flux, mounted within a core, whereby the core has a magnetic permeability selected to guide the magnetic flux generated by the current carrying coil within the core to the magnetostrictive layer in order to maximize the magnetostrictive sensor response to strain, and whereby no air gap or an air gap as small as possible is present at the contact interface between the magnetostrictive layer and the source of magnetic flux.  
      A force or torque applied to the structural element to which the magnetostrictive sensor is attached produces a stress within the structural element which is transferred to the magnetostrictive layer of the magnetostrictive sensor due to its fixed, intimate contact with the structural element, thereby varying the magnetic permeability of the magnetostrictive layer. The varying magnetic permeability of the magnetostrictive layer produces a change in the magnetic flux, thereby producing a change in the inductance and impedance of the coil of the magnetostrictive sensor, and thereby producing a change in the voltage across the coil. The force or torque applied to the structural element and, thus, upon the magnetostrictive sensor can be determined by the produced change in inductance or impedance via the change in the voltage of the coil by techniques well known in the art.  
      The non-invasiveness of the proposed sensor can be further appreciated by considering that with the present invention, the structural element material can be chosen to a large degree independently of the magnetostrictive sensor. For instance, if large stress levels are expected, a material with high yield strength such as steel can be chosen for the structural element, and the magnetostrictive layer can be chosen primarily for its magnetostrictive qualities, such as large permeability change versus stress.  
      Many variations in the embodiments of the present invention are contemplated as described herein in more detail. Other applications of the present invention will become apparent to those skilled in the art when the following description of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The description herein makes reference to the accompanying drawings wherein like reference numerals refer to like parts throughout the several views.  
       FIG. 1A  depicts a prior art magnetostrictive sensor.  
       FIG. 1B  is a representation of the skin depth associated with the prior art magnetostrictive sensor, seen along line  1 B- 1 B of  FIG. 1A .  
       FIG. 2A  depicts a sectional side view of a first preferred embodiment of a non-invasive magnetostrictive sensor according to the present invention.  
       FIG. 2B  depicts an example of a preferable source of magnetic flux of the first preferred embodiment of a non-invasive magnetostrictive sensor according to the present invention presented in  FIG. 2A .  
       FIG. 2C  depicts a top plan view of the first preferred embodiment of a non-invasive magnetostrictive sensor according to the present invention utilizing the source of magnetic flux example presented in  FIG. 2B .  
       FIGS. 2D and 2E  are views similar to  FIG. 2B , showing other aspects of the first embodiment, wherein the magnetic flux penetration is differing.  
       FIG. 3A  depicts a sectional side view of a second preferred embodiment of a non-invasive magnetostrictive sensor according to the present invention.  
       FIG. 3B  depicts a sectional end view of the second preferred embodiment of a non-invasive magnetostrictive sensor according to the present invention seen along line  3 B- 3 B of  FIG. 3A .  
       FIGS. 3C and 3D  are views similar to  FIG. 3A , showing other aspects of the second embodiment, wherein the magnetic flux penetration is differing.  
