User`s guide

3 Linear Model Identification
Commands for Transforming Model Representations (Continued)
Command Model Type to Convert Usage Example
idpoly
Converts single-output
idmodel object to ARMAX
representation.
If you have the Control
System Toolbox product,
this command converts any
single-output L TI object
except
frd.
To get an ARMAX model from state-space
model
m_ss,usethefollowingcommand:
m_p = idpoly(m_ss)
idss
Converts any single- or
multiple-output
idmodel
object to state -space
representation.
If you have the Control
System Toolbox product, this
command converts a ny L TI
object except
frd.
To get a state-space m odel from an ARX
model
m_arx,usethefollowingcommand:
m_ss = idss(m_arx)
Note The ids s conversion produces warnings when the continuous-tim e
disturbance model does not have the required w hite-noise component.
These warnings occur because the underlying state-space model, which
is fo rmed and used by these transformations, is ill defined. In this case,
modify the C-polyn omial such that the degree of the m onic C-polynomial
in continuous-time equals the sum of the degrees of the monic A-and
D-polynomials in continuous-time. For example:
length(C)-1 = (length(A)-1)+(leng th(D)-1)
3-118