Abstract:
A method of mounting in-plane sensors of an inertial measurement unit. The method includes the steps of: providing a structure having first and second planar surfaces oriented orthogonally to one another, positioning a plurality of sensors on the first planar surface such that each of the sensors has a sense axis extending parallel to the first planar surface, positioning at least one other sensor on the second planar surface such that the at lease one other sensor has a sense axis extending parallel to the second planar surface, and orienting the sensors on the first and second surfaces so that the angles formed between any two sense axes are equal.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to systems and methods of mounting in-plane sensors, and more particularly to systems and methods of mounting in-plane sensors on two orthogonal surfaces to form a three-axis inertial measurement unit having tetrahedral angle redundancy. 
         [0003]    2. Description of Related Art 
         [0004]    Inertial measurement units (IMU) are known in the art and have been used in a wide variety of applications. For example, IMUs are commonly used in inertial guidance and navigation systems for all types of vehicles, in particular aircraft and spacecraft. Inertial navigation has the advantage of not being dependent on an external point of reference. Navigation is accomplished by sensing the motion of the vehicle and calculating the change in position with respect to an initial position. The IMU is able to determine the three-dimensional orientation of a body relative to a reference direction absolutely within an inertial system. 
         [0005]    A typical IMU may consist of three equal modules, each including a gyroscopic rotational rate sensor, a linear accelerometer, and associated electronics. Each module is typically oriented on a cube or a similar structure to provide inertial measurements along one of three orthogonal axes, with the gyroscopic rotational rate sensors providing information regarding rotation of the unit and the accelerometers providing information concerning linear movement of the unit. In this way, the IMU is able to determine the position of the vehicle with respect to the vehicle&#39;s initial position to aid in guidance, navigation, and control of the vehicle. 
         [0006]    Three-axis inertial measurement units as described above have been used extensively in aerospace applications. Traditionally, such IMUs included mechanical sensors such as conventional spinning mass gyroscopes and large mechanical accelerometers. However, most current IMUs utilize microelectromechanical systems (MEMS) devices. Many MEMS sensors are mounted on a support substrate made of silicon or a similar material and can detect acceleration by measuring a change in capacitance. Current technologies using MEMS devices encapsulate the accelerometer, gyroscope, and associated electronics into individual packages. These packages are typically soldered to a circuit board, which is then mounted on one plane of an orthogonal assembly, such as a face of a cube. 
         [0007]    Most inertial sensors, including MEMS sensors, are perpendicular sensors or out of plane devices, meaning that the sense axis of the device is oriented at a 90 degree angle with respect to the mounting plane. Some MEMS devices, including accelerometers and gyroscopes, are in-plane sensors. In-plane sensors are inertial sensors having a sense axis that is parallel to the mounting plane. In-plane sensors detect an acceleration or rotation along an axis parallel to the surface of the support substrate. 
         [0008]    Redundant systems of out of plane sensors arranged along non-orthogonal axes are well known in the art. Redundant inertial measurement systems allow for the failure of one or more sensors while still maintaining the ability to determine the essential location of a vehicle in inertial space. Prior art systems have utilized a variety of structures for mounting out of plane sensors to produce redundant systems. For example, out of plane sensors have been placed on the orthogonal faces of a cube, on the faces of a pyramid or wedge, on the faces of a dodecahedron, and on a tetrahedral structure. 
         [0009]    A tetrahedral structure is particularly advantageous for redundant systems. In this configuration, four out of plane sensors are mounted to the four faces of an equilateral tetrahedron, with each of the sensors located symmetrically with respect to the others at an angle of approximately 109.4 degrees between the sense axes of the sensors. This configuration results in the simplifies the equations and resultant equipment necessary to isolate failed sensors and convert output signals from the sensors to a coordinate system fixed in inertial space. 
         [0010]    Although a tetrahedral structure is advantageous for redundant inertial measurement units, such a shape adds complexity to the inertial measurement unit and thus increases costs of manufacturing the unit. Accordingly, there is a need in the art for an inertial measurement unit that can provide the advantages of a tetrahedral configuration without the added complexity required by such a configuration. 
       SUMMARY OF THE INVENTION 
       [0011]    Advantages of the present invention will be set forth in and become apparent from the description that follows. Additional advantages of the invention will be realized and attained by the systems and methods particularly pointed out in the written description and claims, as well as from the appended drawings. 