       FIG. 4  depicts a sectional side view of a third preferred embodiment of a non-invasive magnetostrictive sensor according to the present 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Referring now to the drawing,  FIGS. 2A through 2E  depict a first preferred embodiment of a non-invasive magnetostrictive sensor  200  according to the present invention to sense force or torque  212  applied to a structural element  204 , which may be made of a conducting, or of a non-conducting, material. The magnetostrictive sensor  200  is non-invasively attached to plane surface  202  of the structural element to thereby provide an intimate contact with the structural element, whereby no air gap is present at the plane of contact interface  214  between the magnetostrictive sensor and the structural element.  
      Non-invasive magnetostrictive sensor  200  consists of magnetostrictive layer (by the term “layer” is also meant “coating”)  210  of thickness  216  in intimate contact with a source of magnetic flux  220 , whereby no air gap or as small of an air gap as possible is present at the contact interface  222  between the magnetostrictive layer and the source of magnetic flux. The source of magnetic flux  220  is constructed to effectively and efficiently guide the produced magnetic flux  224 , depicted by way of example in  FIG. 2B , to the magnetostrictive layer  210  in order to maximize the magnetostrictive sensor response to strain.  
       FIG. 2B  depicts an example of a source of magnetic flux  220  which is, preferably, a typical conventional coil  206  mounted inside a core structure  208 , preferably having a predetermined number N of turns wound around a center core member  218  occupying space  226  of the core structure, whose magnetic characteristics and operation are well known in the art. Although many shapes are possible, the core structure  208  will be preferably designed to orient the flux lines according to the direction of the strain, in order to maximize sensitivity. A parallelepiped shape may be desirable in that respect. An alternating current, preferably, sinusoidally alternating current, is applied to the coil to produce a time varying magnetic flux  224  within the core structure  208 . The core structure  208  has a high magnetic permeability selected to guide the magnetic flux generated by the alternating current carrying coil  206  within the cylindrical core structure to the magnetostrictive layer  210  in order to maximize the magnetostrictive sensor response to strain.  
      Force or torque  212  is applied to the structural element  204  to which the magnetostrictive sensor  200  is fixedly attached, and thereby produces a stress within the structural element which is transferred to the magnetostrictive layer  210  of the magnetostrictive sensor due to its fixed, intimate contact with the structural element, and thereby varies the magnetic permeability of the magnetostrictive layer. As is known in the art, the varying magnetic permeability of the magnetostrictive layer  210  produces a change in the magnetic flux  224 , thereby producing a change in the inductance and impedance of the coil  206  of magnetostrictive sensor  200 , and thereby producing a change in the voltage V′ S  across the coil. Force  212  applied to structural element  204  and, thus, upon magnetostrictive sensor  200 , can be determined by the produced change in inductance or impedance of the coil  206  via the change in the voltage V′ S  of the coil by techniques well known in the art.  
       FIG. 2C  depicts a top view of a first preferred embodiment of a non-invasive magnetostrictive sensor  200  according to the present invention utilizing the source of magnetic flux  220  as shown in  FIG. 2B .  
      The depth of penetration  228  of the magnetic flux  224  into the magnetostrictive layer  210  is a function of the thickness of the layer  216  with respect to the frequency of the, preferably, sinusoidal alternating current supplied to coil  206 , the magnetic permeability μ C  of the magnetostrictive layer, the magnetic permeability μ S  of the structural element  204 , the conductivity σ C  of the magnetostrictive layer, and the conductivity σ S  of the structural element, and can be referenced to the skin depth, as defined by equation (1), of the magnetostrictive layer and/or the skin depth of the structural element. The skin depth δ C  of the magnetostrictive layer  210  is given by: 
 
