K=1;
tau=1;
zeta=1;
sis1=tf(K,[tau*tau 2*0.3*zeta*tau 1]);
sis2=tf(K,[tau*tau 2*zeta*tau 1]);
sis3=tf(K,[tau*tau 2*1.5*zeta*tau 1]);
step(sis1,sis2,sis3);
legend('0.3 \zeta','\zeta','1.5 \zeta');
