xfix.hlp

MATLAB在飞行动力学和控制中应用的工具

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XFIX is a standard Simulink gain-block, which is used in the sub-
system 'Aircraft Equations of Motion' (level 3) of the aircraft
model BEAVER to atificially fix states to their initial values.
The value of the constant xfix needs to be defined in the Matlab
workspace before starting a simulation or any other evaluation of
the system BEAVER.

How does it work?
=================
The gain XFIX multiplies the vector with time-derivatives of the
state variables, xdot, with the gain-factor xfix. If the user
does not impose artificial restrictions upon the states, xfix
will be equal to one, and the outputvector from XFIX is equal to
xdot, according to the equations of motions. The same result is
obtained when the constant xfix is equal to a vector of length
12 with elements that are all equal to one.

If, however, the user wants to fix some of the twelve states to
their initial values, xfix needs to be equalled to a vector of
length 12 with elements that are all equal to one, EXCEPT for the
element numbers that correspond with the states that must be
fixed. If the user wants to fix the first, fifth, and last states
to their initial values, the first, fifth, and last element of
xfix must be set to zero. In that case, the input/output structure
around the block XFIX looks like:


xdot_in = [V dot;      \             /    xdot_out = [  0
           alpha dot;  |             |                 alpha dot;
           beta dot;   |             |                 beta dot;
           p dot;      |             |                 p dot;
           q dot;      |             |                  0
           r dot;      |             |                 r dot;
           psi dot;    |>  * xfix   <|                 psi dot;
           theta dot;  |             |                 theta dot;
           phi dot;    |             |                 phi dot;
           xe dot;     |             |                 xe dot;
           ye dot;     |             |                 ye dot;
           H dot]      /             \                  0 ]


Since the first, fifth, and last elements of xdot are always
artificially set to zero, the corresponding states will always be
equal to their initial values!

The constant xfix is, by default, set to 1 if the routine LOADER
is applied to define the system parameters in the Matlab work-
space. Run FIXSTATE.M if you really want to fix a number of state
variables, for instance if you want to analyze symmetrical or
asymmetrical motions only, or if you want to fix the airspeed to
its initial value, representing an 'ideal' autothrottle system.

More info
=========
Type HELP FIXSTATE for more info about the definition of the con-
stant xfix in the Matlab workspace. See EQMOTION.HLP for more in-
fo about the subsystem 'Aircraft Equations of Motion' in the sys-
tem BEAVER. Type HELP LOADER for more info about the definition
of other system parameters.

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The FDC toolbox, Copyright Marc Rauw 1994-1998.