δ C =1/√(π fμ   C σ C )=(π fμ   C σ C ) −1/2    (2) 
 
 where μ C  is the magnetic permeability of the magnetostrictive layer, σ C  is the conductivity of the magnetostrictive layer, and f is the frequency of the current supplied to coil  206 . The skin depth δ S  of the structural element  204  is given by: 
 
δ S =1/√(π fμ   S σ S )=(λ fμ   S σ S ) −1/2    (3) 
 
 where μ S  is the magnetic permeability of the structural element, σ S  is the conductivity of the structural element, and f is the frequency of the current supplied to coil  206 . 
 
      In a first aspect of the first preferred embodiment of the present invention as depicted at  FIG. 2B , the frequency of the alternating current supplied to coil  206 , the magnetic permeability μ C  of the magnetostrictive layer  210 , and the conductivity σ C  of the magnetostrictive layer are such that the thickness  216  of the magnetostrictive layer is greater than the skin depth δ C  of the magnetostrictive layer. In this case, magnetic flux  224  is within magnetostrictive layer  210  having a depth of penetration  228  into the magnetostrictive layer less than the thickness  216  of the magnetostrictive layer.  
      In the first aspect of the first preferred embodiment of the present invention, the reactive part of the voltage of the coil  206  which varies in response to the magnetostriction in layer  210  can be shown to be a function of the square root of the product of the frequency of the current supplied to the coil and the magnetic permeability μ C  of the magnetostrictive layer  210 . Force  212  applied to structural element  204  and, thus, upon the magnetostrictive sensor  200  can be determined by the produced change in inductance or impedance of the coil  206  via the change in the voltage V′ S  of the coil by techniques well known in the art.  
      In a second aspect of the first preferred embodiment of the present invention as depicted at  FIG. 2D , the frequency of the current supplied to coil  206 , the magnetic permeability μ C  of the magnetostrictive layer  210 , and the conductivity σ C  of the magnetostrictive layer are such that the thickness  216  of the magnetostrictive layer is approximately equal to or less than the skin depth δ C  of the magnetostrictive layer and the product of the magnetic permeability μ S  of the structural element  204  and the conductivity of the structural element σ S  is greater than a magnitude of at least ten times the product of the magnetic permeability μ C  of the magnetostrictive layer and the conductivity σ C  of the magnetostrictive layer. In this case, magnetic flux  224  is confined within the thickness  216  of magnetostrictive layer  210  and the depth of penetration  228  of the magnetic flux into the magnetostrictive layer is approximately equal to the thickness of the magnetostrictive layer, serving to increase the sensitivity of the magnetostrictive sensor  200 .  
      As an example of the second aspect of the first preferred embodiment of the present invention, the material of the magnetostrictive layer  210  is a suitable nickel-iron alloy having a thickness  216  of 0.4 millimeters and the material of the structural element  204  is iron, the magnetic permeabilities and conductivities of both materials being well known in the art. For a sinusoidally varying current supplied to coil  206  having a frequency of 1 kHz, the skin depth of the nickel-iron magnetostrictive layer  210  is 0.44 millimeters. Under stress, the magnetic permeability of the stressed nickel-iron magnetostrictive layer  210  decreases resulting in an increase in the skin depth of the stressed nickel-iron magnetostrictive layer, whereas the skin depth of the stressed iron structural element  204  does not change, or changes negligibly compared to the nickel-iron layer. Iron is magnetostrictive, but it is much less so, by orders of magnitude, than suitable nickel-iron alloys. The much smaller skin depth of the stressed iron structural element  204  serves to confine the depth of penetration  228  of the magnetic flux  224  within the thickness  216  of the nickel-iron magnetostrictive layer  210  and is approximately equal to the thickness of the nickel-iron magnetostrictive layer of 0.4 millimeters. Thus, a thickness  216  of 0.4 millimeters of a nickel-iron magnetostrictive layer  210  applied to an iron structural element at a frequency of 1 kHz supplied to coil  206  results in a depth of penetration  228  of the magnetic flux  224  approximately equal to the thickness of the nickel-iron magnetostrictive layer.  
      In the second aspect of the first preferred embodiment of the present invention, the reactive part of the voltage V′ S  of the coil  206  which varies in response to the magnetostriction in layer  210  can be shown to be a function of the product of the frequency of the current supplied to the coil and the magnetic permeability μ C  of the magnetostrictive layer  210 . Force  212  applied to structural element  204  and, thus, upon magnetostrictive sensor  200  can be determined by the produced change in inductance or impedance of the coil  206  via the change in the voltage V′ S  of the coil by techniques well known in the art.  
