Patent Publication Number: US-2007113702-A1

Title: Isolation system for an inertial measurement unit

Description:
FIELD  
      The present invention relates generally to inertial measurement units (IMUs) and more particularly to attenuating shock and vibrational energy using an IMU isolator.  
     BACKGROUND  
      In certain environments, it may be necessary to isolate mechanically sensitive assemblies from shock, vibrational, and acoustic energy. In many applications, this may be accomplished by placing the sensitive components within some form of container or housing. The need to isolate a device from shock, vibrational, and/or acoustic energy may be particularly acute when the device is an inertial sensor assembly (ISA), which may include a sensor suite of an inertial measurement unit (IMU). An ISA typically includes inertial sensors that detect acceleration and/or rotation in three axes. Usually, three accelerometers and three rotational rate sensors are arranged with their input axes in a perpendicular relationship. The sensors may generally be rigidly and precisely mounted within an ISA housing along with related electronics and hardware. Commonly, the housing of the ISA may be mounted to a container of the IMU, and the IMU may be rigidly and precisely mounted to a frame of a vehicle, such as an aircraft, missile, or other object.  
      Some applications expose the IMU to extremely high dynamic environments, such as ballistic applications wherein a projectile including an IMU may be fired from a gun. Traditionally, the inertial sensors were protected to some degree from relatively low level shock and vibration through the use of vibration isolators. However, such extremely high dynamic environments demand a smaller, lighter, and more durable mechanism to withstand the high acceleration, such as 20,000 g&#39;s, associated with these environments. Therefore, it may be desirable to provide a mechanism, particularly a vibration isolator, to attenuate shock and vibrational energy and having integrated features providing protection for the inertial sensors in these high dynamic applications, such as ballistic applications, to increase the performance and reliability of the inertial sensor system.  
     SUMMARY  
      The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention and is not intended to be a full description. A full appreciation of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.  
      The present invention relates generally to inertial measurement units (IMUs) and more particularly to attenuating shock and vibrational energy using an IMU isolator. In one illustrative embodiment, an inertial measurement unit (IMU) having an isolator includes an outer ring having an inner surface with a protruding portion and an inner ring having an outer surface adjacent the inner surface of the outer ring, the outer surface having an recessed portion. The protruding portion is positioned at least partially within the recessed portion and may be interlocking to prevent excessive rotation of the inner ring relative to the outer ring when the IMU experiences a high rotational force. Furthermore, the illustrative IMU may include a sensor suite attached to the inner ring, the sensor suite having at least one inertial sensor, and a cover member and a base member attached to the outer ring forming a cavity for the sensor suite, the cavity defined by cavity walls.  
      Additionally, an elastomer may be positioned between at least a portion of the outer ring and at least a portion of the inner ring to help increase attenuation of shock and vibrational energy in the sensor suite. In some cases, the elastomer may be positioned between at least one side surface of the recessed portion and at least one side surface of the protruding portion. However, in other cases, the elastomer may not be positioned between at least one side surface of the recessed portion and at least one side surface of the protruding portion.  
      In another illustrative embodiment, an inertial measurement unit (IMU) having an isolator includes an outer ring having an inner surface, an inner ring having an outer surface adjacent the inner surface of the outer ring, and an elastomer situated therebetween. The illustrative elastomer may cover substantially the entire height of at least a portion of the inner surface of the outer ring and substantially the entire height of at least a portion of the outer surface of the inner ring providing an elastomer-to-elastomer contact should the outer surface of the inner ring attempt to engage the inner surface of the outer ring during a shock event. In some cases, the elastomer may extend between the outer ring and the inner ring and may have a top gap and a bottom gap between the inner ring and the outer ring.  
      In yet another illustrative embodiment, an inertial measurement unit (IMU) includes a container having a cavity defined by cavity walls, an isolator having an inner ring with an outer surface and an outer ring with an inner surface, and a sensor suite having at least one inertial sensor. The inner ring of the isolator may have a recessed portion and the outer ring of the isolator may have a protruding portion, wherein the protruding portion is positioned in at least part of the recessed portion. The IMU may also include an elastomer position between at least a portion of the inner ring and at least a portion of the outer ring.  
