Patent Publication Number: US-9417294-B2

Title: Current sensors using magnetostrictive material

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 61/726,204 filed Nov. 14, 2012, the content of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     The present invention relates to current sensors which use magnetostrictive material. 
     Known current sensors suffer from a number of drawbacks, including sensitivity to electromagnetic interference (EMI), bulky design, and slow response time. 
     SUMMARY 
     Accordingly, improved current sensor designs are disclosed herein. 
     In one embodiment, the invention provides a current sensor device. The current sensor device includes a strain distribution converter; an optical fiber coupled with the strain distribution converter; and a magnetostrictive material associated with the strain distribution converter such that a change in shape of the magnetostrictive material causes a change in length of the optical fiber. 
     In another embodiment the invention provides a current sensor device. The current sensor device includes a strain distribution converter and a magnetostrictive material associated with the strain distribution converter such that a change in shape of the magnetostrictive material causes a change in shape of the strain distribution converter. 
     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1( a )  shows a chirped fiber optic current sensor (FOCS) in the absence of a magnetic field (H=0) and  FIG. 1( b )  shows a chirped FOCS in the presence of a magnetic field (H≠0). 
         FIG. 2  shows an embodiment of a current sensor including a fiber Bragg grating (FBG) associated with a magnetostrictive composite material in which the FBG is oriented parallel to the magnetic field, shown when the magnetic field is zero (left) or the magnetic field is nonzero (right). 
         FIG. 3 a    shows another embodiment of a current sensor including an FBG associated with a magnetostrictive composite material in which the FBG is oriented perpendicular to the magnetic field in an air gap of a magnetic core. 
         FIG. 3 b    shows directions of expansion and contraction of a current sensor in a transverse orientation relative to the easy axis of a magnetic field as in  FIG. 3   a.    
         FIG. 4 a    shows a FOCS response for H=0 and  FIG. 4 b    shows a FOCS response for large H where FBG 1  has grating period Λ 1 , length L 1  and Bragg wavelength λ B1  and FBG 2  has grating period Λ 2 , length L 2  and Bragg wavelength λ B2 . 
         FIG. 5  shows a transfer function of the FOCS of  FIGS. 4 a    and  4   b.    
         FIG. 6  shows a schematic diagram for a DC measurement setup. 
         FIG. 7  shows a physical layout for a DC measurement setup. 
         FIG. 8 a    shows variation of reflected spectra with microstrain (με) in a chirped FOCS. 
         FIG. 8 b    shows the response of the chirped FOCS. Inset: polymer shape. Black Dotted line: k=0.20 cm −1 ; Green Dashed line: k=0.35 cm −1 ; Red Solid line: k=0.50 cm −1 ; Blue Dash-Dot: known design. 
         FIG. 9  shows variation of spectral width with magnetic field in a chirped FOCS. 
         FIG. 10  shows response of the chirped FOCS. 
         FIG. 11  shows an image of a Terfenol-D composite having a speckle pattern. 
         FIG. 12  shows a 400% zoom of the sample of  FIG. 11  to better show the speckle pattern. 
         FIG. 13  shows a diagram for experimental setup and measurement points. 
         FIG. 14  shows magnetostrictive strain versus magnetic field. 
         FIG. 15  shows application of a magnetic field during composite fabrication. 
         FIG. 16 a    shows a schematic diagram for a setup for using a FOCS for AC current measurements. 
         FIGS. 16 b  and 16 c    show images of a physical layout for measuring an AC current passing through a bus bar. 
         FIG. 17  shows a sensor with a magnetic core. 
         FIG. 18  shows an embodiment of a current sensor as in  FIGS. 1 a  and 1 b    which is stabilized. 
         FIG. 19  shows another embodiment of a current sensor as in  FIGS. 1 a  and 1 b    which is stabilized. 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
     In various embodiments the invention includes various types of current sensors including, in certain embodiments, fiber optic-based current sensor (FOCS) devices. The devices may be envisioned in various embodiments, with two specific embodiments provided but not limiting the scope of the invention. The current sensors measure the current from an adjacent conductor based on a correlation between the current flowing through the conductor and the signal response of the current sensor device. 
     The current sensors may also be described as having an output optical power proportional to the magnitude of the magnetic field in which the sensor is placed. Since a current-carrying conductor generates a magnetic field, the current magnitude may be inferred based on measurements of the magnetic field adjacent the conductor. 
