Abstract:
Apparatus, Systems, and Methods are provided for controlling motion of a spacecraft. One apparatus includes a non-contacting actuator and a passive mechanical system coupled in parallel with one another. A system includes a payload, a bus, and a hybrid actuator including a non-contacting actuator and a passive mechanical system coupled in parallel, and coupled between the bus and the payload. The system also includes an inertial actuator configured to maneuver the bus to maintain a relative position and/or attitude of the bus with respect to the payload. One method includes receiving a signal instructing a first controller to change the position and/or attitude of a payload and utilizing a hybrid system to change the position and/or attitude of the payload. The method also includes receiving the signal at a second controller and utilizing a system to change a position and/or attitude of the bus independent of the payload.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0001]    The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided by the terms of contract number 30218. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention generally relates to spacecraft, and more particularly relates to active control of soft hybrid isolation systems in spacecraft. 
       BACKGROUND OF THE INVENTION 
       [0003]    Traditional agile spacecraft control architectures use an inertial measurement unit (IMU) on the payload to determine the angular orientation of the payload. Spacecraft also include at least one inertial actuator on the bus for applying torques to the spacecraft to control the sensed orientation of the payload (e.g., to follow a prescribed command). The structural connection between the bus and the payload is typically a hexapod of stiff struts. 
         [0004]    To attenuate the transmission of bus vibrations to the payload, these struts are sometimes replaced with passive “isolators,” which provide tuned stiffness and damping to create a mechanical “break.” Because these isolators are located between the inertial actuator and sensor of the attitude control system, the isolators violate co-location protocol and result in an unstable mode. The isolation break frequency must therefore be kept well above the attitude control bandwidth, which minimizes the efficacy of the isolation. 
         [0005]    The introduction of an isolated interface also compromises the agility of the spacecraft because high frequency control torques are not passed to the payload. However, agile spacecraft demand control torques whose frequency content not only exceeds the bandwidth of the attitude control system, but also exceeds the break frequencies of the isolation system. This requires active “feedforwards” to high bandwidth torque actuators that are in stiff contact with all parts of the spacecraft. 
         [0006]    In some cases, the payload pointing agility required exceeds that of the inertial actuator itself, which is the only source of “inertial torque” available. To meet these demands, the payload should be controlled to move relative to the bus, which results in increased stroke (i.e., displacement) requirements between the payload and the bus. 
         [0007]    Accordingly, it is desirable to provide a hybrid actuator for maneuvering a spacecraft payload. In addition, it is desirable to provide a system including a hybrid actuator and an inertial actuator for controlling motion of a spacecraft. Moreover, it is desirable to provide a method for independently controlling a payload and a bus of a spacecraft so that the payload and bus maintain a relative position with respect to one another. Furthermore, 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 this background of the invention. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    Apparatus are provided for maneuvering a spacecraft payload. One apparatus comprises a non-contacting actuator and a passive mechanical system coupled in parallel with one another. 
         [0009]    A system is also provided for controlling motion of a spacecraft. A system includes a payload, a bus, and a hybrid actuator coupled between the payload and the bus, and configured to maneuver a position and/or an attitude of the payload. The hybrid actuator includes a non-contacting actuator and a passive mechanical system coupled in parallel with each other, and coupled between the payload and the bus. The system also includes an inertial actuator configured to maneuver the bus to substantially maintain a relative position and attitude of the bus with respect to the payload. 
         [0010]    A method is provided for controlling a spacecraft payload including a first controller, and a spacecraft bus including a second controller. One method includes the steps of receiving a signal instructing a first controller to change a first position and/or a first attitude of the payload. A hybrid system is then utilized to change the first position and/or the first attitude. A second controller also receives the signal, and a different system is utilized to change a position and/or attitude of the bus independent of the payload. 
     
    
     
       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 a block diagram of a prior art spacecraft; 
           [0013]      FIG. 2  is a block diagram of one exemplary embodiment of a spacecraft including a hybrid actuator; 
           [0014]      FIG. 3  is a schematic diagram of one exemplary embodiment of the hybrid actuator of  FIG. 2 ; and 
           [0015]      FIG. 4  is a block diagram of one exemplary embodiment of control architecture for the spacecraft of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention. 
         [0017]      FIG. 1  is a block diagram of a contemporary spacecraft  100  (e.g., a satellite). Spacecraft  100  includes a payload  110  coupled to a bus  120  via a rigid interface  130  (e.g., a stiff hexapod interface, one or more composite or metal tubes, and/or other similar rigid connecting apparatus). 
