Abstract:
A tunable mass damper (TMD) is provided that can be coupled to a rotating inertia apparatus such as a reaction wheel assembly (RWA). The TMD includes a housing that contains a flexure, a mass, and damping fluid within the housing. The housing is coupled to the flexure, and the flexure is coupled to the mass. The mass is free to sway in the damping fluid. The damping fluid envelops the mass and provides damping between the mass and the housing. At least one of the mass, flexure and the damping fluid can be adjusted to tune the TMD to produce a resonance at or near a mode that is to be mitigated by the TMD such that it operates at a desired or optimal operating frequency.

Description:
TECHNICAL FIELD 
       [0001]    The present invention generally relates to vibration mitigation, and more particularly relates to a tunable mass damper for use with Attitude Control Systems (ACSs). 
       BACKGROUND 
       [0002]    Spacecraft, satellites, or other vehicles in orbit experience a number of factors such as aerodynamic drag that can cause undesirable changes in attitude. Attitude control systems (ACSs) are often utilized to control/adjust the attitude of a spacecraft, satellite, or other vehicle. Such ACSs can include various rotating inertia members such as reaction wheel assemblies (RWAs), control momentum gyroscopes (CMGs) and similar actuators. 
         [0003]    A RWA is a type of attitude control device that can be used in attitude control systems to exchange angular momentum with a space vehicle. A reaction wheel assembly typically includes a very large and heavy flywheel that is fixed in a body frame or housing. An electric motor is used to produce a torque along a spin axis of the flywheel so that the flywheel rotates to produce a force that opposes motion in one plane. The electric motor and wheel are supported on a rotor that acts like an axle. The rotor is positioned between bearings located at opposing ends of the rotor so that the rotor is allowed to spin within bearings. 
         [0004]    A CMG is another type of attitude control device that can be used in attitude control systems. A CMG usually includes a spinning rotor (e.g., flywheel) and one or more motorized gimbals that tilt the rotor&#39;s angular momentum. As the rotor tilts, the changing angular momentum causes a gyroscopic torque that rotates the spacecraft. The spin axis of the CMG can be changed by moving the rotor using the gimbal assembly. The torque produced is orthogonal to the spin axis and the gimbal axis. CMGs differ from RWAs in that the latter applies torque simply by changing rotor spin speed, but the former tilts the rotor&#39;s spin axis without necessarily changing its spin speed. In general, CMGs are more power efficient. 
         [0005]    During launching and/or ascent of a spacecraft vibrations and/or harmonic forces are generated that result in loads that are distributed throughout the load-bearing structure of the spacecraft and its subsystems and components. A portion of these forces are imparted at the bearings of the RWA or CMG, and if the forces exceed the levels that the bearings were designed to accommodate, the bearings could be overstressed. 
         [0006]    As the size of the rotor used in a RWA/CMG increases, loading on the bearings during launching also increases. It would be desirable to reduce the loads on the bearings even though rotor size has increased. 
         [0007]    To handle the increased forces and torques on the bearings, some RWAs/CMGs simply increase the size of the bearings. However, this is not an option or is undesirable in many RWAs/CMGs. The use of bulkier, heavier bearings not only increases the mass/size of the RWA/CMG, it also increases drag torque on the shaft of a RWA/CMG, which can increase power requirements and decrease the life of the RWA/CMG. As such, it is often desirable to use smaller bearings since they generally have lower friction drag. Smaller bearings can also increase the life of the bearings. 
         [0008]    When smaller bearings are used in RWAs/CMGs, a mechanism is needed to ensure that forces and torques on the bearings do not exceed their operating capabilities. As such, there is a need to reduce the loading on the smaller bearings to acceptable levels while maintaining high margins and reliability. 
