Loop transfer recovery design using biased and unbiased controllers

It is argued that an unbiasedness constraint is not a requirement for effective loop transfer recovery (LTR). Controller design procedures are formulated which do not have this restriction and which permit the substitution of other relationships which can assist in the synthesis of the controller. These procedures offer the designer additional freedom in the synthesis of feedback controllers. In particular, they allow the synthesis of either unbiased or biased controllers in a framework that is more general than that offered by standard linear quadratic Gaussian (LQG)/LTR. Application to a simple nonminimum phase example shows that the additional flexibility offered by the methods discussed is useful for shaping sensitivity functions. While reflecting the fundamental limitations inherent in all feedback systems, the results obtained compare more than favorably with LQG/LTR designs