         [0018]    Payload  110  carries the primary mission hardware (e.g., a telescope, a communications system, a tracking system, and/or other devices or systems needing motion stability and control), and also includes at least one inertial measurement unit (IMU)  1110  (e.g. a star-tracker, a gyroscope, an accelerometer, focal plane detector, and the like). IMU  1110  is configured to detect the position and/or attitude of payload  110  with respect to its surroundings or a target (e.g., the earth, a star, a planet, etc.), and transmits a signal to a controller (discussed below). Errors between the IMU  1110  detected position and/or attitude and a commanded (i.e., desired) position and/or attitude cause controller  1205  to command inertial actuator  1210  (as required) to change the position and/or attitude of spacecraft  100  until IMU  1110  is at the commanded position and/or attitude. 
         [0019]    Bus  120  generally stores the items (e.g., power sources, fuel, communications systems, etc.) needed to operate spacecraft  100  properly. A controller  1205  coupled to IMU  1110  is also included on bus  120 . Controller  1205  is configured to receive signals from IMU  1110  and instruct various components (e.g., an inertial actuator or attitude control system) to adjust the position and/or attitude of spacecraft  100 . 
         [0020]    Bus  120  also includes at least one inertial actuator  1210  (e.g., a control moment gyroscope, a thruster, a reaction wheel assembly, a magnetic torquer, a solar sail, and the like) in communication with controller  1205 , and configured to adjust the position of spacecraft  100  in response to instructions received from controller  1205  (and IMU  1110 ). Furthermore, controller  1205  and inertial actuator  1210  form an attitude control system  1220  of spacecraft  100 . 
         [0021]    Since bus  120  includes various components to adjust the position and/or attitude of spacecraft  100 , bus  120  includes the primary sources of vibration in spacecraft  100 , which vibrations are propagated to payload  110  (and the mission hardware) via rigid interface  130 . 
         [0022]    As spacecraft  100  orbits the earth, the position and/or attitude of the mission hardware need to be adjusted to operate properly. When controller  1205  detects that the position and/or attitude of the mission hardware is misaligned and needs to be adjusted (based on an error signal between a commanded position and/or attitude and the IMU  1110  sensed position and/or attitude), controller  1205  transmits a signal to inertial actuator  1210  to maneuver spacecraft  100  to the commanded position and/or attitude. In response, inertial actuator  1210  adjusts the position of spacecraft  100  so that the position and/or attitude of the mission hardware is where it should be, driving the error signal between command and IMU  1110  sensed position and/or attitude to zero. 
         [0023]    Because payload  110  is rigidly coupled (via rigid interface  130 ) to bus  120 , the mission hardware is subject to the vibrations created on bus  120 . Moreover, because inertial actuator  1210  (i.e., a portion of attitude control system  1220 ) adjusts the entire spacecraft  100 , the position and/or attitude of the mission hardware is changed more slowly than it other wise could be changed. 
         [0024]      FIG. 2  is a block diagram of one exemplary embodiment of a spacecraft  200  including a hybrid interface  230 . Spacecraft  200  includes a payload  210  and a bus  220  similar to payload  110  and bus  120 , respectively, discussed above with reference to  FIG. 1 . 
         [0025]    As illustrated in  FIG. 2 , hybrid interface  230  is coupled between payload  210  and bus  220 . Hybrid interface  230  includes multiple hybrid actuators  300  (see  FIG. 3 ) coupled between payload  210  and bus  220 . Hybrid interface  230  includes six hybrid actuators  300  (in a strut or axial element configuration) forming a hexapod assembly (not shown) configured to maneuver payload  210  in six degrees of freedom (e.g., translation along an X-axis, a Y-axis, and a Z-axis, and rotation about the X-axis, the Y-axis, and the Z-axis). Furthermore, various embodiments contemplate that hybrid interface  230  may include more than six hybrid actuators  300  in a strut or axial element configuration (i.e., an over-constrained configuration). 
         [0026]    Hybrid interface also includes a six degrees-of-freedom (6-DOF) relative measurement system  3000  configured to measure the relative position of payload  210  with respect to bus  220 , or vice versa. 6-DOF relative measurement system  3000  may be any of a plurality of non-contacting and/or contacting linear and/or angular measurement devices (e.g., non-contacting displacement sensors or the like built into hybrid actuator  300 ). 
         [0027]      FIG. 3  illustrates a schematic diagram of one exemplary embodiment of hybrid actuator  300  forming a portion of a strut in hybrid interface  230 . Hybrid actuator  300  includes a passive mechanical system  310  coupled in parallel with an active portion  320 . Passive mechanical system  310  includes a spring  3110  coupled in series with a damper  3120  (e.g., a dashpot). Also included in passive mechanical system  310  is a spring  3130  coupled in parallel with spring  3110  and damper  3120 . 
