Transform or eliminate states of mechss object

Dear MATLAB community,

I have a mechss object and I want to fix several states by removing these columns and rows from the system matrices.

However, I encounter the issue that I cannot change the system matrices one by one as this violates the constraints that the system matrix dimensions should coincide.

For first order systems, this issue is circumvented by using the modred function, but this is not applicable to mechss objects.

Can I eliminate states of the system matrices, without having to redefine to redefine the mechss object?

Thank you!

Answer 1

[Paul]

Even garden variety ss objects can't have the system matrices changed one by one

sys = rss(4,2,2)  % model with four states, two inputs, two outputs

sys =
 
  A = 
            x1       x2       x3       x4
   x1   -8.754   -1.154   -3.312     2.16
   x2   0.5769   -0.229    2.056    5.137
   x3   -2.491   -2.232    -1.25    2.006
   x4    3.229   -4.963  -0.1344   -1.076
 
  B = 
            u1       u2
   x1        0    1.129
   x2  -0.7629   0.8949
   x3        0   -1.235
   x4   -1.278    1.064
 
  C = 
            x1       x2       x3       x4
   y1        0   -1.351   -1.069   0.9355
   y2  0.01518    1.068        0   -0.897
 
  D = 
           u1      u2
   y1  -1.096       0
   y2  0.9333       0
 
Continuous-time state-space model.

% sys.a = sys.a(1:3,1:3);  % attempt to eliminate the fourth state from A, results in an error

Instead, use set to eliminate the fourth state by changing all the matrices at once:

set(sys,'a',sys.a(1:3,1:3),'b',sys.b(1:3,:),'c',sys.c(:,1:3));
sys

sys =
 
  A = 
           x1      x2      x3
   x1  -8.754  -1.154  -3.312
   x2  0.5769  -0.229   2.056
   x3  -2.491  -2.232   -1.25
 
  B = 
            u1       u2
   x1        0    1.129
   x2  -0.7629   0.8949
   x3        0   -1.235
 
  C = 
            x1       x2       x3
   y1        0   -1.351   -1.069
   y2  0.01518    1.068        0
 
  D = 
           u1      u2
   y1  -1.096       0
   y2  0.9333       0
 
Continuous-time state-space model.

Maybe a similar approach will work for a mechss object.

[Luuk Poort]

Hi Paul,

Thank you very much for this answer, it indeed also works for mechss models!

In my implementation, where I extended the mechss class, I was also able to acces the Data_ property, which contains all system matrices and thus I could change all system matrices simultaneously. Your method however is much more intuitive and versatile, so thank you!

Best, Luuk