Abstract:
A sensor is disclosed herein. The sensor includes a fiber operable to communicate a light wave. The sensor also includes at least first and second Fiber Bragg Gratings disposed along the fiber. The sensor also includes a structure operable to be deformed in a plane of deformation. The at least first and second Fiber Bragg Gratings are disposed on opposite sides of the structure in the plane of deformation. The sensor also includes an interrogation unit operable to receive first and second signals corresponding to first and second wavelengths from the at least first and second Fiber Bragg Gratings. The first signal is associated with the first Fiber Bragg Grating and the second signal is associated with the first Fiber Bragg Grating. The sensor also includes a processor operably to derive a difference between the wavelengths of the first and second signals and compare the difference with data correlating wavelength differences to extents of deformation of the structure to yield a current extent of deformation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/522,930 for a POSITION SENSOR USING FIBER BRAGG GRATINGS TO MEASURE AXIAL AND ROTATIONAL MOVEMENT, filed on Aug. 12, 2011, which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a fiber optic sensor. 
         [0004]    2. Description of Related Prior Art 
         [0005]    It is known to use sensors to detect strain in a structure. 
       SUMMARY OF THE INVENTION 
       [0006]    In summary, the invention is a sensor. The sensor includes a fiber operable to communicate a light wave. The sensor also includes at least first and second Fiber Bragg Gratings disposed along the fiber. The sensor also includes a structure operable to be deformed in a plane of deformation. The at least first and second Fiber Bragg Gratings are disposed on opposite sides of the structure in the plane of deformation. The sensor also includes an interrogation unit operable to receive first and second signals corresponding to first and second wavelengths from the at least first and second Fiber Bragg Gratings. The first signal is associated with the first Fiber Bragg Grating and the second signal is associated with the first Fiber Bragg Grating. The sensor also includes a processor operably to derive a difference between the wavelengths of the first and second signals and compare the difference with data correlating wavelength differences to extents of deformation of the structure to yield a current extent of deformation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    Advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
           [0008]      FIG. 1  is a perspective view of a first exemplary embodiment of the invention; 
           [0009]      FIG. 2  is a front view of a second exemplary embodiment of the invention with an upper-right portion cut-away; 
           [0010]      FIG. 3  is a partial cross-section taken through section lines  3 - 3  in  FIG. 2 ; 
           [0011]      FIG. 4  is a magnified detail view of the detail circle  4  in  FIG. 2 ; 
           [0012]      FIG. 5  is a graph correlating output of Fiber Bragg Gratings to temperature; 
           [0013]      FIG. 6  is a graph displaying the differential output of a plurality of Fiber Bragg Gratings relative to an extent of deformation of a structure at a first temperature; 
           [0014]      FIG. 7  is a graph displaying the differential output of a plurality of Fiber Bragg Gratings relative to an extent of deformation of a structure at a second temperature; 
           [0015]      FIG. 8  is a graph displaying the quotient of outputs of a plurality of Fiber Bragg Gratings relative to an extent of deformation of a structure at a first temperature; 
           [0016]      FIG. 9  is a graph displaying the quotient of outputs of a plurality of Fiber Bragg Gratings relative to an extent of deformation of a structure at a second temperature; and 
           [0017]      FIG. 10  is a graph displaying the wavelength of outputs of a Fiber Bragg Grating relative to an extent of deformation, with curves for current temperature and for a reference or known temperature. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0018]    The invention, as demonstrated by the exemplary embodiments described below, provides an apparatus and method to measure rotational or linear displacement and temperature using Fiber Bragg Gratings (FBG). In one embodiment, the translation or rotational displacement of a shaft, with an integral wheel and screw, is converted to the bending of a resilient member. The proportional strain (compressive and tensile) induced by the bending can then be measured by two FBGs. In another embodiment, the displacement of a resilient member is induced by a spiral shaft with an integral cam is detected. In various embodiments, the FBGs are fixed to opposing sides of the resilient member such that one experiences tensile strain while the other experiences compressive strain. The design of the exemplary embodiments disclosed below enables accurate displacement measurements while also measuring, and compensating for, any temperature related effects to the sensors. 
         [0019]      FIG. 1  shows a first exemplary embodiment of the invention. An actuation rod  12  is supported in an outer housing by two linear bearings. The rod  12  contacts a wheel  14  and linear movement of the rod  12  results in rotational movement of the wheel  14 . The wheel  14  is fixedly mounted on a threaded axle  16  and movement of the wheel  14  results in movement of the axle  16  as well. As the threaded axle  16  rotates, a forcing block  18 , which has threads that mate with the threads of the axle  16 , moves along an axis referenced at  20 . When the axle  16  rotates in a first angular direction referenced at  22 , the block  18  can move in a first direction referenced at  24  along the axis  20 . When the axle  16  rotates in a second angular direction opposite to the first angular direction, the block  18  can move in a second direction along the axis  20  opposite to the first direction. 
