G=tf(10,[1,10,35,50,24]); Am=[2:5];
f1=figure;  f2=figure; ngrid('new')
for i=1:length(Am)
   [Gc,Kp1,Ti1,Td1]=gphapid(3,[1,Am(i),5,4,10],G);
   G_o=G*Gc; G_c=feedback(G_o,1);
   figure(f1); step(G_c), hold on
   figure(f2); nichols(G_o)
end
figure(f2); axis([-360,0,-40,40])