     Accordingly, the current sensors disclosed herein are based on deformation of a strain distribution converter by a magnetostrictive material when the current sensors are placed in a magnetic field. Disclosed herein are several methods for quantifying the deformation of the strain distribution converter, which can then be used to quantify the current magnitude. 
     In some embodiments, the strain distribution converter deforms in a non-linear fashion along its length in the presence of a magnetic field. In some embodiments, the non-linear deformation is a result of the strain distribution converter having a variable (e.g. tapered) cross-sectional area along its length. In other embodiments, there is a gradation of magnetostrictive material along the length of the strain distribution converter, which leads to the non-linear deformation. In these embodiments, regions of the strain distribution converter having larger amounts of magnetostrictive material deform to a larger degree than regions having smaller amounts of magnetostrictive material. 
     In certain embodiments, deformation of the strain distribution converter is measured optically. In embodiments such as these, the current sensor includes an optical fiber coupled with a fiber Bragg grating (FBG) along with the strain distribution converter and the magnetostrictive material. In one embodiment the FBG is one of a number of known types of optical gratings that may be coupled with (e.g. embedded in) the strain distribution converter. In particular embodiments the FBG is a fiber optic having a long axis. In general, suitable design of the strain distribution converter leads to a non-linear response in which the FBG period is regulated, effectively converting a uniform FBG into a chirped FBG. As magnetic field magnitude increases, the output optical signal is increasingly frequency chirped, i.e. bandwidth-widened. 
     In particular embodiments, the tapered section of the polymer has a width given by the formula w(z)=w 0 /(1+k z), where w 0  is the maximum polymer width, k is a parameter with units of inverse distance, and z is the axial coordinate with origin at the wider end of the polymer block. In this case, the transducer force applied to the small cross sectional end will incur less strain on the FBG as z decreases. With careful control and accurate transfer of the SD on the FBG, the FBG period will be regulated accordingly. As H increases, an FBG with uniform period (see  FIG. 1 a   ) is transformed into a non-uniform one (see  FIG. 1 b   ). 
     In those embodiments which include an FBG and in which the strain distribution converter has a variable cross-sectional area along its length, the cross-sectional area of the strain distribution converter orthogonal to the FBG may vary nonlinearly along the axis of the FBG. In certain embodiments, the magnetostrictive material may include an elongated element (e.g. a rod) that abuts the strain distribution converter at one end, for example at the location where the fiber optic exits the strain distribution converter ( FIG. 1 a   ). In general, this may also be the end of the strain distribution converter having the smallest cross-sectional area. The opposite end of the strain distribution converter and the opposite end of the magnetostrictive element are held fixed. Thus, when the magnetostrictive material changes length it causes changes in the shape of the strain distribution converter. If the strain distribution converter expands due to an increase in magnetic field strength ( FIG. 1 b   ), this leads to compression of the strain distribution converter. As a result of the non-linear (e.g. tapered) shape of the strain distribution converter, the compression would be non-linear, with greater amounts of compression occurring in the regions having a smaller cross-sectional area. Given that the FBG is coupled to the strain distribution converter, changes in shape of the strain distribution converter cause changes in shape of the FBG. Since the changes in shape of the strain distribution converter are non-linear, this causes non-linear changes in the FBG such that the uniform FBG response is converted to a chirped FBG response. 
     In another embodiment, the major components include an FBG, a magnetostrictive element (e.g. a Terfenol-D composite material), a strain distribution convertor (e.g. a polymer with varying cross section area along FOCS axis), and a temperature compensation material (e.g. MONEL 400) ( FIGS. 1 a , 1 b   ). In this embodiment, the magnetostrictive element is supported by a sleeve made of the temperature compensation material; the temperature compensation material is chosen so that it has the same coefficient of thermal expansion as the magnetostrictive material. As above, using suitable materials and shapes the FBG period may be regulated, effectively converting a uniform FBG into a chirped FBG. As the magnetic field magnitude increases, the output optical signal is increasingly frequency-chirped, i.e. bandwidth-widened. The rise in bandwidth translates into increases in return power. In this embodiment the FBG is embedded inside a block of polymer (which acts as the strain distribution converter), allowing the FBG to sense the strain distribution inside the polymer. Owing to the fact that the cross sectional area of the polymer is a function of axial distance (z) from the small cross sectional end, a force applied to the small cross sectional end will cause less strain/compression on the FBG as z increases. 
