Abstract:
In one embodiment, a sensor assembly has a sensor housing forming a fluid chamber having a surface defining a normal axis. A magnetostrictive (MS) core that defines a central longitudinal axis is subjected to stress induced by pressurized fluid in the chamber. An excitation coil is coupled to the core to induce a magnetic flux therein. The central longitudinal axis of the core is coaxial with the axis normal to the fluid chamber surface.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to magnetostrictive (MS) stress sensors. 
       BACKGROUND OF THE INVENTION 
       [0002]    Magnetostrictive stress sensors can be used to measure stress such as might be generated within the sensor by fluid pressure. Typically, a MS stress sensor includes a magnetostrictive (MS) core made of a material, such as a nickel         iron alloy, and a coil that surrounds the core for establishing magnetic flux within the core. The flux loop continues through the medium on the exterior of the coil. The core provides a primary path for the magnetic flux in a first portion of the of magnetic flux loop. A ferromagnetic carrier, either MS or non-MS, can be used to provide an improved return path for the magnetic flux in a second portion of the magnetic flux loop, as the magnetic flux circles the coil through the core and the carrier. The permeability of the MS core, and thus the inductance of the coil, is a function of the stress applied to the core along the axis. The value of the coil inductance therefore represents the magnitude of the stress applied to the core and, hence, the magnitude of the physical phenomenon causing the stress, such as fluid pressure on the core. 
         [0003]    As understood herein, the stress within the MS core optimally should be uniform in the area of the core where stress is to be measured in order to optimize its measurement. 
       SUMMARY OF THE INVENTION 
       [0004]    A sensor assembly includes a sensor housing forming a fluid chamber that in turn defines a surface, with an axis normal to the surface. A magnetostrictive core is provided that defines a central longitudinal axis. The core is subjected to stress induced by pressurized fluid in the chamber. An excitation coil surrounds the core to induce a magnetic flux therein. The central longitudinal axis of the core is coaxial with the axis normal to the fluid chamber surface. 
         [0005]    In some embodiments the MS core may include a designated portion around which the coil is wound. This coil portion of the core defines the central longitudinal axis and can have a circular cross-section or a rectangular cross-section. In other embodiments, the core can be “H”-shaped and the central longitudinal axis extends through the centers of the cross-members connecting the two core leg members rendering them equidistant from the central longitudinal axis. In non-limiting examples the coil portion of the core undergoes only compressive stress during operation. 
         [0006]    In another aspect, a sensor assembly includes a sensor housing forming a fluid chamber and a magnetostrictive core portion subjected to one and only one type of stress due to a force generated by pressurized fluid in the chamber. An excitation coil wound only around the coil portion of the core induces a magnetic flux therein. 
         [0007]    In still another aspect, a sensor is disclosed for outputting a signal representative of stress caused by a source of stress. The sensor includes magnetostrictive (MS) means defining a central longitudinal axis and juxtaposable with the source of stress such that the source of stress causes one and only one type of stress along the central longitudinal axis of the MS means. The sensor further includes signal means configured for carrying a signal representative of the one and only one type of stress along the central longitudinal axis of the MS means. 
         [0008]    The details of the present invention, both as to its structure and operation, can best be understood in reference to the accompanying drawings, in which like reference numerals refer to like parts, and in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  shows a system in accordance with one non-limiting embodiment of the present invention; 
           [0010]      FIG. 2  is a perspective view of a first embodiment of a MS core; 
           [0011]      FIG. 3  is a cut-away perspective view of the core shown in  FIG. 2  in a sensor housing, with the coil and housing cover removed for clarity; 
           [0012]      FIG. 4  is a partial cross-sectional view of an alternate sensor core in a sensor housing, and 
           [0013]      FIG. 5  is a side view of the core and housing shown in  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0014]    Beginning with  FIG. 1 , a general, non-limiting implementation of an MS stress sensor  10  is shown. The MS stress sensor  10  can be coupled to a source of pressure, such as to a fluid container  12 , with the MS stress sensor  10  having the ability to sense fluid pressure in the fluid container  12  in accordance with principles below. Without limitation, the fluid contain  12  may be, e.g., a vehicle fuel rail, an engine combustion chamber, etc., although present principles are not limited to any particular fluid application. 
