My code won't run, error message :Unrecognized function or variable 'ss'. Error in Q4_109 (line 19) Sys=ss(A,B,C,D);

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clear;clc
close all
M=[4 0 0 0;0 3 0 0;0 0 2.5 0;0 0 0 6];
C=[4 -1 0 0 ;-1 2 -1 0;0 -1 2 -1;0 0 -1 1];
K=[500 -100 0 0;-100 200 -100 0;0 -100 200 -100;0 0 -100 100];
x0=[0;0;0;0.01];
v0=[1;0;0;0];
B=[0;0;0;0;0;0;0;1];
t=0:0.01:80;
u=sin(4*t);
A=[zeros(4,4) eye(4);-inv(M)*K -inv(M)*C];
X0=[x0;v0];
C=[eye(4) zeros(4,4)];D=[0;0;0;0];
Sys=ss(A,B,C,D);
[x,tout,X]=lsim(Sys,u,t,X0);
plot(tout,x(:,1),tout,x(:,2),tout,x(:,3),tout,x(:,4))
xlabel('Time');ylabel('x')
legend('x_1','x_2','x_3','x_4')
I'm trying to code and plot this... (see attached)

Answers (2)

Star Strider
Star Strider on 30 Jun 2022
If you are getting an error that MATLAB cannot find the ss function, then you need to license and install the Control System Toolbox.
The code otherwise works as written —
M=[4 0 0 0;0 3 0 0;0 0 2.5 0;0 0 0 6];
C=[4 -1 0 0 ;-1 2 -1 0;0 -1 2 -1;0 0 -1 1];
K=[500 -100 0 0;-100 200 -100 0;0 -100 200 -100;0 0 -100 100];
x0=[0;0;0;0.01];
v0=[1;0;0;0];
B=[0;0;0;0;0;0;0;1];
t=0:0.01:80;
u=sin(4*t);
A=[zeros(4,4) eye(4);-inv(M)*K -inv(M)*C];
X0=[x0;v0];
C=[eye(4) zeros(4,4)];D=[0;0;0;0];
Sys=ss(A,B,C,D);
[x,tout,X]=lsim(Sys,u,t,X0);
plot(tout,x(:,1),tout,x(:,2),tout,x(:,3),tout,x(:,4))
xlabel('Time');ylabel('x')
legend('x_1','x_2','x_3','x_4')
.

the cyclist
the cyclist on 30 Jun 2022
As you can see, I was able to run your code here.
I expect you do not have the ss function, which is in the Control System Toolbox. You need to have that toolbox installed.

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