Abstract:
In a CMG array used to change the attitude of a satellite, noise in the motion of the inner gimbal as it is moved is detected and the speed of the CMG rotors is changed to reduce the noise.

Description:
TECHNICAL FIELD 
     This invention relates to CMG (Control Momentum Gyros), in particular, mitigating gimbal induced disturbances in CMG arrays. 
     BACKGROUND 
     CMGs are commonly used for attitude control in satellites and other spacecraft. A CMG  10  in FIG. 1 on a spacecraft  12  includes an inner gimbal assembly (IGA)  14  and a gimbal torque actuator (GTA) or motor  16  which rotates the IGA. The IGA is an assembly that is free to rotate in one or more axes  17  and stores angular momentum in its rotating inertial mass (rotor)  18 , rotated at a constant speed by a rotor spin motor (RSM)  20 . Output torque to rotate the spacecraft around on axis is obtained from the CMG by rotating the IGA  14 . In practice, several CMGs are aligned along different axis so that the satellite can be oriented spherically. 
     Output torque on axis  22  from the CMG is the result of the mathematical cross product of the gimbal precession rate vector and the angular momentum vector of the IGA. The output torque is in a plane formed perpendicular to each CMG gimbal axis. Spacecraft attitude control is achieved through the coordinated actuation of a plurality of CMGs in a CMG array. This is a well understood mechanical process that has been employed in spacecraft control for some time. As stated, the rotational torque applied to the satellite from the CMG is a cross product of the IGA rate and stored angular momentum of the rotor  16 . An IGA rate can be used to produce high output torque by storing high angular momentum in the IGA rotor. This process is often referred to as torque multiplication because a small input torque to the IGA (input axis in FIG. 1) is multiplied by the stored angular momentum (spin axis) to create a high output torque (output axis). 
     A problem in the practical application of CMG arrays is noise disturbances in the IGA actuation, which also multiplied/amplified and transmitted to satellite motion, producing less than smooth satellite movement. These disturbances are undesirable in the control of precision pointing spacecraft. The most pervasive gimbal disturbances are those associated with the gimbal rate sensor  24 , which is used to feed back IGA motion in a closed loop IGA control that controls the IGA motor speed. Noise in sensors  24  errors can not be easily compensated using conventional control techniques. 
     Another related device is a reaction wheel assembly (RWA), in which the rotor speed is changed to produce rotational torque on the satellite to change its attitude. But the rotors of the RWA array typically are not mounted on gimbals, so their orientations with respect to the spacecraft coordinates do not change like the gimbals of a CMG. 
     SUMMARY 
     An object of the invention is to mitigate the effect of disturbances and noise in IGA motion in a CMG array. 
     According to the invention, to mitigate or reduce the effect of disturbances or noise, such as noise from IGA motion, the IGA rotor is operated like an RWA as a function of the disturbance. 
     According to the invention, the nature of the IGA disturbance is sensed or understood and processed by a signal processor in the CMG system to slightly change the rotor speed to offset the effect of the disturbance. This operation is performed at the array level in a CMG array (CMGA) in an attitude control to mitigate noise in all the CMGs, i.e. all the rotational axis. 
     A feature of the invention is that is can be employed easily in current CMG controls. 
     Other objects, benefits and features will be apparent to one of ordinary skill in the art from the following drawing and description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of one CMG that is used in the present invention. 
     FIG. 2 is a functional block diagram of a CMG attitude control embodying the present invention. 
    
    
     DESCRIPTION 
     Satellite torque control with an array of CMGs typically takes the following form (prior art), where these equations demonstrate a preferred method, among other possible methods, to determine the torque and rotor speed commands for the CMG gimbals and rotors: 
     Definitions 
     {right arrow over (h)}≡Vector representing CMG angular momentum        A   =         ∂     h   ⇀         ∂   δ       ≡     Jacobean of     CMG    array angular momentum geometry                              
     δ≡CMG gimbal angle 
     {dot over (δ)} a ≡ Vector of actual CMG gimbal rates 
     {dot over (δ)} c ≡ Vector of commanded CMG gimbal rates 
     {dot over (h)} a  Actual torque applied to the spacecraft from the CMG array 
     {dot over (h)} c ≡ Commanded torque to the CMG array 
     The torque to a spacecraft from a CMG array is calculated by 
     
       
           {dot over (h)}   a   =A{dot over (δ)}   a   Equation 1 
       
     
     where the subscripts denote the actual values. In the physical satellite, the torque is the actual product of the physical motion (gimbal rate) of the angular momentum vector contained in the IGA including any disturbances. The actual gimbal rate is a function of the commanded gimbal rate and is affected by a variety of factors such as gimbal loop dynamics and the previously mentioned disturbances. This is represented here by a transfer function; 
     