     With careful control and accurate transfer of the strain distribution on the FBG, the FBG period can be regulated accordingly and the FBG assumes a chirped profile. In this embodiment, a thermally compensated Terfenol-D transducer is used to stress the force sensor. The transducer may include a MONEL 400 sleeve (pipe) and a Terfenol-D cylinder. The MONEL 400 sleeve thermally expands along with Terfenol-D cylinder since the two materials have the same thermal expansion coefficient. The cylinder and sleeve may be attached to a rigid and thermally stable support (such as a carbon-fiber composite) at one end while the other end of the sleeve will be adhered to the strain distribution converter. As a result, the cylinder will push the force sensor when there is a magnetic field along the z axis in a manner that is independent of temperature changes. To enable optimal transfer of strain to the FBG, the transducer is placed as close as possible to the FBG. This may be achieved by drilling a hole through the Terfenol-D cylinder to channel the optical fiber through the transducer. For easy machining and fast transducer response, a Terfenol-D composite (discussed further below) may be used to fabricate the cylinder with another appropriate material, such as MONEL 400 composite, that has the same thermal expansion coefficient. 
     In other embodiments, a ‘chirped’ FOCS can be further simplified if it is fabricated from a composite having a gradation of magnetostrictive material (such as Terfenol-D) from one end to the other. In one embodiment, the current sensor may be simplified to a graded composite of magnetostrictive material plus an FBG.  FIG. 2  shows a graded composite of magnetostrictive material in the absence (left) or presence (right) of a magnetic field. 
     In various embodiments, the magnetostrictive material may include Terfenol-D, Galfenol, or Metglas. Particles of Terfenol-D may be used in sizes ranging from 20 to 300 microns in diameter and elongated particles up to 1 mm in length may also be used. The magnetostrictive material is embedded in the strain distribution converter material, e.g. a polymer. The graded composite may include larger particles and/or a higher volume fraction of magnetostrictive materials at one end and smaller particles and/or a lower volume fraction at the other end. Graded composites are discussed further below. 
     When placed in a magnetic field the graded composite material undergoes a non-linear shape change, with the regions having a higher volume fraction and/or larger particle size experiencing a greater shape change than the regions having a lower volume fraction and/or smaller particle size. Since strain scales up with the higher volume fraction region, the region with higher volume fraction has higher strain. In various embodiments, the FBG is embedded inside the graded magnetostrictive composite and the composite material is disposed adjacent to a magnetic field such that the gradient of magnetostrictive material and the FBG are parallel to the magnetic field ( FIG. 2 ). When the strain is transferred fully to the FBG, an initially uniform FBG under zero magnetic field ( FIG. 2 , left) is transformed into a chirped FBG when the magnetic field is nonzero ( FIG. 2 , right), resulting a similar response function as the embodiment having a tapered strain distribution converter. However, unlike the embodiment having the tapered strain distribution converter, in the present embodiment which uses a graded magnetostrictive composite the graded composite functions as a transducer as well as a strain converter and does not require thermal compensation. 
     In yet another embodiment, a current sensor having a graded magnetostrictive composite with an embedded FBG is disposed within the air gap of a magnetic core and is oriented with the magnetic field perpendicular to the direction of the gradient of magnetostrictive material and to the FBG axis ( FIG. 3 a   ). In this embodiment the FBG is not oriented parallel to the easy axis of the magnetic field composite, but instead is oriented orthogonal to the easy axis of the composite ( FIG. 3 b   ). Without being limited as to theory, it appears that the Poisson relationship between the easy axis and an orthogonal or a transverse axis facilitates the function of this embodiment. In this embodiment it is counterintuitive to use a transverse configuration, since typical metals have a ratio of transverse strain magnitude to longitudinal strain of 30%, resulting in a significant decrease in sensor sensitivity. Preliminary results with digital image correlation indicate that the ratio for monolithic Terfenol-D is 60%, while its value for Terfenol-D composite is 46%. Such reduction in strain translates into a slight decrease in sensitivity of the transverse FOCS. On the other hand, the transverse FOCS orientation experiences an increased concentration of the magnetic field which can make up for some or all of the decrease in sensitivity. Additional sensitivity can be provided by increasing the concentration of the magnetic field with a magnetic core. 