       BRIEF SUMMARY 
       [0009]    In accordance with one embodiment, a system is provided comprising a tunable mass damper (TMD) that is physically coupled to a reaction wheel assembly (RWA). The TMD includes a housing that contains a flexure, a mass, and damping fluid within the housing. The housing is coupled to the flexure, and the flexure is coupled to the mass. The mass is free to sway in the damping fluid. The damping fluid envelops the mass and provides damping between the mass and the housing. In one implementation, at least one of the mass, flexure and the damping fluid can be adjusted to tune the tunable mass damper to produce a resonance at or near a mode that is to be mitigated by the TMD such that it operates at a desired or optimal operating frequency. 
         [0010]    Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and the background of the invention provided above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
           [0012]      FIG. 1  is an elevation view of an assembly that comprises a reaction wheel assembly (RWA) and an external tunable mass damper (TMD) in accordance with some of the disclosed embodiments; 
           [0013]      FIG. 2  is an exterior side auxiliary view of the RWA and TMD of  FIG. 1 ; 
           [0014]      FIG. 3  is a cross-sectional view of the RWA and TMD of  FIG. 1  taken along  3 - 3  of  FIG. 1 ; 
           [0015]      FIG. 4  is an exterior side auxiliary view showing the TMD prior to attachment to the RWA; 
           [0016]      FIG. 5  is an exploded exterior side auxiliary view of the TMD just prior to attachment to the RWA; 
           [0017]      FIG. 6  is a cross-sectional view of the RWA of  FIG. 1  prior to attachment of the TMD; 
           [0018]      FIG. 7  is a cross-sectional view of the TMD of  FIG. 1  taken along  3 - 3  of  FIG. 1  without the RWA present in accordance with some of the disclosed embodiments; 
           [0019]      FIG. 8  is an assembly view of the various elements of the TMD in accordance with some of the disclosed embodiments; and 
           [0020]      FIG. 9  is an exterior side auxiliary view showing the mass of the TMD in accordance with some of the disclosed embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. As used herein, the word “exemplary” means “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description. In this regard, although a tunable mass damper (TMD) that is described herein is described as being implemented in conjunction within a reaction wheel assembly (RWA), it will be appreciated that this is merely exemplary, and that the TMD could be used in numerous and varied devices, systems, and contexts including, for example, in conjunction with other rotating inertia members such as control moment gyros (CMGs), energy storage flywheel systems, etc. 
         [0022]      FIG. 1  is an elevation view of an assembly  100  that comprises a reaction wheel assembly (RWA)  110  and an external tunable mass damper (TMD)  150  in accordance with some of the disclosed embodiments.  FIG. 2  is an exterior side auxiliary view of the RWA  110  and TMD  150  of  FIG. 1 . 
         [0023]      FIG. 3  is a cross-sectional view of the RWA  110  and TMD  150  of  FIG. 1  taken along  3 - 3  of  FIG. 1 . The exterior housing of the RWA  110  includes a case  120  (or lower housing) and a cover  140  (or upper housing) that are coupled together using a plurality of fasteners, such as screws  130 . In one implementation, the case  120  and cover  140  can be fabricated from a metal such as aluminum. More details regarding the internal structure of the RWA  110  will be described below with reference to  FIG. 6 . 
         [0024]      FIG. 4  is an exterior side auxiliary view showing the TMD  150  prior to attachment to the RWA  110 . As shown, the cover  140  of the RWA  110  may include an evacuation port  145  that provides an entry point for the RWA to be evacuated as needed. The area marked in the dashed-line circle  410  is illustrated in greater detail in  FIG. 5 . 
         [0025]      FIG. 5  is an exploded exterior side auxiliary view  410  of the TMD  150  just prior to attachment to the RWA  110 . As shown, the TMD  150  includes a plurality of flanges  162 . The cover  140  of the RWA  110  includes a plurality of threaded holes  164 . As illustrated in  FIGS. 1-3 , the housing base  152  of the TMD  150  is coupled or fastened in a stationary position on the cover  140  of the RWA  110  with screws  160 . To install the TMD  150  and attach it to the cover  140  of the RWA  110 , screws  160  are inserted through the flanges  162  and into the threaded holes  164  to secure the TMD  150  to the RWA  110 . 