      In a third aspect of the first preferred embodiment of the present invention as depicted at  FIG. 2E , the frequency of the current supplied to coil  206 , the magnetic permeability μ C  of the magnetostrictive layer  210 , and the conductivity σ C  of the magnetostrictive layer are such that the thickness  216  of the magnetostrictive layer is less than the skin depth δ C  of the magnetostrictive layer and the product of the magnetic permeability μ S  of the structural element  204  and the conductivity of the structural element σ S  is not greater than a magnitude of at least about ten times the product of the magnetic permeability μ C  of the magnetostrictive layer and the conductivity σ C  of the magnetostrictive layer. In this case, the depth of penetration  228  of the magnetic flux  224  exceeds the thickness  216  of the magnetostrictive layer  210  and extends into the structural element  204 , whereby the magnetostrictive sensor  200  has a reduced sensitivity with respect to the second aspect of the first preferred embodiment of the present invention.  
      In the third aspect of the first preferred embodiment of the present invention, the reactive part of the voltage V′ S  of the coil  206  which varies in response to the magnetostriction in layer  210  can be shown to be a function of the product of the frequency of the current supplied to the coil and the magnetic permeability μ C  of the magnetostrictive layer  210 . Force  212  applied to structural element  204  and, thus, upon magnetostrictive sensor  200  can be determined by the produced change in inductance or impedance of the coil  206  via the change in the voltage V′ S  of the coil by techniques well known in the art.  
       FIGS. 3A through 3D  depict a second preferred embodiment of a non-invasive magnetostrictive sensor  300  according to the present invention to sense forces  302 ,  304  and torque  306  applied to structural element  308 , in the form of a shaft or rod comprised of a material which may or may not be a conductor, to which the magnetostrictive sensor is non-invasively attached to the cylindrical surface  310  thereof to thereby provide fixed, intimate contact with the structural element, whereby no air gap is present at the contact interface  312  between the magnetostrictive sensor  300  and the structural element  308 . In  FIGS. 3A and 3B , the structural element  308  is depicted as being hollow with thickness  314 , but the structural element may be alternatively solid.  
      The non-invasive magnetostrictive sensor  300  consists of magnetostrictive layer  316  of thickness  318  in intimate contact with a source of magnetic flux  320 , whereby no air gap or as small of an air gap as possible is present at the contact interface  322  between the magnetostrictive layer and the source of magnetic flux, wherein the source of magnetic flux is constructed to effectively and efficiently guide the produced magnetic flux  324  to the magnetostrictive layer  316  in order to maximize the response of the magnetostrictive sensor  300  to strain.  
      The source of magnetic flux  320  is, preferably, a coil  326  mounted inside a cylindrical core structure  328  encircling the cylindrical surface  310  of the shaft or rod  308 , preferably having a predetermined number N′ of turns wound around the cylindrical surface of the shaft or rod, wherein the magnetic characteristics and operation thereof are well known in the art. An alternating current, preferably, a sinusoidally alternating current is applied to the coil  326  to produce a time varying magnetic flux  324  within the cylindrical core structure, whereby the cylindrical core structure has, as described hereinabove with respect to the first preferred embodiment, a high magnetic permeability selected to guide the magnetic flux generated by the current carrying coil within the cylindrical core structure to the magnetostrictive layer  316  in order to maximize the magnetostrictive sensor response to strain.  
      Force  302 ,  304  or torque  306  applied to structural element  308  to which the magnetostrictive sensor  300  is attached produces a stress within the structural element which is transferred to the magnetostrictive layer  316  of the magnetostrictive sensor due to its fixed, intimate contact with the structural element, thereby varying the magnetic permeability of the magnetostrictive layer. As is known in the art, the varying magnetic permeability of the magnetostrictive layer  316  produces a change in the magnetic flux  324 , thereby producing a change in the inductance and impedance of the coil  326  of magnetostrictive sensor  300 , which can be captured as a change in the voltage across the coil (analogous to V′ S  as depicted in the first preferred embodiment). Force  302 ,  304  or torque  306  applied to structural element  308  and, thus, upon magnetostrictive sensor  300  can be determined by the produced change in inductance or impedance of the coil  326  via the change in the voltage of the coil by techniques well known in the art.  
       FIG. 3B  depicts a side view of the second preferred embodiment of a non-invasive magnetostrictive sensor  300  according to the present invention as shown in  FIG. 3A .  
      The depth of penetration  332  of the magnetic flux  324  into the magnetostrictive layer  316  is a function of the thickness of the layer  318  with respect to the frequency of the, preferably, sinusoidal current supplied to coil  326 , the magnetic permeability μ CC  of the magnetostrictive layer, the magnetic permeability μ SH  of the structural element  308 , the conductivity σ CC  of the magnetostrictive layer, and the conductivity σ SH  of the structural element and can be referenced to the skin depth, given by equation (1), of the magnetostrictive layer and/or the skin depth of the structural element. The skin depth δ CC  of the magnetostrictive layer  316  is given by: 
 