      In another illustrative embodiment, an inertial measurement unit (IMU) having an isolator includes an outer ring having an inner surface with a recessed portion and an inner ring having an outer surface adjacent the inner surface of the outer ring, the outer surface having an protruding portion. The protruding portion may be interlocked with the recessed portion by positioning the protruding portion at least partially within the recessed portion to prevent excessive rotation of the inner ring relative to the outer ring when a rotational force is experienced. Additionally, an elastomer may be positioned between at least a portion of the outer ring and at least a portion of the inner ring. 
    
    
     BRIEF DESCRIPTION  
       FIG. 1  is a perspective view of an illustrative inertial measurement unit (IMU) in accordance with the present invention;  
       FIG. 1A  is a perspective assembly view of the illustrative IMU in  FIG. 1 ;  
       FIG. 2  is a perspective view of the illustrative IMU isolator in  FIG. 1 ;  
       FIG. 3  is a perspective assembly view of the illustrative IMU isolator of  FIG. 2 ;  
       FIG. 4  is a perspective view of the illustrative rotational stop mechanism of the IMU isolator of  FIG. 2 ;  
       FIG. 5  is a perspective view of another illustrative rotational stop mechanism of the IMU isolator of  FIG. 2 ; and  
       FIG. 6  is a schematic diagram of a cross-sectional view of the illustrative IMU isolator of  FIG. 2 . 
    
    
     DETAILED DESCRIPTION  
      The following description should be read with reference to the drawings wherein like reference numerals indicate like elements throughout the several views. The detailed description and drawings show several embodiments which are meant to be illustrative of the claimed invention.  
       FIG. 1  is a perspective view of an illustrative inertial measurement unit (IMU)  10  in accordance with the present invention. The illustrative IMU  10  is designed to help decrease the shock, vibrational, and/or acoustic energy transmitted to the inertial sensors contained in the IMU  10  and may include self-snubbing features when exposed to high dynamic environments, including, for example, gun launches greater than 20,000 g&#39;s, to protect the inertial sensors. Additionally, the illustrative IMU  10  may be able to withstand higher g-forces than conventional IMUs due to its smaller and lighter weight design. That is, the forces realized on the IMU is directly related to the mass of the IMU, thus the smaller size and lighter weight may help improve the illustrative IMU&#39;s  10  performance and durability in these high dynamic environments by decreasing the g-force exerted on the IMU  10 . Additionally, the illustrative IMU  10  may also have a relatively low production costs.  
      The illustrative IMU  10  includes a container, which includes a cover member  14  and a base member  34  forming a cavity defined by the cavity walls. The container may also include an isolator  24  situated between the cover member  14  and the base member  34 . The isolator  24 , in some cases, may define a portion of the cavity walls of the cavity. The cover member  14 , base member  34 , and isolator  24  may be secured together using one or more fasteners  12 , such as, for example bolts or screws  12 . The illustrative embodiment may include four screws  12  to secure the cover member  14 , base member  34 , and isolator  24  together. However, it is contemplated, that any number of fasteners  12 , or any method of securing the cover member  14 , base member  34 , and isolator  24  together may be used, as desired. The container may be used to help mechanically isolate a sensor suite, which may be contained inside the container, from shock, vibration, and/or acoustic energy. In some embodiments, the container may be secured to a projectile, which can be shot from a gun.  
       FIG. 1A  is a perspective assembly view of the illustrative IMU  10  in  FIG. 1 . In the illustrative embodiment, the cover member  14 , the isolator  24 , and the base member  34 , which form the container of the IMU  10 , are shown in an exploded view. The illustrative embodiment may also include a first o-ring seal  16  or gasket situated between the cover member  14  and the isolator  24 . Additionally, a second o-ring seal  32  or gasket may be situated between the isolator  24  and the base member  34 .  
