State space without using the control toolbox - Error using + Matrix dimensions must agree.

I am trying to calculate a continous plant for a unit step input.

Eg dX/dt= AX * Bu y = CX

I get the following error: Error using + Matrix dimensions must agree.

Can anyone please assist me??

My code is below:

   %%Time specifications
   stopTime = 10;
   Fs = 1;
   dt = 1/Fs;
   t = (0:dt:stopTime)';
   N = size(t,1);

   %%State space model
   A = [0,1;-45,-876];
   B = [0,0;1,1];                %Rev 3
   C = [59 99];

   %%Pre-allocation
   u = zeros(2,N);
   x = zeros(2,N);           %Rev 1
   y = zeros(2,N);

   %%Initial conditions
   u(:,1) = [ 1 ; 1 ];
   x(:,1) = [ 0 ; 0 ];       % Rev 1
   y(:,1) = [ 0 ; 0 ];

   %%Simulation - main loop
   for k = 2:N

       x(:,k) = x(:,k-1) + dt.*(A*x(:,k-1) + B*u(:,k-1);      % Rev 2
       y(:,k) = C*x(:,k);

   end

   %%Plot results
figure;
ax(1) = subplot(2,1,1);
plot(t,x);
ax(2) = subplot(2,1,2);
plot(t,y);
linkaxes(ax,'x');

Answer 1

[Azzi Abdelmalek]

The matrix A is 2x2 then x should be 2x1 not 3x1

[George Green]

Hi Azzi,

I correct the errors above for x (Rev 1) however I still get:

Error using + Matrix dimensions must agree.

Error in (line 28) x(:,k) = x(:,k-1) + dt*(A*x(k-1) + B*u(k-1));

Your suggestions would be much appreciated.

Answer 2

[Nobel Mondal]

In this line,

x(:,k) = x(:,k-1) + dt*(A*x(k-1) + B*u(k-1)); % Error

The term A*x(k-1) would give you a 2x2 matrix. You should be using x(:,k-1) instead.

x(:,k) = x(:,k-1) + dt*(A*x(:,k-1) + B*u(k-1));

[George Green]

Thanks for your help Nobel,

Thats what I meant to do but bit of an oversight on my part. I made the changes (Rev2) but got: " Error using * Inner matrix dimensions must agree." for x(:,k) = x(:,k-1) + dt.(A*x(:,k-1) + B*u(:,k-1));"

I then changed B from " B = [0;1] " to " B = [0 0 ;1 1] " ....Does this look right?