         [0012]    In-plane sensors provide a degree of freedom in the angle in which the sensors are mounted on a substrate surface. Two sensors can be mounted on the same planar surface with the sense axis of each sensor pointed in two unique directions to provide sensing along two different axes. By contrast, rotating an out of plane sensor on its mounting plane will not change the orientation of the sense axis. Placing two in-plane sensors on the same surface provides the tetrahedral angles for redundant sensing without the requirement of four unique mounting planes, as in prior art designs. In-plane sensors placed on two orthogonal planes can also be used to sense along three orthogonal axes, removing the need for three unique planes. Prior art designs, which use out of plane sensors, require a separate mounting plane for each sense axis. The present invention provides the redundant sensing using only two planar surfaces. 
         [0013]    To achieve these and other advantages and in accordance with the purpose of the invention, as embodied herein, a method of mounting in-plane sensors is disclosed. The method includes the following steps: providing a structure having first and second planar surfaces oriented orthogonally to one another, positioning a plurality of sensors on the first planar surface such that each of the sensors has a sense axis extending parallel to the first planar surface, positioning at least one other sensor on the second planar surface such that the at lease one other sensor has a sense axis extending parallel to the second planar surface, and orienting the sensors on the first and second surfaces so that the angles formed between any two sense axes are equal. 
         [0014]    An inertial measurement unit is also disclosed. The inertial measurement unit includes a structure having a first planar surface and a second planar surface oriented orthogonally to one another, two or more primary sensors mounted on the first planar surface, and at least one primary sensor mounted on the second planar surface. Each of the primary sensors mounted on the first planar surface has a sense axis that extends parallel to the first planar surface, and the at least one primary sensor mounted on the second planar surface has a sense axis that is parallel to the second planar surface. The angles formed between any two sense axes of the primary sensors on either the first planar surface or the second planar surface are equal. 
         [0015]    A method for providing a tetrahedral angle configuration with sensor redundancy on two mounting surfaces is also provided. The method includes the steps of: providing a structure having a first planar surface and a second planar surface oriented orthogonally to one another, with a linear junction formed at the intersection of the first and second planar surfaces; mounting a first sensor on the first planar surface such that a sense axis of the first sensor is parallel to the first planar surface and oriented at an inner angle of approximately 54.74 degrees from the linear junction; mounting a second sensor on the first planar surface such that a sense axis of the second sensor is parallel to the first planar surface and oriented at an inner angle of approximately 54.74 degrees from the linear junction and 70.53 degrees from sense axis of the first sensor; mounting a third sensor on the second planar surface such that a sense axis of the third sensor is parallel to the second planar surface and is oriented at an inner angle of approximately 54.74 degrees from the linear junction; and mounting a fourth sensor on the second planar surface such that a sense axis of the fourth sensor is parallel to the second planar surface and is oriented at an inner angle of approximately 54.74 degrees from the linear junction and 70.53 degrees from the sense axis of the third sensor. 
         [0016]    It is to be understood by those having ordinary skill in the art that the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention claimed. The accompanying drawings, which are incorporated in and constitute part of this specification, are included to illustrate and provide a further understanding of the method and system of the invention. Together with the description, the drawings serve to explain principles of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    So that those skilled in the art to which the subject invention pertains will readily understand how to make and use the inertial measurement unit disclosed herein without undue experimentation, preferred embodiments thereof will be described in detail below with reference to the following figures: 
           [0018]      FIG. 1  is a perspective view of an inertial measurement unit constructed in accordance with the present invention, showing four in-plane sensors mounted on two perpendicular surfaces as well as an exploded view of a housing that may enclose the two perpendicular surfaces; 
           [0019]      FIG. 2  is an exploded view of the inertial measurement unit of  FIG. 1 , rotated slightly to better show the two perpendicular surfaces; 
           [0020]      FIG. 3  is a perspective view an inertial measurement unit constructed in accordance with a first exemplary embodiment of the present invention, showing four in-plane sensors mounted on two planar surfaces that are perpendicular to one another; 
           [0021]      FIG. 4  is side view of the inertial measurement unit of  FIG. 3 , showing a first planar surface of the inertial measurement unit, the line of sight being perpendicular to the first planar surface, as shown by reference numeral  4  in  FIG. 3 ; 