δ CC =1/√(π fμ   CC σ CC )=(π fμ   CC σ CC ) −1/2    (4) 
 
 where μ CC  is the magnetic permeability of the magnetostrictive layer, σ CC  is the conductivity of the magnetostrictive layer, and f is the frequency of the current supplied to coil  326 . The skin depth δ SH  of the structural element  308  is given by: 
 
δ SH   =1/√(π   fμ   SH σ SH )=(π fμ   SH σ SH ) −1/2    (5) 
 
 where μ SH  is the magnetic permeability of the structural element. σ SH  is the conductivity of the structural element, and f is the frequency of the current supplied to coil  326 . 
 
      In a first aspect of the second preferred embodiment of the present invention depicted at  FIG. 3A , the frequency of the current supplied to coil  326 , the magnetic permeability μ CC  of the magnetostrictive layer  316 , and the conductivity σ CC  of the magnetostrictive layer are such that the thickness  318  of the magnetostrictive layer is greater than the skin depth δ CC  of the magnetostrictive layer. In this case, magnetic flux  324  is within magnetostrictive layer  316  having a depth of penetration  332  into the magnetostrictive layer  316  less than the thickness  318  of the magnetostrictive layer.  
      In the first aspect of the second preferred embodiment of the present invention, the reactive part of the voltage of the coil  206  which varies in response to the magnetostriction in layer  210  can be shown to be a function of the square root of the product of the frequency of the current supplied to the coil and the magnetic permeability μ CC  of the magnetostrictive layer  316 . Force  302 ,  304  and torque  306  applied to structural element  308  and, thus, upon magnetostrictive sensor  300  can be determined by the produced change in inductance or impedance of the coil  326  via the change in the voltage of the coil by techniques well known in the art.  
      In a second aspect of the second preferred embodiment of the present invention depicted at  FIG. 3C , the frequency of the current supplied to coil  326 , the magnetic permeability μ CC  of the magnetostrictive layer  316 , and the conductivity σ CC  of the magnetostrictive layer are such that the thickness  318  of the magnetostrictive layer is approximately equal to or is less than the skin depth δ CC  of the magnetostrictive layer and the product of the magnetic permeability μ SH  of the structural element  308  and the conductivity of the structural element σ SH  is greater than a magnitude of at least about ten times the product of the magnetic permeability μ CC  of the magnetostrictive layer and the conductivity σ CC  of the magnetostrictive layer. In this case, magnetic flux  324  is confined within the thickness  318  of magnetostrictive layer  316  and the depth of penetration  332  of the magnetic flux into the magnetostrictive layer is approximately equal to the thickness of the magnetostrictive layer serving to increase the sensitivity of the magnetostrictive sensor  300 .  
      The example described herein above with respect to the second aspect of the first preferred embodiment of the present invention utilizing nickel-iron as the material of magnetostrictive layer  210  and iron as the material of structural element  204  may be analogously applied to the second aspect of the second preferred embodiment of the present invention.  
      In the second aspect of the second preferred embodiment of the present invention depicted at  FIG. 3C , the reactive part of the voltage of the coil  326  which varies in response to the magnetostriction in layer  316  can be shown to be a function of the product of the frequency of the current supplied to the coil and the magnetic permeability μ CC  of the magnetostrictive layer  316 . Force  302 ,  304  and torque  306  applied to structural element  308  and, thus, upon magnetostrictive sensor  300  can be determined by the produced change in inductance or impedance of the coil  326  via the change in the voltage of the coil by techniques well known in the art.  
      In a third aspect of the second preferred embodiment of the present invention depicted at  FIG. 3D , the frequency of the current supplied to coil  326 , the magnetic permeability μ CC  of the magnetostrictive layer  316 , and the conductivity σ CC  of the magnetostrictive layer are such that the thickness  318  of the magnetostrictive layer is less than the skin depth δ CC  of the magnetostrictive layer and the product of the magnetic permeability μ SH  of the structural element  308  and the conductivity of the structural element σ SH  is not greater than a magnitude of at least about ten times the product of the magnetic permeability μ CC  of the magnetostrictive layer and the conductivity σ CC  of the magnetostrictive layer. In this case, the depth of penetration  332  of the magnetic flux  324  exceeds the thickness  318  of the magnetostrictive layer  316  and extends into the structural element  308 , whereby the magnetostrictive sensor  300 , has a reduced sensitivity with respect to the second aspect of the second preferred embodiment of the present invention.  
