Objective function is undefined at initial point. Fmincon cannot continue.

When executing the code below (in an 's-function'), I get the error message: "Objective function is undefined at initial point. Fmincon cannot continue."

Wearing, when I test the code contained in the "mdloutputs" function, which in this case contains the objective function, it works!!

I'd really like to know what's wrong and get some help.

Thank you !!

function [sys,x0,str,ts] = potential_field(t,x,u,flag)

switch flag,

   case 0,
    [sys,x0,str,ts]=mdlInitializeSizes;

  case 1,
    sys=mdlDerivatives(t,x,u);

    case 3,
    sys=mdlOutputs(t,x,u);

   case { 2, 4, 9 },
    sys = [];

  otherwise
    DAStudio.error('Simulink:blocks:unhandledFlag', num2str(flag));

end

function [sys,x0,str,ts]=mdlInitializeSizes(~,~,~,~)

sizes = simsizes;
sizes.NumContStates  = 0;
sizes.NumDiscStates  = 0;
sizes.NumOutputs     = 5;
sizes.NumInputs      = 6;
sizes.DirFeedthrough = 0;
sizes.NumSampleTimes = 1;

sys = simsizes(sizes);
x0  = [];
str = [];
ts  = [0 0];

function sys=mdlOutputs(~,~,u,~,~,~,~)

a0=0;
av=100;
c=0;           
b1=0;
b2=0;

yub=u(4).*sin(u(4))+10;  %upper boundary
ylb=u(4).*sin(u(4))-10;   %lower boundary
y=.5*(yub+ylb);      %center line

  Urep_1=zeros(1,1);
  Urep_2=zeros(1,1);
  
  if (ylb<=u(5))&&(yub>=u(5))
      
      Urep_1=b1/(u(5)-yub)^2;
      Urep_2=b2/(u(5)-ylb)^2;
  end
  
J=@(X)(a0*(X(1)-u(1))^2 + av*(u(1)*tan(X(2))-u(6))^2 + Urep_1 + Urep_2 + c*(X(2)-u(3))^2);

X0=[1,15];          %Initial point for X
A = [];
b = [];
Aeq=[];            %Matrix for linear equality constraints	 
beq=[];            %Vector for linear equality constraints	 
lb=[0,-pi/2];	   %Vector of lower bounds	 
ub=[90,pi/2];      %Vector of upper bounds

Q=fmincon(J,X0,A,b,Aeq,beq,lb,ub);
  
sys(1)= Q(1);
sys(2)= Q(2);
sys(3)= yub;
sys(4)= ylb;
sys(5)= y;

Answer 1

[Raghavendra Ragipani]

Hi,

While I haven't got a chance to run your code, I have couple of suggestions for you based on the error message.

Please try to correct the bounds for X(2) as tangent function is infinite at bounds. Since, you have initialized it to 15, which is out of specified bounds, matlab might have initialised it to one of the specified bounds and hence the error.

try -pi/2+eps and pi/2-eps as bounds.

and try to initialize within the bounds.

cheers,

raghav

[Valéry Ebogo]

Hi,

First, I would like to thank you for your intervention.

I have tried your suggestions and I always receive the same error message!

[Raghavendra Ragipani]

Another suggestion - please check if variables a0, av etc. defined in J are available in the workspace before J is executed. Variables assigned after defining the anonymous function (J) will not be considered.

[Valéry Ebogo]

Hi,

The problem has been solved using a level-2 matlab s-function. Apparently, the Fmincon function is limited when used in a level-1 matlab s-function because it cannot take into account all types of incoming signals.

Thanks for your help!!!