tk = 1;
zs = 1;
zeta = [1.3 1.0 0.5 0.2];
for i=1:4
  sys{i} = tf(zs, [tk^2 2*zeta(i)*tk 1]);
  [y{i}, t{i}] = step(sys{i}, 15);
end
plot(t{1}, y{1},'k-', t{2}, y{2},'k--', t{3}, y{3},'k-.', t{4}, y{4},'k:')
xlabel('t');
ylabel('y / Z_s');
legend('\zeta = 1.3','\zeta = 1.0','\zeta = 0.5','\zeta = 0.2')

set(1, 'PaperUnits', 'centimeters')
set(1, 'PaperPosition', [3 10 11 9])
print -deps 'obr414.eps'

pause(1)

den = 1;
for i=1:4
  den = conv(den, [tk 1]);
  sys{i} = tf(zs, den);
  [y{i}, t{i}] = step(sys{i}, 15);
end
plot(t{1}, y{1},'k-', t{2}, y{2},'k--', t{3}, y{3},'k-.', t{4}, y{4},'k:')
xlabel('t / T');
ylabel('y / Z_s');
legend('n = 1','n = 2','n = 3','n = 4')

set(1, 'PaperUnits', 'centimeters')
set(1, 'PaperPosition', [3 10 11 9])
print -deps 'obr415.eps'