     This latter embodiment permits the use of a magnetostrictive material having a smaller dimension in the magnetic field direction and hence allows easy integration with a magnetic core and permits a decrease in the air gap to enhance sensitivity. Since the field enhancement from a magnetic core scales up drastically as its air gap decreases, the transverse FOCS can become highly sensitive with a sufficiently small air gap. Moreover, the transverse configuration enables simple fabrication without tedious fine tuning. For example, the uniform FOCS (embodiment of  FIG. 4 a , 4 b   ) requires a dedicated balance of strains on the portions of FBG over Terfenol-D and MONEL-400 to achieve a minimum output at zero magnetic field by tuning the three mounting points. In contrast, a transverse FOCS only requires attaching the FBG to Terfenol-D and MONEL-400 or magnetostrictive composite under a high magnetic field. The procedure provides flexibility in choosing the maximum operating current and improving the sensitivity at low magnetic field. The transverse FOCS output is high with low magnetic fields and vice versa. Additionally, the difficulty in fabricating graded Terfenol-D composite can be greatly alleviated with self-assembly schemes for transverse FOCS. For instance, graded Terfenol-D composites can be easily generated by aligning Terfenol-D particles with a magnetic field H while controlling volume fraction with the assistance of gravity (discussed below). 
     While many of the embodiments disclosed herein refer to the use of Terfenol-D as the magnetostrictive material, those skilled in the art will understand that other such materials may be used along with, or in place of, Terfenol-D, including Galfenol, Metglas, or other suitable magnetostrictive materials. Terfenol-D (chemical formula: Tb x Dy 1−x Fe 2  (x˜0.3)) is particularly suitable as it is a giant magnetostrictive material with high energy density and the highest magnetostriction of any alloy. Terfenol-D expands and contracts as a function of magnetic field strength. 
     In various embodiments, the disclosed current sensors may be used as part of a network of optical sensors for coordinating power system protection, detecting and locating power system faults, as disclosed in Law et al. (C. T. Law, K. Bhattari and D. C. Yu, “Fiber-Optics-Based Fault Detection in Power Systems,”  IEEE Trans. Power Delivery , vol. 23, pp. 1271-1279, 2008; incorporated herein by reference in its entirety). In various embodiments the FOCS can be connected in series by connecting the proximal end of a first sensor to a light source such as a superluminous light emitting diode (SLED) and the distal end of the first sensor to a second sensor, etc. (e.g. see diagrams in  FIGS. 1 a , 1 b   ,  2 ; see also Law et al.). This type of sensor network can be extremely effective in enhancing power system reliability. FOCSs are immune to EMI and have a compact design in comparison to those of the conventional sensors, such as iron-core current transformers, and are suitable for detecting current surges and current direction associated with line faults. For successful interface with an optical sensor network, FOCS convert electrical disturbances into distinctive optical signals for easy detection at a low cost of implementation and maintenance. 
     The magnetostrictive-based FOCSs are cost effective and have a more pronounced response than those of Faraday-effect based FOCSs. The cost and complexity of installing a FOCS network can be further reduced by integrating a fiber Bragg grating (FBG), which also enables easier multiplexing of fault signals in a network of FOCSs. The FBG translates the strain variation induced by a magnetostrictive (e.g. Terfenol-D) transducer to a shift in the FBG resonant wavelength, known as the Bragg wavelength (λ B ). This is manifested in the reflected power spectrum. The operation of the FOCS in  FIGS. 4 a , 4 b    can be easily explained by considering it as a cascade of two FBGs, i.e. the reflected power spectrum R T (λ) of the FOCS (carrying magnetic field intensity (H) information) is a product of the reflected power spectra (R 1 (λ) and R 2 (λ)) of 2 FBGs, FBG 1  and FBG 2 . As MONEL 400 and Terfenol-D experience the same thermal strain, they maintain the same Bragg wavelength λ B1  and grating period Λ 1 . As long as H=0 (as in  FIG. 4 a   ), R 1 (λ) and R 2 (λ) remain identical and, most importantly, the reflective spectral peak locations of the two FBGs are identical. Consequently, the output of the FOCS, i.e. the area under the R T  (λ) curve, is constant and independent of temperature. When H surges with the fault current, Terfenol-D piece elongates and parameters of FBG 2  (attached to the piece) change to L 2 , λ B2  and Λ 2 . These variations are manifested as a modulation in the reflected power and two distinct peaks in the reflective spectrum of the FOCS. Hence, the FOCS output (the area under the R T  (λ) curve) rises proportionally to H (see  FIG. 4 b   ) until R 1 (λ) and R 2 (λ) are entirely apart. 