      The illustrative embodiment may also include a sensor suite  36 , for example, an inertial sensor assembly (ISA), situated in the cavity of the container. The illustrative sensor suite  36  may measure acceleration and/or rotation in three planes. The container may provide protection for the sensor suite  36 . In the illustrative embodiment, the sensor suite  36  is attached to a portion of the isolator  24  in the cavity. The isolator  24  may help attenuate shock and vibrational energy at the sensor suite  36 .  
      The illustrative sensor suite  36  may include one or more inertial sensors, such as, for example, a MEMS gyroscope or a MEMS accelerometer. The one or more inertial sensors may be included in one or more printed wiring assemblies (PWAs)  22  and  26 . In the illustrative embodiment, the sensor suite includes two PWA  22  and  26 . A first PWA  22  is situated above the isolator  24  and a second PWA  26  is situated below the isolator  24 . In the illustrative embodiment, the first PWA  22  has a processor mounted thereon that may provide electronic circuitry and control for the IMU  10 . In some cases, the processor may be a microprocessor. Additionally or alternatively, the first PWA  22  may have an inertial sensor, such as a MEMS gyroscope or MEMS accelerometer, situated thereon. The second PWA  26  has an inertial sensor, such as a MEMS gyroscope or MEMS accelerometer, situated thereon. However, it is also contemplated, that a processor may be situated on the second PWA  26  and the inertial sensor situated on the first PWA  22 , as desired. Furthermore, it is also contemplated that the sensor suite  36  may include any number of PWAs  22  and  26  with any suitable device or component mounted thereon, depending on the desired application.  
      The illustrative sensor suite  36  may also include one or more support members  20 ,  28 . In some cases, the one or more support members  20 ,  28  may include a support ring and/or a center support. In some cases, the center support  21  and  29  may be a washer. In the illustrative embodiment, there is a first support member  20 , including a support ring and a center support  21 , situated above the first PWA  22 , and second support member  28 , including a support ring and a center support  29 , situated below the second PWA  26 . The first support member  20  and the second support member  28  may be adapted to secure the sensor suite  36  together as well as secure the sensor suite  36  to the isolator  24 . The support members  20 ,  28  may include one or more holes for one or more fasteners  30 , such as bolts or screws  30 . Additionally, the isolator  24  may be adapted to secure the sensor suite  36  thereto by providing one or more holes for the one or more fasteners  30 . In the illustrative embodiment, there may be three screws  30  for securing the support rings to the isolator  24  and one screw  30  to secure the center supports,  21  and  29 . However, it is contemplated that any number of fasteners  30  may be used, as desired.  
      Furthermore, the illustrative embodiment may include an input/output flextape  18  adjacent the sensor suite  36  and the cover member  14 . In some cases, a portion of the flextape  18  may be coupled to the sensor suite  36 . Additionally, in some cases, a portion of the flextape  18  may extend through an opening in the cover member  14  and may provide or receive inertial data externally of the IMU  10 .  
      The illustrative IMU  10  may provide inertial data, such as linear and angular acceleration information, about the movement of the IMU  10 . The data may provide information relating to the flight and control of the IMU  10  to a navigational computer. In some cases, the IMU  10  may provide guidance information about the flight of a projectile. In other cases, the IMU  10  may provide information relating to the flight of an aircraft. More generally, the IMU  10  may be used to provide data relating to any movable object, as desired.  
       FIG. 2  is a perspective view of the illustrative IMU isolator  24  in  FIG. 1 . The illustrative IMU isolator  24  may include an inner ring  42  having an outer surface  47 , and an outer ring  40  having an inner surface  49  situated adjacent the outer surface  47  of the inner ring  42 . The IMU isolator  24  may also include an elastomer  44  provided between at least a portion of the inner ring  42  and at least a portion of the outer ring  40 . In some cases, the elastomer  44  may be a silicone rubber, however, it is contemplated that the elastomer  44  may be any material that may absorb energy and that may dampen vibration and shock during normal operation and during the impact between the inner ring  42  and outer ring  40 .  
