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function [u]=chap7f1(u1,u2,u3) 
global s 
persistent errori error1 
 
ts=0.001; 
if u3==0.0 
    errori=0.0; 
    error1=0.0; 
end 
 
s=1 
 if s==1 
       kp=0.4; 
       ki=0.0; 
       kd=0.28; 
   elseif s==2 
       kp=40; 
       ki=0.0; 
       kd=0.28; 
   elseif s==3 
       kp=0.0; 
       ki=0.0; 
       kd=0.28; 
   end 
  error=u2; 
  errord=(error-error1)/ts; 
 errori= errori+error*ts; 
  
 u=kp*error+kd*errord+ki*errori; 
 error1=error;