     The transfer function of the FOCS in  FIGS. 4 a  and 4 b    varies linearly for H between 10 kA/m and 20 kA/m (see  FIG. 5 ), results that have been validated using the setup for DC current measurement shown in  FIGS. 6 and 7 . A superluminous light emitting diode (SLED) was used as the broadband light that interrogated the FOCS. The SLED outputs infra-red light with a central wavelength of 1547 nm and a line width of 63 nm, which was then passed through an optical isolator, enforcing a unidirectional propagation path. The light subsequently entered port 1 of an optical circulator (OC) and reached the FBG through port 2 of the OC. The FBG returned a narrow band of the SLED light near its Bragg wavelength back to port 2. The returned signal from the FBG (output of FOCS) exited the OC through port 3 and was measured by a wavelength meter. The use of an isolator and OC greatly reduced reflection-related instability and maximized the return signal. The FOCS output spectrum was modulated by the H from a coil connected to two 20 A DC power supplies in series. During experiments, the current level was tuned and the magnetic field H and the FOCS output spectrum were recorded. A similar setup was used for 60 Hz AC measurements using a high current source except that the coil and voltage sources were replaced by a bus bar connected to a 1000 A current source. The response of FOCS was similar to that of a conventional Hall effect sensor, except that the FOCS output was unipolar. In particular, the response of the FOCS to the magnetic field H translated to measurements of current in hundreds to thousands of amperes. By using a magnetic core, the FOCS sensitivity was boosted by at least five fold. It is anticipated that the use of a magnetic core, with optimum design, can provide a gain in sensitivity by a factor of up to fifty fold. 
     During fabrication of the FOCS disclosed herein, challenges included the brittleness of Terfenol-D and the fragility of FOCS. To extend the application of FOCS to other uses such as accurate electric current measurements for applications such as motor drives, a current sensor with a short response time and wide dynamic range are required. Although Terfenol-D is a giant magnetostrictive material that produces strain on the order of 1000 ppm, its operational frequency is limited to the order of kHz and its brittleness hinders its use of conventional machining methods for device fabrication. Moreover, Terfenol-D is a rather rare and expensive alloy. To address all these issues, Terfenol-D composites were considered. In a composite with Terfenol-D particles surrounded by a nonmetallic binder (such as epoxy resin or other polymers), the flow of eddy current is interrupted among particles by the increased electrical resistivity of the resin. As a result, the heat generation by eddy current losses is reduced and higher operational frequencies can be reached. In principle, Terfenol-D particles have a response time on the order of microseconds under ideal conditions. Another advantage of a Terfenol-D composite is its flexibility in manufacturing and machining. For instance, complex shapes can be produced with mold injection methods. Since the composite has a volume fraction of Terfenol-D less than one, the FOCS based on composite Terfenol-D will incur less material costs. For the same reason, the strain induced by composite Terfenol-D is expected to be less than that of monolithic Terfenol-D. Moreover, the ability of controlling strain distribution by varying the Terfenol-D particle concentration along the magnetic sensing direction provides opportunities for designing a new type of FOCS that may be able to compensate for reductions in the maximum strain associated with the use of composite materials. 
     Disclosed herein are methods for fabrication of Terfenol-D composites which permit better control of the Terfenol-D particle distribution and which facilitate understanding of interaction between the epoxy and the Terfenol-D particles. FOCSs using these composites are expected to be rugged since the FBG will be embedded inside a polymer or an epoxy. Embedding the FBG inside a block of material allows the FBG to sense the strain distribution inside that material. With careful control and accurate transfer of the strain distribution on the FBG, the FBG period will be regulated accordingly. Such a modulation on the FBG period will introduce frequency chirping to an optical signal, i.e. increase in the bandwidth of the reflected signal. The rise in bandwidth translates into increases in return power. Therefore, the return power can be used to infer the level of magnetic field by converting the return power to a specific magnetic magnitude field level and correlating this to a given strain distribution. 
     Among other applications, composites of magnetostrictive materials such as Terfenol-D may be used with a FOCS such as that shown in  FIGS. 1 a  and 1 b   . In particular, composites of Terfenol-D and MONEL 400 may be placed between the support and the tapered block shown in  FIGS. 1 a  and 1 b   . This arrangement can be used to generate a chirped signal as described above. 