      The illustrative inner ring  42  and outer ring  40  may be adapted to provide a rotation stop mechanism, such as interlocking portions, which may help prevent the inner ring  42  from excessively rotating relative to the outer ring  40 . This may help protect the elastomer during high dynamic events. That is, the rotation stop mechanism may help prevent the inner ring  42  from “spinning out” of the outer ring  40  and tearing the elastomer  44  when exposed to high dynamic environments.  
      The illustrative rotational stop mechanism may include a recessed portion  43  and a protruding portion  41 . In the illustrative embodiment, the inner ring  42  has an outer surface that may be designed to have at least one recessed portion  43 . The outer ring  40  may be designed and machined to have an inner surface with at least one protruding portion  41  corresponding to the at least one recessed portion  43  of the inner ring  42 . Alternatively, it is contemplated that the inner ring  42  may have at least one protruding portion and the outer ring  40  may have at least one corresponding recessed portion. In either case, the protruding portion  41  may be positioned at least partially within the recessed portion  43  so that they are interlocking, which may inhibit or substantially inhibit excessive rotation of the inner ring  42  relative to the outer ring  40  when exposed to high rotational forces. Such high rotational forces may be provided by, for example, the “rifling”of a projectile that includes the IMU with the barrel of a gun or cannon. In the illustrative embodiment, there are four recessed portions  43  in the outer surface of the inner ring  42  and four corresponding protruding portions  41  in the inner surface of the outer ring  40 . However, the use of four recessed portions  43  and protruding portions  41  is only illustrative and it is contemplated that any number of recessed portions  43  and protruding portions  41  may be used, as desired  
      The illustrative IMU isolator  24  inner ring  42  may be adapted to mount the sensor suite thereto and the IMU isolator  24  outer ring  40  may be adapted to secure the cover member and base member thereto. The illustrative inner ring  42  may be adapted to mount the sensor suite thereto by provide one or more holes  48  to facilitate the insertion of fasteners. In the illustrative embodiment, there are three holes  48  provided for fasteners. However, it is contemplated that any number of holes  48  may be used depending on the design of the sensor suite. Furthermore, the outer ring  40  may provide one or more holes  46  for securing the base member and cover member thereto. In the illustrative embodiment, there are six holes  46  provided. However, it is contemplated that any number of holes  46  may be provided to secure the base member and cover member to the outer ring  40 , as desired. Furthermore, it is contemplated that any method of fastening or securing the sensor suite to the inner ring  42  and any method of fastening or securing the cover member and base member to the outer ring  40  may be used, as desired.  
       FIG. 3  is a perspective assembly view of the illustrative IMU isolator  24  of  FIG. 2 . The illustrative IMU isolator  24  assembly includes the inner ring  42 , the elastomer  44 , and the outer ring  40 . The inner ring  42  and outer ring  40  may be designed to provide a recessed portion  43  and a protruding portion  41 . Once the inner ring  42  and the outer ring  40  are machined, or otherwise formed, the inner ring  42  may be positioned within the outer ring  40 . Next the elastomer  44  may be provided between at least a portion of the inner ring  42  and at least a portion of the outer ring  40 . However, it is also contemplated that the elastomer  44  may be provided in the outer ring  40  before positioning the inner ring  42  in the outer ring  40 , the elastomer  44  may be provided around the inner ring  42  before positioning the inner ring  42  and elastomer  44  in the outer ring  40 , or the elastomer  44  may be positioned in the outer ring  40  at the same or substantially the same time that the inner ring  42  is positioned in the outer ring  40 .  
      In some cases, the inner ring  42  and outer ring  40  may be placed in a mold in which the elastomer  44  may be applied. However, the elastomer  44  may be applied between the entire inner ring  42  and the entire outer ring  40  or between a portion of the inner ring  42  and a portion of the outer ring  40 , as desired. In other cases, the elastomer  44  may be provided between the inner ring  42  and the outer ring  40  by spraying, coating, dipping, molding, or by any other suitable method as desired.  
