convert ARMAX to discrete or continiuous transfer function

Question

mohsen 2013-10-6

1
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/89326-convert-armax-to-discrete-or-continiuous-transfer-function

回答： vidya kurada 2016-7-3

hi here is what i have found as armax estimation

>> amx2221
amx2221 =
Discrete-time ARMAX model:  A(z)y(t) = B(z)u(t) + C(z)e(t)
    A(z) = 1 - 1.12 z^-1 + 0.1199 z^-2                      
    B(z) = 0.0004663 z^-1 - 0.0004681 z^-2                  
    C(z) = 1 - 1.154 z^-1 + 0.1929 z^-2                     
Name: amx2221
Sample time: 1.5e-05 seconds
Parameterization:
   Polynomial orders:   na=2   nb=2   nc=2   nk=1
   Number of free coefficients: 6
   Use "polydata", "getpvec", "getcov" for parameters and their uncertainties.
Status:                                           
Estimated using POLYEST on time domain data "p50".
Fit to estimation data: 98.35% (prediction focus) 
FPE: 8.796e-12, MSE: 8.791e-12

while i use tf(amx2221) it answers me:

>> tf(amx2221)
ans =
From input "u1" to output "y1":
0.0004663 z^-1 - 0.0004681 z^-2
-------------------------------
  1 - 1.12 z^-1 + 0.1199 z^-2
Name: amx2221
Sample time: 1.5e-05 seconds
Discrete-time transfer function.

or in the other shape:

>> d2c(tf(amx2221))
ans =
    From input "u1" to output "y1":
      75.11 s - 1.904e04
    -----------------------
    s^2 + 1.414e05 s - 9996
Continuous-time transfer function.