This is machine translation

Translated by Microsoft
Mouseover text to see original. Click the button below to return to the English version of the page.

Note: This page has been translated by MathWorks. Click here to see
To view all translated materials including this page, select Country from the country navigator on the bottom of this page.


Determine whether system is stable


B = isstable(sys)
B = isstable(sys,'elem')


B = isstable(sys) returns a logical value of 1 (true) if the dynamic system model sys has stable dynamics, and a logical value of 0 (false) otherwise. If sys is a model array, then B = 1 only if all models in sys are stable.

B = isstable(sys,'elem') returns a logical array of the same dimensions as the model array sys. The logical array indicates which models in sys are stable.

isstable is only supported for analytical models with a finite number of poles.


collapse all

Create an array of SISO transfer function models with poles varying from -2 to 2. To do so, first initialize an array of dimension [1,length(a)] with zero-valued SISO transfer functions.

a = [-2:2];
sys = tf(zeros(1,1,1,length(a)));

Populate this array with transfer functions of the form 1/(s-a).

for j = 1:length(a)
    sys(1,1,1,j) = tf(1,[1 -a(j)]);
sys.SamplingGrid = struct('a',a);

Examine the stability of the model array.

B_all = isstable(sys)
B_all = logical

By default, isstable returns a single Boolean value that is 1 (true) only if all models in the array are stable. sys contains some models with nonnegative poles, which are not stable. Therefore, isstable returns 0 (false) for the entire array.

Examine stability of each model in the array, element by element.

B_elem = isstable(sys,'elem')
B_elem = 1x5 logical array

   1   1   0   0   0

The 'elem' flag causes isstable to return an array of Boolean values, which indicate the stability of the corresponding entry in the model array. For example, B_elem(2) = 1, which indicates that sys(1,1,1,2) is stable. This result is expected, because sys(1,1,1,2) has a = -1.

See Also

Introduced in R2012a