         [0028]    Although passive mechanical system  310  is illustrated as including one spring in series with one damper, various embodiments contemplate that passive mechanical system  310  may include multiple springs  3110  and/or dampers  3120  coupled in series with one another. Furthermore, passive mechanical system  310  may include multiple series-connected springs and dampers coupled in parallel with spring  3110  and damper  3120 . 
         [0029]    Similarly, passive mechanical system  310  may include multiple springs  3130  coupled in series with one another. Furthermore, passive mechanical system  310  may include multiple springs  3130 , each coupled in parallel with spring  3110  and damper  3120 . 
         [0030]    Active portion  320  includes a non-contacting actuator  3210  (e.g., a voice coil, an electromagnetic actuator, an electrostatic actuator, and the like) coupled in parallel with passive mechanical system  310 . Non-contacting actuator  3210  may include multiple series-connected non-contacting actuators  3210  and/or may include multiple non-contacting actuators coupled in parallel with non-contacting actuator  3210  and passive mechanical system  310 . 
         [0031]    With reference again to  FIG. 2 , hybrid interface  230  includes a controller  2320  in communication with non-contacting actuator(s)  3210 , an IMU (e.g., IMU  2110  discussed below), and a controller (e.g., controller  2205  discussed below) in bus  220  (e.g., bus  220  discussed below). Controller  2320  is configured to receive signals from IMU  2110 , and instruct non-contacting actuator  3210  (and passive mechanical system  310 ) to maneuver a payload (e.g., payload  210  discussed below) to a different position and/or attitude. 
         [0032]    Payload  210  includes an IMU  2110  similar to IMU  1110  discussed above (see  FIG. 1 ). IMU  2110  is coupled to controller  2320 , and error signals between a commanded (i.e., desired) position and/or attitude of payload  210  and the IMU  2110  sensed position and/or attitude result in commands to hybrid actuator(s)  300  in a manner that enables payload  210  to have a new position and/or attitude, driving the error signals to zero. In changing the position and/or attitude of payload  210 , controller  2320  instructs hybrid actuator  300  to make the change with respect to the center of mass of spacecraft  200  instead of the center of mass of payload  210 , thereby minimizing the required stroke of hybrid actuator  300  (and passive mechanical system  310 ). 
         [0033]    Bus  220  includes a controller  2205  that is utilized to control the output of inertial actuator  2210 . Controller  2205  receives a signal from 6-DOF relative motion measurement system  3000 , indicating the relative alignment of payload  210  with respect to bus  220 . Any errors in alignment result in controller  2205  commanding inertial actuator  2210  to change the position and/or attitude of bus  220  such that payload  210  and bus  220  are aligned. Furthermore, controller  2205  receives signals from controller  2320 , for reasons discussed below. 
         [0034]    Inertial actuator  2210  is used to maneuver bus  220  to a new position so that bus  220  maintains or substantially maintains a relative position with respect to payload  210 . Inertial actuator  2210  is also configured to change the position of bus  220  with respect to the center of mass of spacecraft  200  in synchronization with payload  210 . That is, when inertial actuator  2210  receives the new position that payload  210  will change to, inertial actuator  2210  will begin to change the position of bus  220  so that bus  220  follows payload  210 . 
         [0035]    Controller  2205  is configured to receive signals 6-DOF relative motion measurement system  3000  and/or controller  2320 , and instruct inertial actuator  2210  to change the position and/or attitude, respectively, of bus  220  at substantially the same time as hybrid actuator  300  adjusts the position and/or attitude of payload  210 . 
         [0036]    Although payload  210  and bus  220  will not always have the same relative position with one another at all times during the respective changes in position and/or attitude, payload  210  and bus  220  will have the same relative positions and/or attitude when inertial actuator  2210  and attitude control system  2220  are finished changing the positions and/or attitude of bus  220 , respectively. That is, payload  210  and bus  220  will have the same relative position and/or attitude within a pre-determined threshold (e.g., time, displacement of hybrid interface, etc.) of one another during any change in position and/or attitude. 
         [0037]      FIG. 4  is a block diagram of one exemplary embodiment of control architecture  400  for a spacecraft (e.g., spacecraft  200 ). As illustrated, control architecture  400  includes a payload control loop  4120  and a bus control loop  4220 . As one skilled in the art will appreciate, as the isolation break frequency decreases between payload  210  and bus  220 , spacecraft  200  essentially approaches two separate bodies in close formation. Accordingly, control architecture  400  should be such that agile and stable pointing of payload  210  and bus  220  may be achieved with no passive connection (e.g., passive mechanical system  310  of  FIG. 3 ) at all. 
         [0038]    The passive connection then simply imposes a stiffness large enough to substantially prevent payload  210  and bus  220  from drifting out of formation, but small enough to maximize the isolation performance and afford the ability to separate the isolation and pointing functions. Accordingly, the inclusion of a passive connection provides a more fault tolerant spacecraft. That is, if active portion  320  (see  FIG. 3 ) malfunctions, spacecraft  200  (via passive mechanical system  310 ) is still capable of controlling the position and attitude of payload  210 . 