         [0026]      FIG. 6  is a cross-sectional view of the RWA  110  of  FIG. 1  taken along  3 - 3  of  FIG. 1  prior to attachment of the TMD  150  (without the TMD  150  present). 
         [0027]    The RWA  110  of  FIG. 6  includes a motor (e.g., a permanent magnet brushless DC motor) and a flywheel  170  disposed within a housing assembly  120 ,  140 . 
         [0028]    The flywheel  170  includes a rotor shaft  190 , rotor suspension webs  172  and rotor web rings  179 . The flywheel  170  is rotationally mounted within the housing. The rotor shaft  190  has a proximal end, a distal end, and a body between the proximal and distal end. The body of the rotor shaft  190  defines an axis of rotation. 
         [0029]    The motor is mounted within the housing assembly  120 ,  140  and includes a rotor  178  and a stator  176 . The rotor  178  is coupled to the flywheel  170  and is configured to rotate the flywheel  170  in response to electromagnetic excitation generated in the stator  176 . The stator  176  surrounds at least a portion of, and preferably the entirety of, the rotor  178 . The stator  176  is coupled to be appropriately energized from a distribution bus (not illustrated) under control of a motor control unit (not illustrated) that can be disposed, partially or entirely, within the housing assembly  120 ,  140  or external thereto. The rotor  178  of the motor is mounted around the rotor shaft  190 . The motor is used to provide a mechanical torque necessary to rotate the flywheel  170  about the axis of rotation. Because the motor rotor  178  is coupled to the flywheel  170  it is driven thereby at the same rotational speed. 
         [0030]    The RWA also includes several bearings  146  in a fixed upper bearing assembly  182  and a floating lower bearing assembly  184 , which are located proximate the upper and lower ends  192 ,  194  of the rotor shaft  190 , respectively. The flywheel  170  is rotationally mounted within the housing assembly  120 ,  140  via the fixed upper bearing assembly  182  and the floating lower bearing assembly  184 . The bearings  146  are positioned generally at the distal end and proximal end of the rotor shaft  190 , and help stabilize the rotor shaft  190 . In one implementation, the bearing assemblies  182 ,  184  are held in a stainless steel cartridge, and have a diameter between approximately 1.1024 and approximately 2.0472 inches. The bearings  146  allow the rotor  178  to rotate, while constraining it within the housing and minimizing travel or floating in the axial directions. 
         [0031]    In addition to the main housing (i.e., case  120  and the cover  140 ), the RWA  110  includes an evacuation port  145  (e.g., a valve) connected to the cover  140  with a housing cap. 
         [0032]    Other details regarding other reaction wheel assemblies are disclosed in U.S. Pat. No. 5,474,263, entitled “Reaction wheel and method of safing wheel,” assigned to the assignee of the present invention, its contents being incorporated by reference in its entirety herein. 
         [0033]      FIG. 7  is a cross-sectional view of the TMD  150  of  FIG. 1  taken along  3 - 3  of  FIG. 1  without the RWA  110  present (e.g., prior to attachment of the TMD  150  to the RWA  110 ).  FIG. 8  is an assembly view of the various elements  152 - 158  of the TMD  150 . As shown in  FIGS. 7 and 8 , the TMD  150  comprises a housing that includes a housing base  152  and a housing cap  158 , a flexure  154  and a mass  156 . Prior to describing the various elements of the TMD  150  in greater detail, it is noted that the TMD is a passive damping device in that does not include any rotating parts that rotate with respect to the remainder of the assembly  100 . 
         [0034]    The housing base  152  is coupled to clearance holes of housing cap  158  using fasteners such as screws  155  to create a hermetically-sealed volume or vessel for the flexure  154 , mass  156  and silicone or other damping fluid (not illustrated). 