     For simplicity, the response of the chirped FOCS was simulated with a monolithic Terfenol-D transducer rather than a composite. The variation of cross section of the strain distribution converter was designed along the z axis so that FBG period will vary linearly with z and introduce linear chirping for the reflected optical signal, as discussed above.  FIG. 8 a    displays the variation of reflected spectra with maximum strain introduced by the transducer. 
     Expansion of the Terfenol-D caused by a magnetic field with intensity H, denoted by strain ε (H), compresses the polymer with a uniform strain gradient, introducing a linear chirp in the fiber grating period given by 
     
       
         
           
             
               
                 
                   
                     
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     where L T  is the length of the Terfenol-D, L 0  is the length of the tapered section of the polymer, and λ B  is the unchirped Bragg wavelength of the grating. In order to model the strain experienced by the Terfenol-D under the influence of a magnetic field, we have used a third-order polynomial approximation to fit experimental data that we have collected using a FBG as a strain gauge by attaching it to a T-D rod at two points. 
     Numerical simulations are performed to find the reflectance spectrum using two separate methods. Both are based on the “synchronous approximation” to the coupled mode equations describing forward- and backward-propagating modes in the fiber at wavelengths near resonance. The first method involves a numerical fourth-order Runge-Kutta approximation. The second method assumes a piecewise uniform structure for the grating, and the power reflectance at each wavelength is determined by multiplication of a series of transfer matrices. The transfer matrices used in this method are modified to provide a closer approximation of the actual output. Results obtained with the two methods generally agree very well. With these numerical techniques, power spectra of FOCS returned signals under various SDs for certain Hs were obtained. 
     Sensor output as a function of the incident magnetic field is shown in  FIG. 8 b   . Here, values of n eff =1.46547, λ B =1550 nm, L T =3 cm, and L 0 =3 cm were used. It can be seen that the sensitivity is improved at lower field values and that there is over a double relative increase in power over the design in  FIGS. 4 a  and 4 b   . Additionally, the dynamic range is dramatically improved over the original fiber optic current sensor design in  FIGS. 4 a  and 4 b   , as evidenced by the larger quasi-linear range output range and increased saturation field. 
     Note that the non-uniform strain distribution causes a fairly steady change in spectral peak and linewidth as strain increases. However, the analysis focuses only on the linewidth since it is immune from the effect of uniform strain induced by other environmental artifacts, such as thermal expansion of the polymer. In fact,  FIG. 9  depicts a nearly linear relationship between linewidth and magnetic field. Similarly,  FIG. 10  shows the chirped FOCS reflected power as a function of magnetic field. Again, the response is fairly linear with extended dynamic range (compare to  FIG. 5 ) and somewhat enhanced sensitivity. 
     The structure of the chirped FOCS can be further simplified if it is fabricated as a Terfenol-D composite with graded particle size distribution and/or volume fraction, using particles of Terfenol-D ranging from 20 to 300 microns. In some embodiments, elongated particles up to 1 mm in length may also be used (see  FIG. 2 ). In  FIG. 2 , areas with larger particles and/or higher volume fractions of magnetostrictive material are indicated using darker grayscale values. Since strain scales up with the higher volume fraction region, the darker regions will have higher strain than the lighter regions. If the strain can be transferred fully to the FBG, a uniform FBG under zero magnetic field (see  FIG. 2 , left) can be completely transformed into a chirped FBG when the magnetic field is nonzero (see  FIG. 2 , right). 
     Specimens were fabricated using a volume fraction V f =0.4 and a dimension of 1″×1″×0.25″. The resulting Terfenol-D composite, which has the speckle pattern shown in  FIG. 11  and further magnified four times in  FIG. 12 . Two-dimensional digital image correlation (DIC) was used to capture the strains on the surface of the composite specimen. The DIC technique uses a random speckle pattern applied to the specimen that is captured using a couple-charged device (CCD) camera. These images are then processed using correlation algorithms to compare strained maps to a reference image taken before the loading is applied. 
