This paper presents an approach to PID controller tuning based on a simple plant model description; First Order plus Time Delay (FOPTD). The approach is based on the formulation of an optimal approximation problem in the frequency domain for the Sensitivity transfer function of the closed loop. The inclusion of the Sensitivity function allows for a disturbance attenuation specification. The solution to the approximation problem provides a set of tuning rules that constitute a parameterized set that is formulated in the same terms as in [1] and include a third parameter that determines the operating mode of the controller. This factor allows to determine a tuning either for step response or disturbance attenuation. The approach can be seen as an implicit two-degree-of-freedom controller because by using one single parameter the operating mode (servo/regulation) of the control system is determined as well as the appropriate tuning of the controller.