         [0035]    In one non-limiting implementation, the housing base  152  is cylindrical in shape, and includes multiple outer and inner flanges  162  used to attach the housing base  152  to the RWA  110 . The top portion of the housing base  152  has a flange with an o-ring to ensure a hermetic seal within the system when the assembly is filled with damping fluid. A fill port can be located on the side of the housing base  152  to ensure proper filling of the unit with the damping fluid. The fill port can be hermetically sealed using an aluminum ball and setscrew. A plurality of threaded holes are located in the internal and external portion of the top flange to accommodate assembly of the housing cap  158  onto the housing base  152 . The housing base  152  provides one of the sheer plates that interact with the mass, to create damping within the system. In one non-limiting implementation, the housing base  152  is fabricated from a metal, such as aluminum 6061-T6, and has a diameter of approximately 4.95 inches and a length of approximately 1.803 inches. When the material is one such as aluminum, there are little or no difficulties with the lathing and milling of the part. 
         [0036]    In one non-limiting implementation, the housing cap  158  is disc-shaped and is coupled to the housing base  152  using a plurality of hardware at the internal and external portions of the flange. The housing cap  158  provides the secondary, and greater, sheer plate which partially defines the damping of the system. In one non-limiting implementation, the housing cap  158  is fabricated from a metal, such as aluminum 6061-T6, and has a diameter of approximately 4.95 inches and a length of approximately 0.370 inches. When the material is one such as aluminum, there are little or no difficulties with the lathing and milling of the part. 
         [0037]    The flexure  154  is coupled to the housing  152 ,  158  using screws (not illustrated), and functions to provide the stiffness of the TMD  150 . As used herein, the term “stiffness” refers to the resistance of an elastic body to deformation by an applied force. The stiffness, k, of an elastic body is a measure of the resistance offered by the elastic body to deformation (bending, tension or compression). In one implementation, the flexure  154  has a cylindrical shape with cut-out portions that define diagonal beams  155 . The diagonal beams  155  provide the necessary x-axis and y-axis lateral stiffness, while keeping a z-axis translational stiffness and torsional stiffness (e.g., the z-axis moment stiffness) significantly larger than the damping-producing axes. As used herein, the term “translational stiffness” refers to any stiffness that is parallel to the axis, whereas the term “rotational stiffness” refers to any stiffness that is acting rotationally, using x, y and z as the rotational axis, where the x and y axes can be either lateral axis, and the z-axis is orthogonal to the x and y axes. In one non-limiting implementation, the flexure  154  is fabricated from a machined metal such as aluminum 2024-T351, and has a dimensional envelope of 3.50 inch diameter by 1.55 inch length. The flexure  154  is fabricated using standard machining operations and when the material is one such as aluminum, there are no difficulties with the lathing and milling of the part. 
         [0038]    The mass  156  can generally have any shape, dimensions, mechanical properties or other structural features that depend on the specific implementation. In one non-limiting implementation, the mass  156  can be a disk-shaped object, a ring-shaped object, a toroid-shaped object, or any other object having an annular shape generated by revolving a geometrical figure around an axis external to that figure. The mass  156  consists of four holes that are located on each quadrant of its side. These holes are used to allow tenability of the unit by pressing in various density material to increase the mass, or by removing more tungsten alloy volume to decrease the mass to attain better performance within the system. When fully assembled, the upper flat works in conjunction with the housing cap to provide damping within the system, and the lower flat works in conjunction with the housing base to provide damping within the system. In one non-limiting implementation, the mass  156  is fabricated from a metal, such as tungsten or a tungsten alloy, and has a dimensional envelope of approximately 4.12 inches in diameter by 1.00 inch in length. The mass, being an extremely dense material, is more difficult to fabricate compared to the other parts in the system made of aluminum. Extra care would be needed from the machinist to ensure the proper milling and lathing bits are used to properly machine the part. 