     Using the DIC method, it was possible to observe strain distribution and creep effect for these samples. Preliminary DIC data (without calibration) indicate that these samples have average strain values of 400 ppm when they were very close to a pair of N52 rare earth magnets. However, to perform a more accurate measurement of average strain for these samples, an FBG was used as a strain gauge by attaching it along the magnetostriction axis at two points and a 12 gauge copper wire was laid across the top of the sample with adhesive tape to prevent it escaping from the holder. The sample with holder was clamped on one arm of a PC board holder and was placed in between two N52 rare earth magnets (1″ diameter and 0.5″ thick) that were attached at a known spacing apart (using duct tape to the front and back jaws of a bench vice). The distance between the magnets and the sample can be adjusted by moving the jaws of the bench vise and hence, this can be used to control the magnetic field magnitude. Before each measurement, the separation between the magnets was first changed and then the sample was centered between the magnets. After all components were secured and fixed, magnetic field measurements were performed at three locations (see  FIG. 13 ). The reflected power spectrum was recorded from a wavelength meter and the peak power wavelength of the spectrum was also recorded. Magnetic field and power spectrum data were collected for eight magnet separation distances (d&#39;s) varying from 66 mm to 37 mm. 
       FIG. 14  shows a comparison of the magnetostriction of monolithic Terfenol-D to that of its composite counterpart. Since the composite has V f  less than 1, its sensitivity is expected to be diminished compared to the monolithic material.  FIG. 14  confirms the sensitivity dropping to approximately 50% of the monolithic material&#39;s values. The reduction in magnetostriction can be attributed to the somewhat random distribution of Terfenol-D particles. As a result, positive strains are localized around regions with clusters of Terfenol-D particles while negative strains are formed in other sites. On average, the strain of the composite will be lower than that of monolithic Terfenol-D. 
     Fiber optic current sensors generally are compact and simple in construction and immune from EMI and have advantages in measuring high current and flexibility in forming a sensor network. Moreover, they require only simple power measurements without elaborate signal processing for monitoring current. The novel designs disclosed herein using magnetostrictive materials such as Terfenol-D or composites of such materials further enhance the sensor ruggedness, response speed, dynamic range, and sensitivity. In particular, the transverse FOCS configuration allows for easier integration with a magnetic core for sensitivity enhancement than typical FOCS designs. Moreover, this configuration has a facile manufacturing method allowing for fast fabrication of FOCS based on uniform FOCS as well as chirped FOCS. 
     The invention and its embodiments may be applied to a range of current sensing application including such applications as high current power line sensors and in current detection in motor or other power systems, all while not requiring the sensor to be in direct contact with the conductor(s). 
     The above-described embodiments may be varied in a number of ways that are within the scope of the original invention. 
     Several different methods may be used to produce graded composites of magnetostrictive materials and strain distribution converter, whether in a continuous or stepwise gradient. As indicated above, the magnetostrictive material may vary across the gradient by volume fraction and/or particle size. The typical fiber volume fraction ranges will be from 0 to 0.45, although in some embodiments the volume fraction may be as high as 0.65. Sizes of the particles of magnetostrictive materials may range from 20 μm to 300 μm, although in some embodiments elongated (high aspect ratio) particles may be used which range up to 1000 μm (1 mm) in length. 
     In some embodiments the graded composite may have smaller particles at one end and larger particles at the other end, with a graded range of particle sizes in between. These varying particle sizes may be present in a constant volume fraction across the composite or the volume fraction may increase with increasing particle size. Particles may be sorted by size using sieves with or without the use of centrifugal force to separate the particles by size. The particles may be separated within an epoxy or other polymer such that the particles are cured at the time of separation. 
     While the use of one or more sieves can complicate formation of the composites, on the other hand it guarantees the establishment of multiple zones with distinct particle size and volume fraction at each zone. In some embodiments a magnetic field (H) may be applied to the polymer during separation and curing in order to smooth the stepwise gradient into a more continuous distribution and to produce a more even transition of particle distribution between zones by reducing the effect of gravitational force ( FIG. 15 ). 
     In some embodiments individual layers of composite material may be fabricated individually, having different volume fractions and/or particle sizes, in order to produce a stepwise gradient. A first layer may be cast and subsequent layers cast onto the first, second, etc. layers, where the composite undergoes several stages of cure after each layer is added. Alternatively, the layers may be cast separately and joined together after casting, for example using adhesive. In some embodiments the FBG may be cast along with the composite while in others an FBG may be sandwiched between two pre-cast composite layers and joined with adhesives or other methods. The sandwich approach will be easier for mold fabrication and allows the FBG to maintain almost uniform period when the magnetic field H=0, since a nonzero magnetic field is used to align particles during composite fabrication and would otherwise introduce magnetostriction during the curing process. 
