Thread Subject: S-function not working properly

Hello everyone.
I'm writing my first S-function (1 level) but I get unexpected results.

Trying to understand what I'm doing wrong I came up with this two pieces of
code which SHOULD do the same thing but they actually don't.
I really don't understand why: can anyone please help?

Basically what I'm trying to implement is a linear system:
x' = A*x + B*u
y = x

The first function uses matrix and outputs the same result as a "state-space
model" simulink block. The second function uses equations (which should
work as well, shouldn't they?) but the output is completely different.

Thank you!




********** S-FUNCTION #1 **********
function [sys,x0,str,ts] = csfunc(t,x,u,flag,angInit)
         M = 0.2;
        m = 0.3;
         l = 0.1;
        g = 9.81;
        
        A=[0 1 0 0; 0 0 -m*g/M 0; 0 0 0 1; 0 0 (M+m)*g/(M*l) 0];
        B=[0; 1/M ;0 ; -1/(M*l)];
%
% Dispatch the flag.
%
switch flag,

  case 0
    [sys,x0,str,ts]=mdlInitializeSizes(A,B,angInit); % Initialization

  case 1
    sys = mdlDerivatives(t,x,u,A,B); % Calculate derivatives

  case 3
    sys = mdlOutputs(t,x,u,A,B); % Calculate outputs
  
  case { 2, 4, 9 } % Unused flags
    sys = [];
  otherwise
    error(['Unhandled flag = ',num2str(flag)]); % Error handling
end


function [sys,x0,str,ts] = mdlInitializeSizes(A,B,angInit)
sizes = simsizes;
sizes.NumContStates = 4;
sizes.NumDiscStates = 0;
sizes.NumOutputs = 4;
sizes.NumInputs = 1;
sizes.DirFeedthrough = 0;
sizes.NumSampleTimes = 1;
sys = simsizes(sizes);


x0 = [0;0;angInit;0];
str = [];
ts = [0 0];


function sys = mdlDerivatives(t,x,u,A,B)
sys = A*x + B*u;

function sys = mdlOutputs(t,x,u,A,B)
sys = x;





********** S-FUNCTION #2 **********
function [sys,x0,str,ts] = sist_non_lin_1_lev(t,x,u,flag,angInit)
                          
switch flag
    case 0 % initialize
        [sys,x0,str,ts] = mdlInitializeSizes(angInit);
    case 1 % derivatives
        sys = mdlDerivatives(t,x,u);
    case 3 % output
        sys = mdlOutputs(t,x,u);
    case {2 4 9}
        % 2:discrete 4:calcTimeHit 9:termination
        sys =[];
    otherwise
        error(['Error - unhandled flag =',num2str(flag)]) ;
end
 

function [sys,x0,str,ts] = mdlInitializeSizes(angInit)
sizes = simsizes;
sizes.NumContStates = 4;
sizes.NumDiscStates = 0;
sizes.NumOutputs = 4;
sizes.NumInputs = 1;
sizes.DirFeedthrough = 0;
sizes.NumSampleTimes = 1;
sys = simsizes(sizes);

x0 = [0;0;angInit;0];
str = [];
ts = [0 0];


function sys = mdlDerivatives(t,x,u)

    
    % Variabili
    M = 0.2;
    m = 0.3;
    l = 0.1;
    g = 9.81;
    
    x1 = x(1); %posizione
    x2 = x(2); %velocita
    x3 = x(3); %angolo
    x4 = x(4); %vel angolare
    
    F_in = u;
    
    dx1 = x2;
    dx2 = ( -m*g*(x3) + F_in ) / M;
    dx3 = x4;
    dx4 = ( (M+m)*g*(x3) - F_in ) / l*M;
    
    sys = [dx1;dx2;dx3;dx4];
% End of mdlDerivatives.

function sys = mdlOutputs(t,x,u)
sys = x;
% End of mdlOutputs.

Please... can someone help? ;(
Thank you.

"Arantel Bijourt" <gtrive.share@gmail.com> wrote in message
<g3p343$5j7$1@fred.mathworks.com>...
> Please... can someone help? ;(
> Thank you.

Your expression for dx(4) is wrong. From the look of it,
you should have:

dx4 = ( (M+m)*g*(x3) - F_in ) / (l*M)

The parens around l*M were missing, resulting in division by
l only, and the result multiplied by M.

This is what you had:
dx4 = (( (M+m)*g*(x3) - F_in ) / l)*M

This is what you need:
dx4 = ( (M+m)*g*(x3) - F_in ) / (l*M)

I suggest you stick to the State Space representation
because it is easier to understand and debug. This is the
beauty of M-code... A*x +B*u does exactly what you want.

Seth

> This is what you had:
> dx4 = (( (M+m)*g*(x3) - F_in ) / l)*M
>
> This is what you need:
> dx4 = ( (M+m)*g*(x3) - F_in ) / (l*M)
>
> I suggest you stick to the State Space representation
> because it is easier to understand and debug. This is the
> beauty of M-code... A*x +B*u does exactly what you want.
>
> Seth


Thanks so much, Seth: it works!
Thanks for dedicating a few minutes to my problem.

Your name will appear in the "acknowledgements" of my project.
:-D