Copyright © 2010 José Carlos Moreno et al. This is an open access article distributed under the
Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The frequency response is an important tool for practical and efficient design of control systems.
Control techniques based on frequency response are of special interest to dealing with important
subjects such as the bandwidth and the cost of feedback. Furthermore, these techniques are easily
adapted to deal with the uncertainty of the process to control. Quantitative feedback theory (QFT)
is an engineering design technique of uncertain feedback systems that uses frequency domain specifications. This paper analyzes the phase specifications problem in frequency domain using QFT.
This type of specification is not commonly taken into account due to the fundamental limitations
of the linear control given by Bode's integral. An algorithm is proposed aimed at achieving
prespecified closed-loop transfer function phase and magnitude variations, taking into account the
plant uncertainty. A two-degrees-of-freedom feedback control structure is used and a new type of
boundary is defined to satisfy these objectives. As the control effort heavily depends on a good
estimation of these boundaries, the proposed algorithm allows avoiding overdesign.