         [0015]    The MS stress sensor  10  is also electronically connected to a computer  14  which may be, without limitation, an engine control module. The computer  14  receives the signal that is output by the sensor  10  for processing the signal to, e.g., correlate the stress as indicated by the signal to a fluid pressure. Further, the computer  14  may be electronically connected to a component or components  16  such as a fluid pump, injectors, etc., that may be controlled by the computer  14  based on data received from the MS stress sensor  10 . 
         [0016]    A first embodiment of the MS stress sensor  10  is shown in  FIGS. 2 and 3 . Beginning with  FIG. 2 , an MS core  18  is shown. In this first non-limiting embodiment, the body  20  of the core  18  is parallelepiped in shape, but may also be cylindrical in shape to retain a greater degree of symmetry and so that high-stress areas do not occur near the corners of a parallelepiped-shaped core. 
         [0017]    Continuing with  FIG. 2 , an excitation coil  22  is coupled to the core  18 . More specifically, the coil  22  is of a non-magnetic wire, such as e.g. copper, that is wound around core  18 . The coil  22  is energized to induce a magnetic flux  24  in the MS core  18 , and also serves as a transducer providing a signal representing the resulting magnetic flux, which in turn represents the stress of the MS core  18  and, hence, the pressure in the fluid. As shown, the magnetic flux  24  loops around the MS core  18 . As is known in the science of magnetism, the coil can be wound anywhere around the core; it is shown by way of example in  FIG. 2  wound around the body region  20  of the core  18 . Also, a single coil  22  is shown in  FIG. 2 , although several coils can be contemplated for redundancy, temperature compensation, etc. 
         [0018]      FIG. 3  provides another view of the first embodiment of the MS stress sensor  10 , this time representing a cross-section of the MS core  18  shown disposed within a sensor housing  26 . As shown in  FIG. 3 , the sensor housing comprises two cavities or chambers, a chamber  28  designed to receive the fluid under pressure, and a chamber  29  designed to receive the sensor unit (core  18  and coil  22 ). In this example, chambers  28  and  29  have a width Wfluid and Wsensor, respectively, in the plane of the figure (width, or diameter, or length, depending on respective chamber shapes). There is no restrictions on the size of Wfluid and Wsensor. One or both could be large; alternatively, one or both could be very small, for instance if the fluid chamber  28  is a narrow conduit designed to be connected to a relatively distant pressure chamber (such a design may be necessary for instance to isolate the sensing element from extreme temperatures). 
         [0019]    In any event, there is an area designated  25  separating the fluid chamber  28  from the sensor chamber  29 , which transmits the stress from the fluid to the sensor. This part  25  of the sensor housing  26  is where the two chamber areas (widths Wfluid and Wsensor in the plane of the figure) overlap. This part  25  of the sensor housing  26  is also where the housing  26  is at its thinnest, so that the transmitted stress is maximized. The minimum thickness is limited by material strength and the maximum anticipated stress or burst pressure, which could be much larger than the maximum for normal operation. Thicknesses on the order of a fraction of a millimeter to a couple of millimeters are the most common. Further, a point of maximum stress  34  is shown. This point of maximum stress  34  is typically located approximately in the geometric center of area  25 . 