      In the third aspect of the second preferred embodiment of the present invention, the reactive part of the voltage of the coil  326  which varies in response to the magnetostriction in layer  316  can be shown to be a function of the product of the frequency of the current supplied to the coil and the magnetic permeability μ CC  of the magnetostrictive layer  316 . Force  302 ,  304 , and torque  306  applied to structural element  308  and, thus, upon magnetostrictive sensor  300  can be determined by the produced change in inductance or impedance of the coil  326  via the change in the voltage of the coil by techniques well known in the art.  
       FIG. 4  depicts a third preferred embodiment of a non-invasive magnetostrictive sensor  400  according to the present invention to sense force or torque  212  applied to structural element  204  to which the magnetostrictive sensor is non-invasively attached to a surface  202  (for example, planar or cylindrical) of the structural element thereby providing fixed, intimate contact with the structural element, whereby no air gap is present at the contact interface  414  between the magnetostrictive sensor and the structural element.  
      The non-invasive magnetostrictive sensor  400  consists of magnetostrictive sensor  200  or  300  depicted in  FIGS. 2A through 3D  in fixed intimate contact with a conductive layer  410  of thickness  416 , whereby no air gap is present at the contact interface  420  between the magnetostrictive layer  210  and the conductive layer. By example, a magnetostrictive sensor  200 , having a source of magnetic flux  220  and coil  206 , is depicted in  FIG. 4 , wherein no air gap or as small of an air gap as possible is present at the contact interface  222  between the magnetostrictive layer and the source of magnetic flux. The operation of magnetostrictive sensor  400  utilizing magnetostrictive sensor  300  would be analogous to that described hereinabove.  
      A force  212  applied to structural element  204  to which the magnetostrictive sensor  400  is attached produces a stress within the structural element which is transferred to magnetostrictive layer  210  of the magnetostrictive sensor, via the conductive layer  410 , due to its fixed intimate contact with the conductive layer, thereby varying the magnetic permeability of the magnetostrictive layer. As is known in the art, the varying magnetic permeability of the magnetostrictive layer  210  produces a change in the magnetic flux  224 , thereby producing a change in the inductance and impedance of the coil  206  of magnetostrictive sensor  200 , and thereby producing a change in the voltage V″ S  across the coil. Force  212  applied to structural element  204  and, thus, upon magnetostrictive sensor  400  can be determined by the produced change in inductance or impedance of the coil  206  via the change in the voltage V″ S  of the coil by techniques well known in the art.  
      The depth of penetration of the magnetic flux  224  into the magnetostrictive layer  210  is a function of the thickness of the layer  216  with respect to the frequency of the, preferably, sinusoidally alternating current supplied to coil  206 , the magnetic permeability μ C  of the magnetostrictive layer, the magnetic permeability μ CN  of the conductive layer  410 , the conductivity σ C  of the magnetostrictive layer, and the conductivity σ CN  of the conductive layer and can be referenced to the skin depth, given by equation (1), of the magnetostrictive layer and/or the skin depth of the conductive layer. The skin depth δ C  of the magnetostrictive layer  210  is given by equation (2), where now μ C  is the magnetic permeability of the magnetostrictive layer, σ C  is the conductivity of the magnetostrictive layer, and f is the frequency of the current supplied to coil  206 . The skin depth δ CN  of the conductive layer  410  is given by: 
 
δ CN =1/√(π fμ   CN σ CN )=(π fμ   CN σ CN ) −1/2    (6) 
 
 where μ CN  is the magnetic permeability of the conductive layer, σ CN  is the conductivity of the conductive layer, and f is the frequency of the current supplied to coil  206 . 
 