           [0022]      FIG. 5  is a side view of the inertial measurement unit of  FIG. 3  showing a second planar surface of the inertial measurement unit, the line of sight being perpendicular to the second planar surface, as shown by reference numeral  5  in  FIG. 3 ; 
           [0023]      FIG. 6  is a top view of the inertial measurement unit of  FIG. 3  showing both the first and second planar surfaces of the inertial measurement unit, the line of sight being shown by reference numeral  6  in  FIG. 3 ; 
           [0024]      FIG. 7  is a perspective view an inertial measurement unit constructed in accordance with a second exemplary embodiment of the present invention, in which six in-plane sensors are mounted on three planar surfaces; 
           [0025]      FIG. 8  is a side view of the inertial measurement unit of  FIG. 7 , showing a first and a second in-plane sensor mounted on a first planar surface of the inertial measurement unit, the line of sight being perpendicular to the first planar surface, as shown by reference numeral  8  in  FIG. 7 ; 
           [0026]      FIG. 9  is a side view of the inertial measurement unit of  FIG. 7  showing a third, fourth, and fifth in-plane sensor mounted on a second planar surface of the inertial measurement unit, the line of sight being perpendicular to the second planar surface, as shown by reference numeral  9  in  FIG. 7 ; 
           [0027]      FIG. 10  is a top view of the inertial measurement unit of  FIG. 7  showing both the first and second planar surfaces of the inertial measurement unit, the line of sight being shown by reference numeral  10  in  FIG. 7 ; 
           [0028]      FIG. 11  is a bottom view of the inertial measurement unit of  FIG. 7 , showing a sixth sensor mounted on the third planar surface and showing the remaining four sensors in phantom; 
           [0029]      FIG. 12  is a perspective view of a third exemplary embodiment of an inertial measurement unit according to the present invention, showing three in-plane sensors mounted on two planar surfaces; 
           [0030]      FIG. 13  is a side view of the inertial measurement unit of  FIG. 12 , showing a first and a second in-plane sensor mounted on a first planar surface, the line of sight being shown by reference numeral  13  on  FIG. 12 ; 
           [0031]      FIG. 14  is a side view of the inertial measurement unit of  FIG. 12 , showing a third in-plane sensor mounted on a second planar surface, the line of sight being shown by reference numeral  14  on  FIG. 12 ; and 
           [0032]      FIG. 15  is a top view of the inertial measurement unit of  FIG. 12 , showing both the s first and second planar surfaces of the inertial measurement unit, the line of sight being shown by reference numeral  15  in  FIG. 12 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0033]    The subject invention provides a method of mounting in-plane sensors on non-parallel planar surfaces to provide the full three-axis coverage required for an inertial measurement unit. In a first exemplary embodiment, four in-plane sensors are mounted on two non-parallel surfaces in a tetrahedral configuration to provide three-axis redundant coverage; the fourth sensor provides enhanced reliability and fault detection. In a second exemplary embodiment, six in-plane sensors are mounted on three non-parallel surfaces to provide a six-axis redundant configuration. In a third exemplary embodiment, three in-plane sensors are mounted on two non-parallel planar surfaces to provide a three axis orthogonal alignment of three planar sensors. 
         [0034]    A tetrahedral configuration has been recognized as an ideal configuration for mounting in-plane sensors for redundant three axis coverage. In a tetrahedral configuration, each of four sensors is typically oriented perpendicular to a different one of the four faces of an equilateral tetrahedron. In this configuration, all of the sensor are equiangular, that is, the sense axis of each sensor is oriented at an outer angle of 109.47 degrees and an inner angle of 70.53 with respect to the sense axis of every other sensor. This configuration is advantageous because it provides equal redundancy of the sensors in all directions. 
         [0035]    Typically, a tetrahedral mounting surface is required for a tetrahedral sensor configuration. However, for in-plane sensors, the inventors have determined that a tetrahedral configuration for the sensors can be achieved using only two mounting planes having a 90° dihedral angle. Using two planes oriented at 90° with respect to one another advantageously allows the same two planar surface to be used for either the non-redundant orthogonal alignment of three in-plane sensors, or for creation of the tetrahedral angle between four in-plane sensors. 
         [0036]    With this background, reference will now be made in detail to the present preferred embodiments of the inertial measurement unit, examples of which are illustrated in the accompanying drawings. In each of the side views shown in the drawings, sensors of the inertial measurement unit that are not on the plane perpendicular to the line of sight have been omitted for simplicity. Any reference to an angle in the specification or claims that includes the qualifier “approximately” encompasses an angle within ±1 degree of the stated angle, and preferably within ±0.5 degrees of the stated angle. 
         [0037]    For purposes of explanation and illustration, and not limitation, a first exemplary embodiment of the inertial measurement unit is shown in  FIGS. 1-6  and is designated generally by reference character  10 .  FIG. 1  shows an exploded view of a removable housing that may be placed over inertial measurement unit  10 .  FIG. 2  shows a rotated and exploded view of the two perpendicular surfaces of inertial measurement unit  10 . 