         [0039]    The flexure  154  surrounds the mass  156  of the TMD  150 , and the mass  156  can be attached to the flexure  154  via screws (not illustrated). In one implementation, the TMD  150  is assembled by attaching the flexure  154  and mass  156  to the housing base  152 . The housing cap  158  can then be torqued onto the housing base  152  to hermetically seal the housing of the TMD  150 . 
         [0040]    The housing base  152  includes a fill-port (not shown) that allows a damping fluid to be filled into the TMD  150  after it is assembled. The damping fluid provides viscous damping between the flat surfaces of the mass  156  and housing  152 ,  158 , and in one implementation can be silicone fluid or silicone-based fluid. As will be explained further below, the TMD  150  employs a damping fluid to damp certain structural modes of the RWA  110 . 
       Operation of the TMD 
       [0041]    During launch of a vehicle (e.g., spacecraft), vibrations excite lateral or rocking modes of the rotor of the RWA  110 . These lateral or rocking modes refer to the rotor&#39;s natural tendency to rotate about two of the three axes (e.g., if the x-axis were aligned to the rotor&#39;s spin axis, then the rotor&#39;s lateral or rocking modes occur about the y-axis and the z-axis.) Because the lateral or rocking modes are the primary loads into the bearings (i.e., have the most influence or contribution to the loads the bearings), it is desirable to reduce these modes. 
         [0042]    To reduce these modes, the TMD  150  applies a force that cancels a portion of the excitation force during launch, which means lower forces and torques occur at the bearings. More specifically, the TMD  150  attenuates the rotor rocking resonance of the RWA  110  by applying a reactive force through the case cover to the spin bearings and rotor shaft. The reacting force of the TMD  150  is produced by the excitation of its mass  156  and accompanying flexure  154  stiffness by the rotor rocking motion. (It is noted that the TMD  150  uses a mass  156  smaller than the RWA  110  on which it is mounted to produce a resonant frequency near that of the mode it is trying to counteract.) Inspection of a typical response curve of a system with a resonance and a TMD shows two response peaks at reduced amplitudes with frequencies surrounding the unmitigated resonant frequency. The damping fluid acts to damp lateral or rocking modes that occur during launch or ascent of the vehicle and thus reduce the two response peaks of the combined dual-mode resonant system. 
         [0043]    The efficacy of the TMD  150  is determined by (1) the proximity of its damped natural frequency to the rotor excitation frequency, (2) the amount of damping generated in the shearing of the damping fluid, and (3) its mass. As will now be described below, the damped natural frequency of the TMD  150  can be tuned so that it is near the natural frequency of the rotor&#39;s structural modes (i.e., rocking or lateral modes), and the damping fluid can be adjusted to change the amount of damping. 
       Tuning the TMD 
       [0044]    Prior to attaching the TMD  150  to the RWA  110 , the TMD  150  can be tuned to alter the target damping, stiffness, and/or frequency to help ensure that the TMD  150  reduces the modes in an optimal manner. To tune the TMD  150 , a variety of different parameters of the TMD  150  that can be changed or altered, including, for example, the physical mass of the mass  156 , stiffness of the flexure  154 , and the viscosity of the damping fluid. One or more of these parameters can be altered while keeping one or more of the other parameters constant. 
         [0045]    The flexure  154  allows a stiffness (Ka) parameter to be adjusted or tuned for a particular implementation. For example, the flexure  154  can be re-machined to create a flexure  154  with a different stiffness that fits in the same housing. To facilitate altering of the stiffness of the TMD  150 , the flexure  154  is made of a material that can be easily machined (e.g., Aluminum 6061). This way it is relatively easy to alter the existing flexure  154  in little time. 
         [0046]    The damping constant (c) parameter of the TMD  150  can be tuned by increasing or decreasing the damping constant (c) of the viscous fluid. In other words, the viscosity of the damping fluid can be changed to increase or decrease the damping effects. This could be done, for example, by draining the viscous fluid and remixing it so that it has a new viscosity. 
