I am using the method of Ziegler-Nichols in the the following form: Kp = 3*Ku/5; Ki = 6*Ku/(5***Pu); Kd = 3***Ku*Pu/40, but it seems it does not work well for object with the following transfer function: W(z)=(b1*z + b2)/(z^2+a1*z+a2), ts is 2s.

Can someone defie me accurately the equations for Kp, Ki and Kd depending on the transfer function of the objects? Any web links will be very appreciate.

Thanks in advance, zlatko