balred - Reduce model order (requires Control System Toolbox product)
Syntax
MRED = balred(M)
MRED = balred(M,ORDER,'DisturbanceModel','None')
Description
This function reduces the order of any model M given
as an idmodel object. The resulting reduced-order
model, MRED, is an idss model.
The function requires routines from the Control System Toolbox product.
ORDER: The desired order (dimension of the
state-space representation). If ORDER = [], which
is the default, a plot shows how the diagonal elements of the observability
and controllability Gramians of a balanced realization decay with
the order of the representation. You are then prompted to select an
order based on this plot. The idea is that such a small element has
a negligible influence on the input-output behavior of the model.
We recommend that you choose an order such that only large elements
in these matrices are retained.
'DisturbanceModel': If the property DisturbanceModel is
set to 'None', then an output-error model MRED is
produced: that is, one with the Kalman gain K equal
to zero. Otherwise (default), the disturbance model is also reduced.
The function recognizes whether M is a continuous-
or discrete-time model and performs the reduction accordingly. The
resulting model, MRED, is similar to M in
this respect.
There are several options for how the reduction is performed: AbsTol,
RelTol, Offset, Elimination.
Algorithm
The function uses the balred algorithm
in Control System Toolbox. The plot, in case ORDER = [],
shows the vector g returned by balreal.
Examples
Build a high-order multivariable ARX model, reduce its order
to 3, and compare the frequency responses of the
original and reduced models:
M = arx(data,[4*ones(3,3),4*ones(3,2),ones(3,2)]);
MRED = balred(M,3);
bode(M,MRED)
Use the reduced-order model as an initial condition for a third-order
state-space model.
M2 = pem(data,MRED);
See Also
 | arxstruc | | bj |  |
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