         [0038]    Referring to  FIGS. 3-6 , an exemplary embodiment of inertial measurement unit  10  having four in-plane sensors in a tetrahedral configuration is shown. In this exemplary embodiment, inertial measurement unit  10  includes a first planar surface  12  and a second planar surface  14 . First planar surface  12  is orthogonal to second planar surface  14 , that is, the angle between first planar surface  12  and second planar surface  14  is approximately 90°. The structure of inertial measurement unit  10  may include additional surfaces and other elements. Inertial measurement unit  10  may include additional planar surfaces at a variety of orientations. For example, inertial measurement unit  10  may be a cube structure having additional planar surfaces that are parallel to either first planar surface  12  or second planar surface  14 . However, only two perpendicular planar surfaces are required to construct inertial measurement unit  10  having a tetrahedral configuration. 
         [0039]    As shown in  FIGS. 3-6 , inertial measurement unit  10  may include four in-plane sensors mounted on the two orthogonal planar surfaces. The in-plane sensors may include one or more microelectromechanical systems (MEMS) sensors, such a MEMS gyroscopic rotational rate sensor or a MEMS linear accelerometer. A first sensor  16  is mounted to first planar surface  12  and has a first sense axis  18  that is parallel to first planar surface  12 . A second sensor  20  is also mounted to first planar surface  12  and has a second sense axis  22  that is parallel to first planar surface  12 . First sense axis  18  and second sense axis  22  are positioned approximately 54.735 degrees from a linear junction or line  24  intersecting first planar surface  12  and second planar surface  14 . 
         [0040]    Mounting first sensor  16  and second sensor  20  on first planar surface  12  such that their respective sense axes  18 ,  22  are both oriented at 54.735 degrees from intersection line  24  creates the supplementary angles of 109.47 and 70.53 degrees between sense axes  18  and  22 , as illustrated in  FIG. 4 . That is, the outer or obtuse angle formed between sense axes  18  and  22  is approximately 109.47 degrees and the inner or acute angle formed between sense axes  18  and  22  is approximately 70.53 degrees. 
         [0041]    A third sensor  26  is mounted on second planar surface  14  and has a third sense axis  28  that is parallel to the second planar surface  14 . A fourth sensor  30  is also mounted to second planar surface  14  and has a fourth sense axis  32  that is parallel to second planar surface  14 . Third sensor  26  and fourth sensor  30  are mounted on second plane  14  such that their respective sense axes  28 ,  32  are oriented at an angle of 54.735 degrees from intersection line  24 , which creates the supplementary angles of 109.47 and 70.53 degrees between sense axes  28  and  32 , as shown in  FIG. 5 . 
         [0042]    Mounting sensors  16 ,  20 ,  26 ,  32  in this manner forms the supplementary angles of 109.47 and 70.53 degrees between each of the sense axes  18 ,  22 ,  28 ,  32 . That is, first sense axis  18  is oriented at an angle of 70.53 degrees with respect to not only second sense axis  22 , but also with respect to third sense axis  28  and fourth sense axis  32 . 
         [0043]    Mounting sensors  16 ,  20 ,  26 ,  32  in the configuration described above simplifies the fabrication and assembly of inertial measurement unit  10  by requiring only two surfaces for mounting the sensors. Consequently, inertial measurement unit  10  can be manufactured more efficiently and at a lower cost that prior art inertial measurement units. 
         [0044]    For purposes of explanation and illustration, and not limitation, a second exemplary embodiment of the inertial measurement unit is shown in  FIGS. 7-11  and is designated generally by reference character  40 . 
         [0045]    In this exemplary embodiment, as shown in  FIG. 7 , inertial measurement unit  40  includes a first planar surface  42 , a second planar surface  44 , and a third planar surface  46 . First planar surface  42  is orthogonal to second planar surface  44 . In other words, the angle between first planar surface  42  and second planar surface  44  is approximately 90°. Third planar surface  46  is oriented at approximately 45° with respect to both first planar surface  42  and second planar surface  44 . The structure of inertial measurement unit  40  may include other surfaces and elements as well. 
         [0046]    As shown in  FIGS. 7-11 , inertial measurement unit  40  may include six in-plane sensors mounted on the three planar surfaces, with two of the surfaces mounted at a 90° with respect to each other, and one planar surface mounted at a 45° with respect to each of the other planar surfaces. The in-plane sensors may include one or more MEMS sensors, such a MEMS gyroscopic rotational rate sensor or a MEMS linear accelerometer. A first sensor  48  is mounted on first planar surface  42  in such a way that a first sense axis  50  of first sensor  48  is parallel to first planar surface  42 . A second sensor  52  is oriented in such a way that a second sense axis  54  of sensor  52  is also parallel to first planar surface  42 . First sense axis  50  and second sense axis  54  are each positioned approximately 54.735 degrees from the line  56  intersecting first planar surface  42  and second planar surface  44 . 