      The illustrative IMU isolator  24  may have a temperature range of −55 degrees Celsius to 90 degrees Celsius. Over this temperature range, the IMU isolator  24  may have a natural frequency of about 260 hertz (Hz). The natural frequency temperature variation may have a range of 50 Hz. In one case, the IMU isolator  24  may have a natural frequency of 225 Hz with an allowable range of 35 Hz over the temperature range. The illustrative IMU isolator  24  may also have a transmissibility that ranges from 3.0 to 7.0 over the temperature range. However, it is contemplated that any suitable temperature range, frequency, or transmissibility may be used, depending on the application.  
       FIG. 4  is a perspective view of the illustrative rotational stop mechanism of the IMU isolator  24  of  FIG. 2 . The illustrative rotational stop mechanism includes the recessed portion  43  of the outer surface of the inner ring  42  and the protruding portion  41  of the inner surface of the outer ring  40 . The recessed portion  43  and the protruding portion  41  may be situated so that the protruding portion  41  is at least partially within the recessed portion  43 . With such an alignment, the side surface  52  of the recessed portion  43  of the inner ring  42  may come into contact with the side surface  50  of the protruding portion  41  of the outer ring  40  when the IMU isolator  24  experiences a significant rotational force. The illustrative rotation stop mechanism may cause the inner ring  42  and outer ring  40  to interlock, preventing or substantially preventing rotation of the inner ring  42  relative to the outer ring  40 .  
      In some cases, to help improve the interlocking of the inner ring  42  with the outer ring  40 , the side surface  52  of the recessed portion  43  may recede at an angle of 90 degrees. The side surface  50  of the protruding portion  41  may be protruding at an angle similar to that of the recessed portion  43 , in this case, 90 degrees. Additionally, in some cases, the side surface  52  of the recessed portion  43  may be recessed at an angle greater than 90 degrees. The side surface  50  of the protruding portion may also protrude at a corresponding angle. However, it is contemplated that the side surface  52  of the recessed portion  43  and the side surface  50  of the protruding portion  41  may have any suitable angle greater than or less than 90 degrees, as desired.  
      As illustrated, the elastomer  44  may be positioned between a portion of the inner ring  42  and the outer ring  40 , and in some cases, may be adjacent to the side surface  52  of the recessed portion  43  and the side surface  50  of the protruding portion  41 , shown in region  54 . The elastomer  44  may, for example, be provided in region  54  to help attenuate the shock and vibrational energy when the IMU isolator  24  bottoms out during a high rotational event. In this case, there is an elastomer-to-elastomer contact between the inner ring  42  and the outer ring  40 . However, it is also contemplated that the elastomer  44  may be provided only on one surface causing an elastomer-to-metal contact or on no surface, as illustrated in  FIG. 6 , having a metal-to-metal contact. One advantage of providing elastomer  44  on at least one surface  50  or  52  may be the increased attenuation of shock and vibrational energy in the sensor suite of the IMU during a high rotational event.  
       FIG. 5  is a perspective view of another illustrative rotational stop mechanism of the IMU isolator  24  of  FIG. 2 . Similar to  FIG. 4 , the illustrative IMU isolator  24  has a rotational stop mechanism that includes the recessed portion  43  of the inner ring  42  and the protruding portion  41  of the outer ring  40 . The recessed portion  43  and the protruding portion  41  may be positioned so that the protruding portion  41  is at least partially within the recessed portion  43 . In such an alignment, the side surface  52  of the recessed portion  43  of the inner ring  42  may come into contact with the side surface  50  of the protruding portion  41  of the outer ring  40 , preventing or substantially preventing excessive rotation of the inner ring  42  relative to the outer ring  40  during a high rotational event.  
      However, in some cases, the elastomer  44  is not provided between the side surface  52  of the recessed portion  43  and the side surface  50  of the protruding portion  41 , shown in region  55 . Two reasons for not providing the elastomer  44  in region  55  may be the design constraints and the increased cost. When no elastomer  44  is provided, the side surface  52  of the recessed portion  43  and the side surface  50  of the protruding portion  41  may come into contact as a metal-to-metal contact when the illustrative IMU isolator  24  is exposed to high rotation forces. However, a metal-to-metal contact may produce a reduced attenuation of shock and vibrational energy in the senor suite during such events, as compared to  FIG. 4 .  
