Patent Publication Number: US-2005134253-A1

Title: Current sensor

Description:
This application claims the benefit of U.S. Provisional Application No. 60/461,924 filed on Apr. 10, 2003. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates generally to the field of current sensing devices and more particularly to an improved magneto-striction based, passive optical current sensor that is located at high voltage and communicates to ground level via an optical fiber.  
      2. Description of Related Art  
      Various current sensors are known including iron-cored transformers commonly referred to as CT&#39;s. Additionally, optical current sensors are also known that utilize the Faraday magneto-optic effect.  
      While the prior art arrangements may be generally useful as current sensors, it is desirable to provide a current sensor of minimal size and with improved operating parameters.  
     SUMMARY OF THE INVENTION  
      Accordingly, it is a principal object of the present invention to provide an improved magneto-striction based, passive optical current sensor that is located at high voltage and communicates to ground level via an optical fiber.  
      These and other objects of the present invention are efficiently achieved by a magneto-striction based, passive optical current sensor that is located at high voltage and communicates to ground level via an optical fiber. The optical current sensor includes a ferromagnetic core, a modulator of magnetostrictive material, e.g. Terfenol-D in a preferred embodiment, that responds to the magnetic field, and two or more matched fiber Bragg gratings that convert this response into a wavelength modulated optical signal that is transmitted via an optical fiber to ground level electronics. To linearize the output of the optical current sensor, the optical sensor includes arrangements to provide both mechanical and magnetic bias to the modulator.  
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
      The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the specification taken in conjunction with the accompanying drawing in which:  
       FIG. 1  is a schematic and diagrammatic representation of a current sensor in accordance with the present invention;  
       FIGS. 2   a  and  2   b  are respective plots of magnetic field versus magnetostrictive strain for no bias and with bias respectively of the current sensor of  FIG. 1 ;  
       FIG. 3  is a plot of transmittance versus wavelength of the current sensor of  FIG. 1 ;  
       FIG. 4  is an elevational view, partly in section, of an illustrative embodiment of a current sensor of the present invention in accordance with  FIG. 1 ;  
       FIG. 5  is a plot of output current as measured by the current sensor of  FIGS. 1 and 4  versus the measured conductor current;  
       FIG. 6  is a plot of the output of the current sensor of  FIG. 4  and the conductor input current versus time; and  
       FIG. 7  is an elevational view, partly in section, of another embodiment of a current sensor in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION  
      Referring now to  FIG. 1 , a current sensor  10  in accordance with the present invention is illustrated to function as a magneto-striction based, passive optical current sensing device for high voltage applications, e.g. to sense the current in a conductor  12 . The current sensor  10  includes a sensor head  14  having a ferromagnetic core  16  in the shape of a yoke, a modulator  18  of magnetostrictive material, e.g. Terfenol-D in a preferred embodiment, that responds to the magnetic field generated by the current in the conductor  12 , and one or more tunable fiber optical filters at  20 , e.g. Bragg gratings, that convert this response into a wavelength modulated optical signal at  22  that is transmitted via an optical fiber  24  to ground level electronics at  26 . For example, two tunable fiber optical filters  20   a  and  20   b  are illustrated in the illustrative embodiment of  FIG. 1  although it should be realized that the number of tunable fiber optical filters  20  may be one or several in various other embodiments. A broad-band optical source  28  is used over path  30  and through a coupler at  32  to interrogate the tunable optical filters at  20 , e.g. the output  29  of the optical source  28  being distributed equally between the two illustrated tunable fiber optical filters at  20  by the coupler at  32 . The tunable fiber optical filters  20   a  and  20   b  reflect back a narrow characteristic spectrum at  21   a ,  21   b  respectively, e.g. with a center frequency determined by the pitch of the grating that changes in response to applied strain. The reflected spectrum travels down the same path used to illuminate the tunable fiber optical filters  20   a  and  20   b . Accordingly, the modulator  18  and the tunable fiber optical filters  20   a  and  20   b  in combination produce a spectral output that is proportional to the current in the conductor  12  in the form of a wavelength-modulated output at  24 , i.e. picking up the strain generated by the modulator  18  by the magnetic field of the conductor  12  and transmitting the information as a reflected wavelength modulated signal back at  21   a ,  21   b.    
      In accordance with important aspects of the present invention and referring now additionally to  FIG. 2 , it is useful to bias the operating point of the of the strain versus magnetic field characteristic of the modulator  18  to achieve a linearization of the strain output  31  versus the magnetic field input  33 . Specifically, it has been found useful to utilize both dc magnetic bias, e.g. via the permanent magnets at  25 ,  27  in the core  16 , and mechanical prestress of the modulator  18  as shown at  23  in  FIG. 1  and as will be explained in more detail hereinafter.  FIG. 2   a  illustrates this relationship without magnetic bias and  FIG. 2   b  illustrates the linearization achieved with magnetic bias so as to produce a strain output at  31  that is proportional to the magnetic field strength in the modulator  18 . This has also been found to reduce hysteresis and prevent the modulator  18  from having excessive tension applied thereto.  