       
         {dot over (δ)} c =ƒ({dot over (δ)} a )  Equation 2 
       
     
     The commanded gimbal rates can be determined by a variety of methods, however the most efficient and most popular are variations of the Moore-Penrose pseudo-inverse; 
     
       
         δ c   =A   T ( AA   T ) −1   {dot over (h)}   c   Equation 3 
       
     
     If we consider the disturbances in the CMG transfer function, ƒ({dot over (δ)} a ), to be represented at the CMG output axis by 
     {dot over (h)} d ≡Gimbal disturbance related output torque 
     then the disturbance torque can be mitigated using this invention which is derived as follows; 
     A new Jacobean is generated that contains the geometry defining the output torque directions as well as the spin torque directions                A   *     =       [         ∂     h   ⇀         ∂   δ                         ∂     h   ⇀         ∂   Ω         ]     =     [       A     CMG                       A   RTR       ]               Equation                 4                                
     The Rotor Spin Motor commands for the respective IGAs are generated similarly to Equation 3 with the exception of using the rotor portion of the Jacobean 
     
       
         {dot over (Ω)} c   =A   RTR   T ( A   RTR   A   RTR   T ) −1   {dot over (h)}   d   Equation 5 
       
     
     As was the case as for the CMG, the spin motor loop dynamics influence the actual torque that the spin motors can produce. 
     
       
         {dot over (Ω)} a =ƒ({dot over (Ω)} c )  Equation 6 
       
     
     and Equation 1 becomes                  h   .     a     =       A   *          [             δ   .     a               -       Ω   .     a             ]               Equation                 7                                
     which represents the torque from the array to the spacecraft from the precession of the stored angular momentum vector and the change in that vector due to using the spin motors to cancel the gimbal induced disturbances. The net torque from the array contains fewer disturbances and therefore is “smoother”, or in other words has a higher torque quality. 
     The disturbance torque from the gimbal can be derived through a variety of methods that can include direct measurement, indirect measurement, and estimation or through an approximation such as a look-up table. A signal representing the disturbance torque is applied in Equation 5 to arrive as the RSM acceleration command, that is the signal to change the rotor speed to offset the disturbance torque. 
     For clarity and simplicity, in FIG. 2 mathematical terms for various rates and accelerations are shown in the drawing but identified below by reference numerals. The following discussion generally describes the control of one CMG  10 , but it will be understood that it typically is used individually with a plurality of CMGs in an array, each CMG preferably having a system as shown in FIG.  2 . In FIG. 2, a signal processor  26  provides signals  28 ,  31  respectively to rotor spin motor controller  30  and gimbal motor controller  32  of a CMG  10  (FIG.  1 ). The signal  28  controls the spin motor (RSM) control  20  to control the speed of rotor  18  producing angular momentum  29 . With signal  30 , the gimbal motor controller  32  moves the gimbal  14  with gimbal motor  16 , producing the gimbal torque  37  on axis  22 . The torque  37  is combined  39  with the rotor  18  torque, produced by the spin motor (RSM)  20 . The resultant composite torque  40  is applied to rotate the spacecraft or satellite  12  around axis  22 . 
     Attitude rate sensors  41  detect the attitude change  42  to produce a measured rate  44  which is supplied to an attitude control system (ACS)  46  for the satellite (to control the three dimensional orientation of the satellite), which also receives a commanded attitude rate for the CMG. The ACS produces an attitude acceleration/deceleration  50  for the signal processor  26 , which using known routines, changes the position of gimbal  14  so that, over time, the difference between the measured attitude  44  and command attitude  48  are the same. 
     Normally, the signal processor maintains a constant rotor spin rate  29 , but according to the invention it receives a gimbal disturbance rate  52  from a disturbance signal processor  54 , which responds to the output  55  from the disturbance sensor  24 . The signal processor  54 , which may be separate as shown or its function a program in signal processor  26 , may compute the rate  52  in real time or use a look-up table or other technique. The signal processor  26  uses the rate  52  to produce a spin acceleration/deceleration  28 , causing the spin motor controller to change the spin speed up or down in relation to the disturbance rate  52 . This change slight increases/decreases the rate  40  to mitigate the effect of the disturbance. 
     Even though the invention has been described to mitigate the effects of gimbal disturbances, changing the rotor speed as explained can also be used to mitigate other disturbances or noise in an attitude control system. In addition to any modifications and variations described previously, one skilled in the art may be able to make modifications to invention and its components and functions, in whole or in part, without departing from its true scope and spirit.