      More generally, it is contemplated that elastomer  44  may be provided on both the side surface  50  and  52  of the recessed portion  43  and the protruding portion  41 , on only the side surface  52  of the recessed portion  43 , on only the side surface  50  of the protruding portion  41 , or not provided at all between the side surfaces  50 ,  52  of the recessed portion  43  and protruding portion  41 , as desired.  
       FIG. 6  is a schematic diagram of a cross-sectional view of the illustrative IMU isolator of  FIG. 2 . The illustrative IMU isolator includes inner ring  42 , outer ring  40 , and elastomer  44  extending therebetween. The illustrative elastomer  44  may be disposed over the entire height of the inner ring  42  and the entire height of the outer ring  40 , if desired. The elastomer has a top gap  60  and a bottom gap  63 . Under some circumstances, the IMU isolator may be exposed to translational forces causing a compression between the inner ring  42  and the outer ring  40 . Under this compression, elastomer area  61  on the inner ring  42  may come into contact with elastomer area  62  on the outer ring  40 . Likewise, elastomer area  64  on the inner ring  42  may come into contact with elastomer area  65  on the outer ring  40 . One advantage of having full surface coverage may be to help prevent a metal-to-metal or metal-to-elastomer contact. An elastomer-to-elastomer contact may have a higher attenuation of shock and vibrational energy, which may be advantageous when operating in high dynamic environments. However, it is also contemplated that the elastomer  44  may provide full surface coverage only on either the inner ring  42 , such as areas  61  and  64 , or only on the outer ring  40 , such as areas  62  and  65  or on neither the inner or outer rings, as desired.  
      Furthermore, the inner surface  72  of the outer ring  40  may be similarly shaped to the outer surface  70  of the inner ring  42  and not have any pointed regions. One advantage of a non-pointed region is that when exposed to translational forces that cause a compression of the inner ring  42  and outer ring  40 , a non-pointed region may spread out the force over the entire surface area of the inner ring  42  and the outer ring  40  and may not cause the elastomer  44  to tear or cut, whereas a pointed region may centralize the force in one point and cause the elastomer  44  to tear or cut, resulting in a metal-to-metal contact.  
      Additionally, the illustrative elastomer  44  may be symmetrical or nearly symmetrical in shape in the isolator. The symmetrical shape may provide an advantage of having the elastic center of the elastomer  44  at the geometric center of the isolator similarly located. Furthermore, the elastomer  44  may balance the axial and radial stiffness of the isolator at a ratio of approximately 1:1. Moreover, in some cases, the elastomer  44  may have a linear spring rate over an expected deflection range associated with shock and vibrational energy during a high dynamic event. In one case, the expected deflection range may be approximately 0.003 inches. However, it is contemplated that the elastomer  44  may have a linear spring rate or stiffness up to a deflection of approximately 0.014 inches in the radial direction and an even greater deflection range in the axial direction.  
      The illustrative isolator may provide protection and/or attenuation of the inertial sensors in all six degrees of freedom, such as the longitudinal, lateral, axial, roll, pitch, and yaw degrees of freedom. In some cases, the illustrative elastomer  44  may provide the protection and/or attenuation of the inertial sensor in three degrees of freedom, such as, for example, the longitudinal, lateral, and the axial degrees of freedom.  
      Having thus described the preferred embodiments of the present invention, those of skill in the art will readily appreciate that yet other embodiments may be made and used within the scope of the claims hereto attached. Numerous advantages of the invention covered by this document have been set forth in the foregoing description. It will be understood, however, that this disclosure is, in many respect, only illustrative. Changes may be made in details, particularly in matters of shape, size, and arrangement of parts without exceeding the scope of the invention. The invention&#39;s scope is, of course, defined in the language in which the appended claims are expressed.