I have an error: Error in 'nonlinear​_LQR/S-Fun​ction' while executing MATLAB S-function 'fcn_sfcn', flag = 1 (derivatives), at time 0.0." Can someone help me? This is the code:

function [sys,x0,str,ts]=...
                 fnc_sfcn(t,x,u,flag,xinit)

switch flag

case 0

str=[]

ts=[0 0]

s=simsizes

s.NumContStates=6;

s.NumDiscStates=0;

s.NumOutputs=6;

s.NumInputs=3;

s.DirFeedthrough=0;

s.NumSampleTimes=1;

sys=simsizes(s);

x0=[xinit];

case 1

tau=u;

sys=fnc(t,x,tau)

case 3

sys=x;

case {2 4 9}

sys=[];

otherwise

error(['unhandked flag=',num2str(flag)]);

end

Function File:

function xdot= fnc(t,x,tau)

l1=1;l2=1;l3=1;m1=1;m2=1;m3=1;g=10;

xdot = zeros(6,1);

xdot(1)=x(4);

xdot(2)=x(5);

xdot(3)=x(6);

xdot(4)= 1/(l1^2*(m1+m2+m3))*[-( l1*l2*(m2+m3))*xdot(5) - (m3*l1*l3)*xdot(6) + (m1+m2+m3)*g*l1*sin(x(1)) + tau(1) - tau(2)];

xdot(5) = 1/(l2^2*(m2+m3))* [-(l1*l2*(m2+m3))*xdot(4) -( m2*l2*l3)*xdot(6) + (m2+m3)*g*l2*sin(x(2)) + tau(2) - tau(3)];

xdot(6)= 1/(m3*l3^2)*[-(m3*l1*l3)*xdot(4)- (m3*l3*l2)*xdot(5)+(m3*g*l3)*sin(x(3))+tau(3)];

end

[Walter Roberson]

You do not construct x0 or str or ts for any case other than 0. That might be a problem.

Have you considered invoking the debugger?