Error using InputOutpu​tModel/fee​dback (line 137)

Error using InputOutputModel/feedback (line 137)

The first and second arguments of the "feedback"

command must have compatible I/O sizes.

A = [-0.040493 9.8689 0 -9.81;

-0.0021967 -0.030908 1 0;

-0.013639 -0.015207 -0.42801 0;

0 0 1 0];

B = [3.2952 0;

0 -0.0020488;

0 -0.068204;

0 0];

C = [1 0 0 0;0 1 0 0;0 0 1 0;0 0 0 1];

D = [0 0;0 0;0 0;0 0];

hzpk = zpk(ss(A,B,C,D))

pole(hzpk);

t = [0:0.01:10];

step(0.2*hzpk,t);

axis([0 10 0 0.8]);

ylabel('pitch angle (rad)');

title('Open-loop Step Response');

sys_cl = feedback(hzpk,1);

step(0.2*sys_cl);

ylabel('pitch angle (rad)');

title('Closed-loop Step Response');

Answer 1

[Bhavana Ravirala]

Hi Tenzing,

From my understanding, you are trying to give positive feedback to your system and gain of feedback block is one. But your system has 2 inputs and 4 outputs. So, you need to mention from which output you would like to give feedback to which input of the system.

For example:

feedin=[1]; % feedback should be given to 1st input of the block 
feedout=[1]; %feedback is taken from 1st output of the block 
sys_cl=feedback(hzpk,1,feedin,feedout,1); % first one represents the gain of the feedback block and the other one represents the positive feedback

For knowing no.of inputs and outputs of the system

size(hzpk);

For more information refer the documentation below:

https://www.mathworks.com/help/control/ref/lti.feedback.html#:~:text=Consider%20a%20state%2Dspace,of%20G%20and%20K.

Hope this helps!