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about why in pole placement gives me error?

Asked by azam ghamari on 26 Dec 2018
Latest activity Commented on by azam ghamari on 29 Dec 2018
Hi guys
Does every body knows why in the system :
s=tf('s');
G=(s-2.44)/((s+5)*(s+6)*(s+7));
[A,B,C,D]=tf2ss([1 2.44],[1 18 107 210]);
k_max=1001;
for j=1:k_max
p(j,:)=[-j-1 -3*j -j-2];
K(j,:)=place(A,B,p(j,:));
end
with differeing the p value it gives me this error?
Error using place (line 78)
The "place" command cannot place poles with multiplicity greater than rank(B).
Error in test2 (line 33)
K(j,:)=place(A,B,p(j,:));

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2 Answers

Answer by Aquatris
on 27 Dec 2018
 Accepted Answer

It is just the error says. Place command cannot place poles with multiplicity greater than rank(B). In your case, rank(B) is 1. However when j=1, you want to place your poles at [-2 -3 -3]. You see the multiplicity of pole at -3 is 2. So place command cannot be used. If you start j from 2 instead of 1, there won't be an issue.

  6 Comments

As I mentioned, it is exactly the same. You call place() function with the state space A and B matrices. What exactly are you asking?
No it doesn't work with more than 1 input witj place order in matlab.
Is there any direct order in matlab we can do that or we shall writ ean algorithm?
Thanks
I dont know why you think it does not work. You do realize place is for state feedback not output feedback so you need an observer right?

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Answer by azam ghamari on 28 Dec 2018

no, i used it for state feedbackany way when we have somethink like this:
P=p(j,:)=[-j-1 -3*j -j-2];
and B is for example 4x2;
it doesn't work and give me the result.
It gives me this error:
Subscripted assignment dimension mismatch.
Error in test2 (line 31)
K(j,:)=place(A,B,p(j,:));
A=[0.5 0 0.025 0;0 -0.01 0 0.017;0 0 -0.025 0;0 0 0 -0.0178];
B=[0.48 0;0 0.35;0 0.077;0.055 0];
C=[0.5 0 1 0];
D=0;
sys=ss(A,B,C,D);
G=tf(sys)
k_max=1001;
for j=1:k_max
p(j,:)=[-10*rand(1) -20*rand(1) -15*rand(1) -rand(1)];
K(j,:)=place(A,B,p(j,:));
end

  2 Comments

This error is not because of place function. Just check the size of the matrix place() function return with given A and B matrice and you will realize the error.
Since this is a 2 input 4 state system, place() will return a 2x4 matrix. With the line "K(j,:) = place(...)" you are trying to assign 2x4 matrix to a 1x4 matrix, hence the error dimension mismatch. A quick fix is to change the line to;
K(j,:,:)=place(A,B,p(j,:))
it works. thanks

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