         [0039]    To control both portions of spacecraft  200 , full, active control of the nine relevant degrees of freedom of payload  210  and bus  220  (e.g., the rotation of payload  210  (three degrees of freedom), the rotation of bus  220  (three degrees of freedom), and translation between payload  210  and bus  220  (three degrees of freedom)) should be achieved. As such, spacecraft  200  includes three torques from inertial actuator  2210  and six forces from the six hybrid actuators  300  of hybrid interface  230 . 
         [0040]    Spacecraft  200  also includes nine sensors (not shown): three sensors for the three axes of IMU  2110 , and six sensors for the six deflections of the hexapod of hybrid actuators  300 . The differential motion between payload  210  and bus  220  may be measured at the struts of hybrid interface  230  or other location on spacecraft  200  which enables the differential motion to be accurately measured or derived. 
         [0041]    Feedback systems  4210  and  4220 , which manage the nine degrees of freedom, may be implemented in such a way that they are decoupled and independent of one another. This is achieved through the use of geometric transformations and “feedforwards”  2250  and  2251 . To accomplish this, geometric transformations and feedforward control are implemented with their own bandwidth. 
         [0042]    The three degrees of freedom needed to rotate payload  210  are controlled by applying torques to payload  210  using the collection of strut forces from hybrid interface  230 . In this situation, the inertia of payload  210  is being controlled, not the inertia of bus  220 . 
         [0043]    The three degrees of freedom needed to rotate bus  220  are controlled to achieve a “follow-up” to the rotation of payload  210 . The angular information between payload  210  and bus  220  is obtained from the collection of strut deflections, and the torques are applied to bus  220  using inertial actuator  2210 . In this situation, the inertia of bus  220  is being controlled, not the inertia of payload  210 . 
         [0044]    The three degrees of freedom needed for translation of payload  210  with respect to bus  220  may be actively controlled using the collection of strut forces from hybrid interface  230  to minimize translation at hybrid interface  230 . The translation information is obtained from the collection of hybrid actuator  300  strut deflections, and forces are applied using the collection of hybrid actuators  300 . Note that if the maneuver feedforwards  2250  are sufficiently accurate, passive mechanical system  310  (see  FIG. 3 ) could do this entire job. 
         [0045]    To achieve the necessary independence between control loops  4210  and  4220 , payload  210  and bus  220  should be able to react against each other without causing coupling between payload  210  and bus  220 . To accomplish this, the following feedforwards and transformations may be utilized: 
         [0046]    Payload reaction feedforward  2251 —inertial actuator  2210  is instructed to apply torque to oppose the control torque reacting into bus  220  through hybrid actuators  300 . If the majority of these reactions are cancelled, that which remains will appear simply as a disturbance to bus  220  rotation, and not a stability consideration; 
         [0047]    Bus reaction feedforward (not shown)—since the bus torque is produced by inertial actuator  2210 , the bus torque does not react into payload  210 . Accordingly, a bus reaction feedforward is not required here; and 
         [0048]    Geometry transformations—the collection of six hybrid actuators  300  (struts) should be controlled to work together to produce the desired forces and torques on payload  210 . 
         [0049]    To enable maneuvers whose frequency content exceeds the bandwidth of control loops  4210  and  4220 , high-bandwidth actuators may receive feedforward commands  2250 , which would result in substantially the desired maneuver even if the underlying loops were not present. Control loops  4210  and  4220  then act only to eliminate residual error. Examples of such feedforwards include: 
         [0050]    Payload acceleration feedforward  2252 —the control torques on payload  210  are computed based only on the inertia of payload  210 ; 
         [0051]    Bus acceleration feedforward  2253 —the control torques on bus  220  are computed based only on the inertia of bus  220 ; and 
         [0052]    The final outcome of any useful maneuver should be a rotation of the entire spacecraft  200  about its collective center of mass. Accordingly, in addition to the payload control torques, appropriate hybrid actuator  300  strut forces are also applied to null translation at hybrid interface  230 , which is done with geometric transformations. 
         [0053]    If hybrid actuators  300  are sufficiently higher in bandwidth than inertial actuator  2210 , payload  210  may be moved to a new position and/or attitude faster than bus  220 . As such, hybrid interface  230  is configured to include a sufficient amount of displacement or stroke such that bus  220  may be allowed to “lag” behind (i.e., follow-up error). Accordingly, bus control loop  4220  will continue to work to remove any follow-up error after payload  210  has reached its desired position and/or attitude. 
         [0054]    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 and their legal equivalents.