      In the third preferred embodiment of the present invention, the frequency of the current supplied to coil  206 , the magnetic permeability μ C  of the magnetostrictive layer  210 , and the conductivity σ C  of the magnetostrictive layer are such that the thickness  216  of the magnetostrictive layer is less than the skin depth δ C  of the magnetostrictive layer, whereas the frequency of the alternating current supplied to coil, the magnetic permeability μ CN  of the conductive layer  410 , and the conductivity σ CN  of the conductive layer are such that the thickness  416  of the conductive layer is approximately equal to or larger than the skin depth δ CN  of the conductive layer and the product of the magnetic permeability μ CN  of the conductive layer and the conductivity σ CN  of the conductive layer is greater than a magnitude of at least about ten times the product of the magnetic permeability μ C  of the magnetostrictive layer and the conductivity σ C  of the magnetostrictive layer. In this case, magnetic flux  224  is confined within the thickness  216  of magnetostrictive layer  210  and the depth of penetration of the magnetic flux into the magnetostrictive layer is approximately equal to the thickness of the magnetostrictive layer serving to increase the sensitivity of the magnetostrictive sensor  400 . The reactive part of the voltage V″ S  of the coil  206  which varies in response to the magnetostriction in layer  210  can be shown to be a function of the product of the frequency of the current supplied to the coil and the magnetic permeability μ C  of the magnetostrictive layer  210 . Force  212  applied to structural element  204  and, thus, upon magnetostrictive sensor  200  can be determined by the produced change in inductance or impedance of the coil  206  via the change in the voltage V″ S  of the coil by techniques well known in the art.  
      The non-invasiveness of the proposed sensor can be further appreciated by considering that with the present invention, the structural element material can be chosen to a large degree independently of the magnetostrictive sensor. For instance, if large stress levels are expected, a material with high yield strength such as steel can be chosen for the structural element, and the magnetostrictive layer can be chosen primarily for its magnetostrictive qualities, such as large permeability change versus stress.  
      It is to be understood that forces  212 ,  302 , and  304  and torque  306  applied to structural elements  204 ,  308  impose stresses upon the structural elements and, in particular, surface stresses upon the structural elements. The surface stresses imposed upon the surfaces  202 ,  310  of the structural elements  204 .  308  in  FIGS. 2A-2E  and  3 A- 3 C result in surface strains upon the structural elements which are transferred to the magnetostrictive layers  210 ,  316  due to their fixed, intimate contact with the structural elements, thereby varying the magnetic permeabilities of the magnetostrictive layers by which the forces and torque applied to the structural elements can be determined as previously described. The surface stress imposed upon the surface  202  of the structural element  204  in  FIG. 4  results in a surface strain upon the conductive layer  420  which is transferred to the magnetostrictive layer  210  due to its fixed, intimate contact with the conductive layer, thereby varying the magnetic permeability of the magnetostrictive layer by which the forces and torque applied to the structural element can be determined as previously described. As such, the present invention is, in this sense, a magnetostrictive sensor to sense strain imposed upon a structural element as previously described.  
      It is, also, to be understood that the terms “force and “torque” are applicable to, and inclusive of, all causes of stress, including for example pressure, vacuum, impact, acceleration, deceleration, and are, as such, within the scope of the present invention.  
      To those skilled in the art to which this invention appertains, the above described preferred embodiment may be subject to change or modification. Such change or modification can be carried out without departing from the scope of the invention, which is intended to be limited only by the scope of the appended claims.