% ex4_4
% 二階環節的奈魁斯特圖
figure('pos',[30 100 260 260],'color','w'); 
axes('pos',[0.15 0.2 0.7 0.7]);
wn=1; 
w=[0,logspace(-2,5,200)];
for zeta=[0.5 1 5]
    G=tf(1,[wn^-2  2*zeta/wn  1]);
    nyquist(G,w);
    hold on;
end;
axis([-0.5 1 -1.1 1.1]);
set(gca,'xcolor','k','ycolor','k');