Path: SC/Dynamics

% Plant for a momentum bias spacecraft with flexible solar arrays.
   Computes the state space description of a spacecraft
   with two flexible appendages where the internal momentum is fixed.
   If dT is not input it returns the continuous time plant.
   If dT is input it returns the discrete time plant.
   In either case, the plant is linearized.
   The state vector is


   Where the state vector is with respect to the rotating frame
   defined by wo.

   The outputs are the angles, i.e. the first three elements in the
   state vector. The flex model is in the physical frame with each
   mass having a vector stiffness and damping

   If h and wo are not in the same direction or inr is not diagonal,
   dxdt will be nonzero and you will get constant disturbance torques.

   If only one output is specified it gives the right hand side 
   of the nonlinear dynamical equations of motion. You need to use
   the kinematics routines to get the nonlinear kinematics equations.

   Since version 1.
   [a,b,c,d,dxdt] = FXModel(inr,h,wo,m,r,wF,zF)
   [a,b,c,d,dxdt] = FXModel(inr,h,wo,m,r,wF,zF,dT)
   [wDot]         = FXModel(inr,h,w,m,r,wF,zF,fT,mT,v,x)

   inr           (3,3)          Inertia matrix
   h             (3,1)          Internal momentum vector
   wo            (3,1)          Orbit rate vector
   m             (1,n)          Mass of the flex masses
   r             (3,n)          Positions of the flex masses
   wF            (3,n)          Frequencies of the flex masses
   zF            (3,n)          Damping ratios of the flex masses                      
   dT            (1,1)          Sampling period


   fT            (3,1)          Total external force on the spacecraft c.m.
   mT            (1,1)          Total spacecraft mass
   v             (3,n)          Flex mass velocity
   x             (3,n)          Flex mass position

   a             (6+6*n,6+6*n)  Plant matrix
   b             (6+6*n,3)      Input matrix
   c             (3,6+6*n)      Output matrix
   d             (3,3)          Feedthrough matrix
   dxdt          (6+6*n,1)      Steady state derivative


   wDot          (3,1)          State derivative of rates



Common: Control/C2DZOH
Math: Linear/SkewSymm

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