Following this question, I am trying to comprehend the functionality of MATLAB's ss()
and tf2ss()
functions and their differences. Consider the code below:
Kf = 0.2;
J = 0.02;
h2 = tf(1, [J Kf]);
dcm1 = ss(h2);
dcm2 = tf2ss(h2.Numerator{1}, h2.Denominator{1});
partially copied from here. Now I expect the dcm1
and dcm2
to be identical but here is what I get:
>> dcm1 dcm1 = A = x1 x1 -10 B = u1 x1 8 C = x1 y1 6.25 D = u1 y1 0 Continuous-time state-space model. >> dcm2 dcm2 = -10
I would appreciate it if you could help me understand why am I getting two different results? how I can have identical results using the tf2ss()
function? In other words, I want to create a dcm2
which is identical to dcm1
but using the tf2ss()
function.
ss
returns a state-space system
, similar to tf
they both return continuous systems in ss
and tf
representation respectively.
ss2tf
returns the A, B, C, D matrices which are the result of converting the transfer function tf
into its state-space representation.
There's one caveat though, the state-space representation of a transfer function is not unique. It seems like Matlab
uses two different algorithms for ss
and tf2ss
. You can verify that both systems return the same transfer function:
Kf = 0.2;
J = 0.02;
h2 = tf(1, [J Kf]);
dcm1 = ss(h2);
[A, B, C, D] = tf2ss(h2.Numerator{1}, h2.Denominator{1});
dcm1 =
A =
x1
x1 -10
B =
u1
x1 8
C =
x1
y1 6.25
D =
u1
y1 0
[A, B, C, D]
A = -10
B = 1
C = 50
D = 0
Now let's compare transfer functions:
[b, a] = ss2tf(A, B, C, D)
% yields
b = [0 50]
a = [1 10]
[b, a] = ss2tf(dcm1.a, dcm1.b, dcm1.c, dcm1.d)
% yields
b = [0 50]
a = [1 10]