function [sys,x0,str,ts]=exp_pidf(t,x,u,flag)
switch flag,
case 0           % initializations
    [sys,x0,str,ts] = mdlInitializeSizes;
case 2           % discrete states updates
    sys = mdlUpdates(x,u);
case 3           % computation of control signal
    sys=mdlOutputs(t,x,u);
case {1, 4, 9}   % unused flag values
    sys = [];
otherwise        % error handling
    error(['Unhandled flag = ',num2str(flag)]);
end;

%==============================================================
% when flag=0, perform system initialization
%==============================================================
function [sys,x0,str,ts] = mdlInitializeSizes
sizes = simsizes;        % read default control variables
sizes.NumContStates = 0; % no continuous states
sizes.NumDiscStates = 3; % 3 states and assume they are the P/I/D components
sizes.NumOutputs = 1;    % 2 output variables: control u(t) and state x(3)
sizes.NumInputs = 2;     % 4 input signals
sizes.DirFeedthrough = 1;% input reflected directly in output
sizes.NumSampleTimes = 1;% single sampling period
sys = simsizes(sizes);   % 
x0 = [0; 0; 0];          % zero initial states
str = []; 
ts = [1 0];             % sampling period
%==============================================================
% when flag=2, updates the discrete states
%==============================================================
function sys = mdlUpdates(x,u)
T=1;
sys=[ u(1); 
      x(2)+u(1)*T;
      (u(1)-u(2))/T];

%==============================================================
% when flag=3, computates the output signals
%==============================================================
function sys = mdlOutputs(t,x,u)

kp=0.02;
ki=0.05;
kd=0.0001;

%sys=[kp,ki,kd]*x;
sys=kp*x(1)+ki*x(2)+kd*x(3);