Configuring the output of an S-Function

I have the following codes in Matlab.

function dx=massamola(t,x,u)
m=1;
k=1;
b=1;
x1 = x(1);
x2 = x(2);
dx1 = x(2);
dx2 = -(k/m)*x1-(b/m)*x2+(1/m)*u;
dx=[dx1;dx2];

soon after I have a specific code of an S-Function.

function [sys,x0,str,ts]=massamola_sfcn(t,x,u,flag,x10,x20)
switch flag
    case 0      %initialization
        str=[];
        ts = [0 0];
        s = simsizes;
            s.NumContStates=2;
            s.NumDiscStates=0;
            s.NumOutputs=3;
            s.NumInputs=1;
            s.DirFeedthrough=0;
            s.NumSampleTimes=1;
         sys = simsizes(s);
         x0 = [x10;x20];
    case 1      %derivatives computation
            ;
        sys = massamola(t,x,u);
    case 3          %output
        sys(1) = x(1); %position
        sys(2) = x(2);

    case {2 4 9}    %2:discrete
                    %4:calctimeHit
                    %9:termination
        sys = [];
    otherwise
        error(['unhandled flag=',num2str(flag)]);
end

In case 3 seen in the code above, I extract to Simulink the position of the mass through state x(1) and the derivative of state position x(2). As shown below:

I want to extract to Simulink through the S-Function the system's response to the unit step, as shown in the figure:

Below is a picture of the system system in Simulink:

Answer 1

[Jesús Zambrano]

Hi Vagner,

I think it is exactly you already have. Since you put x(1) and x(2) in the S-function output, and these are the unknown described with the two ODEs as described in massamola function, then it is the system response.

The only difference is that you might need to change the initial conditions (values x10 or x20). From the plot, I see it starts from 0 (is it the position or the velocity?).

[Vagner Martinelli]

thanks, modified the initial conditions.

[Jesús Zambrano]

Great! I assume it works, or?

[Vagner Martinelli]

yes.I changed the initial conditions and it worked.thanks.

[Jesús Zambrano]

Great! Please then mark this as an accepted answer (there is a button named "Accept this Answer") so this question can be closed.