         [0020]    It is noted that the block  18  is generally mounted on a rail  36 . The rail  36  is received in a notch  38  defined by the rail  36 . Engagement between the notch  38  and the rail  36  limits movement of the block  18  along the axis  20  but does not prevent all movement. 
         [0021]    Movement of the forcing block  18  in the first direction  24  imparts a load on a spring member  26 . Distal ends (one referenced at  28  and the other hidden) defined by a pair of arms  40 ,  42  of the spring member  26  is elastically deform in the first direction  24  relative to a base portion  30  of the spring member  26  when the block  24  moves in the first direction  24 . The arms  40 ,  42  project from the base portion  30 . As the forcing block  18  displaces the distal ends of the spring member  26  relative to the base portion  30 , strain is created along a length of the spring member  26  between the base portion  30  and the distal ends, in the arms  40 ,  42 . 
         [0022]    Two Fiber Bragg Gratings (hereafter FBG) are attached to the spring member  26  to sense conditions that can be electronically communicated, measured, and correlated to the strain in the spring member  26 , as well as correlated to the extent of movement of the block  18 , the wheel  14 , and the rod  12 . A first FBG  32  is attached to the first arm  40  of the spring member  26  to sense conditions corresponding to compressive strain. A second FBG  34  is attached to the second arm  42  to measure tensile strain. 
         [0023]    The FBGs  32 ,  34  are in electronic communication with an interrogation unit (referenced schematically at  44 ) through the fibers  46 ,  48 . An electronic processor  45  can be integral with or separate from the interrogation unit  44 . The processor  45  can process the signals received from the FBGs  32 ,  34 . Each fiber  46 ,  48  is operable to communicate a light wave and each can extend at least partially through a sheath, such as sheath  50 . It is noted that the fiber  46 ,  48  are integral with one another and also with loop portion  52 , to define a continuous wave guide. As temperature affects the wavelength of a FBG, it is difficult to differentiate between wavelength change due to physical strain and the change  20  induced by thermal strain. The use of two FBGs in the exemplary embodiment of the invention allows for temperature compensation in strain measurement. By finding the difference between the wavelength changes arising from FBGs  32  and  34 , the effect of thermal strains can be cancel, leaving only the strain due to mechanical deformation. The resultant strain is an accurate representation of the true strain in the spring member  26 . This strain can then be scaled into the desired engineering units of measure. 
         [0024]    Deriving the differential wavelength as set forth above also reveals thermal strain. The “cancelled” portion of strain corresponds to the temperature calibration of either of the FBGs  32  or  34 . Thus, the temperature of either FBG  32  or  34  can be calculated by detracting the known mechanical strain. This allows the embodiment of the invention to measure both a position of one of the structural components (derived from strain) and the temperature. 
         [0025]      FIG. 2  is a planar view of a second embodiment of the invention with a portion cut-away. An actuation tube  54  can be supported by an outer tube  56 . The tubes  54 ,  56  can be concentric. As shown in  FIG. 3 , internal to the actuation tube  54 , two bearings support an internal precision spiral transfer shaft  58 . A nut  60  encircles the spiral transfer shaft  58  and is fixed to the actuation tube  54 . The nut  60  forces the spiral transfer shaft  58  to rotate in response to linear movement of the actuation tube  54 . Rotation of the spiral transfer shaft  58  causes rotational movement of a cam  62 . As the cam  62  rotates it applies a load to a spring member  64 , causing the spring member  64  to bend. As the cam  62  displaces a tip  66  (referenced in  FIG. 2 ) of the spring member  64 , strain is created along a length of the spring member  64  between the tip  66  and a base portion  68 . 
         [0026]      FIG. 4  is taken in a plane of deformation of the spring member  64 ; the deformation of the spring member  64  is visible in this plane. First and second FBGs  70 ,  72  are attached to the spring member  64 . The first FBG  70  can be attached to a top of the spring member for sensing conditions corresponding to compressive strain as the spring member  64  is deflected away, upward (relative to the perspective of  FIG. 2 ) by the cam  62 . The second FBG  72  can be attached to bottom of the spring member  64  to sense conditions corresponding to tensile strain as the spring member  64  is deflected upward by the cam  62 . 
         [0027]    As with the first embodiment, the FBGs  70 ,  72  can be connected to an interrogation unit. Also, the operation of the second embodiment is similar to the operation of the first embodiment in that the use of two FBGs allows for temperature compensation in strain measurement. This allows the second embodiment of the invention to measure both position and temperature. 
         [0028]    The method of measuring strain will now be described. In  FIG. 5 , the outputs of the two FBGs for an embodiment of the invention are shown for three separate temperatures as a function of the sensor position. Each curve (the straight lines in the graph of  FIG. 5  are designated herein as curves) represents an extent of deformation of the structure being monitored. The two FBGs are distinguished from one another by the designations “A” and “B.” The horizontal axis defines the position of the sensor as a percentage and corresponds to a range over which the spring member is expected to deform in a particular operating environment. In other words, at 50% for example, the spring member will have deformed approximately 50% of the maximum amount the spring member is expected to possibly deform. Thus, the position of sensor is analogous to the extent of deformation of the structure being sensed, a spring member in the exemplary embodiments. 
