Robust plant by plant control design using model-error tracking sets

This paper presents a method for designing a robust two-degree-of-freedom control scheme, capable of satisfying multiple model-error specifications on a plant by plant basis. Traditional quantitative feedback theory methods generally use a single model-error or above-below magnitude tracking specification, which can result in overdesign for plants located away from the bounding conditions. The performance specifications are also generally hand-tuned, or iteratively adjusted to keep the underlying time-domain signals within permissible levels. Our method aims to perform a model-error design on a per-plant basis, such that each plant's corresponding model tracking has equal weighting given the plant's inherent feedback requirements and capability. The quantitative feedback theory method allows this per-plant approach to be undertaken with ease. Additionally, sufficiently low-order model specifications are designed using simple optimisation, which take into account performance limiting effects, such as non-minimum phase behaviour and signal constraints. A worked example is presented, showing the viability and transparency of the proposed method.