      In the illustrative embodiment, the reflected signals at  24  are split into two paths via a second coupler  34  of the ground level electronics at  26 . A first path  36  out of the coupler is connected through a linear transmission filter  38  having a transmittance that varies linearly with wavelength as illustrated in  FIG. 3  so as to function as a passive wavelength demodulator, i.e. since the transmittance of the linear transmission filter  38  varies linearly with wavelength, any changes in the reflected spectrum at  36  with respect to wavelength results in a change in the transmitted intensity at the output of the filter  38 . The output of the filter  38  and the second path from the second coupler  34  are each to respective photodiode arrangements  42  and  44  so as to produce at respective output signals at  46 ,  48  that are have a voltage proportional to the input intensity of the light to the respective photodiode arrangements  42 ,  44 . Accordingly, any changes in intensity to the photodiode arrangement  42  is translated into a proportional change in the voltage output at  46 . The reference path and output at  48  is useful to stabilize the sensor  10  against intensity variations produced by drift in the optical source  28  or other environmental effects, e.g. by normalizing the output at  46  by the output at  48 . In this way, it has been found that the output at  46  linearly tracks the current in the conductor  12  within the limits of the linearity of the modulator  18  and the optics of the sensor  10 . It should also be noted that the use of two or more tunable fiber optical filters, e.g.  20   a  and  20   b , not only provides more accurate pickup of the strain of the modulator  18  via area coverage but also improves the signal to noise ratio of the signal at  46 . This is achieved via the presence of the two reflected signals and also via the further reflection of each reflected signal through the other tunable fiber optical filter. The sensor  10  in one arrangement was found to have a useful linear range of 200-1000 amperes in response to 60 Hz current in the conductor  12 . It should also be noted that the optical fiber path at  24  is the only path traversing the high voltage barrier or area of the sensor head  14  and the ground-level electronics at  26 , thus reducing insulation requirements. It should also be noted that the wavelength demodulation arrangement shown and discussed is illustrative only since various other methods are also possible in particular applications.  
      With additional reference now to  FIGS. 4-6 , an illustrative embodiment of a current sensor  100  in accordance with the principles of the present invention and the current sensor  10  of  FIGS. 1-3  illustrates specific structure and configuration of a sensor head  114 , e.g. for use in the current sensor  10  of  FIG. 1 . The modulator  118  with one or more attached tunable fiber optical filters at  120  is positioned between two pole pieces  130 ,  132 . The pole pieces  130 ,  132  are attached to the core  116 . A cylindrical shell  140  is utilized in combination with a locknut assembly at  142  to provide mechanical prestress to the modulator  118 , i.e. mechanical strain to perform a bias function as discussed hereinbefore. In a specific arrangement, a Beliville-type washer  144  is utilized to apply this prestress to the pole pieces  130 ,  132  within the cylindrical shell  140 , e.g. the Beliville-type washer  144  being deformed to produce the desired force and then maintaining this deformation via the locknut assembly  142 . A permanent magnet  125  provides the desired magnetic bias to the sensor head  114 . The output of the tunable optical filters at  120  is via an optical fiber at  122 . The annular space at  150  in the cylindrical shell  140  is utilized in specific embodiments for a DC bias coil (not shown here but illustrated in the arrangement of  FIG. 7 ) either in lieu of or in addition to the magnet  125 .  FIG. 5  illustrates the rms magnitude and phase response at the output  124  of the current sensor  100  in response to 60 Hz currents in the range of 50-1000 amperes. Additionally,  FIG. 6  illustrates the linear response of the current sensor 100 to 60 Hz input current, e.g. current in the conductor  12  as shown in  FIG. 1 .  
      Another illustrative embodiment of a current sensor  200  in accordance with the present invention is shown in  FIG. 7  wherein a DC bias coil  202  is provided in the sensor head  214  in lieu of magnets in the core  216  to provide the magnetic bias. A mechanical prestress arrangement  223  is shown that is similar to that of  FIG. 4 . In the current sensor  200 , the modulator  218  is held and strained between pole pieces  230  and  232  that are shaped to provide desired flux concentrations to the modulator  218 .  
      While there have been illustrated and described various embodiments of the present invention, it will be apparent that various changes and modifications will occur to those skilled in the art. Accordingly, it is intended in the appended claims to cover all such changes and modifications that fall within the true spirit and scope of the present invention.