G=tf(1,[1,3,3,1]); P=[0.1:0.1:1];
for i=1:length(P)
   G_c=feedback(P(i)*G,1); 
   step(G_c), hold on
end
figure; rlocus(G), axis('square');
k=rlocfind(G)

Kp=1; Ti=[0.7:0.1:1.5];
for i=1:length(Ti)
   Gc=tf(Kp*[1,1/Ti(i)],[1,0]); G_c=feedback(G*Gc,1);
   step(G_c), hold on
end
axis([0,20,0,2])

Kp=1; Ti=1; Td=[0.1:0.2:2]; 
for i=1:length(Td)
   Gc=tf(Kp*[Ti*Td(i),Ti,1]/Ti, [1,0]);
   G_c=feedback(G*Gc,1); step(G_c), hold on
end
axis([0,20,0,1.6])