         [0029]    The graph of  FIG. 5  reveals that as the temperature of the FBGs increase, the outputs of the FBGs (in wavelength) also increase. Since the common increase in wavelength due to temperature affects the output of both FBGs, taking the difference of the two outputs or taking a ratio of the two outputs allows for a cancelation of thermal effects on the measurement of position change, leaving only the change due to mechanical strain. 
         [0030]      FIGS. 6 and 7  correlates the difference in wavelengths between FBGs A and B with the position of sensor.  FIGS. 6 and 7  also show the difference between exemplary FBGs A and an FBG B when the pairs of FBGs are at two different temperatures. In  FIG. 6  the FBGs A and B are at 10° C. and in  FIG. 7  the FBGs A and B are at 38° C. A comparison between the two graphs shows that the differential output at different temperatures yields the same position of sensor for either temperature. In other words, the graphs of  FIGS. 6 and 7  show that mechanical strain can be accurately determined regardless of temperature. 
         [0031]      FIGS. 8 and 9  are analogous to  FIGS. 6 and 7 .  FIGS. 8 and 9  alternatively show the quotient of a FBG A and a FBG B with at two different temperatures, each FBG having the same temperature. As shown in  FIGS. 8 and 9 , the calculated difference between the outputs of the FBGs A and B at the different temperatures yields the same position of sensor at either temperature. 
       EXAMPLE 1 
       [0032]    A point  74  referenced on the graph of  FIG. 5  corresponds to the FBG A at 10° C. and at 10% of the position of sensor. A point  76  referenced on the graph of  FIG. 5  corresponds to the FBG B at 10° C. and at 10% of the position of sensor. The coordinates of point  74  are (10%, 1558.6 nanometers) and the coordinates of point  76  are (10%, 1559.6 nanometers). The vertical, differential distance between points  74  and  76  is 1 nanometer. This value is confirmed by reference to  FIG. 6 , in which coordinates of point  78  are (10%, 1 nanometer). The quotient of the wavelength values (1559.6 divided by 1558.6) is equal to 1.0006. This value is confirmed by reference to  FIG. 8 , in which coordinates of point  80  are (10%, 1.0006). 
         [0033]    It is noted that the value of the position of sensor would be the value being pursued. After the differential wavelength or quotient is known,  FIG. 6  or  8  would be consulted to derive the position of sensor. In an embodiment of the invention, the data graphically shown in  FIGS. 6 and 8  could be in the form of a table stored in the memory of an electronic processor. An electronic processor can receive the signal inputs from the FBGs, determine the differential wavelength and/or quotient, access a table of data analogous to the data in  FIG. 6  or  8 , and obtain the position of sensor. An electronic processor in an embodiment of the invention can be component of the interrogation unit. 
         [0034]    Once the position of the sensor is calculated the temperature of the sensor can be determined. The measured output of one of the FBGs at the known position can be referenced against known output for a FBG at the same position and at a known temperature. The dashed line in  FIG. 10  represents the output of an FBG at an unknown temperature. The solid line in  FIG. 10  represents the output of an FBG at a known temperature. Data associated with FBG output at one or more known temperatures can be stored as data in an electronic processor that receives and processes signals from the FBGs.  FIG. 5  shows a plurality of curves/lines representing observed FBG output at various temperatures; an electronic processor can retain such data in memory. 
       Example 2 
       [0035]    The observed output of an FBG is referenced at point  82  in  FIG. 10 . It has been previously determined that the position of sensor is 50%. Several alternative methods can be applied to derive the temperature of the FBG. In one embodiment of the invention, the vertical position of the point  82  relative to other, known curves can be the basis of interpolation. For example, if the point  82  were vertically equidistant between a curve associated with 0° C. and a curve associated with 20° C., the temperature of the FBG could be determined to be 10° C. if the relationship between the 0° C. curve and the 20° C. curve was known to be parallel. Alternatively, the difference in wavelength can correspond directly to the temperature difference. In  FIG. 10 , the point  82  is approximately 0.4 nanometers vertically distance from a point  84  on 0° C. curve. In an embodiment of the invention, the distance 0.4 nanometers can correspond to a 40° C. temperature difference. The FBG operating at point  82  would thus be operating at a temperature of 40° C. 
         [0036]    Embodiments of the invention can be applied to methods and apparatus related to monitoring the position and temperature of mechanical components such as, by way of example and not limitation, variable valve positions, actuator stroke length, flow control devices, inlet guide vane positions, automation feedback loops, thermal growth of structures, gate position and component deflection. 
         [0037]    While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Further, the “invention” as that term is used in this document is what is claimed in the claims of this document. The right to claim elements and/or sub-combinations that are disclosed herein as other inventions in other patent documents is hereby unconditionally reserved.