         [0047]    Mounting first sensor  48  and second sensor  52  on first planar surface  42  such that their respective sense axes  50 ,  54  are both oriented at 54.735 degrees from intersection line  56  creates the supplementary angles of 109.47 and 70.53 degrees between sense axes  50  and  54 . That is, the outer or obtuse angle formed between sense axes  50  and  54  is approximately 109.47 degrees and the inner or acute angle formed between sense axes  50  and  54  is approximately 70.53 degrees. 
         [0048]    A third sensor  58  having a third sense axis  60  is mounted on second planar surface  44  such that third sense axis  60  is parallel to second planar surface  44 . A fourth sensor  62  having a fourth sense axis  64  is also mounted to second planar surface  44  such that fourth sense axis  64  is parallel to second planar surface  44 . Third sense axis  60  and fourth sense axis  64  are also oriented at an angle of approximately 54.735° with respect to the line  56  intersecting first planar surface  42  and second planar surface  44 . Third sensor  58  and fourth sensor  62  are mounted on second plane  44  such that their respective sense axes  60 ,  64  are oriented at an angle of approximately 54.735 degrees from intersection line  56 , which creates the supplementary angles of 109.47 and 70.53 degrees between sense axes  60  and  64 . 
         [0049]    A fifth sensor  66  having a fifth sense axis  68  is mounted to third planar surface  46  and oriented such that fifth sense axis  68  is parallel to second planar surface  46 . A sixth sensor  70  having a sixth sense axis  72  may also be mounted to any one of the first, second, or third planar surfaces  42 ,  44 , or  46 . In the exemplary embodiment shown in  FIGS. 7-11 , sixth sensor  70  is mounted to second planar surface  44 . Sixth sensor  70  is mounted in such a way that sixth sense axis  72  is oriented at approximately 90° with respect to fifth sense axis  68 . For example, in the exemplary embodiment shown, sixth sensor  70  is oriented such that the projection of fifth sense axis  68  onto third planar surface  46  forms a 90° angle with sixth sense axis  72 , as shown in  FIG. 11 . 
         [0050]    Advantageously, two planar surfaces mounted at an angle of 90° with respect to one another can also be used form an inertial measurement unit having a non-redundant orthogonal alignment of three planar sensors, in addition to forming a tetrahedral configuration using four planar sensors. 
         [0051]    For purposes of explanation and illustration, and not limitation, a third exemplary embodiment of the inertial measurement unit is shown in  FIGS. 12-15  and is designated generally by reference character  80 . 
         [0052]    Inertial measurement unit  80  includes a first planar surface  82  and a second planar surface  84 . First planar surface  82  and second planar surface  84  are orthogonal to one another, that is first planar surface  82  and second planar surface  84  are oriented at an angle of approximately 90° with respect to one another. A first sensor  86  having a first sense axis  88  is mounted on first planar surface  82  such that first sense axis  88  is parallel to first planar surface  82 . A second sensor  90  having a second sense axis  92  is also mounted to first planar surface  82  such that second sense axis  92  is parallel to first planar surface  82 . A perspective view of inertial measurement unit  80  is shown in  FIG. 12 . 
         [0053]    First sensor  86  and second sensor  90  are mounted on first planar surface  82  such that first sense axis  88  and second sense axis  92  are each oriented at an angle of approximately 45° with respect to a line  94  intersecting first planar surface  82  and second planar surface  84 . Mounting first sensor  86  and second sensor  90  in this manner creates an angle of approximately 90° between the two sense axes  88  and  92 . A third sensor  96  having a third sense axis  98  is mounted on second planar surface  84  at an angle of approximately 90° with respect to the intersection line  94 . 
         [0054]    The above configuration allows inertial measurement unit  80  to provide a non-redundant orthogonal alignment of three planar sensors on only two surfaces. First sense axis  88 , second sense axis  92 , and third sense axis  98  are mutually orthogonal. 
         [0055]    As with the first exemplary embodiment, mounting sensors  86 ,  90 , and  96  in the configuration described above simplifies the fabrication and assembly of inertial measurement unit  80 . 
         [0056]    The inertial measurement unit of the present invention, as described above and shown in the drawings, is a device that requires only two surfaces for mounting MEMS sensors such as accelerometers and gyroscopes. This configuration allows the inertial measurement unit to be manufactured more efficiently and at a lower cost that prior art inertial measurement units. It also requires less space than prior art designs. It will be apparent to those skilled in the art that various modifications and variations can be made to the device of the present invention and to the methods of making the device without departing from the scope of the invention as described in the appended claims and their equivalents.