     In addition to the methods listed above, continuous gradients may be formed using a controlled segregation approach. In this approach, separation is caused by the force of gravity acting on the difference in density of the powders in the polymer. This can be accomplished through processes such as sedimentation forming, slip casting, centrifugal casting, and thixotropic casting. The volume fraction will be different depending on the location ( FIG. 15 ). 
     The strain distribution converter (either alone or as part of a composite) may include resilient polymers or epoxy, including various thermosetting and/or thermoplastic materials. While epoxy was used for many of the embodiments disclosed herein, different polymers may be selected with different stiffnesses in order to modulate the behaviors of the final products. 
     In some embodiment, composites were fabricated using a range of fiber volume fractions, V f =0.3-0.45 and a dimension of 25×25×6 mm using the following procedures. The monolithic Terfenol-D (T-D) bar was crushed into fine powders. The T-D particle range was from 100 to 300 microns. The powder was then poured into a mold and mixed with an epoxy which has a very low viscosity (cps=65) and which allows sufficient powder wetting and void reduction. Care has to be taken with lower volume fractions, as the larger density of the particles will result in stratification. The mixture was degassed under a vacuum for 30 min to eliminate air bubbles. Following this step, the mold with the mixture was placed between a pair of rare earth magnets for alignment of T-D particles along the maximum magnetostriction direction. The whole assembly was placed inside a 70° C. oven for 12 h to ensure full cure of the epoxy. 
     In general the epoxy should have low viscosity so that all of the bubbles come out when the composite is put inside a vacuum chamber and allowed to cure. Owing to the limitation on packing T-D particles, the maximum V f  is generally limited to 65% (0.65). Typical T-D particle sizes are 100 to 300 microns when T-D particles are produced by crushing. The particle sizes may be as small as 20 microns. Currently, 20 micron to 300 micron particles are available commercially but will require more labor to sort out particles. In various other embodiments V f  may range from 15% to 65% (0.15 to 0.65). 
     The output of the sensor depends of the ratio of the maximum and minimum V f  in the graded density profile. With V f  from 15% to 65%, this ratio is about 4.3 if particles of the same size are used. The output response can be further enhanced by using smaller particles for the low end (e.g. V f  15%) and larger particle for the high end (V f  65%), since the smaller particles have weak magnetostrictive strain. 
     Another factor to consider is the alignment field which is required to line up particles. However, it will also affect the uniformity of particle distribution. Moreover, gravitational force will limit uniformity. With these considerations, the dimension of the sensor of a graded T-D composite may be up to 2 cm long (generally long enough to cover the full length of a typical FBG), 5-6 mm wide, and 2-3 mm thick. The sensor may be secured at the middle by a holder which lies on the top of a bus bar or conductor (see  FIGS. 16 a , 16 b , and 16 c   ). Another way is to put the sensor in the air gap of a magnetic core (see  FIG. 17 ). This configuration will be good for the transverse configuration of the sensor since the FBG will be not lie across the gap of the magnetic core while the magnetic core will amplify the magnetic field. 
     Although FBGs are used in most examples herein as a means to measure strain and deformation of the strain distribution converter, in various embodiments other methods may be used, for example optical methods such as the DIC method outlined above in which tracking and image registration techniques, such as digital image correlation and photogrammetry. Alternatively, optical techniques which measure the changes of birefringence in the material such as photoelasticity may be used. 
     Contact-based methods for measurement can also be used to indicate the strain levels. Examples of such contact methods are those employing extensometers or bonded electrical resistance strain gauges. 
     Finally, magnetically-induced strains can also be measured using brittle coatings, i.e. polymeric substances designed to fracture at given strain levels. Application of brittle coatings to the surface of the sensor may be used in conjunction with optical techniques to permit direct visual indication if a certain magnetic field is experienced by the magnetostrictive composite material. 
     In some embodiments, the tapered strain distribution converter block may be stabilized with one or more blocks in parallel ( FIGS. 18, 19 ). In addition to the strain distribution converter containing the FOCS, one ( FIG. 18 ) or more ( FIG. 19 ) additional tapered strain distribution converters may be attached between a pair of fixed supports. The additional tapered strain distribution converters are held in place by a temperature compensation material (e.g. MONEL 400) having the same coefficient of thermal expansion as the magnetostrictive material to compensate for temperature fluctuations. 
     Various features and advantages of the invention are set forth in the following claims.