         [0047]    Increasing or decreasing the mass  156  affects the damped natural frequency, the damping constant (c) parameter, and flexure stiffness. As such, increasing or decreasing the mass  156  allows the damped natural frequency, the damping constant (c) parameter, and flexure stiffness to be adjusted or tuned for a particular implementation. To tune the damped natural frequency of the TMD  150  rotational modes (i.e. about the same axis as the rotor as defined in my prior comment above), the weight of the mass  156  can be increased or decreased as shown in  FIG. 9 . 
         [0048]      FIG. 9  is an exterior side auxiliary view showing the mass  156  of the TMD  150  in accordance with some of the disclosed embodiments. To alter/tune the frequency of the TMD, the mass  156  of the TMD  150  has N pre-drilled holes  157  (e.g., in each quadrant). The pre-drilled holes  157  can be drilled to a larger size to decrease the weight of mass  156 . In other words, the size of the pre-drilled holes  157  can be increased to further decrease the weight of  156 . To increase the weight of the mass  156 , any number of plugs  158  can be inserted (or press-fit) into the pre-drilled holes  157  in the mass  156 . The particular material used to create the plugs  158  can vary depending on the implementation and the increase in weight that is desired. The plugs  158  can be made of any material with whatever density that will increase the mass to a specific number to achieve the optimal mass. 
         [0049]    The damping constant (c) parameter of the TMD  150  can also be tuned by increasing or decreasing the dimensional gap between the mass and the housing. In this regard, it is noted that replacing the nominal fluid with a different viscosity fluid is a much simpler operation than redesigning the mechanical components to alter the gap between the mass and housing. 
         [0050]    In some other implementations, other components, such as portions of the housing (e.g., internal shelves of a housing cap and housing base can be altered or adjusted to tune the tunable TMD). 
         [0051]    Any of the tuning techniques described above (or any combination of them) can be used to achieve proper tuning of the TMD  150 . 
       CONCLUSION 
       [0052]    During launch and/or ascent, rotor structural resonance places heavy loads on the bearings  146  used in the RWA  110 . When the TMD  150  is installed on a RWA  110 , the TMD  150  reduces/mitigates the launch forces and torques and reduces the energy communicated to the bearings  146 . As a result, the bearings  146  will last longer, which increases the useful lifetime or survivability of the RWA  110 . In addition, because loading on the bearings  146  is decreased, the RWA  110  can utilize smaller, high-performance bearings. Moreover, smaller bearings  146  have less drag, which means there is less drag torque and therefore the robustness of the ACS system is improved. In addition, the use of smaller bearings  146  reduces the overall cost. 
         [0053]    Thus, by implementing the TMD  150  in conjunction with the RWA  110 , smaller bearings  146  can be used in the RWA  110 . As a result, the RWA  110  experiences less drag, and because drag torque is reduced, power consumption is also reduced. The smaller bearings  146  also have lower cost, and allow the total size of the RWA  110  to be decreased since the smaller bearings  146  take up less volume and are lighter in weight. In addition, because loading on the bearings  146  is decreased, the RWA  110  will last longer and need to be serviced less often. 
         [0054]    In this document, relational terms such as first and second, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Numerical ordinals such as “first,” “second,” “third,” etc. simply denote different singles of a plurality and do not imply any order or sequence unless specifically defined by the claim language. The sequence of the text in any of the claims does not imply that process steps must be performed in a temporal or logical order according to such sequence unless it is specifically defined by the language of the claim. The process steps may be interchanged in any order without departing from the scope of the invention as long as such an interchange does not contradict the claim language and is not logically nonsensical. 
         [0055]    Furthermore, depending on the context, words such as “connect” or “coupled to” used in describing a relationship between different elements do not imply that a direct physical connection must be made between these elements. For example, two elements may be connected to each other physically through one or more additional elements. 
         [0056]    While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention. It being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.