         [0020]    The area  25  of the sensor housing  26  includes a surface  21  on which the fluid under pressure applies stress. The surface  21  defines a direction normal to itself, and more specifically an axis  30  normal to surface  21  and located at the approximate geometric center of area  25 , i.e. at or close to point  34  where the stress is maximum. Further, the MS core  18  defines a central longitudinal axis  32  that is coaxial with the axis  30  normal to surface  21 . The stress caused by the fluid pressure travels through the point of maximum stress  34 , along the axis  32 , and transfers to the body  20  of the core  18 . In the presence of stress, the magnetic permeability of the body  20  of the core  18  changes affecting the inductance of the coil. To achieve better performance and accuracy from the sensor  10 , stress may be applied to the body  20  of the core  18 , but the rest of the core  18  is essentially not stressed. The rest of the core  18  is not stressed because the axis  32  extending through the body  20 , i.e. the axis  32  of the core  18  region around which the coil  22  is wound, is collinear with the axis  30  normal to the surface  21  of the fluid chamber  28 , causing stress to be transferred substantially only along the axes. 
         [0021]    A second embodiment of the MS stress sensor  10  is shown in  FIGS. 4 and 5 . Beginning with  FIG. 4 , a sensor housing  36  is shown, which is substantially similar to the sensor housing  26  from the first embodiment above, with the following exceptions. Distinguishing from the first embodiment, a MS core  38  upon which the coil  40  is wound is “H”-shaped in the second embodiment. That is, the MS core  38  includes two leg members  50  connected by cross-members  48 . The cross-members  48  are located at some distance from the top and bottom of the leg members  50 . Moreover, the MS core  38  defines a central longitudinal axis  44 . 
         [0022]    As shown in  FIGS. 4 and 5 , the sensor housing  36  defines a fluid chamber  42 , a sensor chamber  49 , and includes an area  45  where these two chambers  42  and  49  overlap. Further, the area  45  has a surface  41  separating area  45  and fluid chamber  42 . In turn, this surface  41  defines an axis  46  normal to surface  41  and located approximately where the stress within area  45  is maximum. This axis  46  is coaxial with the central longitudinal axis  44  of the MS core  38 . Specifically, as can be appreciated in cross-reference to  FIGS. 4 and 5 , the axis  46  normal to the surface  41  of the fluid chamber  42  extends through the center of upper and lower cross-members  48  each of which connects the two leg members  50  of the MS core  38 . The leg members  50  are equidistant from the central longitudinal axis  44  and are perpendicular to the cross-members  48 . The coil  40  is shown wound around one of the leg members  50  between the cross-members  48 , although coil  40  could also be wound around one of the two cross-members  48  between the leg members  50 . 
         [0023]    The H-shaped MS core  38  is shown in greater detail in  FIG. 5 .  FIG. 5  shows a pressure action arrow  52 , representing fluid pressure from the fluid chamber  42 . The pressure action arrow  52  indicates the direction of the force caused by the fluid pressure, the fluid pressure being caused by fluid in the fluid chamber  42 . Further, magnetic flux predominantly remains in the inner area of the core  38  as indicated by the vertical hatched areas  54  and horizontal hatched areas  56 . The area hatched areas may be referred to as a skin effect area. 
         [0024]    As long as the cross-members  48  are located at some distance from the top and bottom of the leg members  50 , they can be designed to experience essentially no stress. It can therefore be appreciated that stress affecting the core  38  is then confined substantially only to the vertical areas  54 , which are under a single, compressive stress. The magnetic flux traverses these areas of uniform stress  54  as well as areas  56  under essentially no stress, resulting in a uniform stress-dependent signal. 
         [0025]    While the examples shown herein have assumed compressive stress, hoop stress and tensile stress may be employed in other embodiments. 
         [0026]    The materials used for the MS core may include but are not limited to nickel/iron alloys, pure nickel, terfenol, galfenol. Preferred non-limiting materials include maraging steel (steel with about 18% nickel content) and nickel-iron alloys with 30%-70% nickel content. 
         [0027]    While the particular MAGNETOSTRICTIVE SENSOR WITH UNIFORM STRESS is herein shown and described in detail, it is to be understood that the subject matter which is encompassed by the present invention is limited only by the claims.